EMULSION PHASE MATCHING
(EPM) TECHNIQUE FOR
PREDICTING OPTIMAL
EMULSIFERS
By
Teresa J. Harding
A thesis submitted in fulfilment of the requirements for
the award of Doctor of Philosophy
2006
Centre for Applied Colloid and Biocolloid Science
School of Chemical Engineering and Science
Swinburne University of Technology
Melbourne, Australia
ACKNOWLEDGEMENTS
Perplexed by the trial and error approach that emulsifier selection usually
required before a stable emulsion system could be formed, I had an idea for
an alternative approach to emulsifier selection which I wanted to investigate.
A special thank-you to Dr. Ian Harding for supporting the idea and giving
me the opportunity to undertake this work.
I would like to thank all three supervisors, Dr. Ian Harding, Dr. Russell
Crawford and Dr. Ian Bowater for their time, guidance and contributions
throughout the study.
Thank-you to my employer Cognis Australia, who offered me the flexibility
in my role to carry out this work.
Finally, to my husband, Ron, whose unyielding support, makes absolutely
anything possible. Thank-you for being there as always!
i
PREFACE
I hearby declare that, to the best of my knowledge, this thesis contains no
material previously written or published by another person, except where
reference is made in the text. I also declare that none of this work has been
previously submitted for a degree or similar award at another institution.
Teresa Harding
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS
i
PREFACE
ii
TABLE OF CONTENTS
iii
LIST OF TABLES
viii
LIST OF FIGURES
xi
ABBREVIATIONS AND SYMBOLS
xiii
CHEMICAL FORMULAS OF MATERIALS USED
xv
ABSTRACT
xvii
CHAPTER 1
LITERATURE REVIEW AND INTRODUCTION
1.1
SURFACTANT OVERVIEW
1
1.1.1 History and Development
3
1.1.2 Classification of Surfactant Types
6
1.2
7
EMULSIONS
1.2.1 Formation
8
1.2.2 Nonionic Surfactants in Emulsions
10
1.2.3 Stability
11
1.2.4 Considerations for Cosmetic Emulsions
13
1.3
TECHNIQUES TO PREDICT AND MEASURE EMULSION STABILITY 16
1.3.1 Particle Size Determination
18
1.3.2 Visual Assessment
21
iii
1.4
THEORIES TO AID SURFACTANT SELECTION
24
1.4.1 Hydrophile-Lipophile Balance (HLB)
24
1.4.1.1 Limitations of the HLB System
28
1.4.2 Solubility Parameter
29
1.4.2.1 Component Solubility Parameters
30
1.4.3 Phase Inversion Temperature
32
1.4.4 New Method Introduction – Emulsion Phase Matching (EPM)
35
1.4.5 Summary and Comparison of Theories
36
1.5
38
TECHNIQUES TO CLASSIFY EMULSIFIER MOIETIES
1.5.1 Surface Tension
38
1.5.2 Interfacial Tension
40
1.5.3 Application of the Solubility Parameter
41
CHAPTER 2
2.1
EPM TECHNIQUE FOR EMULSIFIER SELECTION
DEVELOPMENT OF EPM THEORY
42
2.1.1 Hydrophobic and Hydrophilic Moieties of Emulsifiers
44
2.1.1.1 Example of the Splitting Process as applied to Laureth-4
44
2.1.2 EPM Database
46
2.1.3
EPM Theory
49
2.1.4
Benefits of EPM Technique
52
CHAPTER 3
3.1
MATERIALS AND METHODS
MATERIALS
54
3.1.1 Emulsifiers Selected for EPM Technique Evaluation
54
3.1.1.1 Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
55
3.1.2 Oils Selected for EPM Technique
55
iv
3.1.3 Auxiliary Materials Selected for EPM Technique
57
3.2
59
METHODS
3.2.1 The First Ten Angstroms (FTÅ) 200 Instrument
59
3.2.1.1
Instrument Standardisation and Calibration
61
3.2.1.2
Interfacial Tension Measurement
64
3.2.2 Emulsion Preparation
65
3.2.2.1
Oil Concentration
66
3.2.2.2
Order of Mixing
66
3.2.2.3
Degree of Mixing
67
3.2.2.4
Emulsifier Concentration (Theoretical Determination)
68
3.2.2.5
Temperature of Emulsification
71
3.2.2.6 Summary of Emulsion Preparation Parameters
73
3.2.3 Emulsion Evaluation
73
3.2.3.1
Visual Determination
74
3.2.3.2
Malvern Mastersizer
74
3.2.3.2.1 Instrument Validation
75
3.2.3.2.2 Commercial Emulsion Evaluation
79
CHAPTER 4
4.1
MATERIALS CHARACTERISATION
CHARACTERISATION OF EMULSIFIERS
80
4.1.1 Interfacial Tension Data Determination for EPM Technique
80
4.1.1.1 Measured Interfacial Tension Data for Selected Emulsifier Moieties
82
4.1.1.2 FTÅ 200 Droplet Contrast
84
4.1.1.3 Data Extrapolation
85
4.1.2 Classical HLB and Solubility Parameter Classification
92
4.1.3 Alternative Solubility Parameter Classification using EPM Concept 93
4.2
CHARACTERISATION OF OILS
95
4.2.1 Interfacial Tension Data
95
4.2.2 HLB and Solubility Parameter Classification of Oils
97
v
CHAPTER 5
5.1
EMULSION PREPARATION
99
EMULSIFIER CONCENTRATION EFFECTS ON EMULSION
FORMATION AND STABILITY
100
5.1.1 Effect of Emulsifier Concentration on Interfacial Tension
100
5.1.2 Effect on Emulsifier Concentration on Particle Size
102
5.1.3 Effect on Emulsion Stability
104
5.2
DEGREE AND TIME OF MIXING
106
5.2.1 Effect on Particle Size and Stability
106
CHAPTER 6
6.1
CHARACTERISATION OF EMULSIONS
DESIGN AND STABILITY OF TEST EMULSION SYSTEMS
108
6.1.1 ‘Ideal’ Emulsion System Design
108
6.1.2 EPM vs HLB Comparison: Test Emulsion 4
110
6.1.3 EPM and EPM vs HLB: Test Emulsion 5
111
6.1.4 Stability of Commercially Representative Emulsion Systems
113
6.1.5 Stability Comparison of HLB and EPM Derived Emulsion Systems 114
6.1.6 Confirmation of Optimal Oil Blends for EPM Technique
CHAPTER 7
7.1
7.2
117
PREDICTING OPTIMAL EMULSIFIERS
USING THE EPM TECHNIQUE FOR EMULSIFIER
PREDICTION
120
DESIGN AND DEVELOPMENT OF ‘MODEL’ EMULSION
121
7.2.1 Product Requirements and Design
121
7.2.2 Application of the EPM Technique
122
vi
7.2.3 Emulsion Stability
124
7.2.4 Model Emulsion System Using Alternative EPM Emulsifier
126
7.2.5 Second Model Emulsion System Using Alternative EPM Emulsifier 127
CHAPTER 8
CONCLUSIONS AND RECOMMENDATIONS
FOR ADVANCEMENT OF EPM TECHNIQUE
8.1
OUTLINE SUMMARY OF EXPERIMENTATION
131
8.2
POSITIVE ATTRIBUTES OF THE EPM TECHNIQUE
133
8.2.1 Discussion of Positive Attributes
133
8.3
135
NEGATIVE ATTRIBUTES OF THE EPM TECHNIQUE
8.3.1 Discussion of Negative Attributes
135
8.4
FUTURE REQUIREMENTS TO VALIDATE EPM THEORY
136
8.5
FUTURE REQUIREMENTS OF COSMETIC EMULSIONS
137
8.6
RECOMMENDATIONS FOR FURTHER WORK
139
REFERENCES
140
APPENDICES
Appendix 1
Examples of Ethoxylated Surfactant Types (ICI)
152
Appendix 2
Mastersizer Standard Analysis Results
155
vii
LIST OF TABLES
Chapter 1
Table 1.1
Chemical Classification of Surfactants
Table 1.2
Typical Cosmetic Emulsion Ingredients
Table 1.3
Group HLB Numbers
Table 1.4
HLB Values Related to Surfactant Application
Chapter 3
Table 3.1
Emulsifiers Selected for EPM Technique Evaluation
Table 3.2
Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
Table 3.3
Selected Oils - Simple Esters
Table 3.4
Selected Oils - from other Chemical Groups
Table 3.5
Auxiliary Ingredients Selected for EPM Technique Evaluation
Table 3.6
FTÅ 200 – Instrument Repeatability
Table 3.7
Calibration Results for Diethylene Glycol and Hexane
Table 3.8
Surface Tension Validation
Table 3.9
Cloud Points of Selected Emulsifiers
Table 3.10
Emulsion Preparation Parameters
Table 3.11
Droplet Size Evaluation of Commercial Emulsion Products
Chapter 4
Table 4.1
Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
Table 4.2
Interfacial Tension Data (mN m-1 @ 20°C) Between Ranges of
Alkanes and Glycol Materials (Selected Emulsifier Moieties)
Table 4.3
Interfacial Tension Results (mN m-1 @ 20°C) with Dye Added
Table 4.4
Interfacial Tension Data (mN m-1 @ 20°C) Between Ranges of
Alkanes and Glycol Materials (Selected Emulsifier Moieties)
Summary Data with Extrapolated Data Included
viii
Table 4.5
EPM Values (Expressed and Measured as Interfacial Tension) for
Selected Emulsifiers
Table 4.6
HLB and Solubility Parameter Values for Selected Emulsifiers
Table 4.7
Solubility Parameter Values as Applied to the Hydrophobic and
Hydrophilic Moieties of the Selected Emulsifiers and to EPM
Technique
Table 4.8
Interfacial Tension Results for Selected Oils against Purified Water
Table 4.9
Summary of Interfacial Tension, Required HLB and Solubility
Parameter Values for the Selected Oils
Chapter 5
Table 5.1
Effect of Emulsifier Concentration on Droplet Size
Table 5.2
Effect of Emulsifier Concentration on Emulsion Stability
Table 5.3
Summary of Optimal Emulsifier Concentration Determination
Table 5.4
Shear Time vs Emulsifier Concentration to Achieve 2.5 µm Droplet
Size
Chapter 6
Table 6.1
Proposed ‘Ideal’ Test Emulsion Systems
Table 6.2
EPM / HLB Comparison: Test Emulsion 4
Table 6.3
Results and Stability of Test Emulsion 4
Table 6.4
EPM Test Series 5
Table 6.5
Test Series 5 Emulsion Systems Matched with HLB Values
Table 6.6
Results and Stability of Test Emulsion 5
Table 6.7
HLB and EPM Emulsion Comparison
Table 6.8
Ceteareth Emulsifier Systems Emulsified at 75ºC
Table 6.9
EPM Emulsion Screening 1
Table 6.10
EPM Emulsion Screening 2
ix
Chapter 7
Table 7.1
Model Formulation Ingredients
Table 7.2
Model Emulsion Stability Results at 20ºC and 40ºC
Table 7.3
Model Emulsion (Using Ceteareth-12) Stability Results at 20ºC and 40ºC
Table 7.4
Second Model Formulation Ingredients
Table 7.5
Second Model Emulsion Stability Results at 20ºC and 40ºC
x
LIST OF FIGURES
Chapter 1
Figure 1.1
Simplified Diagram of a Surfactant Acting as a Detergent
Figure 1.2
Schematic Diagrams of O/W and W/O Emulsion Types
Figure 1.3
Laser Particle Size: System Overview
Figure 1.4
Schematic Diagram of the EPM Technique
Figure 1.5
Surface Tension Derived from Hanging Pendant Drop
Chapter 2
Figure 2.1
Example of Emulsifier ‘Splitting’ in EPM Technique
Chapter 3
Figure 3.1
Photograph of the FTÅ Instrument Set-up
Figure 3.2
Malvern Mastersizer – Histogram and Graphical Standard Results
Figure 3.3
Histogram and Graphical Results for Mixed 5 μm and 14 μm Standards
Chapter 4
Figure 4.1
Diagrammatic Outline of the EPM Technique
Figure 4.2
Example of Emulsifier ‘Splitting’ in EPM Technique
Figure 4.3
Effect of Alkane Chain Length on Interfacial Tension
Figure 4.4
Interfacial Tension Values for Selected Emulsifier Moieties
Figure 4.5
Interfacial Tension Data for Selected Emulsifier Hydrophiles with
MW > 150 g mol-1
xi
Chapter 5
Figure 5.1
Effect of Laureth-4 Concentration on Interfacial Tension
Water + Laureth-4: Capric / Caprylic Triglyceride System
Chapter 7
Figure 7.1
Marketing Brief for a New Product Development
Chapter 8
Figure 8.1
Example of a Polymer Displaying ‘Self-Emulsifying’ Properties
xii
ABBREVIATIONS AND SYMBOLS
Abbreviations
DPD
Dissipative Particle Dynamics
EIP
Emulsion Inversion Point
EO
Ethylene Oxide
EPM
Emulsion Phase Matching
FTÅ
First Ten Ångstroms
HLB
Hydrophile-Lipophile Balance
HLBE
Extended HLB Scale
INCI
International Nomenclature Cosmetic Ingredients
O/W
Oil-in-water
PEG
Polyethylene Glycol
PIT
Phase Inversion Temperature
POE
Polyoxyethylenated
TGA
Therapeutic Goods Administration
W/O
Water-in-oil
Symbols
ΔA
Change in Interfacial Area
ΔHv
Heat of Vaporisation
δ
Solubility Parameter
δa
Solubility Parameter due to Lewis Acid Interactions
δb
Solubility Parameter due to Lewis Base Interactions
δd
Solubility Parameter due to Dispersion Forces
δh
Solubility Parameter due to Hydrogen Bonding
δo
Solubility Parameter due to Orientation Effects
δp
Solubility Parameter due to Polarity Effects
xiii
δt
Total Solubility Parameter
η
Viscosity
ρ
Density
γ
Surface Tension/Interfacial Tension
a
Radius of Droplet
c
Cohesion Parameter
D
Statistically determined Diameter value
di
Diameter of particle size class i
g
Local acceleration due to gravity
H
Drop Height
H
Hydrophile
L
Lipophile
p
Capillary Pressure
r
Radius
R
Alkyl Group
RO
Cohesive Energy Ratio
SA
Surface Area
SE
Equatorial Diameter
SW
Drop Width at Height H
V
Rate of Sedimentation (Velocity)
Vi
Relative Volume for particle size class i
[V]
Molar Volume
W
Work
xiv
CHEMICAL FORMULAS OF MATERIALS USED
INCI Name
Chemical Formula
Laureth-2
CH3(CH2)10CH2(OCH2CH2)2OH
Laureth-3
CH3(CH2)10CH2(OCH2CH2)3OH
Laureth-4
CH3(CH2)10CH2(OCH2CH2)4OH
Ceteareth-12
50% CH3(CH2)14CH2(OCH2CH2)12OH
50% CH3(CH2)16CH2(OCH2CH2)12OH
Ceteareth-20
Ceteareth-30
50% CH3(CH2)14CH2(OCH2CH2)20OH
50% CH3(CH2)16CH2(OCH2CH2)20OH
50% CH3(CH2)14CH2(OCH2CH2)30OH
50% CH3(CH2)16CH2(OCH2CH2)30OH
Dodecane
CH3(CH2)10CH3
Hexadecane
CH3(CH2)14CH3
Octadecane
CH3(CH2)16CH3
Diethylene Glycol
H(OCH2CH2)2OH
Triethylene Glycol
H(OCH2CH2)3OH
PEG 200
H(OCH2CH2)4OH
PEG 300
H(OCH2CH2)6OH
PEG 400
H(OCH2CH2)8OH
PEG 600
H(OCH2CH2)12OH
PEG 1000
H(OCH2CH2)20OH
PEG 1500
H(OCH2CH2)30OH
xv
INCI Name
Chemical Formula
Ethyl Hexyl Palmitate
CH3(CH2)14COO CH2CH(CH2CH3)(CH2)3CH3
Hexyl Laurate
CH3(CH2)10 COO(CH2)5CH3
Decyl Oleate
CH3(CH2)7CHCH(CH2)7 COO CH2(CH2)8CH3
Ethyl Hexyl Stearate
CH3(CH2)16COO CH2CH(CH2CH3)(CH2)3CH3
Iso-Propyl Myristate
CH3(CH2)12COO CH(CH3)2
Iso-Propyl Palmitate
CH3(CH2)14COO CH(CH3)2
Dibutyl Adipate
CH3(CH2)3OOC(CH2)4COO(CH2)3CH3
Capric/Caprylic Triglyceride
CH2(OCOR)CH(OCOR)CH2(OCOR)
R = C7H15 and C9H19 (present in all possible
combinations)
Cocoglycerides
General formula as capric / caprylic triglyceride with also
mono- and di- glycerides from- C8 – C16
Octyl dodecanol
CH3(CH2)9CH(CH2(CH2)6CH3)CH2OH
Dioctyl cyclohexane
1,3 di-C8H17 C6H4
Mineral Oil
CH3(CH2)XCH3 X = 16-18
Dicapryl Ether
CH3(CH2)6CH2OCH2(CH2)6CH3
xvi
ABSTRACT
The formation of a ‘stable’ emulsion is more of an art than a science. Emulsions are
inherently unstable heterogenous systems such that a truly stable emulsion is not feasible.
However, commercial emulsions are very common indicating that it is possible to achieve a
‘commercially stable’ emulsion. A commercially stable emulsion can be defined as one
that remains homogenous throughout its commercial life. Emulsions provide an
aesthetically pleasing and practical method of forming a homogenous mixture of oil and
water. This type of product is very common with examples including milk, mayonnaise,
cosmetic creams and paints.
Despite the very many emulsion products available and the considerable time and effort
spent on research, there is still no definitive technique available that can pinpoint an
emulsifier or emulsifier combination that will form a guaranteed ‘commercially stable’
emulsion. Trial and error is still required in emulsion formulation.
The techniques that are available to help reduce the number of emulsifier options that
should be considered have been outlined in this thesis. One, the Hydrophile-Lipophile
Balance (HLB) is well known to the emulsification industry and very popular. The benefits
and the limitations of this technique are reviewed along with the modifications and
extensions of the technique that have been proposed throughout its 50 year life.
It is important to note that the HLB technique is over 50 years old. The commercial
demands of emulsion products have developed considerably in this time and emulsion
systems tend to be somewhat more complex with more ingredients required than when this
technique was first formulated.
A fresh approach to emulsifier selection is desirable and this work proposes a new concept
called the Emulsion Phase Matching or EPM technique. The name arises from the fact that,
in this technique, each phase of the emulsion is considered independently of the other,
although with all auxiliary ingredients included. A quantitative measure of the ‘difference’
between the two phases to be emulsified (in this case interfacial tension) is ‘matched’ to the
xvii
difference between the hydrophobic and hydrophilic moieties of the emulsifier to be used.
A good match should correlate to a ‘stable’ emulsion. The data should ultimately be listed
in a database to make this task simpler. However, the scope of the current work was to test
the concept, in general, and the use of interfacial tension, in specific, as the measure of
difference.
Interfacial tension was, indeed, found to be linked to emulsion stability. Test emulsions
were designed to test the proposed EPM technique and to compare the results with those
emulsions that would be achieved if the HLB technique was used. Results for both ‘ideal’
and ‘commercial’ emulsions (refer Chapter 6) did show superior results for the EPM
proposed emulsion, both in terms of initial droplet size of the emulsion as well as its longer
term stability.
Although the work completed in this thesis only touches the surface of a full validation of
the proposed method, it does show encouraging signs and raises some interesting questions
regarding emulsion formation, resulting stability and the mechanisms involved with each
process. This is an area where further work is justified in the quest to remove the trial and
error approach to emulsifier selection.
xviii
Chapter 1
CHAPTER 1
LITERATURE REVIEW AND INTRODUCTION
1.1
SURFACTANT OVERVIEW
Materials that we now know by the relatively recent term ‘surfactant’ have actually been
in use for thousands of years. For example, the use of the alkali metal soaps as articles of
trade by the Phoenicians has been documented as early as 600 BC (Markoe, 2000).
A
soap like material was found in clay pots during excavation of ancient Babylon giving
evidence that soap making was known even as early as 2800 BC (Unilever, 2002). Alkali
metal soaps function as cleaning and foaming agents and are still used today. Essential to
their efficiency as detergents is their ability to act as surfactants. This is only one of
many possible surfactant applications, but the most commonly known.
Surfactants obtain their special properties due to the molecular structure of their
component molecules. This is composed of two distinct parts, or moieties. Each of the
two parts possess opposing properties: one part is water loving (hydrophilic) and the
other is water hating (hydrophobic), thus overall the surfactant is ampiphilic.
Schematically, surfactants are typically displayed as being tadpole like with a small
hydrophilic head group and a long non-polar hydrophobic tail:
Hydrophilic head group
Hydrophobic tail
Because of this ‘dual personality’ surfactants prefer to position themselves at surfaces or
interfaces, since it is only here that each part can be in contact with its preferred
environment.
At higher concentrations, they may additionally self-assemble into
aggregrates which form distinct aqueous and oil-like phases.
-1-
Chapter 1
Figure 1.1 demonstrates one way in which surfactants are able to function as detergents;
the long hydrophobic tails of the alkali metal soap are soluble in the non-polar grease or
dirt, and the polar, hydrophilic head group is soluble in water. The result is that the
surfactant lies at the interface as shown, lowering the interfacial tension between these
materials. This reduces the work of adhesion between the dirt and the substrate and
increases the ease with which the dirt particle lifts off by agitation, usually mechanical.
Figure 1.1
Simplified Diagram of a Surfactant Acting as a Detergent
This detergent application is so well known, both in the home and in industry, that almost
everybody has some appreciation for surfactants, although they may not know the
product by this name. Without surfactants, all cleaning processes would be significantly
more difficult, less efficient and more time consuming.
Detergency covers many
different aspects of cleaning from liquid laundry, high acid or alkaline hard surface
cleaners and also automated process cleaners which require different surfactants again to
optimize performance. Several texts are available that are dedicated to the various
aspects of detergency (Showell, 1998, Broze, 1999, Friedli, 2001). The comprehensive
detail of this area, although of considerable interest, is outside the scope of this work.
A less well known mechanism of detergency is through the formation of mesomorphic
phases, into which grease can be solubilised. Nonionic surfactants are generally required
and the mechanism involves the formation of a ‘liquid crystal’ type phase which, in turn,
requires a low temperature. Although less well known, this mechanism is considered
very important and is the basis behind ‘cold power’ laundry detergents (Kirk-Othmer,
1993).
-2-
Chapter 1
The term ‘surfactant’ is very much a general term for all products that possess surfaceactive properties and is, in fact, a contraction of the term ‘surface active agent’. Because
there are so many surfactant types they are often further sub-divided according to their
most suitable application(s).
Classifications include detergents, wetting agents, solubilisers and emulsifiers. It is the
subgroup known as emulsifiers that is the focus of this work.
Emulsifiers are surfactants that have found specific application in the formation and more
importantly, in the stabilisation of emulsions.
It is interesting to note that many
surfactants (including soap) can have application in many areas and can occur in more
than one of the above classification areas. Detergents interact at the surface to dislodge
the dirt or grease, but they must also be able to emulsify or solubilise that dirt ensuring
that it stays in solution and does not become redeposited. Soap can contribute to the
whole detergency process but is a relatively poor emulsifier compared to other materials.
Consequently, most detergent products (i.e., laundry, dishwashing etc) sold commercially
are made up of a mixture of several surfactants each offering different surface active
properties.
1.1.1 History and Development
Soap is the oldest known surfactant and has been in use for more than 4500 years
(Sellington, 1998). Soap is formed when natural fats and oils are heated with an alkali; a
process known as saponification. Although there is no documented evidence, soap is
believed to have been discovered as a result of fat from an altar, on which burnt sacrifices
were made, running into a nearby stream and causing foam formation. The alkali used
would have been potash (potassium carbonate) which is formed when wood and plants
are burned. The ashes from the firewood would have partly saponified the sacrificial
animal’s fat.
-3-
Chapter 1
By the nineteenth century the making of soap had become a major industry (Sellington,
1998). Soap was cheap, readily available, highly effective, and was the only widely
known cleaning agent.
The first non-soap material specifically used for its surface active properties was a
sulphonated castor oil, which was first introduced in the late nineteenth century as a
dyeing aid, and is still used today in the textile and leather industries (Schick, 1980).
In Germany during World War I, a shortage of available animal and vegetable fats forced
companies to investigate the use of synthetic materials. The broad range of possibilities
and performance benefits for synthetic surfactants was quickly realised and this initiated
rapid and thorough development in the area of surfactant technology.
Initially this was driven around the development of synthetic anionic surfactants. A
major disadvantage of soap is evident when used in hard water. Water insoluble salts are
formed (calcium or magnesium stearate) which are responsible for a ‘ring’ around the
bathtub or surface being cleaned and this renders the product ineffective as a detergent.
This ‘ring’ is caused by the carboxylate end group of the soap which complexes with
metal ions, such as calcium found in hard water, forming a precipitate. Synthetic anionic
surfactants could be created to largely overcome this problem by using materials whose
calcium and magnesium salts possess a small degree of water solubility. One example is
sodium lauryl sulphate (SLS) which forms calcium sulphate in hard water. The water
solubility of calcium sulphate is small, at only 0.2% at 18°C (Merck, 2001), but this is
sufficient to almost eliminate the formation of the visible ‘ring’. This was the start of
major development in this area which has meant that today, anionic surfactants still make
up the largest proportion of surfactants produced (around 49% of all surfactants made
according to the Key Centre for Polymer Colloids (KCPC, (Anionic Surfactants), 2003)
and 60% according to Holmberg et al ( 2003)).
Anionic surfactants were not the only major class of synthetic detergent to be developed
in the early 20th Century. Schoeller & Wittwer (1930), in the mid to late 1930’s,
synthesised the entire range of ethoxylated surfactants and launched a new area of
-4-
Chapter 1
nonionic surfactant science. For a man who had created what is at least a multi-billion
dollar industry, he remains virtually unknown (Sellington, 1998)! These surfactants were
relatively simple to vary in composition making it easy to systematically study the effect
of different hydrophobic and hydrophilic compositions. The range and use of nonionic
surfactants grew from this point onwards making them now the largest class used in the
area of surface science (KCPC (Nonionic Surfactants), 2003).
Nonionic surfactants differ from anionic (and cationic) surfactants in that the molecules
are uncharged.
Traditionally, polyethylene oxide chains are used as the hydrophilic group. Polyethylene
oxide is a water soluble polymer and the polymers used in emulsion technology are
typically 3 – 30 units long. Alcohol ethoxylates and alkylphenol ethoxylates are the two
most common classes of nonionic surfactants.
Because of the importance of these
surfactant classes in emulsion technology, further detail on the scope of the emulsifiers
available and the properties they can offer will be covered in section 1.2.2.
More recently a newer class of nonionic surfactant, the alkyl polyglucosides (APGs), has
been introduced. For the last 20 years surfactants within this group have been dubbed
“new generation nonionic surfactants” (KCPC (Nonionic Surfactants), 2003). In these
molecules the hydrophilic group is a glucose derivative, generally with only one or two
glucose molecules in the chain. These surfactants can challenge the anionics in terms of
detergency and foamability and the high chain APGs (C12 to C18) do possess
emulsification properties. In terms of emulsion formation, however, APGs do not offer
the flexibility of the nonionic ethoxylates and so have a more limited application in this
area. APGs possess excellent mildness for personal care cleansing applications as well as
improving biodegradability for home and industrial applications (Hill et al, 1997). There
is good reason to believe that in the future these materials will take a considerable portion
of the anionic surfactant market share, particularly if dermatological and environmental
factors continue to grow in importance at the current rate.
-5-
Chapter 1
1.1.2 Classification of Surfactant Types
The hydrophobic part of a surfactant is generally a long-chain hydrocarbon moiety. The
hydrophilic part is a polar or ionic group that imparts some water solubility to the
molecule. The most useful chemical classification of surfactants, for aqueous systems, is
primarily based upon the nature of the hydrophile and is outlined in Table 1.1.
Although the major surfactant classification is determined by the hydrophile, the
hydrophobe can also vary considerably in composition. Most commonly, it is simply a
long chain alkyl group which can range from C8 to C20 with either a straight or branched
chain. This alone provides hundreds of potential surfactant possibilities. In addition,
many
other
hydrophobic
groups
are
available,
including
alkylbenzenes,
alkylnaphthalenes and polydimethylsiloxanes.
Table 1.1
Chemical Classification of Surfactants
Hydrophile
Classification
Anionic
Cationic
Properties / Uses
Negatively charged.
Largest used surfactant class; mainly for
e.g., sulphate (ROSO3-M+),
detergents and cleansing products.
carboxyl (COO-M+).
Minor application in emulsification.
Positively charged.
Bacteriocide and conditioning agent in
e.g., quarternary ammonium
textiles and personal care.
halides (R4N+X-).
Incompatible with anionic surfactants.
Rarely used in emulsions.
Amphoteric
The molecule contains, or can
Possess good synergies with anionic and
potentially contain, both a
cationic surfactants and are commonly
negative and a positive charge
used as co-surfactants in detergent and
e.g., sulphobetaines
cleansing preparations.
(RN
Nonionic
+
(CH3)2CH2CH2SO3-).
Rarely used in emulsion applications.
No charge.
Largest surfactant class used in
Water solubility derived from
emulsification.
polar groups such as
High compatibility with ionics.
polyoxyethylene (-OCH2CH2O-)n. Low sensitivity to electrolytes and pH.
-6-
Chapter 1
From the wide range of surfactants available, the choice of an optimal surfactant for a
given application can become a tedious and lengthy process.
It is the aim of this work to provide a summary of the currently available techniques to
aid surfactant selection and then to postulate a new, potentially more suitable, technique
for the selection of today’s emulsifiers.
1.2
EMULSIONS
The classic definition of an emulsion was originally advanced by Becher, (1965):
“a heterogeneous system, consisting of at least one immiscible liquid dispersed in another
in the form of droplets, whose diameters, in general, exceed 0.1 μm. Such systems
possess a minimal stability, which may be accentuated by such additives as surface active
agents, finely divided solids, etc.”
Emulsions generally consist of water, or an aqueous solution, as one immiscible phase
and an insoluble oil as the other immiscible phase. Two emulsion types dominate:
1.
Oil-in-water (O/W) emulsion, where the oil phase is dispersed in the aqueous phase.
2.
Water-in-oil (W/O) emulsion, where the aqueous phase is dispersed in the oil phase.
These emulsion types are shown diagrammatically in Figure 1.1 along with an illustration
of the terms dispersed and continuous phases which are also commonly used in emulsion
terminology (Poly, 2002).
There are many practical examples of emulsions in everyday life that use both natural and
synthetic surfactants. Milk and mayonnaise are naturally formed emulsions with lecithin
present as the natural surfactant. Paint and cosmetic creams or lotions are examples of
products made using essentially synthetic emulsifiers.
-7-
Chapter 1
O/W and W/O Emulsification
Hydrophilic
Micelle Formation
Continuous aqueous
phase: O/W emulsion
Continuous
Aqueous
Phase
Lipophilic Emulsifier
Inverse Micelle
Continuous oil phase:
W/O emulsion
Dispersed
Aqueous
Phase
Figure 1.2
1.2.1
Dispersed
Oily
Continuous
Oily
Phase
Schematic Diagrams of O/W and W/O Emulsion Types
Formation
If oil is tipped into water, the two phases will separate almost immediately. If the two
phases are shaken vigorously the mixture becomes more opaque in appearance and
droplets can be seen. On standing the two phases will quickly re-separate into two
distinct layers once again. If enough energy is used, however, smaller droplets will form,
slightly prolonging the separation process. The separation of the two phases even when
small droplets have been formed, show that the emulsion is not yet stable. This indicates
the importance of stabilisation, as well as formation mechanisms.
The work required, W, to generate one square centimetre of new interface is given by:
W = γ ΔA
where γ is the interfacial tension between the two liquid phases and ΔA is the change
(increase) in the interfacial area.
-8-
Chapter 1
The formation of the small droplets generates a large amount of surface area, which is
why a large amount of work is required.
For example, if 15 mL of mineral oil is emulsified in water giving an average droplet
diameter of 2.5 µm then the interfacial area would have increased to just under 10 m2 (see
3.2.2.4 for a detailed calculation to determine the number of droplets formed). The
interfacial tension between mineral oil and water is 44 mN m-1, and so the work required
to disperse this oil in water is of the order of 400 J. Since this amount of work remains in
the emulsion system as potential energy, the system will be thermodynamically unstable
and will rapidly undergo whatever transformations are possible to attain minimum
potential energy (e.g., minimise interfacial area). This is why the oil and water phases
quickly separate again following mixing.
If a material can be added that reduces the interfacial tension to, say, below 10 mN m-1,
then there is a substantial reduction in the work required to form the emulsion and
subsequent reduction in potential energy of the system. The result is that a significantly
more stable system can be achieved. This is the function of the surfactant.
Surfactants have lower energy at an interface than they do in either the oil or the water
phase and act to lower the overall energy of the emulsion. In the case detailed in the
calculation above, the remaining energy in the emulsion system would be reduced from
~400 J to <10 J if a surfactant was utilised which reduced the interfacial tension to < 1
mN m-1.
Surfactants aid the initial mixing of an emulsion system as discussed (by lowering the
work required) and also contribute to emulsion stability (by providing a kinetic barrier to
emulsion coalescence).
The emulsifying agent forms an adsorbed film around the
dispersed droplets which helps to prevent coagulation and coalescence.
The actual
stabilisation mechanism is usually very complex and varies from system to system.
Many texts are available that cover this area in the detail it warrants (e.g., Sjoeblom,
2001, Hunter, 2001, Becher, 2001, Morrison & Ross, 2002, Rosen, 2004) and, indeed, it
is an area still under development. The important point with regard to this work is that
-9-
Chapter 1
surfactants serve two purposes: to aid in the formation of the emulsion and to help
stabilise that emulsion.
1.2.2 Nonionic Surfactants in Emulsions
Nonionic emulsifiers have advantages over ionic emulsifiers, which make them the
emulsifier of choice in many applications.
Nonionic systems are substantially free of affects arising from the insolubility of specific
salts e.g., water hardness, and therefore more latitude is possible in formulations (Becher
& Schick, 1987). This is of great importance in agricultural formulations, where the
water hardness varies significantly with location (Becher, 1985). They are, however, not
free from all affect arising from the presence of electrolyte in the formulation, despite
popular belief to the contrary. The cloud points and micellar properties of nonionic
surfactants are affected by electrolyte. In general, high electrolyte concentration makes
nonionics less water soluble with increasing temperature (Meguro et al, 1997, Mackay,
1997) reducing their efficiency.
APG’s) are an exception.
The newer sugar based nonionic surfactants (i.e.,
Like anionic surfactants, their solubility increases with
temperature (Holmberg et al, 2003).
The most striking advantage of nonionic emulsifiers (ethoxylated) is that they allow the
formulator to systematically control the polarity of the emulsifier.
Ethoxylated surfactants can be tailor made with high precision with regard to the average
number of oxyethylene units added to a specific hydrophobe (e.g., a fatty alcohol). The
vast arrays of sequential nonionic surfactants have led to the systematic study of
emulsion stability and were used to develop the well known HLB approach (covered in
greater depth in Section 1.4.1). The development of these nonionic surfactants must, in
some part, be also responsible for the detailed knowledge now available in most areas of
surface science including solubilisation, emulsion technology and wettability.
- 10 -
Chapter 1
The chemical company ICI produced the most comprehensive range of ethoxylated
surfactants on a global basis and Appendix 1 displays details and properties of the vast
range of Teric nonionic surfactants (alcohol ethoxylates) that were available from ICI
before they were disbanded in Australia in the late 1990s. Similar ranges of surfactants
are now available from many of the surfactant suppliers (i.e., Huntsman, Cognis, Orica
etc) although the range available is now a little less extensive.
1.2.3 Stability
Emulsions undergo several destabilising mechanisms which are covered in detail in
numerous surface and colloidal chemistry texts including Hunter (2001) and Rosen
(2004). However, in brief, the major destabilising mechanisms include;
i)
coagulation, where droplets form close-packed aggregates which are difficult to
re-disperse,
ii)
flocculation, where droplets form loose-packed aggregates which are relatively
easy to re-disperse, and
iii)
coalescence, where droplets merge to form larger droplets of greater total volume
but lesser total surface area.
Random Brownian motion is sometimes all that is required for coagulation to occur.
Coalescence often follows coagulation and eventually the dispersed phase can become a
continuous phase, separated from the dispersion medium by a single interface. The time
taken for such phase separation may be anything from seconds to centuries, depending on
the emulsion formulation and manufacturing conditions. It is this length of time that is
used to define emulsion stability and varies according to the application. For example,
cosmetic lotion products usually require a two year shelf life (Therapeutic Goods
Administration (TGA), 1994) and so anything that shows signs of separation within two
years is considered unstable. Pesticide emulsion concentrates, on the other hand, are
designed to be added directly to the farmer’s spray tank to form the emulsion
- 11 -
Chapter 1
immediately prior to spraying.
The so-formed emulsion is only usually tested for
stability for up to 24 hours only because these product types are recommended by the
manufacturers for immediate spraying only.
Good selection of emulsifiers and stabilisers enables them to adsorb strongly at the
interface between the continuous and disperse phases. By their presence, such materials
can reduce the energetic driving force to coalescence by forming a mechanical or
chemical barrier between drops. This is a key factor to achieving emulsion stability.
Emulsifiers have been mentioned already but stabilizers are typically polymers which
form a steric barrier to coalescence. Other examples are hydrophobic solid particles
which stabilize W/O emulsions or hydrophilic solid particles which stabilize O/W
emulsions ( Aveyard & Clint, 2003).
As interfacial tension values fall, the ease of emulsion formation increases and the droplet
size achievable decreases (Everett, 1988). Systems in which the interfacial tension falls
to near zero (<10-3 mN m-1) may emulsify spontaneously under the influence of thermal
energy and produce droplets so small (<100 nm diameter) that they scatter little light and
give rise to clear dispersions. The microemulsions so formed occupy a place between
macroemulsions and micelles and are thermodynamically stable. The interfacial tension
is so low that only thermal energy is required for emulsification (the main ‘driving force’
is the configurational entropy gain in having so many droplets). Microemulsions are
extensively used for low viscosity emulsions like commercial spray emulsion products.
Macroemulsions generally do not provide sufficient stability for the desired life of such a
low viscosity product.
Microemulsions were first reported by Hoar and Schulman (Hoar and Schulman, 1943),
who described transparent or translucent systems that formed spontaneously when oil and
water are mixed with relatively large amounts of an ionic surfactant combined with a cosurfactant. The systems described have been shown to be dispersions of very small drops
(radius ~100 nm) of water in oil.
- 12 -
Chapter 1
The formation of a microemulsion is, like that of conventional emulsions, still rather an
art. Science has not advanced to the point where one can predict with accuracy what is
going to happen in the beaker or reaction vessel with all mixtures of ingredients, let alone
what is going to happen in large scale manufacture.
As previously stated, microemulsions (which form spontaneously), can only be formed if
the interfacial tension is so low that the remaining free energy of the interface can be
overcompensated by the configurational entropy of the dispersion process. Although a
single surfactant lowers the interfacial tension, this is usually insufficient to enable
microemulsion formation – even when using a high shear mixer and processing at
elevated temperatures. The addition of a second surfactant of a different chemical nature
is usually necessary to lower the interfacial tension still further (Luckham, 1986). This
second surfactant is termed a co-surfactant, is commonly an alcohol (Everett, 1988) and
need not be, by itself, a powerful surfactant.
Although microemulsions are undoubtedly the most stable emulsion type, three to ten
times the level of total surfactant is required in their formation as compared to a standard
macroemulsion. When cost and, in the case of cosmetic emulsions, dermatological
compatibility, are considered, these emulsion types are often unsuitable for commercial
preparations.
1.2.4 Considerations for Cosmetic Emulsions
Materials used for cosmetic application must demonstrate excellent skin compatibility as
well as superior aesthetic properties. Specific marketing claims are frequently a major
driving force behind the materials selected and these claims often require the addition of
many more auxiliary ingredients than would commonly be found in other emulsion
applications.
- 13 -
Chapter 1
Table 1.2 provides an overview of typical raw materials listed in cosmetic emulsion
formulations (Cognis GmbH, Care Chemicals Division CD-Rom, 1999).
Table 1.2
Typical Cosmetic Emulsion Ingredients
Ingredients
Emulsion Phase
Oil Phase
Emollients
Hydrophilic, oil soluble consistency factors
Lipid soluble emulsifiers
Oil soluble additives / active ingredients
Aqueous Phase
Water
Water soluble moisturisers
Water soluble emulsifiers (e.g., solubilisers)
Polymer stabilisers and thickeners
Water soluble additives /active ingredients
It is not uncommon for a cosmetic formulation to include ten to fifteen separate
ingredients. Frequently several emollients are used, for example to achieve the desired
sensory properties; or several herbal active ingredients are used to increase marketability.
All of these materials may need to be formulated into a stable product.
Any auxiliary ingredient that possesses surface activity itself, or can affect the solubility,
polarity or hydrophobicity of either phase, can influence emulsion formation and
stability.
There is no current technique for surfactant selection that satisfactorily
considers all the auxiliary ingredients in the selection process. In fact, it is often the
practise to formulate a stable emulsion in the absence of crucial additives (particularly
colour, fragrance and preservatives). When these are then added, much of the initial
research and development is rendered invalid. It is common practice to carry out very
little further development leading to a product which is no longer at is optimum. With
the complexity of today’s emulsion systems, particularly in the cosmetic area, this is an
area that needs to be addressed.
- 14 -
Chapter 1
The use of polymers in cosmetic formulations continues to grow. Sodium polyacrylate
and acrylate co-polymers are commonly used to regulate viscosity and provide additional
stability to the emulsion.
Even in sprayable emulsion systems, a small amount of
polymer provides some steric stabilisation to improve the shelf-life of commercial
formulations.
Self-emulsifying polymers are also now on the market to act as co-emulsifiers or even to
offer an alternative to conventional emulsifiers (by trapping the oil phase within voids of
polymer gel matrix offering excellent stability (see Chapter 8)). As long chain materials
that sit on, but do not penetrate into, the dermal layers polymers have an additional
benefit in that they are a milder option for skincare.
A so called ‘emulsifier-free’
marketing concept is beginning to appear in advertisements (www.polcopharma.com.au).
It is very possible that these products may become important in terms of future direction
of cosmetic emulsions and so are of interest to this work. The possible part polymers can
play in cosmetic emulsion technology will be discussed further in Chapter 8 where the
future directions of the Emulsion Phase Matching Technique, the topic of this study, are
put forward.
- 15 -
Chapter 1
1.3 TECHNIQUES TO PREDICT AND MEASURE EMULSION
STABILITY
At the formulation development stage it is important to be able to predict the life of a test
emulsion, ahead of its usual shelf life. The usual technique to measure this (Therapeutic
Goods Administration (TGA), 2003 and 2004) is by means of ‘ageing’ samples by
storing them at a range of different temperatures (known as ‘accelerated’ storage tests).
The Arrhenius equation (Arrhenius, 1889) is often used to calculate the factor for
increased ageing (Turner, 2002). The Arrhenius equation states that a rise in temperature
of 10°C will cause approximately a doubling of the rate of chemical reaction. It is
assumed that doubling the rate of chemical reaction doubles the rate of decay of
packaging and product (Turner, 2002). Therefore, 6 months stability storage at 40°C is
assumed to indicate the equivalent chemical stability of a product stored for 2 years at
20°C (and 1 year at 30°C although this storage condition is rarely used). In terms of the
product itself, this decay as far as the TGA is concerned, applies to the chemical stability
of actives i.e., sunscreen or pharmaceutical actives, and not necessarily the physical
condition of the emulsion itself. However, if the emulsion has destabilized then there is
no doubt that this will be evident in the analysis of the actives.
The accelerated storage tests at elevated temperatures make use of a lower viscosity
component (from Stokes Law (see section 1.3.4), which increases the rate of
sedimentation. Higher temperatures also speed up the rate of Brownian motion and the
consequent number of droplet collisions. The higher temperatures give particles more
energy such that, when they collide, they are more likely to coagulate and/or coalesce.
This, in turn, increases their particle size and thus their rate of sedimentation. Any
tendency for the formulation to undergo flocculation and/or creaming effects will usually
show up more quickly under accelerated storage testing. However, depending on the
stabilization mechanism, increasing temperatures sometimes (not often) increases
stability. This may occur, for example, when the increased energy allows a partially
flocculated system to de-flocculate, giving rise to a more stable emulsion. Thus stability
tests should also be carried out at lower temperatures.
- 16 -
Chapter 1
Storage temperatures typically used include:
•
4°C
•
Room temperature (RT) which can be 20 or 25°C
•
40 or 45°C
•
50 or 54°C
•
A continuous cycling temperature, where the temperature covers a cycle from - 5ºC
to + 40ºC over each 24 hour period
•
- 10, - 15 or even - 20ºC is sometimes also used when the product is destined for
colder climates
These temperatures are the most common temperatures but others can be used, providing
a justification for the use of the temperature employed is submitted to the regulatory
body.
Depending on the product’s final application and selected storage temperatures, the test
samples are evaluated at designated time periods. Typically these can include 2 and 24
hours, 1 week and 3, 6, 12 and 24 months.
One, or more, of the many techniques available to identify the first signs of emulsion
instability caused by accelerated ageing can be used. These methods include particle (or
droplet) size analysis and the convenient and effective method of visual examination.
Both techniques were used in the current work and are described below. Conductance,
centrifuge and ultrasound evaluations are other common techniques that could have been
used as an alternative to droplet size analysis.
Development of new and alternative techniques to speed up the analysis of emulsion
stability continues. Currently, 3 and 6 month successful accelerated storage test results
are the minimum required to obtain approval for a product to be commercialised within
TGA guidelines (TGA, 2003 and 2004). If a technique was approved that could more
quickly predict emulsion stability and product shelf life, then product commercialisation
could be sped up by several months. This is an attractive proposition for manufacturers
- 17 -
Chapter 1
and drives the quest to find a new technique that can shorten the stability requirements
before approval is given. Recent work includes the techniques of differential scanning
calorimetry (DSC), (Yamamoto, 1994), high frequency spectroscopy (Kageshima et al,
2003) and continued developments of existing ultrasound techniques (Daniels, 2002).
Fairhurst et al, (2002) have combined acoustic spectroscopy with electro-acoustics to
measure both particle size and zeta potential on O/W and W/O emulsions without the
need for dilution. It has been the requirement for dilution in many techniques that has
thus far prevented them gaining widespread approval with regulatory bodies. Dilution of
the system changes it in an irreversible manner and so there can never be full confidence
that the results are sufficiently representative to accurately predict stability.
It should also be noted that assumptions are frequently used in instrumental data analysis
e.g., many particle size instruments assume that the test material is spherical. When
measuring emulsion stability, an instrumental technique is preferably backed up by a
visual technique which provides different but complimentary information (e.g., can
differentiate between flocculation and coalescence).
Two such techniques used in
conjunction can provide a more comprehensive picture of the material under test and this
is the reason why two techniques were selected in the current work.
1.3.1 Particle Size Determination
Comparison of the particle size distributions of fresh and aged emulsion samples is a very
good way of determining information on the stability of an emulsion system. For
example, both Ostwald Ripening (Hunter, 2001) and droplet coalescence reduces the
number of droplets observed and increases the average particle size. An increase in
particle size is, of course, expected since all emulsions are inherently unstable. It is the
rate of this increase which is of importance. Frequently, the rate is sufficiently fast to be
a problem to medium term stability, but sufficiently slow to be undetected by the human
eye. Under these conditions, particle sizing is an effective means for early detection of
emulsion instability.
- 18 -
Chapter 1
The particle size instrument used in this project was a laser light scattering device
(Malvern Mastersizer S). A schematic diagrammatic representation is given in Figure
1.3. All light scattering based particle sizers are comprised of an optical measurement
unit which forms the basic part of the sensor, and a computer which manages the
measurement and performs result analysis and presentation.
Spatial
Filter
Collimating
Lens
Flow Cell
Focussing Lens
Laser
Detector
Interlace Electronics
Sample Dispersion
Unit
Computer
Figure 1.3 Laser Particle Size: System Overview
The light scattered by the particles, as they pass through the flow cell, and the unscattered
remainder, are incident on a receiver lens known as the focussing lens. This operates as a
Fourier transform lens forming the far field diffraction pattern of the scattered light at its
focal plane. Here, the detector gathers the scattered light over the range of angles of
scatter. The unscattered light is brought to a focus on the detector and passes through a
small aperture in the detector and out of the optical system. The total laser power passing
out of the system in this way is monitored, allowing the sample volume concentration to
be determined.
- 19 -
Chapter 1
Laser light scattering can be used to measure the size distribution of any one material
phase dispersed in another. The only qualification of the technique is that each phase
must be optically distinct and the medium must be transparent to the laser wavelength.
This means that the refractive index of the test material must be different from the
medium in which it is supported.
For laser light scattering particle sizers, the range of scattering angles measured is
typically 0.01 – 15º. The size ranges covered are typically from 1 μm upwards in such
cases. For these types of size and angle ranges, the scattering properties are largely
independent of the internal optical properties of the sample material. The theory used to
model the scattering in such an instrument is the ‘Fraunhofer’ scattering theory
(Woodruff and Delchar, 1994), which requires no assumptions of the particles optical
properties.
When interpreting results from laser light scattering instruments a few key points must be
considered:
•
The result is volume based. This means that when the instrument reports, for
example, 11% of the total distribution in the size category 6.97 – 7.75 μm this
means that the total volume of all particles with diameters in this range represents
11% of the total volume of all particles in the distribution. A simplistic example
can be used to show the importance of defining whether the distribution is number
or weight (or other) averaged. Consider a sample consisting of only two sizes of
particle - 50% by number having diameter 1 μm and 50% by number 10 μm.
Assuming spherical particles, the volume of each of the larger particles is 1000
times the volume of one of the smaller ones. Thus, as a volume distribution, the
larger particles represent 99.9% of the total volume.
•
The result is often expressed in terms of equivalent spheres. The instrument does
not consider the shape of the particle being measured (i.e., cuboid, cylindrical
etc). The results are measured as a volume and the diameter values quoted are
calculated based on the particle being completely spherical (or some other limited
geometrics).
- 20 -
Chapter 1
•
The result is derived from particle size classes, optimised to match the detector
geometry and optical configuration that gives the best distribution.
When analysing particulate matter, the particle shape is rarely, if ever, spherical.
Information on the particle shape cannot be gained and is not considered in this
technique. For this reason, visual microscopic analysis, is recommended in parallel with
instrumental particle size analysis. Emulsion droplets, on the other hand, are spherical
and well represented by this technique.
1.3.2 Visual Assessment
Although this method is a simple technique not requiring sophisticated equipment, visual
assessment of emulsions following accelerated ageing is a highly effective method to
evaluate emulsion stability.
An aged, stable emulsion looks exactly as it did immediately after preparation - a
homogenous dispersion throughout the cylinder.
An emulsion showing signs of
instability displays a distinct layer, either at the top or bottom of the cylinder. This is
caused by separation between the dispersed and continuous phases of the emulsion and it
depends on the density of the two phases whether the layer appears at the bottom (termed
sedimentation) or the top (termed ‘creaming’) of the cylinder. At this stage the separate
layer appears whiter (more concentrated) than the body of the emulsion.
The phenomenon of creaming receives its name from its most common instance: the
separation of unhomogenised milk. What occurs in this case, and in all cases of true
creaming, is not so much a breaking of the emulsion as a separation into two emulsions,
one of which is richer in the disperse phase, the other poorer, than the original emulsion.
The emulsion which is more concentrated is the ‘cream’. In the case of milk, the cream
represents an emulsion much richer in the dispersed butter fat phase than the depleted
milk phase. This is why the cream phase is often removed to provide milk that has a
lower fat content.
- 21 -
Chapter 1
It should be noted that while creaming may be undesirable in many cases, it does not
represent a ‘breaking’ of the emulsion where free oil or water can be seen as distinct,
clear layers. On the other hand, since creaming is favoured by large droplet sizes, it may
well be a manifestation of processes that will eventually lead to breakdown of the
emulsion and phase separation.
The process followed for visual assessment testing is commonly as follows:
1. A quantity of the emulsion is prepared and transferred into a graduated cylinder (e.g.,
100 mL).
Its appearance is examined and recorded, and the cylinder is left to stand
at room temperature or in water baths or ovens at the required test temperature.
Common storage test temperatures used include 4°C, RT (20 or 25°C), 40°C, 45°C
and 50 or 54°C, depending on the climatic conditions of the country where the
product will be sold.
2. After a defined time period, which may be hours, days, weeks or months, the cylinder
is carefully removed (without agitation) from the water bath or oven and examined
with the aid of a bright light (placed behind the cylinder).
3. The physical appearance of the emulsion is recorded as a percentage of free oil,
cream or sediment. The presence of any of these layers is an indication of instability.
Decisions have to be made as to the significance of the results achieved in terms of
the shelf life of the product. For example, 0.5% ‘cream’ after 1 month at 50 °C may
be considered tolerable but 0.5% ‘cream’ after 24 hrs at room temperature might not.
This visual technique is especially useful when optimising emulsifier ratios because the
effect of several emulsions can easily be compared at one time. The effect of increasing
or decreasing emulsifier content, or ratios of different emulsifiers, can be clearly
observed.
In many emulsion systems, creaming, although undesirable, is allowable to some extent.
Adjustments in manufacturing technique (e.g., shear rate or duration) or formulation
- 22 -
Chapter 1
(e.g., emulsifier content or concentration of thickener) can reduce the rate of creaming to
a point where it can be considered to have a negligible effect.
This may be understood from a consideration of sedimentation phenomenon using Stokes
Law (Sjoeblom, 1996):
V =
2 g a 2 (ρ2 − ρ)
9η
where V = rate of sedimentation
g = local acceleration due to gravity
a = radius of droplet
ρ = density of continuous phase
ρ2 = density of droplet (dispersed phase)
η = viscosity
The value of V may be positive or negative depending on the direction the particles /
droplets will move. This depends on the relative values of the densities of each phase. In
an O/W emulsion, for example, the oil density (ρ2) is usually the smaller, hence upward
sedimentation (creaming) will occur.
Examination of Stokes Law leads to the conclusion that emulsion stability (with respect
to creaming) is favoured by small droplet size, small density differences between the
disperse and continuous phases, and by high viscosity of the continuous phase.
Therefore, if a formula is changed to increase the level of thickener in a formula then
stability will be improved. In the same manner if a smaller droplet size is achieved by
increasing the rate or duration of shear or increasing the emulsifier then again improved
stability should be expected.
The accelerated storage tests at elevated temperatures make use of a lower viscosity
component, which increases the rate of sedimentation and will thus show any instabilities
sooner than would appear at room temperature.
- 23 -
Chapter 1
1.4
THEORIES TO AID SURFACTANT SELECTION
The formulating chemist has an array of surfactants available to choose from and the
selection of a suitable surfactant for a given application can, as already stated, become a
tedious and lengthy process. Assistance is required to make this process easier and there
is one, widely used, theory that formulation chemists use when developing emulsions.
This is the Hydrophile-Lipophile Balance (HLB) theory described below (Griffin, 1949
and 1954). The system was a breakthrough in its time although it has its limitations.
These limitations have prompted further theories to be developed and are a major driving
force behind the proposal of the Emulsion Phase Matching theory, which is the basis of
this project.
1.4.1 Hydrophile-Lipophile Balance (HLB)
Whilst working in the laboratories of Atlas Powder Company (which became ICI) in the
late 1940’s William C. Griffin was studying the emulsification properties of the newly
synthesized, ethoxylated surface-active agents. In this work, Griffin noted the striking
relationship between emulsion stability and ethylene oxide content.
He correlated
emulsion stability to the weight percent of ethylene oxide in the molecule and founded
the HLB (Hydrophile-Lipophile Balance) system (Griffin, 1949 and 1954).
The HLB of an emulsifier is an expression of the balance of the polar (water-loving) and
non-polar (water-hating) groups of the emulsifier.
Griffin defined HLB as:
weight percent hydrophile
HLB =
5
which for nonionic, ethoxylated surfactants is simply:
HLB =
weight percent ethylene oxide (EO)
- 24 -
5
Chapter 1
For example, laureth-4 has a molecular weight of 362 g mol-1. It has four moles of
ethylene oxide per molecule giving 48% EO on a weight basis. The HLB of laureth-4 is:
48
5
= 9.6
The divisor was used for convenience only - values then range from 0 to 20 on an
arbitrary scale. HLB is related to solubility; an emulsifier with a low HLB number has a
low proportion of hydrophilic groups and tends to be ‘oil soluble’ while an emulsifier
with a high HLB number has a high proportion of hydrophilic group and tends to be
‘water soluble’.
Following Griffin’s work the HLB theory was developed by Davies & Rideal (1961),
who proposed the following equation:
HLB = 7 + ∑ (hydrophobic group numbers) - ∑ (hydrophilic group numbers)
The group numbers are given in Table 1.3 with Table 1.4 illustrating how the HLB
number relates to solubility and also to surfactant application.
Table 1.3 Group HLB Numbers
Hydrophilic
Groups
-SO4Na
-COOK
-COONa
Sulfonate
-N (tertiary amine)
Ester (sorbitan ring)
Ester (free)
-COOH
-OH (free)
-O-OH (sorbitan ring)
HLB
Lipophilic Groups
38.7
21.1
19.1
~11
9.4
6.8
2.4
2.1
1.9
1.3
0.5
-CH-CH2-CH3-CH=
-(CH2-CH2-CH2-O-)
- 25 -
HLB
-0.475
-0.15
Chapter 1
Ever since the introduction of the HLB technique, there has been research into alternate
methods to accurately determine the HLB number. Some are equations to link other
techniques for emulsifier selection to the HLB number which will be covered in more
depth in sections 1.4.2 and 1.4.3. Others include the determination of HLB number by
the use of the ‘phenol index’ (Leszak et al, 1981) or by utilizing reversed phase thin layer
chromatography (Trapani et al, 1995). This continued development reflects the need for
more accuracy in determining the HLB number.
In more general terms, table 1.4 illustrates how the HLB number relates to solubility and
also to surfactant application. This is a useful guide which is widely known and reported
in almost all texts relating to the HLB technique.
TABLE 1.4
HLB Value Related to Surfactant Application
HLB RANGE
USE
SOLUBILITY
4-6
W/O Emulsifiers
Oil soluble
7-9
Wetting Agents
8-18
O/W Emulsifiers
13-15
Detergents
10-18
Solubilisers
Water Soluble
In addition to the HLB of the emulsifiers themselves, data tables are also available for the
materials that are to be emulsified. These are characterised by an individual ‘Required
HLB’ value which are based on emulsification experience (method to determine
‘Required HLB’ is given in Becher, 1973) rather than structural considerations. The
Required HLB is a number which should be matched to the HLB of an emulsifier or
emulsifier blend for optimal emulsion stability.
For example, iso-propyl myristate (IPM) has a Required HLB of 11 and so a nonionic
emulsifier, or blend of nonionic emulsifiers, having an HLB of 11 will generally make a
more stable O/W emulsion than emulsifiers of any other HLB value within the same
- 26 -
Chapter 1
chemical class. To determine the optimal emulsifier combination, however, various
mixtures of other types of emulsifying agents with the same weighted average HLB
number must then be screened (as explained in section 1.3.2) to determine which
structural types of emulsifiers give the best restuls for the particular system to be
emulsified.
The HLB number is indicative only of the type of emulsion to be expected, not the
efficiency or effectiveness with which it will be accomplished (Rosen, 2004, Griffin,
1954, Becher 1973). This should be noted because formulators’ expectations of the HLB
system are often greater than this.
It has been pointed out (Shinoda, 1968, Boyd et al, 1972, Kloet & Schramm, 2002) that a
single surfactant can produce either an O/W or a W/O emulsion, depending on the
temperature at which the emulsion is prepared, the shear rate, or, at high oil
concentrations, on the ratio of surfactant to oil. O/W emulsions can be prepared with
certain surfactants over the entire range of HLB numbers from 2 to 17 (Rosen, 2004)
HLB and Required HLB values are available for both O/W and W/O emulsions, although
the data available for W/O systems is limited. Neither HLB nor ‘Required HLB’ give
any indication of the quantities or chemical nature of emulsifier or oil required – these are
left largely up to the formulator to determine.
Application of the principles of the HLB concept greatly aids the formulation chemist by
vastly reducing the number of candidate emulsifiers to be screened in a particular system.
It cannot guarantee that the emulsifier(s) selected will give a stable emulsion but there is
no system currently available that can offer this assurance. There are situations where the
HLB system is over-extended and these should be known to avoid inaccurate results.
- 27 -
Chapter 1
1.4.1.1
1.
Limitations of the HLB System
Temperature and HLB play a prominent role, particularly in the case of nonionic
emulsifiers, in determining the partitioning of the emulsifier between phases.
Raising the temperature reduces the HLB of the emulsifier while decreasing the
temperature raises it. Tables of HLB values, however, rarely involve more than
one temperature.
2.
Any specific, chemical interactions of the emulsifier with the oil (as well as
interactions with any other formulated raw material) are disregarded; only water
and/or oil solubility and the physical properties of surfactants at interfaces are
accounted for.
With the increased number of materials now included in
formulations this has become more important than when this method was first
developed.
3.
The matching of the emulsifier chemical type to that of the oil is left open to trial
and error, albeit on a much less extensive basis.
4.
A fundamental tenet of Griffin’s definition of HLB (Griffin 1949 and 1954) is
that the HLB of a mixture of nonionic emulsifiers is the weight average of the
HLB of each of its components. This rule does successfully predict trends in
stability, but only when the chemical types are similar and the dissimilarity in
HLB between the emulsifiers is not too large (<5 HLB units).
Solubility
differences can have a major effect when the HLB difference between the
emulsifiers is too large.
5.
The HLB value is assumed to be the arithmetic mean of the surfactant
components. This is not the case for other important parameters for nonionic
surfactants e.g., the critical micelle concentration (cmc), and this may contribute
to the limitations of this system. Furthermore, it is frequently not true for ionic
surfactants (Shaw, 1993).
- 28 -
Chapter 1
6.
Commercial nonionic surfactants are rarely single components. For instance,
commercial ethoxylated surfactant laureth-4 possesses a range in the number of
ethylene oxide moles present. This range is typically from 1 – 7 moles of
ethylene oxide (Tesmann, 1988). Lower molecular weight components of the
surfactant (i.e., components with 1 – 3 moles ethylene oxide) tend to partition into
the oil phase, and raise the required HLB by effectively changing the oil phase
composition.
Despite its shortcomings, the HLB system remains very well accepted as an aid to
emulsifier selection, particularly for simple nonionic emulsifier systems. Provided its
limitations are known, the HLB system is a valuable tool for the formulation chemist.
Complex nonionic or ionic emulsifiers are not within the scope of Griffin’s system and
should not be used. The HLB of ionic surfactants, for example, are not fully additive and
are concentration dependent.
Nevertheless, the system is commonly used for all
emulsifier types.
1.4.2 Solubility Parameter
The solubility parameter theory (Hildebrand, 1915, 1916 and Hildebrand & Scott, 1950)
is based on the premise that when the solubility parameters of two materials are equal, the
materials are infinitely soluble (Burke, 1984). The solubility parameter is a numerical
value that indicates the relative solvency behaviour of a specific solvent. It is derived
from the heat of vapourisation.
The intermolecular forces that cause materials to dissolve are the same forces that prevent
those materials from boiling away until a specific temperature is reached (Vaughan,
1985).
The sum of all these intermolecular attractive forces has been defined by
Hildebrand (1915, 1916 and Hildebrand & Scott, 1950) as the solubility parameter (δ).
The concept was originally postulated by Scatchard in 1915 and greatly extended by
Hildebrand (1915, 1916 and Hildebrand & Scott, 1950) who used his own earlier work
- 29 -
Chapter 1
combined with Scatchard’s concept to mathematically define the solubility parameter,
and showed it to be empirically related to the extent of mutual solubility of many
chemical species. The ‘character’ of chemicals is related to the way they interact.
Interaction energy increases when character is similar, regardless of strength (Martin et
al. 1985), i.e., ‘like attracts like’.
The solubility parameter has many applications. It indicates to an analytical chemist
what order the test materials will come through the HPLC column (Alessi et al. 1975).
To biochemists, it is a value related to membrane penetration and binding (Sloan et al.
1986). For coatings chemists, it indicates which materials will readily adhere and to
formulators it indicates which materials will mix, and which will separate.
In mathematical terms, Hildebrand originally defined the solubility parameter as:
δ = ( ΔHv / [ V ] )1/2
where ΔHv is the heat of vaporisation, and [ V ] is the molar volume.
The energy of vaporisation forms the foundation of the expression because when the
material vapourises it is no longer held together by its intermolecular forces.
1.4.2.1
Component Solubility Parameters (or Cohesion Parameters)
Originally Hildebrand (1950) was only concerned with non-polar materials, i.e., those in
which only dispersion forces act between the molecules. To be of universal use the
theory had to be extended to include polar materials. This was first achieved by Hansen
(1967) who included molecules interacting by dipolar and hydrogen bonding forces (as
well as dispersion forces), by making the assumption that the solubility parameter could
be represented by an additive function of the three components, giving a squared form of
the solubility parameter which became known as the cohesion parameter (c):
δt2= (c) = δd2 + δp2 + δh2
- 30 -
Chapter 1
where the subscripts t, d, p and h refer to the total, dispersion, polar, and hydrogen
bonding contributions respectively.
For complete miscibility, two liquids need each of these parameters to be similar. Other
researchers found that even Hansen’s expanded parameter was insufficient for their
applications and further expansion was necessary. The Hansen expression did not take
into account the unsymmetrical nature of the hydrogen-bonding interactions. Beerbower,
Martin & Wu (1984) developed an alternative equation, which is becoming a generally
accepted improvement (in the polymer and paint industries) to the Hansen expression:
δt2 = δd2 + δo2 + 2 δaδb
where the subsript o refers to contributions due to orientation effects and a and b refer to
contributions to Lewis acid and Lewis base interactions respectively.
The solubility parameter offers a far more comprehensive system than the HLB system
but has the disadvantage of being very complex with several alternative expressions
available. This has been sufficient to deter formulators from applying the solubility
parameter in emulsion development. In order to overcome this, some researchers (e.g.,
Vaughan, 1991) have put forward simpler methods to achieve estimates of solubility
parameter values, which are sufficient in some cases for initial experiments.
One such method is the quick and easy ‘Drop Weight’ test;
Ten or twenty drops (dispensed from the same pipette) of a pure non-volatile liquid with
unknown drop weight are weighed and an average value for a single drop calculated.
This average weight is then bracketed against the average drop weight for two
nonvolatile reference liquids with known solubility parameters. Although crude, this
method has been reported by Vaughan (1991) to show both linear and reproducible
results. Results are completely dependent on the type of dropper used and the method is
suitable for comparative testing only.
- 31 -
Chapter 1
The solubility parameter is quite widely known in some industries, e.g., polymer
technology, but it has not gained wide acceptance in the field of emulsion technology.
Presumably, this is due to difficulties in its interpretation and use compared to the HLB
system. The solubility parameter theory is based on the premise that when the solubility
parameters of two materials are equal, the materials are infinitely soluble. By definition
this does not apply to emulsifiers, which cannot be fully soluble in either component if
they are to function as emulsifiers.
It was shown (see Table 1.4) that the solubility of surfactants and their HLB values are
inter-related. The activity of a surfactant is greatly affected by its bulk solubility so it is
quite logical to develop the solubility parameter further to determine its relationship to
the HLB system.
An expression for a ‘Cohesive Energy Ratio’ was defined by
Beerbower (1972) which did link the solubility parameter and HLB. However, it did not
achieve any simple method to determine the solubility parameter of a surfactant from its
HLB number. Thus the full potential of the solubility parameter to emulsification has not
yet been realised.
1.4.3 Phase Inversion Temperature
One major limitation of the HLB method for selecting emulsifiers is that it makes no
allowance for the change in HLB value with changes in the conditions for emulsification
(temperature, nature of the oil and water phases, presence of co-surfactants or other
additives). For example, the degree of hydration of an ethoxylated nonionic surfactant
decreases as the temperature is raised and the surfactant becomes less hydrophilic.
Consequently, its HLB must decrease.
An O/W emulsion made with ethoxylated
surfactant may invert when the temperature is raised and a W/O emulsion may invert to
an O/W emulsion when the temperature is lowered.
Shinoda and Arai (1964, 1967) recognised the importance of temperature on surfactant
properties (particularly nonionic surfactants) and introduced the concept of the ‘Phase
- 32 -
Chapter 1
Inversion Temperature’ (PIT) as a quantitative approach to the evaluation of surfactants
in emulsion systems.
The temperature at which a test emulsion (consisting of oil,
aqueous phase, and 3-5% surfactant prepared by shaking at various temperatures)
inverted from O/W emulsion to W/O emulsion was defined as the PIT of that particular
emulsion system. This temperature (usually a range rather than a specific temperature)
occurs when the interfacial properties of the system are balanced and generally emulsions
of very fine droplet size are produced. The PIT temperature (or temperature range) is
often easy to identify because the emulsion becomes clear due to the formation of a
microemulsion.
The sensitivity of emulsions, particularly nonionic emulsions, to temperature led
(Shinoda & Saito, 1969) to suggest that the PIT method be used as a possible method for
emulsion preparation. In such a procedure, an emulsion would be prepared near the PIT
of that particular system (± 4ºC), where minimum droplet sizes can be achieved and then
cooled to its normal storage or use temperature. Droplet sizes of the final, cooled
emulsion are not as small as the microemulsion formed at the PIT itself but are
significantly smaller (of the order of 0.1 – 0.3 μm (Ansmann et al. 1995)) than if the
same emulsion was prepared using Becher’s traditional method described in Section 1.3.2
(in the order of 1 to 10 μm).
The phase inversion temperature depends on the concentration of the emulsifier mixture
(Kunieda & Ishikawa, 1985) and the nature of the emulsified oils (Kunieda & Miyajima,
1989), as well as on the HLB of the emulsifier.
The PIT appears to reach a constant value at 3-5% w/w surfactant concentration when an
ethoxylated surfactant containing a single POE (polyoxyethoxylated) chain is used.
When there is a distribution of POE chain lengths in the surfactant, the PIT decreases
very sharply with increase in the surfactant concentration when the degree of
ethoxylation is low. As the oil to water ratio increases in an emulsion with a fixed
surfactant concentration, the PIT increases. Additives such as mineral oil, that decrease
the polarity of the oil phase, increase the PIT; whereas those that increase the polarity,
such as triglycerides, esters or oleic acid, lower the PIT. The addition of salts to the
- 33 -
Chapter 1
aqueous phase decreases the PIT of emulsions made with POE nonionics (Shinoda &
Takeda, 1970).
By varying the composition of these ingredients it is possible to alter the PIT to suit
manufacturing conditions (Foerster et al. 1990).
Two options for the formation of PIT emulsions are as follows:
1.
hot/hot process: as with conventional emulsions both the oil and the water phases
are separately heated to the temperature at which they will be mixed (~85 - 90°C).
They are then mixed at this temperature, which is in the PIT range, and the
resulting emulsion is cooled rapidly to room temperature.
2.
hot/cold process: the oil phase and a portion of the water phase are heated to the
PIT temperature where they are mixed to form a microemulsion. The remaining
cold water is then added to aid rapid cooling to room temperature.
The second method is favoured because there is less volume of liquid to be heated, which
saves time and energy usage for manufacturers and the microemulsion phase enables easy
visualisation of the required phase inversion. PIT technology for the manufacture of
emulsions is known but not commonly used. As low viscosity emulsion products gain
commercial popularity (e.g., sprayable emulsions), then it is expected that this theory will
become more frequently utilised. Marketers are continually requesting lighter feeling
products and the PIT technology meets this need perfectly although marketing trends for
EO-free products from the growing ‘natural’ products area (Aubrey Organics, Purist
Company) may limit its use to a degree.
Gasic et al (1998 a & b) researched the possibility of using PIT as a parameter for the
selection of an appropriate nonionic emulsifier. Some linear dependence between the
PIT and HLB value of the emulsifier was found for lower ethoxylates but when placing
the corresponding values into an empirical equation connecting these values, the expected
results were not obtained.
- 34 -
Chapter 1
1.4.4 New Method Introduction – Emulsion Phase Matching (EPM)
Despite the very many methods detailed in Chapter 1.4 to aid emulsifier selection when
formulating emulsions, there is still no definitive technique available that can pinpoint an
emulsifier or emulsifier combination that will form a guaranteed ‘commercially stable’
emulsion. Trial and error is still required in emulsion formulation and this remains a
source of frustration to formulation chemists. The idea behind this current work was to
investigate a different approach that perhaps may take us a step nearer to finding a more
comprehensive method for emulsifier selection.
To the best knowledge of this student, and her supervisors, a new method called EPM is
introduced, in this thesis, for the first time.
The Emulsion Phase Matching method involves an alternative approach to surfactant
selection; the surfactants, which possess two separate parts (or entities) and situate
themselves between two different phases, are evaluated as two individual entities. For a
required emulsion system, the separate surfactant entities are selected based upon their
comparison with the two phases that are to be emulsified.
Consider Figure 1.4 which displays a diagrammatic summary of the EPM technique.
Emulsifier
Value 1
Emulsion
Value 2
Value 2 – Value 1 = ΔValue Surfactant
Value 3
Value 4
Value 4 – Value 3 = ΔValue Emulsion
Figure 1.4 Schemetic Diagram of the EPM Technique
- 35 -
Chapter 1
A given physical property is chosen and measured for each of the two surfactant entities
as well as for each of the emulsion phases. The difference (ΔValue) between each pair is
measured and compared. Where ΔValueSurfactant and ΔValueEmulsion are matched, then the
emulsion will be the most stable according to the EPM technique.
It should be noted that the EPM technique does not rely on a causal link between the
selected physical parameter and emulsion stability.
It does rely, however, on a
correlation to exist. A low interfacial tension, for example, is thought not to directly
result in emulsion stability; in fact, it should result in easier breaking of that emulsion. A
low interfacial tension may, however, correlate to emulsion stability because both result
from a high packing density of emulsifier at the interface.
1.4.5 Summary and Comparison of Theories
The HLB technique is quite simple to use but has a large number of limitations for
today’s emulsions. The solubility parameter addresses several of these limitations but is
too complex for everyday use and has not achieved widespread acceptance in the area of
emulsion development. PIT technology is viewed by many manufacturers as requiring a
change in both processing procedures and equipment and has also not been widely
embraced. The EPM technique may provide a more practical approach.
In this thesis, the Emulsion Phase Matching (EPM) technique is being proposed to offer a
different approach to the current systems. It is designed to be more encompassing than
the HLB system but just as easy to use. There is, deliberately, no attempt to inter-relate
the proposed EPM technique to the existing HLB technique. It was felt that the HLB
system has too many limitations to meet today’s emulsion requirements (particularly in
the cosmetic area) and that a completely fresh approach is required.
It is believed that the Emulsion Phase Matching technique is the first technique designed
to select an emulsifier based on the individual properties of both the hydrophilic and
- 36 -
Chapter 1
lipophilic moieties. As already described, surfactants are made up of two completely
distinct parts, or moieties, and yet traditionally, the emulsifier is treated as a whole. The
EPM technique requires the formulator to determine a measure of ‘difference’ between
the oil and water phases to be emulsified. An available database then enables the
formulator to match their ‘difference’ value to the same value for an emulsifier. It is
possible that experimentation may show that a scaling factor is required to provide this
match.
If the difference in properties between the hydrophobic and hydrophilic moieties of the
emulsifier is the same as those between the oil and water phases of the emulsion, the
emulsifier is ideally balanced between the two phases at the interface and does not favour
one phase over the other (i.e., is not more soluble in one phase than the other). There is,
therefore, a reduced tendency for phase migration and a high possibility of maintaining a
strong emulsifier layer at the interface. This will aid emulsion stability. The EPM offers
two key differences from the other systems so far described:
1.
It allows for consideration of the separate emulsion phases, in their entirety, to
gain and utilise information about the system as a whole, including any effects of
auxiliary materials.
2.
It splits emulsifiers into their corresponding hydrophobic and hydrophilic entities
and considers the individual effects each will have on its relevant phase of an
emulsion.
Full details of the EPM technique, including the theory behind it, are given in Chapter 2.
The aim of this current work is to evaluate the concepts behind the EPM technique and
assess its feasibility and usefulness in practical terms.
- 37 -
Chapter 1
1.5
TECHNIQUES TO CLASSIFY EMULSIFIER MOIETIES
The emulsifier moieties and oil phase ingredients used in the EPM method need to be
systematically measured in order to build up an EPM database. The main techniques
used to obtain such measurements in this study are surface and interfacial tension.
The short range intermolecular forces which are responsible for surface/interfacial
tensions include van der Waals forces (particularly London dispersion forces) and also
hydrogen bonding for aqueous systems as well as metal bonding for metallic surfaces.
The relatively high value of the surface tension of water (72.8 mN m-1 @ 20°C) reflects
the contribution of hydrogen bonding. The forces represented by surface and interfacial
tension are, according to Shaw (1993), assumed to be additive and are not appreciably
influenced by one another. The surface tension of water can be considered as the sum of
its dispersion forces and its hydrogen bonding forces. In the case of hydrocarbons, the
surface tension is entirely the result of its dispersion forces.
Surface and interfacial tension values, therefore, provide valuable information about the
different intermolecular forces possessed by different materials and the way they interact,
which is important in assessing how materials mix when forming an emulsion. The ease
of emulsion formation increases, and the droplet size achievable decreases, as the
interfacial tension falls (Everett, 1988)
An EPM classification based on solubility parameter was also considered. A review of
literature relating HLB to solubility parameters is given in section 1.5.3.
1.5.1 Surface Tension
Molecules located at the surface of a liquid are not completely surrounded by other
molecules like they are in the bulk solution. This causes a net inward force of attraction
exerted on a molecule at the surface from the molecules in the bulk solution, which
results in a tendency for the surface to contract. This contraction is spontaneous i.e., it is
- 38 -
Chapter 1
accompanied by a decrease in free energy and explains why droplets of liquid and
bubbles of gas tend to naturally form a spherical shape.
Surface tension (γ) is defined as ‘the force acting at right angles to any line of unit length
at the liquid surface’ (Shaw, 1993). A more favoured and clearer definition is ‘the work
required to increase the area of a surface isothermally and reversibly by a unit amount’
(Shaw, 1993). The unit commonly used is mN m-1 although many older texts give
surface tension values in dynes cm-1. The two units are equivalent.
The tension of a surface must be balanced by an equal and opposite force. For an isolated
droplet the balancing force must come from stresses generated within the droplet by the
surface tension itself. The stresses so generated (known as capillary pressure) depend,
for liquids, on the surface tension and on the curvature of the surface. This simple
experimental fact was discovered in 1709 (Hawksbee, 1709) but nearly a century passed
before Young (1805) deduced the correct theoretical relationship between capillary
pressure and surface curvature. Working independently from Young, Laplace (Greene,
1964) defined the first algebraic equation linking capillary pressure (p) and curvature:
p = γ[(1/r1) + (1/r2)]
where γ is surface tension, and r1
and r2
are the radii of curvature of any two normal
sections of the surface perpendicular to one another.
This equation is now known as the Young-Laplace equation and describes the shape of a
fluid drop under equilibrium conditions. A hanging pendant drop can be analysed more
reliably than can a sitting sessile drop, since axial symmetry can be safely assumed for
the pendant drop.
Unfortunately, the Young-Laplace equation cannot be solved
analytically and solutions rely on numerical techniques and interpolations. The classic
solution was provided by Bashforth and Adams (1883) although today, the more modern
recalculated tables of Padday (1969) are used.
- 39 -
Chapter 1
Figure 1.5 illustrates a pendant drop and the Bashforth-Adams technique for solving the
Laplace-Young equation to determine surface tension values.
SE = Equatorial diameter (maximum)
H = Distance SE from lowest point of drop shape
SW
SW = line parallel to SE at top point of H
SE
Given fluid density, the value of SW/SE is an
entry into a reference table (Padday, 1969),
H = SE
Figure 1.5
which yields surface tension.
Surface Tension Derived from a Hanging Pendant Drop
1.5.2 Interfacial Tension
There is no fundamental distinction between the terms surface and interface, although it
is customary to describe the boundary between two phases, one of which is gaseous as a
surface and the boundary between two non-gaseous phases as an interface.
At the interface between two liquids there is, like that with surface tension, an imbalance
of intermolecular forces. This imbalance is generally of a lesser magnitude than between
a liquid and a gas. Consequently, interfacial tension (γ) values are considerably lower
than surface tension values for similar materials. Interfacial tension is measured
according to the same pendant drop technique as described for surface tension.
- 40 -
Chapter 1
1.5.3 Application of the Solubility Parameter
A criticism levelled against the HLB concept has been that it disregards the effect of the
chemical nature of the hydrocarbon moiety of the surfactant (Vaughan, 1993). Schott
(1984) investigated the application of the solubility parameter concept to nonionic
surfactants. He compared solubility parameters determined from heat of vaporisation, as
well as using a calculated method derived by Small (1953), with relevant HLB values.
He initially found a poor comparison between the methods but after correction for the
hydrogen-bonding component he found that plots of HLB versus solubility parameter
were nearly linear and parallel for the three series of polyethoxylated nonionic surfactants
studied (based on dodecanol, octyl phenol and sorbitan).
According to Vaughan (1993), Schott (1984) also appears to be the first person to utilise
the different solubility parameters of the nonpolar and polar ends of surfactants to
increase their efficiency in emulsions.
He suggested altering oil or water phase
ingredients to make them better match the emulsifier tails.
Schott’s approach has some similarity to the concept of the EPM technique whilst not
quite treating the emulsifier moieties as separate entities.
It appears sensible to use the solubility parameter values for the separate emulsifier
moieties if they are available. Therefore, alongside the characterisation of materials for
the EPM technique (in Chapter 4) the solubility parameter values of the separate entities
will also be evaluated.
The EPM technique is developed throughout the course of this work. The following
Chapter (Chapter 2) outlines the philosophy behind the technique and how it might work
in practice.
- 41 -
Chapter 2
CHAPTER 2
EPM TECHNIQUE FOR EMULSIFIER SELECTION
2.1 DEVELOPMENT OF EPM THEORY
The EPM technique is an emulsifier selection technique suitable to meet the emulsion
formulation requirements of the current day. It is designed to be a more encompassing
and accurate method for the prediction of optimal emulsifiers than the popular HLB
technique but one that is just as easy to use.
Cosmetic formulations have increased in complexity over recent years:
•
Increased competition in the marketing of cosmetic formulations has meant that
many more materials must be included to satisfy consumer demands or offer a
product that has a perceived edge over the competition products. These additives
are not easily accounted for (with regard to their effect on emulsion systems)
using the HLB system.
•
It is rare to include just one oil (emollient) ingredient in the formulation; mixtures
of three or four oils are not uncommon, to meet sensorial needs or for better
solubility of an active ingredient, making it more difficult to predict the most
suitable emulsifier.
•
Safety considerations of raw materials, including their impurities, are more
prominent in the public domain. This is forcing some restriction in the use of
some materials. Recent publications (Koopman, 2004, Begoun, 2002, Sittig,
2002) and numerous 2006 websites (www.hairsite.com, www.1stholistic.com,
www.aubreyorganics.com, www.purist.com) have resulted in email chain letters
or newspaper advertisements discussing the dangers of sodium lauryl sulfate
(SLS), paraben preservatives, ethylene oxide surfactants or nitrosamines as
potential carcinogens. Although there are many other articles where SLS is
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Chapter 2
defended (NICNAS, 2002) and the information on the dangers of these materials
is somewhat limited, from a marketing perspective the damage is done and SLS
and parabens, at the very least, are rarely used in current cosmetic and toiletry
developments. Unfortunately, this list of ‘labelled potentially harmful’ materials
continues to grow resulting in limitations of the use of some traditionally very
commonly used raw materials.
This further makes the job harder for the
formulating chemist.
The importance of being able to evaluate the oil and water phases, with all of their
additives, cannot be over-stressed. It is common practice, for example, for a formulating
chemist to select (on the basis of HLB numbers) a suitable surfactant for a given O/W
emulsion, and this can often result in the preparation of a suitably stable emulsion.
However, it is also common practice, after the emulsion has been stabilised, for the
chemist to be required to add, for example, a herbal extract, a fragrance, preservative or
dye. Not surprisingly, these additional components often destabilise the emulsion and
hence their addition becomes a limiting factor in the resulting commercial viability of
that product. It is difficult (although not impossible) to account for such additives using
the HLB procedure.
Gasic et al (2002) were able to prove that it is possible to account for additives using the
HLB procedure by studying the influence of three additives (ethanol, ethylene glycol and
glycerol) on the HLB values of nonionic emulsifiers. This work made the conclusions
that ethanol and ethylene glycol could contribute to an increase in the stability of
emulsions whilst glycerol has the opposite effect. Glycerol is a common additive in
cosmetic emulsions as a humectant / moisturising agent and so this work is very relevant.
Unfortunately, there are a very large number of other additives used and to do this work
on each additive for each series of emulsifiers is not really feasible.
The EPM procedure, to be developed here, adopts a slightly different approach. In this
process, a suitable property is directly measured by the formulating chemist.
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The
Chapter 2
property is a particular measure of ‘difference’ of intermolecular forces between the oil
and water phases to be emulsified. An available database then enables the formulator to
match their ‘difference’ value to the same value for an emulsifier.
Only a simple
measurement of a physical property is required making the system quick and easy to use.
Each moiety of a surfactant does posess very different physical and performance
properties.
The EPM technique is aimed to promote consideration of how these
individual moieties influence the stability at the interface of a specified emulsion system.
2.1.1
Hydrophobic and Hydrophilic Moieties of Emulsifiers
Within the EPM technique each of the surfactant moieties are considered as individual
entities, rather than a whole molecule.
Here, an emulsifier can be split into its corresponding hydrophobic and hydrophilic
moieties before any further characterisation. The properties of the separate ‘sections’ are
then assumed to be identical to analogous compounds which have genuine physical
existence and can be tested. The analogous compounds are chemically identical to the
above-mentioned moieties except that they possess an additional hydrogen atom.
2.1.1.1 Example of the Splitting Process as applied to Laureth-4
Figure 2.1 displays diagrammatically an example of splitting the emulsifier Laureth-4.
This splitting process is an important part of the EPM Technique so more detailed
explanation is also given below.
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Chapter 2
Example of ‘splitting’ the emulsifier laureth-4
Hydrophobic tail
Hydrophilic head group
CH3(CH2)10CH2(OCH2CH2)4OH
Add one Hydrogen
to make stable materials
CH3(CH2)10CH3 = C12H26
Dodecane
Figure 2.1
H(OCH2CH2)4OH
PEG 200
Example of Emulsifier ‘Splitting’ in EPM Technique
The emulsifier laureth-4 has the molecular formula:
CH3(CH2)10CH2(OCH2CH2)4OH
The hydrophobic portion of the emulsifier laureth-4 is its lauryl (C12) chain,
CH3(CH2)10CH2 but this is not a stable material on its own. The addition of one hydrogen
atom forms the stable C12 alkane - dodecane C12H26. Thus dodecane is the hydrophobic
moiety for this emulsifier and for all ethoxylated, laureth emulsifiers.
The hydrophilic portion of the emulsifier laureth-4 is -(OCH2CH2)4OH, which again is
not a stable material.
The addition of a hydrogen atom forms the stable entity
polyethylene glycol (PEG) 200, and this is the hydrophilic moiety for this emulsifier.
The addition of a hydrogen atom (H), in this case, is likely to render the moiety slightly
more hydrophobic than it really is. An alterative would be to add a hydroxyl (OH) group
resulting in a diol.
However, this would only serve to render the moiety more
hydrophilic than it really is, and would probably be a greater deviation from ‘reality’ than
the addition of an H atom. It would also be problematic in terms of synthesizing the
resulting compounds, particularly with the other surfactants used later in this thesis. The
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Chapter 2
use of H to complete the molecule, rather than OH, will be used for the sake of
conformity in all future compounds.
It should be noted that the number that appears in the PEG 200 name, as is the case for all
the PEG materials, indicates the approximate molecular weight of the substance. For
example, commercial grade PEG 200 has a molecular weight range of 190 – 210 g mol-1
(Merck, 2001). The chemical formula states 4 moles of ethylene oxide for this substance
but in reality the number ‘4’ is an average value and commercial samples will have a
value around this number. Commercial grade materials were used in this work because
this ‘average’ number of moles of ethylene oxide also applies to the commercial
emulsifiers used here, and the EPM technique is designed for commercial use. The
broader distribution in EO numbers actually produce more stable O/W emulsions than
those with a narrower distribution due to improved packing at the interface. They are
also more stable over a larger temperature range (Saito, 1990)
2.1.2 EPM Database
An EPM database must contain data characterising a comprehensive range of commercial
emulsifiers. This is an essential requirement of any technique if it is to obtain widespread
acceptance and use.
For the purposes of this project, where the feasibility and use of the technique is being
investigated, six nonionic emulsifiers were studied.
Nonionic emulsifiers are the
dominant surfactant type used in emulsification and this emulsifier group would also
provide the best comparison to the HLB technique.
To obtain a range of values, emulsifiers were selected to be two sets of three emulsifiers:
Three ‘laureth series’ emulsifiers and three ‘ceteareth series’ emulsifiers. The two sets of
emulsifiers allow for a variation in the hydrophobic groups whilst, within each set, the
hydrophilic group is varied. Many more emulsifiers will need to be added to the database
as the next stage in the development of the technique.
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Chapter 2
The ultimate goal of the EPM technique is that the formulating chemist will determine
the EPM value for their emulsion system (prior to emulsification) and then be able to
match their value to the emulsifier EPM value in the database. In this case, there is no
need for a second database comprising the required values for the oil phase, or to ‘adjust’
the value to account for additives. Indeed a potentially helpful by-product to the EPM
Technique is that a complimentary section in the database could very easily, with time, be
put together to list auxiliary materials that ‘generally’ do not alter the interfacial tension
of the emulsion system. This would provide the formulator with options that have
historically been added to a system without influencing the interfacial tension and
resulting emulsion stability.
Previous experience with other selection systems has shown that formulators want to
have the data available for the other materials before they will fully use the system (the
solubility parameter system is an example where insufficient data is readily available and
is therefore under utilized in industry). Data for a system similar to the one they are
using gives the formulator confidence and encourages them to use the system. It also
shows that the system has been thoroughly tested and utilised. Unfortunately, to build up
a database for all the possible oils and auxiliary materials that could be used in an
emulsion is beyond the scope of this thesis. The aim here is to evaluate the feasibility of
the proposed EPM technique and to put forward the next steps to advance the technique
further in the future. It is recognized that further work will be required before any
recommendations of the use of the EPM technique can be implemented.
Interfacial tension has been selected as the parameter to classify the emulsifiers, oils and
auxiliary materials to build up the EPM database.
As explained in Chapter 1.5,
interfacial tension values give a large amount of information about the intermolecular
forces in a system which, in some circumstances, can be very different when considering
the phase boundaries rather than bulk properties.
Emulsions display one such circumstance where the phase boundary area is so large
compared to the volume of the system that a substantial fraction of the total mass of the
system is present at boundaries. In this case, surfactants can always be expected to play a
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Chapter 2
major role in the system (Rosen, 2004) and interfacial tension is an important measure to
attempt to understand what is going on in a particular system.
Most significantly, interfacial tension is a technique that can be carried out using the
entire oil and the entire water phases (i.e., including all the auxiliary materials). Many
auxiliary materials do have an effect on surface activity whether it is an effect on micelle
formation, or to consume surfactant by adsorption on their own surfaces. This important
point is not covered by any current emulsifier selection technique, but is implicit in the
use of the entire aqueous (or oil) phase when measuring its characteristic property
(interfacial tension).
Common auxiliary materials include essential oils for natural
fragrance or aromatherapy claims, starches and polymers for sensorial adjustment, oil
based vitamins or amino acids and peptides for skincare claims. All of these ingredients
can impact on the adsorption and consequent availability of emulsifier at the emulsion
interface.
Interfacial tension has not been chosen on the basis of any currently recognised emulsion
stability theory. Indeed, whilst it is known that lowering the interfacial chemistry makes
it easier to form an emulsion, it is also known that under many conditions it also makes it
easier to break that emulsion. However, a correlation between interfacial tension and
surfactant concentration at the interface may be found. Indeed, work by Evans et al,
(2002) proposes a model for surfactant adsorption kinetics using dynamic interfacial
tension (also using the pendant drop technique). This work will be described in a little
more detail in the next section (2.1.3).
There is considerable work ongoing on the effect of surfactant structure on interfacial
properties. Due to the large number and types of surfactants available it is a very
complex area to make generalisations. Rekvig et al (2003) found that for a given
interfacial tension, a double-tail surfactant is more efficient than a single-tail isomer only
if the head repulsion is sufficiently strong. For a given concentration in the bulk water
phase, the single tail surfactants are more efficient in both cases. In general, since highly
purified surfactants produce interfacial films that are not close packed and hence not
mechanically strong, good emulsifying agents are usually a mixture of two or more
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Chapter 2
surfactants rather than an individual surfactant. A commonly used combination consists
of a water soluble surfactant and an oil soluble one to increase the lateral interaction
between the surface active molecules in the interfacial film and condenses it to one that is
mechanically stronger than where a single surfactant only is used (Rosen, 2004). More
recent work by Li et al (2005), states that there is a close correlation between the
interfacial activity and the adsorption of the surfactant at the interface. This work also
found a beneficial decrease in interfacial tension value if the hydrophobic chains of the
surfactant and the oil have a similar structure which supports to a degree the idea behind
the EPM technique.
If the conditions are achieved that are the most energetically favourable for a surfactant to
be at the interface between the oil and water droplets, then the highest possible
concentration of emulsifier will be at the interface. A strong interfacial film should be
achieved.
This, coupled with the fact that cosmetic emulsions are thickened, thus
reducing the number of droplet collisions that occur, should provide systems with
enhanced stability. Interfacial tension measurement is also a widely known technique
that is relatively simple to perform.
2.1.3 EPM Theory
The principle behind the EPM technique is to create an environment where it is much
more energetically favourable for the emulsifier to be at the oil/water droplet interface
than anywhere else in the emulsion system. The emulsifiers selected using the EPM
technique should result in a maximum concentration of emulsifier molecules at the
interface, thus providing a strong defence against droplet collisions, thinning, coagulation
and coalescence.
The interfacial tension measurements taken for the specific oil and aqueous phases
against one another provide a measure of the imbalance of attractive forces that exists at
the interface between them.
Similar measurements between the hydrophobic and
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Chapter 2
hydrophilic moieties of the emulsifier, provides a measure of imbalance for the
emulsifier. If these two levels of imbalance are matched then the belief is that it should
be energetically more favourable for the emulsifier to go to, and remain, at the droplet
interface. This would reduce the tendency for phase migration and maintain a strong
emulsifier layer at the interface.
Newton’s Third Law of Motion states that ‘for every action there is an equal and opposite
reaction’. The relevance to cosmetic chemistry is clearly seen in dermatology when soap
is applied to the skin during the cleansing process. Soap possesses a high pH and the
surfactant cleansing effects also results in a removal of the natural sebum required to
maintain the condition of the epidermis.
The sebaceous glands immediately act to
counteract the condition left by the soap and restore the correct balance (the ‘reaction’ to
the ‘action’ of the high pH on the skin). The higher the pH of the soap and the more
aggressive the surfactant on the skin (the ‘action’), the greater the potential damage to the
skin and the stronger the ‘reaction’ of the sebaceous glands to compensate the effect.
However, as stronger re-actions are required, it is more difficult for the sebaceous glands
to produce the correct amount of sebum. This can then lead to skin disorders until the
system can once again be stabilised.
It is always the case that the greater the imbalance or change to any system, the harder it
is to judge the appropriate re-action and the longer the system takes to stabilise.
The proposal of the EPM technique is that by measuring an imbalance of the two phases
to be emulsified and matching this to an emulsifier whose moieties possess the same level
of imbalance, then the emulsifier is ‘balanced’ at the emulsion interface. If a good
balance can be achieved then there should be a lower tendency for ‘disturbance’ effects at
the interface which therefore, requires less ‘reaction’ of the emulsifier at the surface and a
stronger interfacial film at the emulsion interface.
No other emulsifier prediction technique has attempted to achieve a balance between the
produced emulsion and the emulsifier which is situated at the interface. If the situation is
- 50 -
Chapter 2
not balanced, this must promote activity to attain a balance which may well weaken the
emulsion system.
The emulsifier needs to be strongly attracted to the interface upon initial mixing of the
emulsion phases in order to promote a small initial droplet size. It is also very important
for the emulsifier to form a strong elastic barrier to resist droplet coalescence when the
droplets collide. A continually high concentration of emulsifier at the interface is one
such condition that supports both of these criteria – the high packing of emulsifier at the
interface providing an increase in surface viscosity, as well as providing a steric barrier to
coalescence.
Theories to explain surfactant adsorption, resulting surface concentration at the interface
and the strength of the resulting interfacial layer are only known to a limited degree in
colloid science and this remains an area of future development. Sjoeblom, 2001 and
Rosen, 2004 provide a comprehensive summary of the dynamics of surfactant adsorption
currently known.
According to Sjoeblom, the most popular surfactant adsorption
isotherms are those of Langmuir (1917 & 1918), Volmer (1925), Frumkin (1925) and van
der Waals (Brunauer et al., 1940). There are also expressions for the Gibbs (1928)
elasticity of adsorption monolayers which correspond to these isotherms. Because of the
complexity of the adsorption processes, the proposed theories are based around
monolayer formation. This may or may not be the true case.
Evans et al., (2002), proposed a model for surfactant adsorption kinetics which
incorporated ‘random sequential adsorption (to account for difficulty associated with
packing at the interface) and mass transfer theory (to calculate the concentration of
surfactant near the interface). The associated adsorption isotherm, when combined with
the Gibbs equation and some micellisation equilibria, gave good agreement between the
equilibrium values of the interfacial tension versus the total surfactant concentration.’
The agreement was not so good at concentration levels above the critical micelle
concentration and Evens suggested that the micelles near the interface collapse to provide
free surfactant for rapid adsorption at the interface.
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Chapter 2
The work by Evans highlights the difficulty in understanding the adsorption process at
the interface, even in a single surfactant system. Considering that in cosmetic emulsions
the surfactant level would most definitely be above the critical micelle concentration and
there may also be two emulsifiers, it would be a very complex exercise to understand the
mechanism for the formation of interfacial layer using the EPM technique. Instead, for
the current work, the focus has been on forming the emulsion and measuring the resulting
stability to establish the merit of the technique and identify any obvious improvements
which can be made.
If the correlation proves fruitful, then the next steps for
advancement of the technique should include an understanding of the actual mechanisms
involved. Computer simulation is becoming an effective tool for the study of interfacial
systems on a detailed molecular level. The work by Li et al., (2005) used a mesoscopic
level simulation named dissipative particle dynamics (DPD) to investigate the behaviour
of surfactants at the water/oil interface. This technique has now been used by many
researchers (Rekvig et al., 2004, Dominguez, (2002 & 2004), Dong (2004)) and is
gaining credence, showing good agreement with experimentation. It is hoped that as this
method, or ones like this develop, they can be used to understand, or perhaps prove, that
the EPM technique can provide a strong emulsifier barrier at the interface.
2.1.4 Benefits of the EPM Technique
The benefits of the EPM technique, as discussed in this Chapter, can be listed as follows:
•
All ingredients to be included in the emulsion can be taken into consideration before
the emulsifier selection is made.
•
→
Method is all encompassing
→
Even the newest materials can be considered
One simple measurement only is required to be carried out by the formulator to
determine the EPM value of that system.
→
Little work required from formulator
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Chapter 2
•
The value measured by the formulator is matched to a corresponding value in the
EPM database to identify the optimum emulsifier(s) to be selected.
→
•
Quick, easy reference system will be available
Where materials are changed or added as the formula develops, a repeat of the same
simple measurement only is required to see if modification to the emulsifier system is
required.
→
Offers flexibility to the formulator
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Chapter 3
CHAPTER 3
MATERIALS AND METHODS
3.1
MATERIALS
Materials were selected to prepare emulsions in both ideal and commercial systems.
Emulsifiers, oils and other auxiliary materials were used to form the foundations of an
EPM database.
3.1.1 Emulsifiers Selected for EPM Technique Evaluation
The emulsifiers selected were from the broad ranging group of ethoxylated fatty alcohol
surfactants, which have the general chemical formula:
R(OCH2CH2)nOH where R = alkyl and n = 2 to 60
Table 3.1 provides a summary of the emulsifiers selected for use in this study. The
International Nomenclature Cosmetic Ingredient (INCI) name, chemical formula,
tradename and supplier details are given along with HLB value (explained in Section
1.4.1) to give an indication of the range of emulsifier properties covered.
Table 3.1 Emulsifiers Selected for EPM Technique Evaluation
Emulsifier
(INCI Name)
Chemical Formula
Tradename &
Supplier
HLB
Value
Laureth-2
CH3(CH2)10CH2(OCH2CH2)2OH
Dehydol LS2 (Cognis)
7
Laureth-3
CH3(CH2)10CH2(OCH2CH2)3OH
Dehydol LS3 (Cognis)
8
Laureth-4
CH3(CH2)10CH2(OCH2CH2)4OH
Dehydol LS4 (Cognis)
9.6
Ceteareth-12
50% CH3(CH2)14CH2(OCH2CH2)12OH
50% CH3(CH2)16CH2(OCH2CH2)12OH
Eumulgin B1 (Cognis)
13
Ceteareth-20
50% CH3(CH2)14CH2(OCH2CH2)20OH
50% CH3(CH2)16CH2(OCH2CH2)20OH
Eumulgin B2 (Cognis)
15
Ceteareth-30
50% CH3(CH2)14CH2(OCH2CH2)30OH
50% CH3(CH2)16CH2(OCH2CH2)30OH
Eumulgin B3 (Cognis)
17
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Chapter 3
3.1.1.1
Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
To apply the EPM concept, the selected emulsifiers were first split into their hydrophobic
and hydrophilic segments as explained in Chapter 2. The hydrophobic and hydrophilic
segments for each of the selected emulsifiers are summarised in Table 3.2 along with the
supplier and grade used.
TABLE 3.2
Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
Emulsifier
(INCI Name)
Hydrophobic
Moiety
Grade and
Supplier
Hydrophilic
Moiety
Laureth-2
Dodecane
Analar (BDH)
ICI (Huntsman)
Laureth-3
Dodecane
Analar (BDH)
Laureth-4
Dodecane
Analar (BDH)
Diethylene
Glycol
Triethylene
Glycol
PEG 200
Supplier
ICI (Huntsman)
ICI (Huntsman)
Ceteareth-12 50% Octadecane
50% Hexadecane
Analar (BDH)
Analar (BDH)
PEG 600
ICI (Huntsman)
Ceteareth-20 50% Octadecane
50% Hexadecane
Analar (BDH)
Analar (BDH)
PEG 1000
ICI (Huntsman)
Ceteareth-30 50% Octadecane
50% Hexadecane
Analar (BDH)
Analar (BDH)
PEG 1500
ICI (Huntsman)
3.1.2 Oils Selected for EPM Technique Evaluation
Oils commonly used in commercial applications were required that would display a wide
range of interfacial tension values, when measured against water. A range of oils from
the same chemical type was preferred in the first instance, to avoid the probability of
specific and variable chemical interactions affecting the stability of the emulsion. Esters
are the largest single family of cosmetic oils commercially available and those selected
are detailed in Table 3.3. All oils were supplied by Cognis (formerly Henkel).
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Chapter 3
TABLE 3.3
Selected Oils - Simple Esters
Simple Esters
(INCI Name)
Tradename
General Chemical Formula R1COOR2
R1
R2
Ethyl Hexyl Palmitate
Cegesoft® C24
Palmityl – C16
2-Ethyl hexyl – C8
Hexyl Laurate
Cetiol® A
Lauryl – C12
Hexyl – C6
Decyl Oleate
Cetiol® V
Oleyl – C181
Decyl – C10
Ethyl Hexyl Stearate
Cetiol® 868
Stearyl – C18
2-Ethyl hexyl – C8
Iso-Propyl Myristate
IPM
Myristyl – C14
Iso-propyl – C3
Iso-Propyl Palmitate
IPP
Palmityl – C16
Iso-propyl – C3
It is a requirement by law (Federal Bureau of Consumer Affairs, 1991) that all
ingredients present in a commercial formulation are listed on the label of these products
for information to the consumer. If time is taken to look at the ingredients of several
cream and lotion products available, it can be seen that although the most common oil is
an ester, it is rarely the only oil present. The majority of products contain oils from more
than one different chemical group. For example, Dove Hand Lotion contains mineral oil
in combination with an ester whilst Nivea Hand Cream contains octyldodecanol and
capric/caprylic triglyceride in combination with an ester
It is therefore appropriate to consider a second set of oils from other classes in the scope
of this project. Oils covering a much wider range of polarities are needed. A second
group of oils of different polarity were, thus, also tested. Their details are given in Table
3.4.
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Chapter 3
TABLE 3.4
Selected Oils - from other Chemical Groups
Name of Oil
(INCI Name)
Tradename
Chemical Type
Chemical Formula
Dibutyl Adipate
Cetiol® B
di-ester
CH3(CH2)3OOC(CH2)4COO(CH2)3CH3
Capric/Caprylic
Triglyceride
Myritol®
318
mixed tri-ester
CH2(OCOR)CH(OCOR)CH2(OCOR)
R = C7H15 and C9H19
Cocoglycerides
Myritol®
331
(mono-, di-, and tri-esters)
mixed glyceryl ester
General formula as above with also
mono- and di- glycerides from C8 - C16
Octyl dodecanol
Eutanol®G
alcohol (Guerbert)
CH3(CH2)9CH(CH2(CH2)6CH3)CH2OH
Dioctyl
cyclohexane
Cetiol® S
cycloalkane
(aromatic)
1,3 di-C8H17 C6H4
Mineral Oil
OP6A
mixed aliphatic alkanes
Dicapryl Ether
Cetiol® OE
ether
CH3(CH2)XCH3
X = 16-18
CH3(CH2)6CH2OCH2(CH2)6CH3
3.1.3 Auxiliary Ingredients Selected for EPM Technique Evaluation
In addition to emulsifiers and oils, commercial formulations also contain a large number
of water-soluble additives. For example, aqueous systems require preservation to ensure
they have a useful shelf-life. Fragrances and viscosity regulating materials are added for
consumer acceptance and other ‘active’ materials (e.g. herbal extracts, sunscreen filters
etc) are also often included to make a marketable, desirable product. For the purposes of
this work, just a few of these auxiliary materials were selected for study and these are
listed in Table 3.5.
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Chapter 3
Table 3.5 Auxiliary Ingredients Selected for EPM Technique Evaluation
Material
(INCI Name)
Tradename and
Supplier
Function
Water
Purified
Aqueous phase solvent
Glycerine
Glycerine USP
Humectant and solvent
Carbomer
Carbopol ETD 2001
(BF Goodrich)
Consistency giving
agent
Cetostearyl Alcohol
Lanette MY
(Cognis)
Consistency giving
agent
Tocopherol Acetate
(Vitamin E)
Copherol 1250
(Cognis)
Moisturising Agent
5-Chloro-2-methyl- 4isothiazolin-3-one
Kathon® CG
(Rohm & Haas)
Preservative
Arginine (and) Disodium
Adenosine Triphosphate
(and) Mannitol (and)
Pyridoxine HCl (and) RNA
(and) Histidine HCl (and)
Phenylalanine (and) Tyrosine
Photonyl LS
(Laboratoires
Serobiologiques)
Active ingredients for
protection against
photoageing
Santalum Album
Sandalwood Oil
(Dragoco)
Fragrance, essential oil
active
The function that these raw materials perform in the formulation is given as well as the
tradename and supplier. All these suppliers listed provide their own technical supporting
data and guide formulations to help the formulation chemist achieve the desired
performance.
All emulsifiers, oils and auxiliary materials used throughout this project have been
summarised within Chapter 3.1. The next section details the main methods used to
characterise these materials.
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Chapter 3
3.2 METHODS
The methods used and the accuracy of the instruments employed during this project, were
vital to determine the validity of the EPM technique. Hence, a detailed description is
given below.
3.2.1 The First Ten Ångstroms (FTÅ) 200 Instrument
The FTÅ 200 is an instrument that uses rapid video capture of images and automatic
image analysis to accurately determine properties such as surface tension, interfacial
tension and contact angle. Surface and interfacial tension can be measured from a drop’s
shape using the Bashforth-Adams technique as explained in Chapter 1.5.
The FTÅ’s efficient video imaging system is capable of measuring many separate images
per second, which can be used to statistically improve the precision of the results. The
manufacturer (First Ten Ångstroms) recommends a minimum of 10 image analyses be
used. For optimum results, First Ten Ångstroms also suggest that the image of the drop,
as viewed on the computer screen, occupies at least one half of the screen. This also
helps the user to determine the best definition of the image and provide a clear, sharp
image necessary for analysis. The FTÅ optimally fits curves to the regions of interest in
the pendant drop image to minimise noise effects and to provide accurate measurements.
This has previously been difficult to achieve with any other method without large
associated errors.
An image is taken to check that the conditions are suitable for analysis. Error messages
are displayed if the image is not sufficiently defined to enable accurate measurements to
be taken. If this is the case the following adjustment options are available:
a)
focal length (by physical movement of the camera)
b)
focus (on the camera lens)
c)
brightness scale
d)
contrast scale.
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Chapter 3
A picture of the FTÅ 200 instrument is given in Figure 3.1.
Computer Image of Droplet
Figure 3.1
Instrument
Photograph of the FTÅ 200 Instrument Set-up
Once an image has been successfully taken and analysed, a series of images of the drop
(a ‘movie’) can then be taken for the actual measurement. Throughout the project, for
both instrument calibration and sample measurement, 25 images were taken to enable a
measurement to be recorded.
The FTÅ 200 software package reduces the grey-scale computer image to a set of
equations describing the drop’s edges. Resolution to less than the size of one pixel is
available and a least-squares fit is used to derive the curve equation. The least-squares fit
effectively averages many data points into one equation, smoothing the inevitable noise
in the video image.
The image analysis software is next utilised to solve the analytical equations applied to
the images and determine the result of the analysis. In the case of surface and interfacial
- 60 -
Chapter 3
tension, this equation was the Laplace-Young equation which is solved using the classical
Bashforth-Adams technique with the more modern recalculated tables of Padday (1969).
Extensive statistical analysis of the result was provided by the FTÅ 200 software
package. The output values that were of most interest for the project include the average
(arithmetic mean - ξ), minimum and maximum values as well as the standard deviation
(σ). Surface and interfacial tension values were all reported by the FTÅ in the units of
dynes which is equivalent to the more modern unit of mN m-1.
The surface and interfacial tension results are reported throughout this study as the mean
value with its 95% confidence limit, calculated according to basic statistical analysis.
3.2.1.1 Instrument Standardisation and Calibration
Before making any interfacial tension measurements, it was necessary to standardise the
FTÅ 200 using surface tension measurements. The standardisation was required prior to
each use of the instrument and again during use if the focus control, or focal length was
adjusted at any point during the analysis. Purified water was the standard material for the
standardisation procedure, which was carried out in the ‘Calibration’ mode of the
software. On completion of the analysis, verification of the calibration was made using a
fresh, purified water droplet. The FTÅ’s syringe pump (driven by a stepper motor to
dispense the test drops) was used to pump one millilitre of water through the syringe.
The ‘slow’ pump control was then used to deliver a large sized drop as portrayed on the
computer screen. An analysis of this drop was made to confirm that the instrument was
correctly prepared for the measurement of unknown materials. It should be noted that the
purity of the water used was not validated using this technique, so an alternative
technique (Du Noüy Ring) was used to carry out the validation. Only one source of
purified water was used throughout the project and each time, water was taken from this
source, it was tested using the Du Noüy Ring apparatus. The water was tested and only
used when the measured result agreed (to ± 0.1 which is limit of instrument) with the
CRC Handbook (Lide, 1994) literature value of 72.8 mN m-1 at 25ºC.
- 61 -
Chapter 3
One syringe and needle type only was used throughout the project. These were a 10 mL
B-D plastic disposable, sterile, single use syringe with a lock tip and a 1 inch length, grey
collar, 27 guage needle. Both were supplied from Livingstone International, Laboratory
Supplies.
An example of purified water standardisation analysis, taken on different days over a
period of several months, is given in Table 3.6, to demonstrate typical variations given by
the FTÅ 200 instrument.
TABLE 3.6
FTÅ 200 - Instrument Repeatability
No.
of
measure
-ments
Temp
°C
Lit.
Value*
(mN m-1)
Mean of Data
(mN m-1)
95 %
Confidence
Limit
(±)
Standard
Deviation
1
50
15
73.5
73.10
0.10
0.37
2
50
16
73.3
73.44
0.11
0.39
73.2 (large drop)
0.12
0.42
72.7 (small drop)
0.16
0.42
73.1 (small drop)
0.13
0.41
72.5 (large drop)
0.12
0.31
72.4 (small drop)
0.09
0.23
Analysis
50
3
25
16
73.3
98
4
25
99
20
72.8
* Barton, A., “Handbook of Solubility Parameters and Other Cohesion Parameters”, CRC Press, Boca Raton, Florida (1990)
Included in the data set are results taken on the same date, within a few minutes of each
other, where the influence of the size of the droplet was investigated. The recommended
droplet size was a minimum of one half of the screen, as viewed by the user. However, it
was observed that with some liquids this was not possible – only a small, circular drop
could be formed. If droplet size had an influence on the results then this required
identification. Analyses 3 and 4 represent an example of data where the size of the drop
was varied. It was clear that the results for the smaller drop size were farther from the
literature value than with the larger drop. However, if more measurements were taken of
the smaller drop this did appear to give more consistency between larger and smaller drop
- 62 -
Chapter 3
size measurements. The recommended larger drop size was used wherever possible. If
only a small droplet was achievable then close to 100 measurements were made to
improve the accuracy of the result.
To calibrate and validate the instrument over the expected range of use, surface tensions
of other materials were required. Materials were selected whose surface tension literature
values fell towards the two extremes of the expected values for test materials, i.e. those of
diethylene glycol and hexane. A selection of the results obtained is given in Table 3.7.
Table 3.7
Temp.
°C
Calibration results for Diethylene Glycol and Hexane
Lit.
Value*
(mN m-1)
Mean of Data
(mN m-1)
95 %
Confidence
Limit (±)
Standard
Deviation
45.65
45.21
45.21
45.57
45.33
44.80
0.03
0.05
0.11
0.10
0.13
0.28
18.91
18.40
18.91
19.12
18.74
19.15
0.07
0.07
0.02
0.17
0.163
0.064
Diethylene Glycol
15
20
20
Hexane
15
20
15
*
Barton, A., “Handbook of Solubility Parameters and Other Cohesion Parameters”, CRC Press, Boca Raton, Florida (1990)
The FTÅ 200 is a relatively new technique and even though its accuracy of pure test
substances show good agreement with literature values, the instrument and its method
required validation against an alternative proven instrument. The Du Noüy Ring method
of surface tension measurement is widely recognised as a suitable method for
determining surface tension (Hunter, 2001). Identical samples of both purified water and
hexane, were measured (as sample analyses) on the two instruments. Results are given in
Table 3.8.
- 63 -
Chapter 3
Table 3.8
Temp.
°C
Surface Tension Validation
Lit. Value
(mN m-1)
(Hunter
2001)
Du Noüy Ring
Result
(mN m-1)
95 %
Confidence
Limit
(±)
FTÅ
Result
95 %
Confidence
Limit
(±)
(mN m-1)
High Quality Distilled Water
22
72.4
71.5
0.21
72.9
0.12
18.4
18.9
0.17
18.7
0.07
Hexane
20
Both sets of results compare well with the literature values. The FTÅ 200 has advantages
over the Du Noüy Ring instrument in that it has a sophisticated software package for
analysis, provides a statistical analysis, and can measure many values in a very short
period of time. The Du Noüy Ring instrument used, in this case an Analite Surface
Tension Meter, displays its results to one decimal place only. This is a considerable
disadvantage when measuring low surface tension values where 0.1 mN m-1 is a
significant portion of the result.
3.2.1.2
Interfacial Tension Measurement
Interfacial tension measurements between two immiscible liquids, at least one of which is
clear enough to transmit light, is determined using the FTÅ 200 by exactly the same
Pendant Drop technique as that used to determine surface tension.
In this technique, one of the immiscible liquids is formed into a drop, via a syringe (as for
surface tension determination), into a cell or bath containing the liquid against which the
interfacial tension is to be tested. The interfacial tension cell attachment has two parallel
windows: one to allow light to shine on the as-formed droplet (from behind) and the other
to allow the camera to view the image (from the front).
The liquid with the highest density (generally the hydrophilic material) was dropped into
the lower density liquid (generally the hydrophobic material). This ensured that the
- 64 -
Chapter 3
resulting drop was naturally ‘hanging’ from the needle, as required for the analysis, and
not malformed as it attempted to rise to the surface, as would be the case if the less dense
material was placed in the syringe.
The resulting drop was viewed through the second liquid and consequently the resulting
image was optically different to that of a drop viewed alone (e.g. when formed in air).
The image tended to be less clear than those achieved for surface tension measurement
and so the ‘Contrast’ and ‘Brightness’ controls of the instrument were adjusted to obtain
an image that was suitably defined for analysis.
Throughout the project, a “check” surface tension analysis was made between each
analysis using distilled water. This ensured that there had been no drift during the test
measurement, which was an effect that had been observed during early measurements. If
drift was apparent, then the previous measurement was discarded and the sample retested.
3.2.2 Emulsion Preparation
There are many variables involved in the preparation of emulsions. In fact, this area has
been developed into a graduate degree course, and postgraduate studies on optimising
and controlling the many variables involved in Emulsion Formulation Engineering are
now published i.e., Salager et al, 2002 and Perez et al, 2002. For the purposes of this
thesis, consistency of the emulsion preparation is of paramount importance to provide
accurate, comparative results. Standard procedures as described and outlined in the
following paragraphs were employed to ensure this consistency.
The choice of the emulsifier was the parameter under investigation in this thesis. The
remaining variables, which are listed below, only require standardisation before any
testing can be commenced.
1.
Oil Concentration
2.
Order of Mixing
- 65 -
Chapter 3
3.
Degree of Mixing
4.
Emulsifier Concentration
5.
Emulsification Temperature
3.2.2.1
Oil Concentration
The best oil concentration to use in emulsion testing is arguably a value typical of those
used in commercial formulations. O/W cream and lotion formulas can, and do, cover a
wide range of oil concentrations and there is no single, definitive value which is used as a
standard. The oil concentration of a cosmetic cream or lotion determines the degree of
moisturisation offered by the product and can be required as a carrier for pigments,
sunscreens or other oil soluble actives. As a guide, the oil concentrations from twenty
O/W body or hand lotion recipes, featured in the Cognis Personal Care Formulation CDROM (Cognis GmbH, Care Chemicals Division, 1999) and shown in appendix 3, were
evaluated. The oil concentration ranged from 8 to 20% with an arithmetic mean value (to
the nearest integer) of 15% and so this was the concentration of oil chosen for use
throughout this thesis.
3.2.2.2
Order of Mixing
There are two distinct phases in an emulsion; the oil phase and the aqueous phase. The
emulsifier is premixed with one of the phases before mixing. Becher (1965) researched
this area to show which pre-mix option is preferable:
a) Emulsifier-in-water method
In this method, the emulsifying agent is dissolved directly in the water, and the oil is then
added, with considerable agitation. This procedure prepares O/W emulsions directly.
b) Emulsifier-in-oil method
- 66 -
Chapter 3
Here, the emulsifying agent is dissolved in the oil phase. The emulsion may then be
formed in two ways:
i)
By adding the mixture directly to the water.
In this case, an O/W
emulsion forms.
ii)
By adding water directly to the mixture. In this case, a W/O emulsion is
formed but if the volume of the water phase becomes high enough it is
possible to spontaneously invert the emulsion to an O/W emulsion.
According to Becher (1965), the emulsifying agent-in-water technique usually results in
quite coarse emulsions, with a wide range of particle size. These emulsions tend to be
unstable and homogenisation is essential to produce a reasonable emulsion.
For the emulsifying agent-in-oil method, Becher’s findings were that this results in
uniform emulsions, with a small particle size range. This technique usually results in the
most stable form of emulsion. Where the water concentration is above 50%, as is usual
for O/W emulsions, the addition of water directly to the mixture is preferred because the
process of inverting the emulsion forms a smaller droplet size.
The emulsifier in oil option (ii), adding water directly to the oil, was selected for use in
this work.
3.2.2.3
Degree of Mixing
The amount of shear applied affects the mechanical energy input into the system and so
can influence the initial droplet size formed. Shear applied to emulsion systems can
range from 30 rpm for a traditional paddle stirrer mixer (used in traditional hot process
emulsion manufacture) to 30,000 rpm for a high shear mixer (used in lower temperature
emulsion preparations). In the case of this work many emulsions were prepared at room
temperature and due to the absence of heat energy a high shear mixer was required.
9000 rpm was selected for 90 second duration.
- 67 -
Chapter 3
3.2.2.4
Emulsifier Concentration (Theoretical Determination)
In terms of interfacial tension reduction, the emulsifier concentration should be sufficient
to saturate all the available interfaces.
The actual particle size distribution of the
emulsion is never known prior to mixing so formulators tend to act conservatively and
add a slight excess. This allows for possible higher interface availability (smaller particle
size distribution) and additionally for rapid repair or broken or stretched interfaces, or to
account for any unexpected loss of surfactant.
A large excess would be wasteful
however, and also inefficient. Furthermore, the excess forms micelles in solution, which
can alter the apparent stability of the emulsion by causing solubilisation of lipophilic
components.
There are several interfaces in the case of an O/W emulsion that are available for
surfactant adsorption. The most significant interface is that between the dispersed oil
droplets and the aqueous media (i.e. the emulsion interface). Its area varies depending on
the droplet size and number. Other minor interfaces include those between the aqueous
phase and air and between the vessel and solution surfaces. The emulsifier will position
itself at all these interfaces and will also be present free in solution as well as in micelles
and/or other self-assembly aggregates.
It is possible to calculate the theoretical level of emulsifier required to saturate all the
interfacial area between the oil droplets and the solution. It is also possible to estimate
the free surfactant level in solution from its critical micelle concentration (cmc). The
sum of these two values, plus a small excess to allow for loss of surfactant at, for
example, the air/solution interfaces, would be a close approximation for the minimum
level of emulsifier required to saturate all surfaces. This may then be equated with the
minimum level of emulsifier required to give reasonable degree of emulsion stability. In
practice, of course, emulsion stability may require more than a single monolayer so this
value is strictly a ‘minimum required’ value.
Laureth-4 is an emulsifier that is used throughout this work, with the chemical formula
CH3(CH2)10CH2(OCH2CH2)4OH.
It can be used as an example to calculate the
theoretical emulsifier concentration required to ensure surface saturation is achieved.
- 68 -
Chapter 3
The following information is required:
1.
Average droplet size of test emulsion – in this case the aim is 2.5 µm which falls
inside the typical droplet size range for macroemulsions of 0.1 – 10 μm (Shaw,
1993) and was the average value for three commercial body lotions (Nivea,
Vaseline Intensive Care and Dove brands, see Section 3.2.3.2.2).
2.
Surface area of the surfactant molecule plus its molecular weight (for laureth-4,
Schick (1962) measured a value of 40 Å2 (40 x 10-20 m2 )). Laureth-4 has a
molecular weight of 360 g mol-1.
Note: The surface area taken up by the surfactant molecule is dependent on the chemical nature of the oil
and the presence of other competing materials. It will vary with different emulsion systems. Thus 40 Å2
can only be taken as an indicative figure and is one reason why the following calculation should be treated
as an approximation only.
The calculation is carried out in the following steps:
Step 1: Determination of the amount of laureth-4 per average emulsion droplet.
If an average emulsion droplet is assumed to possess a diameter of 2.5 μm then its
surface area (SA) is calculated as follows:
SA = 1.96 x 10-11 m2
SA = 4πr2 and since r = 1.25 x 10-6 m,
(1)
Given that the SA of each laureth-4 molecule is 40 x 10-20 m2 (Schick, 1962),
(2)
1.96 x 10-11 ⁄ 40 x 10-20
(3)
4.9 x 107 laureth-4 molecules
=
that can be packed per average emulsion droplet or,
8.13 x 10-17 moles of emulsifier per droplet
(4)
Given the molecular weight of laureth-4 is 362 g mol-1, there are:
362 x (8.13 x 10-17)
=
32.93 x 10-14 g emulsifier per droplet
- 69 -
(5)
Chapter 3
Step 2: Emulsifier required to saturate emulsion droplet surfaces
As discussed in Section 3.2.2.1, a typical oil phase concentration used in O/W emulsions
is 15%. Hence for the calculation, 15 mL (15 x 10-6 m3) of oil phase were used per 100
mL of final emulsion.
The volume of one droplet of oil V = (4/3)πr3
and since
r = 1.25 x 10-6m
V
= 8.18 x 10-18 m3
1.83 x 1012 droplets
Therefore, per 15 mL there are:
(6)
(7)
With 2.93 x 10-14 g emulsifier per droplet (from equation (5)),
this gives:
0.054 g emulsifier per 100 mL of emulsion (0.054%)
(8)
It would be expected that the emulsifier required to adequately cover the dispersed oil
droplets would be, by far, the most significant portion of the total concentration of
emulsifier required.
Step 3: Total emulsifier required to cover all available surfaces
The next portion would be expected to be any micelles formed.
This figure is the cmc, which is according to Rosen (1987) and Meguro (1982), for
laureth-4 is 0.03 g L-1 or 0.003 %.
Even with a slight excess to allow for the remaining interfaces as well as errors in the
assumptions made (i.e. radius of emulsion droplet and SA of emulsifier molecules), the
above combined results give a concentration of less than 0.1% emulsifier.
In conclusion, for an emulsion formed using laureth-4, with an average droplet size of 2.5
μm, the theoretical level of emulsifier required to guarantee interface saturation has been
calculated to be less than the order of:
0.1 % laureth-4
- 70 -
(9)
Chapter 3
An emulsifier concentration of 0.1 % is, in practice, very low. In commercial emulsion
preparations the figure is commonly 2 – 4 %.
As already explained, commercial
cosmetic emulsions frequently contain a number of additives, many of which can be oil
soluble. A large excess of emulsifier is often added to allow a high enough concentration
to rapidly supply surfactant to the interface to promote stability. In addition micelles,
formed by the excess emulsifier, help solubilise any oil soluble additives and keep these
additives from separating from the emulsion. No currently available technique that
assists with surfactant selection allows the formulator to easily consider the effects of
auxiliary materials. The formulator, therefore, errs on the side of caution and uses more
surfactant to keep the material in solution. This deficiency is one that is addressed in the
proposed new technique for surfactant selection covered in this thesis.
Many emulsifiers are quite large and bulky molecules so they can also contribute to steric
stabilisation and to the rheological properties of the overall emulsion.
The excess
emulsifier utilised may provide a more viscous and congested system, which could serve
to aid emulsion stability by retarding the rate of droplet collisions. It should be noted,
however, that there are many, cheaper materials that can fulfill exactly the same function.
Using excess emulsifier is not the most efficient method to sterically stabilise an
emulsion or to control its rheology.
Taking all the above factors into consideration, as well as the experiments carried out in
Chapter 5, an emulsifier concentration of 0.5% w/v was selected as the standard for this
work. This is still an excess over the theoretical value but significantly less than the
quantity frequently found in today’s emulsions to try to test the proposed technique and at
the same time offer a commercial advantage to its use.
3.2.2.5
Temperature of Emulsification
As discussed in Chapter 1.2, heat is often employed to aid the initial mixing of the
emulsion phases.
- 71 -
Chapter 3
Heat cannot be used in every surfactant system. Ethoxylated nonionic surfactants, for
example, possess a ‘cloud point’ which needs to be considered. The cloud point is the
temperature (or temperature range) at which the surfactant begins to lose sufficient water
solubility to affect its normal function as a surfactant. Cloud points for the laureth and
ceteareth emulsifiers used in this work are given as examples in Table 1.3.
TABLE 3.9 Cloud Points of Selected Emulsifiers
Emulsifier
Cloud Point (°C)
Laureth-2
23
Laureth-3
23
Laureth-4
24
Ceteareth-10
90-97
Ceteareth-20
90-94
Ceteareth-30
94-96
Emulsions can be prepared using the ceteareth emulsifiers at the usual processing
temperatures (typically 70 - 80ºC). The laureth emulsifiers, however, are only truly
effective in cold processable preparations. Forming emulsions at cold temperatures is, of
course, quite advantageous as it eliminates the heating step altogether, saving both time
and energy costs. Usually however, it is more difficult to achieve a stable emulsion when
processing cold and the selection of the emulsifier, as well as high shear mixing, are very
important.
Due to their low cloud point, the laureth series of emulsifiers could not be processed
efficiently at high temperatures. All mixing involving these emulsifiers was carried out
at 20°C. The ceteareth series of emulsifiers, however, were solid at room temperature
and hence, required heating to around 50°C to melt. Since it was preferred to have a
single processing temperature, (to limit the number of variables involved), the emulsifier
was dissolved in the oil phase at 50°C and the oil product cooled to 20°C before mixing
- 72 -
Chapter 3
with the water phase to form the emulsion. Since 20°C was selected as the temperature
of emulsification, there was no need to cool the emulsions prior to their analysis.
3.2.2.6
Summary of Emulsion Preparation Parameters
The emulsion preparation conditions so far explained can be summarised in Table 3.10
Table 3.10
Emulsion Preparation Parameters
Parameter
Value used throughout Project
Oil Concentration
15% (w/v)
Order of Mixing
Emulsifier in Oil Method
Degree of Mixing
90 secs @ 9000 rpm shear (IKA)
Emulsifier Concentration
0.5% (w/v)
Emulsification Temperature
20° C
3.2.3 Emulsion Evaluation
The emulsions prepared were analysed using two techniques; particle size analysis using
the Malvern Mastersizer and visual assessment as discussed in Chapter 1.4.
Particle size analysis was carried out on the freshly homogenised emulsion to determine
how effectively the emulsifier system had emulsified the oil droplets and as a reference
point to later determine if there is growth / coalescence with ageing. At this initial stage
all emulsions appeared as white, homogenous liquids and visual determination could not
differentiate between them.
- 73 -
Chapter 3
For aged samples, where the visual indicators of cream and free oil were easily observed,
the visual assessment method was preferred. When these visual indicators are apparent,
visual analysis gives more information on the stability, or more particularly, instability of
the product. Particle size analysis was carried out alongside, if required, but where
differences in the emulsions could be clearly observed, the visual method was a clearer
indicator of emulsion stability.
3.2.3.1
Visual Determination
Once the emulsions were prepared, they were stored in 100 mL graduated stoppered
cylinders 40 °C. An incubator was used to maintain the temperature.
The emulsions were inspected after defined time periods of 24 hours, 72 hours and 1
week and their characteristics were recorded as described in Section 1.4.2. Tests were
continued until there was a clear differentiation between the emulsions under
investigation. Not all emulsions were tested for the full week of the trial because, in
some cases, the emulsions had broken well before this time.
3.2.3.2
Malvern Mastersizer
The Mastersizer is based on the principle of laser light scattering. It falls into the
category of non-imaging optical systems due to the fact that the sizing is accomplished
without forming an image of the particle/droplet onto a detector.
In addition to using the Fraunhofer scattering theory, the Mastersizer has the option of
extending its range of detection up to 135º in order to measure sizes down to 0.05 μm.
The scattering from such small particles at such large angles becomes dependent on the
optical properties of the material to a degree that should not be ignored. For such an
extended performance it is necessary to use the ‘Mie Theory’ (Schramm, 2001) model of
light scattering.
This theory is a complete description of the light scattering from
optically homogenous spheres and requires assumptions regarding the optical nature of
- 74 -
Chapter 3
the particles. Interestingly, the Fraunhofer scattering model is fully encompassed within
the Mie Theory by appropriate optical constant settings and fully agrees with it over its
applicable range.
The statistics of the distribution for the Malvern Mastersizer are calculated from the raw
result using the derived diameters D[m,n] – an internationally agreed method of defining
the mean of particle size (British Standard BS2955:1993).
The derived diameters are defined (Malvern, 1997):
D[m,n]
=
m-3
Σ V i di
Σ Vi din-3
1
m-n
where Vi is the relative volume in class i with mean class diameter di.
m and n are integer values which describe the type of derived diameter. The more
common of which are:
D[4,3] – the volume averaged mean diameter
D[3,2] - the surface averaged mean diameter
D[1,0] - the arithmetic mean diameter
3.2.3.2.1
Instrument Validation
Emulsion systems can be quite broad ranging in terms of their particle size distribution.
For the purposes of this project, commercially viable emulsion systems were being
evaluated, i.e. those that could be used for creams or lotions. Generally, a particle size
range from ~1 μm to 60 μm would fully encompass the majority of these commercial
systems (actual analysis examples are given in section 3.2.3.2.2).
Three standards were used to validate the Malvern Mastersizer:
0.9 μm - monodisperse polystyrene (Polysciences Incorporated)
5 μm - silica (ex Phase Sep (distributed by Alltech))
45 – 62 μm – glass beads (ex Polysciences Incorporated)
- 75 -
Chapter 3
Particle size distributions for the individual standards, pairs of the standards, and all three
standards in the one sample were measured to ensure instrument validity. The data is
shown in Appendix 2 where it can be seen that all standards and mixtures thereof gave
results within the appropriate specification (± 0.1 µm).
The results for the three
standards in one sample are also shown in Figure 3.2 where it can be seen that all three
are clearly differentiated.
Evaluation of a range of standards, using the Malvern Mastersizer and displayed in
Figure 3.2, has shown that accurate measurement of the average particle size of
monodisperse standards ranging from 0.9 μm to ~60 μm could be obtained. Even when
all three standards were mixed together, the instrument could successfully isolate the trimodal system.
To determine if the Mastersizer could separate standards that are even closer than a 5-fold
difference in average particle size, an additional monodisperse standard (14 μm) was
introduced. Figure 3.3 show the results, both in graphical and tabulated form, from
mixed 5 µm and 14 μm standards. A bi-modal distribution was obtained for the mixed
standards with a split peak showing that the instrument could just distinguish between
them. The high degree of overlap suggests that this ~3-fold particle size difference
between standards is the limit of differentiation for this instrument.
- 76 -
Chapter 3
Res ult: His togram Table
ID: mixed stds 0.9/5/55
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 14.1 %
Analy sis: Poly disperse
Residual: 1.226 %
Conc. = 0.0111 %Vol
Distribution: Volume
D(v , 0.1) = 0.98 um
Span = 2.646E+00
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.25
1.04
8
7
Measured: 30/8/1999 14:23
Analy sed: 30/8/1999 14:23
Source: Analy sed
Density = 0.960 g/cm^3
D[4, 3] = 31.17 um
D(v , 0.5) = 26.20 um
Unif ormity = 1.032E+00
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
2.37
4.32
6.76
9.11
11.15
12.60
13.53
13.77
13.93
14.23
14.86
16.22
18.89
23.26
29.17
35.65
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 1.6751 m^2/g
D[3, 2] = 3.73 um
D(v , 0.9) = 70.32 um
Volume
Under %
41.28
45.24
47.55
48.68
49.13
49.28
49.32
49.32
49.38
49.54
49.99
51.04
53.24
57.38
64.71
74.29
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
83.85
91.27
95.86
98.20
99.18
99.53
99.67
99.77
99.88
99.96
99.99
100.00
100.00
100.00
100.00
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
100.0
1000.0
0
Particle Diameter (µm.)
Figure 3.2
Malvern Mastersizer – Histogram and Graphical Standards Results
- 77 -
Chapter 3
Result: Histogram Table
ID: 5/14 stds
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 15.6 %
Analy sis: Poly disperse
Residual: 1.162 %
Conc. = 0.0135 %Vol
Distribution: Volume
D(v , 0.1) = 2.60 um
Span = 2.253E+00
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.01
0.01
0.02
0.04
0.08
0.14
0.24
0.39
0.61
0.88
1.21
1.57
1.97
10
9
Measured: 3/9/1999 09:58
Analy sed: 3/9/1999 09:58
Source: Analy sed
Density = 0.970 g/cm^3
D[4, 3] = 9.76 um
D(v , 0.5) = 7.74 um
Unif ormity = 7.150E-01
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
2.39
2.79
3.14
3.44
3.70
3.97
4.34
4.93
5.87
7.37
9.61
12.76
16.92
22.06
27.94
34.17
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 1.5340 m^2/g
D[3, 2] = 4.03 um
D(v , 0.9) = 20.04 um
Volume
Under %
40.36
46.31
52.03
57.70
63.53
69.73
76.33
82.99
89.04
93.94
97.39
99.42
99.99
100.00
100.00
100.00
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
100.0
1000.0
0
Particle Diameter (µm.)
Figure 3.3
Histogram and Graphical Results for Mixed 5 µm and 14 µm Standards
- 78 -
Chapter 3
3.2.3.2.2
Commercial Emulsion Evaluation
The EPM Technique is being evolved around today’s commercial emulsion requirements.
The determination of the average droplet size found in commercial emulsions is therefore
necessary to provide a droplet size value to aim for when the emulsions are prepared.
Three commercial lotion products where analysed for droplet size using the Malvern
Mastersizer. The products, used and the corresponding results, are given in Table 3.11.
Table 3.11
Droplet Size Evaluation of Commercial Emulsion Products
Commercial Emulsion
Nivea Body Lotion
Average Droplet Size
(μm)
2.2
Droplet Distribution Range
(μm)
0.1 – 9
Vaseline Intensive Care
Body Lotion
Dove Body Lotion
2.8
0.2 – 15
2.6
0.1 – 10
Redwin Sorbolene Lotion
4.8
0.2 – 55
The first three systems gave very similar results for both the average droplet size and the
distribution range. The results for the Redwin product showed a wider distribution and
consequent larger average droplet size. It was decided to use the results from the first
three emulsions as a tighter distribution and smaller particle size generally favours overall
emulsion stability. The desired droplet size selected for use in this project was 2.5 μm.
This Chapter has detailed the important materials, methods and instrumentation necessary
to continue with this study. The basis has now been set to investigate the EPM concept
and this will be outlined in the following chapters.
- 79 -
Chapter 4
CHAPTER 4
MATERIALS CHARACTERISATION
4.1 CHARACTERISATION OF EMULSIFIERS
This section details the characterisation of the selected emulsifiers according to the EPM
technique. For comparison, the classical HLB characterisation scale and the solubility
parameter technique are also given.
4.1.1 Interfacial Tension Data Determination for EPM Technique
Figure 4.1 displays a diagrammatic summary of the EPM technique, which was detailed
in Chapter 1.4.4.
Emulsifier
Value 1
Emulsion
Value 2
Value 2 – Value 1 = ΔValue Surfactant
Figure 4.1
Value 3
Value 4
Value 4 – Value 3 = ΔValue Emulsion
Diagrammatic Outline of the EPM Technique
Once each selected emulsifier (from the laureth and ceteareth emulsifier series) had been
split into its corresponding hydrophobic and hydrophilic entities, the interfacial tension
between the hydrophilic and hydrophobic entities was measured. By way of reminder of
the emulsifier ‘splitting’ process, the example given in Chapter 2 is shown again as
80
Chapter 4
Figure 4.2 and the tabulated hydrophobic and hydrophilic entities of the selected
emulsifiers shown in Table 4.1.
EPM Technique
Swinburne University of Technology
Example of ‘splitting’ the emulsifier laureth-4
Hydrophobic tail
Hydrophilic head group
CH3(CH2)10CH2(OCH2CH2)4OH
Add one Hydrogen
to make stable materials
CH3(CH2)10CH3
Dodecane
H(OCH2CH2)4OH
PEG 200
24th ACSSSC, SA, 2-6 February 2004
Figure 4.2
Example of Emulsifier ‘Splitting’ in EPM Technique
Table 4.1
Hydrophobic and Hydrophilic Moieties of Selected Emulsifiers
Emulsifier
(INCI Name)
Hydrophobic Moiety
Laureth-2
Laureth-3
Hydrophilic Moiety
Diethylene Glycol
Dodecane
Triethylene Glycol
Laureth-4
PEG 200
Ceteareth-12
PEG 600
Ceteareth-20
50% Octadecane
50% Hexadecane
PEG 1000
Ceteareth-30
PEG 1500
81
Chapter 4
4.1.1.1 Measured Interfacial Tension Data for Selected Emulsifier Moieties
The split emulsifier moieties, from the laureth emulsifier series, were liquid at room
temperature but the ceteareth emulsifier moieties were waxy solids and consequently,
much more difficult to use. Neither the room nor unit housing the FTÅ 200, were fully
temperature controllable. To eliminate the need for numerous data adjustments when
measurements were taken at different temperatures, care was taken to measure all data at
a room (and consequent FTÅ unit) temperature of 20°C (± 0.5ºC) by judicious choice of
which days to carry out experimentation and reliance on the air conditioning of the
building.
Two of the three hydrophilic moieties and the hydrophobic moiety of the ceteareth
emulsifiers selected (PEG 1000, PEG 1500 and the 50:50 mix of C16 and C18 alkanes),
were solid at 20°C, and hence their interfacial tensions could not be directly measured.
The interfacial tension of the C12, C14 and C16 alkanes and all PEG materials up to a
molecular weight of 600 (PEG 600 has a melting point of 18ºC (Lide, 1994)) could be
measured directly, so experimental data was obtained for these materials and the
remaining required results for the ceteareth emulsifiers were then determined by
extrapolation.
Additional PEG materials (PEG 300 and PEG 400), with intermediate molecular weights,
were tested to extend the number of data points for the extrapolation. The interfacial
tension of a 50:50 mix of C12/C14 alkanes was measured to confirm that the alkanes could
be mixed without any anomolous effects. To complete the data set for the alkanes, a
50:50 mix of C14/C16 alkanes was also included. For commercial emulsifiers, the alkane
moiety is rarely, if ever, a pure material. Mixed chain lengths are usual and for the tests
to be realistic it is important to include mixed alkane moieties within the data.
Table 4.2 provides a concise summary of the emulsifier moieties tested and the results
that were obtained by experimentation.
82
Chapter 4
Table 4.2
Interfacial Tension Data (mN m-1 @ 20°C) Between Ranges of
Alkanes and Glycol Materials (Selected Emulsifier Moieties)
Chemical Name
Molecular
Weight
(g mol-1)
Dodecane (C12)
Mixed System
(50:50 C12:C14)
Tetradecane
(C14)
Mixed System
(50:50 C14:C16)
Hexadecane
(C16)
DEG
TEG
PEG 200 PEG 300 PEG 400 PEG 600
106
150
2001
3001
4001
6001
12.6
12.7
12.5
12.1
11.9
12.1
11.1
10.9
10.8
10.6
10.4
10.5
10.3
10.2
10.1
9.9
9.9
9.9
12.6
12.0
10.9
10.5
10.2
9.9
12.8
12.9
12.8
12.6
12.7
12.6
11.3
11.2
11.5
11.0
10.8
10.9
10.4
10.5
10.4
10.1
10.2
10.2
12.8
12.6
11.3
10.9
10.4
10.2
13.5
13.3
13.2
12.8
12.8
12.9
11.8
11.8
11.8
11.5
11.5
11.3
10.6
10.5
10.7
10.4
10.5
10.4
13.3
12.8
11.8
11.4
10.6
10.4
RI too close
to see drop Refer
Table 4.3
13.6
13.7
13.6
12.3
12.5
12.5
12.0
11.8
11.9
10.8
10.9
10.9
10.6
10.6
10.5
14.1
13.6
12.5
11.9
10.9
10.6
RI too close
to see drop Refer
Table 4.3
RI too close
to see drop Refer
Table 4.3
13.1
13.2
13.1
12.2
12.4
12.3
11.4
11.3
11.4
10.8
10.7
10.8
14.8
14.2
13.1
12.3
11.4
10.8
Notes: 1The molecular weights of the PEG materials are given as average, approximate values because these materials are not
manufactured commercially as pure components. The number of moles of ethoxyl groups present is an average value (Merck,
1999). Although it is possible to purchase these particular materials in a pure form, they are representing moieties of
emulsifiers which are only commercially available as average number of moles of ethoxyl groups only. Diethylene glycol and
triethylene glycol are both commercially available as pure materials.
Average values in bold indicate arithmetic mean values of measured data. The remaining data was obtained using the dye test
explained in the next section. For reference, interfacial tension values for many of these alkanes against water can be found in
Table 4.7.
83
Chapter 4
4.1.1.2 FTÅ 200 Droplet Contrast
The refractive indices of the two substances under analysis need to be sufficiently
different to observe a contrast between the drop itself and the liquid in the bath. When
the refractive indices of the two test materials were very similar (i.e. ≤ 0.02 difference) it
took considerable optimisation of FTÅ 200 settings to obtain a clearly defined image
suitable for analysis and was sometimes impossible. For example when attempting to
measure hexadecane (R.I. of 1.434) against diethylene glycol (R.I. of 1.447) or
triethylene glycol (R.I. of 1.453), it was not possible for a droplet image to be clearly
observed. These data points were needed however, so a method to deal with this problem
was required.
The addition of a dye to the material forming the droplet to be viewed was used to
overcome the difficulty. A water soluble dye (Sicovit Patent Blue 85 ex BASF) was
added to the test materials.
Diethylene glycol and triethylene glycol were the test
materials with PEG 200 included as a known reference to show whether or not the dye
would influence the resulting interfacial tension.
Interfacial tensions of the three systems, with dye added, are shown in Table 4.3. The
results for PEG 200 were comparable to those obtained previously (Table 4.2) and it was
therefore assumed that the results for diethylene glycol and triethylene glycol were
reliable.
Although a ‘faint’ droplet could just be made out for the interfacial tension of the mixed
alkane system with diethylene glycol, the definition was not such that the FTÅ 200 could
utilise more than 80% of the datapoints to generate the result. It was decided to use the
addition of the dye in this case also as the accuracy of the result was much higher due to
the higher droplet definition achieved.
84
Chapter 4
Interfacial Tension Results (mN m-1 @ 20°C) with Dye Added
Table 4.3
Diethylene Glycol
Hexadecane
(C16)
1.
2.
3.
14.8
14.8
14.8
Av. 14.8
1.
2.
3.
50:50
C14:C16
Alkane
Triethylene Glycol
1.
2.
3.
14.2
14.3
14.2
Av. 14.2
PEG 200
1.
2
3
13.2
13.2
13.2
Av. 13.2
14.1
14.0
14.1
Av. 14.1
Confirmation
test
+ 0.05% dye 14.2
+ 0.1% dye 14.2
+ 0.5% dye 14.2
As a final test to check that the dye was not influencing the interfacial tension; three
different levels of dye (0.05%, 0.1% and 0.5%) were added to triethylene glycol. The
interfacial tensions of these dye solutions were measured against hexadecane to
determine if a difference could be observed. All average results obtained were the same
(14.2 mN m-1), indicating the absence of interference in the measurement upon the
addition of dye. These results are also displayed in Table 4.3.
All results determined using the dye addition are displayed in Table 4.1 in non-bold
format.
4.1.1.3
Data Extrapolation
To determine the remaining values for the ceteareth emulsifier moieties (those which
could not be measured under experimental conditions) it was necessary to extrapolate the
data that had been measured.
85
Chapter 4
In the first instance, interfacial tension values from Table 4.2 were plotted against alkane
chain length for each of the PEG hydrophiles.
This is displayed in Figure 4.3.
Extrapolation of these simple regression lines of best fit resulted in the relevant data point
for the mixed C16/C18 alkane (displayed as the point where each series line meets the
green vertical line).
16
Diethylene Glycol Series
15.2
15
Interfacial Tension Value (mN m
-1
)
14.6
Triethylene Glycol Series
14
13.6
P E G 200 S eries
13
12.8
P E G 300 S eries
12
11.6
PE G 400 S eries
11
11.0
P E G 600 S eries
10
9
C12
C14
Do decane
M Ixe d
C 12 - C 14
C16
T etradecane
M Ixe d
C 14 - C 16
H exadec ane
P r e d Ic t e d
C 16 - C 18
Alka ne Cha in Le ngth
Figure 4.3
Effect of Alkane Chain Length on Interfacial Tension
Linear plots of alkane chain length versus interfacial tension are expected when the
second phase is water (Schoenfeldt, 1969, Drelich & Miller, 2000), but could not be
assumed in this case, where the second phase was a glycol. The data points from all the
PEG series plots shown in Figure 4.3, however, do form a near straight line, thus giving
confidence in the use of linear extrapolation to derive the C16/C18 alkane values required
to continue the work. The data points for the diethylene glycol and triethylene glycol
however, deviate more from the line of best fit giving less confidence as discussed in
coming pages.
86
Chapter 4
The interfacial tension data shown in Table 4.2 is summarized in Table 4.4 along with the
extrapolated results for the C16/C18 system.
This table (Table 4.4) now shows all
hydrophobic components of the selected emulsifiers (Table 4.1) but not yet all the
hydrophilic components. Again, data for the remaining hydrophilic components can be
obtained by extrapolation (Figure 4.4).
Table 4.4
Interfacial Tension Data (mN m-1 @ 20°C) Between Ranges of
Alkanes and Glycol Materials (Selected Emulsifier Moieties)
SUMMARY DATA WITH EXTRAPOLATED RESULTS INCLUDED
Chemical Name
DEG
TEG
Molecular
Weight (g mol-1)
106
150
200
300
400
600
12.6
12.0
10.9
10.5
10.2
9.9
Mixed System
(50:50 C12:C14)
12.8
12.6
11.3
10.9
10.4
10.2
Tetradecane
(C14)
13.3
12.8
11.8
11.4
10.6
10.4
Mixed System
(50:50 C14:C16)
14.1
13.6
12.5
11.9
10.9
10.6
Hexadecane
(C16)
14.8
14.2
13.1
12.3
11.4
10.8
Mixed System
(50:50 C16:C18)
Extrapolated
results
15.2
14.6
13.6
12.8
11.6
11.0
Dodecane (C12)
PEG 200 PEG 300 PEG 400 PEG 600
In the first instance, all data in Table 4.4 was used to plot interfacial tension as a function
of the molecular weight of the hydrophile, but not yet extrapolated to higher molecular
weight.
87
Chapter 4
Using a log scale of the molecular weights, lines of best fit were added for each data set.
However, upon viewing these curves it was evident that the first two points for most data
sets (to the left of the red (bold), vertical line) were higher than the line of best fit, whilst
the remaining points (to the right of the red, vertical line) were below it. Whilst this may
indicate curvature, in this case it probably indicates two separate trends (separated by the
red, bold line). The effect is more pronounced for the lower chain length alkanes but still
quite obvious for the hexadecane results. An explanation for this effect was required
before the results could be validated and used for a meaningful extrapolation plot.
16
Dodecane
MIxed
MIxed
Hexadecane
C14-C16
C12-C14
Tetradecane
Log. (P r e d I c t e d
C16-C18)
Interfacial Tension Value (mN m
-1
)
15
14
13
12
11
10
9
100 DEG
TEG
PEG 200
PEG 300
PEG 400
Molecular Weight of Hydrophile (g mol
Figure 4.4
-1
PEG 600
) Log Scale
Interfacial Tension Values for Selected Emulsifier Moieties
Diethylene glycol (MW = 106) and triethylene glycol (MW = 150), whose points are to
the left of the red (bold) line, are pure components. The PEG materials, on the other
hand, are not. They are mixtures of similar polymer homologous members in the PEG
series (Huntsman Literature, 2001). The molecular weight distribution of PEGs 400 –
4000 corresponds to the Poisson distribution (Flory, 1940) that demonstrates an
88
1000
Chapter 4
increasing polydispersity of the molecular weight distribution with increasing mean
molecular weight.
The polydispersity of the PEGs probably results in different surface activity of these
materials as compared with pure materials. High molecular weight material dominates
any adsorption process because it has a disproportionately greater hydrophobic affect (its
desire to leave the water phase). If two surfactants having the same molecular weight,
but different degrees of polydispersity are compared, the more polydisperse surfactant
will have more, high molecular weight material, and thus will absorb into the oil phase
more strongly. Indeed, there is a large amount of experimental evidence to support
similar phenomena for a host of physical properties when comparing a polydisperse and a
monodisperse material possessing the same molecular weight; these include for a
polydisperse system, a lower cmc (Warr et al., 1983), greater partitioning into the oil
phase (Cowell et al., 2000) and lower surface tension between microemulsions and air
(Kegel, 1997).
The materials tested are meant to represent moieties of commercial surfactants.
Commercial ethoxylated surfactants are, like the PEG materials, comprised of a mixture
of polymer homologues.
Indeed, PEGs are one of the starting materials in the
manufacture of ethoxylated surfactants. It is, therefore, logical to use the data points
from the PEGs to extrapolate to the higher molecular weight PEG’s, rather than to use the
data points for diethylene and triethylene glycol which have less relevance to
commercial, highly ethoxylated surfactants.
The data for diethylene and triethylene glycol interfacial tension were therefore not used
during the extrapolation process to obtain data for high molecular weight PEGs.
The data (Figure 4.4) obtained for each of the alkanes shows a general trend of reducing
interfacial tension value as the molecular weight of the hydrophile increases. This is due
to an effective decrease in the influence of the polar hydroxyl functionality as the number
of ethylene oxide units increases. The result is that the hydrophilic entity becomes more
89
Chapter 4
oil-like with increasing molecular weight and so the interfacial tension between the two
moieties becomes less.
This, general trend, of reducing interfacial tension with
increasing molecular weight of the hydrophilic entity, has been observed by many
authors (Schick, 1980 and Jasper, 1972).
All data sets, however, need to be extended up to a hydrophile molecular weight of 1500
g mol-1 (to yield the data required for the emulsifier ceteareth-30), which is a significantly
higher molecular weight than the values actually measured. The lines displayed in Figure
4.4 are lines of best fit and were extended to a molecular weight of 1500 g mol-1 as
shown in Figure 4.5.
14
Dodecane
MIxed
MIxed
Hexadecane
C14-C16
C12-C14
Tetradecane
Log.
(P r e dC16-C18
icted C16-C18)
Predicted
Interfacial Tension Value (mN.m-1)
13
12
10.9
11
10
9.6
9
8.6
y = -2.5161Ln(x) + 26.956
8
100
1000
10000
Molecular Weight of Hydrophile (g.mol-1) Natural Log (Ln) Scale
Figure 4.5
Interfacial Tension Data for Selected Emulsifier Hydrophiles with
MW > 150 g mol -1
The data, as shown in Figure 4.4 to the right of the red line, results in lines that all follow
a similar trend but the lines for each data set, as shown in Figure 4.5, do not form a
similar, simple pattern.
90
Chapter 4
All lines do show the same trend of a linear decrease in interfacial tension with
hydrophile molecular weight, but have increasingly steeper gradients with increasing
hydrophobe molecular weight such that a point of convergence is reached. At the point
of the highest molecular weight under consideration, 1500 g mol-1, the order of the curves
has fully reversed, and this must cast some doubt on the extrapolation at this molecular
weight. However, it is true to say that the trend of all lines is to converge at a point very
near to the molecular weight of 1000 g mol-1 and at this molecular weight, the potential
for error in interfacial tension is not large.
The points required to continue the project are the hydrophile (or PEG) values for 600,
1000 and 1500 g mol-1 for the C16/C18 alkane mixture. These are indicated by the arrowed
boxes with values 10.9, 9.6 and 8.6 respectively in Figure 4.6.
These figures, (from Figure 4.6 boxed arrows), complete the data required to give the
EPM values (in terms of interfacial tension) for the selected emulsifier moieties. Table
4.5 shows a summary of the data points taken from Figure 4.6 as well as the relevant data
values from Table 4.4.
Table 4.5 EPM Values (Expressed and Measured as Interfacial Tension) for
Selected Emulsifiers
Hydrophilic
Entity
Density
(ρ) g cm-3
(20°C)
Interfacial
Tension
(mN m-1)
Diethylene Glycol
1.121
12.6
Triethylene Glycol
1.128
12.0
Laureth-4
PEG 200
1.129
10.9
Ceteareth-12
PEG 600
1.130
10.9
PEG 1000
1.170
9.6
PEG 1500
1.212
8.6
Emulsifier
Hydrophobic
Entity
Density
(ρ) g cm-3
(20°C)
Laureth-2
Laureth-3
Ceteareth-20
Ceteareth-30
Dodecane
50% Octadecane
50%Hexadecane
0.749
0.775*
91
Chapter 4
*Arithmetic mean of density values for Octadecane and Hexadecane.
These values in Table 4.5 are those used in the practical evaluation of the EPM technique
detailed in the following chapters. The density value (Lide, 1994) for each component is
also given, as this is a required input for the FTÅ 200 instrument.
4.1.2 Classical HLB and Solubility Parameter Classification
From Table 4.5 the EPM values for laureth-4 and ceteareth-12 are the same
(10.9 mN m-1). These emulsifiers have quite different HLB values as shown in Table 3.1
and so will be a good choice of emulsifier to test and compare the EPM and HLB
techniques.
The HLB and Solubility Parameter theories were discussed in Chapter 1.4. Values for
these theories, where available, for the selected emulsifiers are given in Table 4.6 below.
The EPM values (from Table 4.5) are also included for comparison.
Table 4.6 HLB and Solubility Parameter Values for Selected Emulsifiers
EPM Value
Emulsifier
HLB Value
Solubility Parameter
(MPa)
(based on Interfacial
Tension (mN m-1))
Laureth-2
Laureth-3
Laureth-4
7
8.0
9.6
7.8*
8.1*
8.3
12.6
12.0
10.9
Ceteareth-12
Ceteareth-20
Ceteareth-30
13
15
17
(8.9*)
9.1
(9.3*)
10.9
9.6
8.6
* Denotes Solubility Parameter determination carried out using Drop Weight Technique (refer 1.4.2)
HLB values were obtained from the manufacturer and solubility parameter figures from
Vaughan (1988). For surfactants that fall into the emulsifier category HLB values are
92
Chapter 4
widely available but solubility parameter values are not. Although solubility parameters
of these emulsifiers can be calculated, it is a complex process and for the reasons
explained in Section 4.1.3 cannot be justified within the scope of this project. However,
estimates in the form of drop weight measurements (as described in Section 1.4.2) were
measured and results for laureth-2 and laureth-3 have been added to Table 4.6 in italics.
Due to the fact that ceteareth-12 and ceteareth-30 are solid at room temperature; these
materials had to be heated to 50°C to conduct the drop weight test. Because no literature
support is available for the accuracy of this method at elevated temperatures the figures
obtained are shown in brackets.
The two theories show opposite trends with increasing molecular weight; increasing
solubility parameter figures but decreasing EPM values with increasing molecular
weight.
4.1.3 Alternative Solubility Parameter Classification using EPM Concept
The solubility parameter is a measure of ‘likeness’ of one material to another and is an
excellent technique when comparing simple molecules (refer Section 1.4.2 for further
detail).
When applied to emulsifiers, however, it is reasonable to assume that the
hydrophobic and hydrophilic entities each have component solubility parameter values
which would be considerably different to each other. Literature values listed in texts
offer only one value for each emulsifier. This lies between the two values that would be
achieved if each entity was measured separately. Moreover, it is possible to have a solute
with the ‘correct’ solubility parameter for a solvent but not be soluble because its
component (see Section 1.4.2.1) solubility parameter terms are too disparate.
The EPM concept of applying values to the separate emulsifier entities may be a suitable
tool to apply to solubility parameter values. Table 4.7 gives the split emulsifier moieties
with their relevant solubility parameter values. Data was again obtained again from
Vaughan (1988) and Barton (1990). Values for PEG 600 and PEG 1500 were not
93
Chapter 4
available, so were measured using the Drop Weight method described in Section 1.4.2
and are marked with an asterisk to indicate the difference.
The data for the hydrophilic entity in Table 4.7 show a clear trend for the solubility
parameter, reducing as the material becomes more non-polar. As the material increases
in carbon chain length it becomes less like water and less water soluble for this reason.
Consequently the solubility parameter value decreases (for reference, water has a value of
~24 MPa). The interfacial tension values in Table 4.5 also show the same trend.
Table 4.7: Solubility Parameter Values as Applied to the Hydrophobic and
Hydrophilic Moieties of the Selected Emulsifiers and to EPM Technique
Solubility
Emulsifier
Hydrophobic
Entity
Dodecane
7.59
Ceteareth-20
Ceteareth-30
50%
Octadecane
50%
Hexadecane
EPM
Value
Hydrophilic
Entity
(MPa)
(from Sol.
Parameter
Data)
Diethylene Glycol
13.61
5.0
12.6
Triethylene Glycol
12.21
4.8
12.0
PEG 200
11.61
4.0
10.9
PEG 600
11.1*
3.8
10.9
PEG 1000
10.95
3.6
9.6
PEG 1500
10.85*
3.5
8.6
Laureth-4
Ceteareth-12
EPM
Value
Parameter
(MPa)
Laureth-2
Laureth-3
Solubility
Parameter
7.29
(from
Interfacial
Tension
Data)
* Denotes Solubility Parameter determination carried out using Drop Weight Technique (refer 1.4.2)
If applying the EPM concept to the solubility parameter, then it would take the form of
the ‘difference’ between the hydrophilic and the hydrophobic entity solubility parameter
values (i.e. one value subtracted from the other). The values for the EPM from the
solubility parameter of the hydrophobic entity subtracted from the solubility parameter of
the hydrophilic entity are shown in bold in Table 4.7.
94
Chapter 4
It is clear from Table 4.7, that there is a strong similarity in the pattern of the EPM values
derived from both interfacial tension and solubility parameter. With further data, a
scaling factor could be calculated and confirmed to link the EPM values determined from
these different methods. A scaling factor may also be required when matching the
surfactant data with the oil/water data. Even with the limited results shown in Table 4.7,
it can be seen that the EPM value from interfacial tension is ‘very approximately’ 2.5
times the solubility parameter derived value.
4.2 CHARACTERISATION OF OILS
4.2.1 Interfacial Tension Data
Interfacial tension values between possible oil and aqueous phases are also required to
prepare the EPM database and to compare these with the emulsifier data. Average
interfacial tension results (arithmetic mean of three measurements) of the selected oils,
were measured against purified water, and the data is given in Table 4.8 on the following
page.
Table 4.8 displays a clear, known trend (Shaw, 1993) in that the interfacial tension data
reduces with increasing oil polarity. The trend is most easily observed when comparing
materials from the same family where only the chain length differs (e.g. hexadecane
compared with dodecane and IPP compared with IPM).
For reference purposes,
structures of all the chemicals used in this thesis are displayed on page xv.
Dioctylcyclohexane (Cetiol S) has a low value for an alkane due to its cyclic structure
and two ‘octyl’ sidegroups. Octyl dodecanol (Eutanol G) has a high value for an alcohol.
This material is a branched alcohol made according to the Guerbet reaction (Clayden et
al. 2000). Steric factors result in a higher interfacial tension value.
95
Chapter 4
Table 4.8
Interfacial Tension Results for Selected Oils against Purified Water
Chemical Name
Density
(g cm-3)
20°C
Average Interfacial
Tension / mN m-1
(γI)
(+/- 0.2 mN m-1)
Analar Grade
Hexadecane
0.773
53.8
Analar Grade
Dodecane
0.749
52.9
OP61A
Mineral Oil
0.837
44.1
Cetiol OE
Dicaprylyl Ether
0.808
29.2
Cetiol S
Dioctylcyclohexane
0.830
28.4
IPP
Iso-Propyl Palmitate
0.853
25.2
IPM
Iso-Propyl Myristate
0.853
24.0
Eutanol G
Octyl Dodecanol
0.840
23.1
Cegesoft C24
Octyl Palmitate
0.860
23.0
Cetiol 868
Octyl Stearate
0.860
23.0
Cetiol A
Hexyl Laurate
0.859
20.8
Myritol 318
Caprylic/Capric
Triglyceride
0.940
20.2
Cetiol V
Decyl Oleate
0.865
19.5
Cetiol B
Dibutyl Adipate
0.960
15.0
Myritol 331
Cocoglycerides
0.935
8.3
Trade Name or
Grade
The most ‘non-polar’ oil used in cosmetic formulation is mineral oil; hexadecane or
dodecane are not used and interfacial tension values are included for reference only. The
use of mineral oil, although is used less these days, does creates a challenge for the EPM
technique in that emulsifiers will be required to have an EPM value (using ‘absolute’
interfacial tension) of ~44. Table 4.4 showed EPM values for the selected nonionic
emulsifiers to be in the order of 9 – 13. By the nature of the ethoxylated nonionic
surfactants, it will not be possible to achieve EPM values of ~40 using this surfactant
type alone (unless a scaling factor can be applied). However, if anionic surfactants are
considered then higher emulsifier EPM values can be achieved. These have a water
96
Chapter 4
soluble hydrophile and in the case of Sodium Lauryl Sulphate (SLS), dodecane as the
hydrophobe. EPM values in the order of 40 are possible if the hydrophile is allowed to
be measured in solution because the actual hydrophilic entity for SLS would be a solid
salt (sodium hydrogen sulphate).
The fact that there are surfactants available that could emulsify the most non-polar oil is
as far as this direction will be taken in the present work. An investigation of anionic
surfactants on their own or blended with nonionic surfactants is a significant amount of
work which would need to be carried out in the future development of the EPM
technique.
The current work will concentrate on the use of the most commonly used
materials in cosmetic emulsions; polar oils and nonionic emulsifiers. The possibility of
the use of scaling factor for EPM values derived by interfacial tension is, however,
investigated a little more in the current work in Chapter 6.
4.2.2 HLB and Solubility Parameter Classification of Oils
For comparison, Table 4.9 on the following page, lists the measured interfacial tension
(against water) data of many materials from Table 4.8 together with literature values for
required HLB and classical solubility parameter data values, where available (Vaughan,
1991).
The trend observed with the interfacial tension data is not followed with either the
solubility parameter or the Required HLB values of the oils. However, as shown in
Section 1.5 the two systems (solubility parameter and HLB) are related and do exhibit a
correlation in the data in Table 4.9. Presumably, the trends seen only apply to oils of a
similar class. This has already been identified as a major weakness in the application of
HLB values and may or may not prove to be a similar weakness in the EPM approach.
97
Chapter 4
Table 4.9
Summary of Interfacial Tension, Required HLB and Solubility
Parameter Values for the Selected Oils
Trade Name
or Grade
Chemical Name
Interfacial
Tension
(mN m-1)
Required
HLB Value
Solubility
Parameter
Analar Grade
Dodecane
52.9
11.06
7.59
OP 61A
Mineral Oil
44.1
9.62
7.09
Cetiol OE
Dicaprylyl Ether
29.2
10.21
7.30
IPP
Iso-Propyl Palmitate
25.2
11.65
7.78
IPM
Iso-Propyl Myristate
24.0
12.43
8.02
Eutanol G
Octyl Dodecanol
23.1
15.66
8.92
Cegesoft C24
Octyl Palmitate
23.0
10.61
7.44
Cetiol 868
Octyl Stearate
23.0
-
7.53
Myritol 318
Caprylic/Capric
Triglyceride
20.2
13.34
8.29
Cetiol V
Decyl Oleate
19.5
9.17
6.92
Myritol 331
Cocoglycerides
8.3
13.4*
-
*Myritol 331 is a relatively new material (1999). The HLB result was supplied by the supplier Cognis.
The preceding tables conclude the characterisation of the materials used throughout this
work.
Considerably more data would be required for a meaningful database to be
constructed. However, the materials that have been evaluated can provide the feasibility
tests required to decide on the usefulness of the proposed EPM technique.
98
Chapter 5
CHAPTER 5
EMULSION PREPARATION
The aim of the EPM technique is to select the optimal emulsifier (or emulsifier
combination) to achieve the best emulsion stability. A further benefit of the EPM
technique is that by measuring the interfacial tension of the phases to be emulsified, the
emulsion system as a whole is being considered and this could also be utilized be used to
help determine the correct level of the optimal emulsifier(s) to be used. This is the
subject for investigation in this Chapter. The HLB technique makes no attempt to predict
emulsifier concentration.
An added advantage of utilising interfacial tension is that prediction of the minimum
concentration of emulsifier required to achieve the lowest interfacial tension can be
obtained (by plotting interfacial tension versus concentration using the particular
surfactant / surfactant blend). No current emulsifier prediction technique offers any
guidance on emulsifier concentration to use. Although, invariably an excess will be used,
it is useful (from a cost, solubility as well as skin compatibility perspective) to ensure that
this excess is not too large and only enough to ensure adequate stability.
In Section 3.2.2.4 a theoretical level of laureth-4 emulsifier of 0.1% was shown to be all
that would be required to emulsify 15% of oil into water.
In practice, however,
considerably more is required. This chapter begins to investigate the effect of emulsifier
concentration on emulsion formation and stability in practice. It provides background,
practical data on some elements of the emulsion preparation used in this work, as well as
determining the feasibility of the EPM technique to predict emulsifier concentration.
For consistency, laureth-4 was chosen as the emulsifier to be used in these experiments to fully investigate these effects for all emulsifiers being studied was beyond the scope of
this study. The parameters to be used in the preparation of the emulsions in this chapter
- 99 -
Chapter 5
were summarized in Table 3.10. In this chapter, concentration, the effect of shear on
initial droplet size and emulsion stability is also investigated.
5.1 EMULSIFIER CONCENTRATION EFFECTS ON EMULSION
FORMATION AND STABILITY
This section investigates the influence of laureth-4 concentration on interfacial tension,
initial emulsifier droplet size and resulting emulsion stability.
5.1.1
Effect of Emulsifier Concentration on Interfacial Tension
Solutions of varying emulsifier concentrations were prepared using laureth-4. Using the
interfacial cell of the FTÅ instrument, a drop from each emulsifier solution was formed,
via a syringe, into a bath of oil (capric/caprylic triglyceride). The interfacial tension for
the two materials could then be measured using the methods described in Chapter 3.2.
Capric/caprylic triglyceride was the oil selected because it is a common emollient in the
cosmetic and pharmaceutical areas. A hanging drop was only achievable with the water
phase dropped into the oil phase due to the higher density of the water.
The concentrations used and the interfacial tension results obtained are summarised in
Figure 5.1 in both graphical and tabulated formats.
Note: with the low emulsifier dilutions used the x-axis scale of Emulsifier Concentration ‘% m/m’ is the
same as ‘% w/w’
- 100 -
Chapter 5
Interfacial Tension mN m-1
Series1Concentration in water
Emulsifier
Interfacial Tension
(% m/m)
(mN m-1)
25
20
15
0
20.2
0.01
8.2
0.1
2.0
0.3
1.6
0.5
0.9
1
0.74
2
0.72
3
0.72
10
5
0
0
0.5
1
1.5
2
2.5
Emulsifier Concentration (%m/m)
Figure 5.1
Effect of Laureth-4 Concentration on Interfacial Tension
Water + Laureth-4 : Capric/Caprylic Triglyceride System
There are two clear effects on the interfacial tension as the emulsifier concentration
increases;
1) The interfacial tension values decrease strongly with initial increasing
concentration.
2) A plateau is reached, which in this case is between 0.5 to 1.0%, where no further
significant reduction in interfacial tension can be measured, even when two or
three times the emulsifier concentration is reached. For this particular case many
more data points would be required to accurately determine this point of inflexion
in the results curve but for the scope of this work an approximate figure is
sufficient.
- 101 -
3
Chapter 5
This data now gives us three identifiable methods that can be useful in predicting
emulsifier concentration as follows:
1.
Theoretical quantity (for laureth-4 ~0.1% required to emulsify 15 mls of oil
into water (Section 3.2.2.4)).
2.
Cmc value (for laureth-4 ~0.003% (Section 3.2.2.4))
3.
Interfacial tension cmc value (for laureth-4 solutions into capric / caprylic
triglyceride ~0.6%)
The interfacial tension cmc value for emulsifier concentration (~0.6%) is very high
compared to the other values. The experiment above in this Section 5.1.1 was repeated
and values were again achieved within +/- 0.1 mN m-1. The high value must be due to the
fact that laureth-4 is soluble in the oil phase as well as the water phase and some
emulsifier must be diffusing into the oil phase prior to the interfacial tension value
stabilizing.
5.1.2 Effect of Emulsifier Concentration on Particle Size
If the lowest interfacial tension value is achieved then the smallest droplet sizes should be
formed.
By measuring resulting droplet size formed using different emulsifier
concentration it may be possible to correlate smallest droplet size to lowest interfacial
tension and use this to help identify an optimal concentration.
In order to verify this, emulsions were prepared using the conditions defined in Chapter
3.2. The emulsifier and oil materials, as well as the emulsifier concentrations are as in
section 5.1.1. Particle size measurements were taken on emulsions prepared both with
and without shear. Results are given in Table 5.1.
- 102 -
Chapter 5
Table 5.1
Effect of Emulsifier Concentration on Droplet Size
Emulsifer
Concn
% (m/m)
Emulsion Appearance and/or
Droplet size – No Mechanical Shear
Emulsion Appearance and/or
Droplet size – Shear (90 sec @ 9000 rpm)
Initial
Initial
0.1
Coarse–not spontaneous (free oil after 5 min)
Not determined #
0.3
Spontaneous (57 μm)
3.3 μm
0.5
Spontaneous (34 μm)
2.7 μm
1
Spontaneous (18 μm)
2.1 μm
2
Spontaneous (18 µm)
2.2 µm
3
Spontaneous (18 µm)
2.2 µm
Note: The ‘initial’ droplet size measurement was taken immediately after emulsification of the system.
For the case where no shear was applied the measurement was taken after 30 seconds inverting the cylinder
to sufficiently homogenise.
For the samples where shear was applied, the measurement was taken
immediately after the 90 seconds of shear had been applied.
#
The droplet size for the 0.1% concentration with shear applied was not measured because the
emulsion was already breaking (clearly visible) during the 30 seconds after emulsification. It was not
possible to measure an accurate value for the droplet size due to this high instability.
It is evident that the particle size data confirms the trend found from interfacial tension
results. Also, despite the fact that the initial droplet sizes measured were very different
between the “shear” and “no shear” cases, the overall trend is the same for both sets of
data.
In section 1.3.4, the theoretical level of emulsifier required to achieve a droplet size of
2.5 μm was determined. The result indicated that 0.1% emulsifier would be sufficient.
This is clearly not the case in practice. From the data presented in Table 5.1 it can be
seen that an emulsifier concentration of between 0.5 and 1 % was required to achieve 2.5
μm droplets.
- 103 -
Chapter 5
The theoretical calculation for emulsifier concentration assumed that only a monolayer of
emulsifier is required – a fact that is not necessarily correct. The calculation also used a
specific value for the surface area of an emulsifier molecule – a value determined
experimentally by Schick (1980). It is known that this surface area can vary quite
considerably depending on the conditions and emulsifier concentration used so was only
taken as an indication.
It should be stressed, however, that one reason for the use of interfacial tension
measurement in the development of the EPM technique was that it should correlate with
surfactant concentration at the interface. The lower the interfacial tension value, the
greater the tendency for the surfactant molecules to go to the interface and the more
efficient the surfactant packing at the interface.
5.1.3 Effect on Emulsion Stability
To conclude this set of experiments, the stability of the test emulsions (from Table 5.1)
was examined visually using the method defined in Section 1.4.2 and the results are given
in Table 5.2. The 24-hour results show emulsions with both ‘no shear’ and ‘shear’
applied. The ‘no shear’ samples (with considerably less energy input into the systems)
had all separated after 24 hours and so these tests were not continued for the later time
periods.
Note: * ‘Phased out’ indicates that the emulsion displayed two distinct phases – an aqueous emulsion
phase as well as > 5 mL clear, free oil. Where the previous time point clearly displayed distinct
separation, this sample was not tested at the following time point. The term ‘cream’ indicates first signs of
instability with a higher density layer appearing at the top of the emulsion.
- 104 -
Chapter 5
Table 5.2
Effect of Emulsifier Concentration on Emulsion Stability
Emulsifier
Emulsion Appearance
Conc (%)
After 24 hours
Emulsion Appearance
Emulsion Appearance
After 72 hours
After 1 Week
Emulsion with shear
Emulsion with shear
No Shear
Shear
0.1
Phased out*
Phased out*
Phased out*
Not determined
0.3
Phased out*
5 mL cream
1 mL free oil /8 mL cream
Not determined
0.5
Phased out *
2 mL cream
trace free oil /4 mL cream
2 mL free oil/10 mL cream
1
Phased out*
homogenous
0.5 mL cream
68 mL cream
2
Phased out*
homogenous
trace cream
5 mL cream
3
Phased out*
homogenous
trace cream
5 mL cream
The stability of the emulsions increases with increasing emulsifier concentration. Again,
as with the earlier interfacial tension and particle size results, only minimal improvement
was observed after approximately 1% emulsifier concentration was reached. The optimal
concentration in this case, was slightly higher at between 1 and 2%. At emulsifier levels
of 2% or higher, there is no improvement in the stability. It should be noted that laureth4 does have a slight thickening effect on aqueous media so it is likely that any stability
improvement as concentration increases is due to viscosity increases only. Increasing
viscosity causes a slowdown in the occurrence of droplet collisions and thus shows an
increase in emulsion stability but via a different mechanism to that under consideration.
The three results determined in this chapter for the optimal emulsifier concentration when
using laureth-4 as the emulsifier can now be easily compared alongside the theoretical
determination in Section 3.2.2.4. This data is summarized in Table 5.3.
Table 5.3
Summary of Optimal Emulsifier Concentration Dermination
Method Used
Optimal Emulsifier Concentration (% m/m)
Theoretical Determination
0.1
Interfacial Tension
0.5 – 0.6
Particle Size
1.0
Observation (Visual Determination)
1.0 – 2.0
- 105 -
Chapter 5
Clearly, a value well in excess of 1% is required. However, studying systems that fail
often tell us more than those which are stable so a level at the lower end of the
requirement should be used to continue this work.
It was decided to use an emulsifier concentration of 0.5%. This level is sufficient to form
an ‘adequate’ emulsion close to 2.5 µm but not so much emulsifier as to hide signs of
initial separation. This should also avoid viscosity effects becoming significant and
influencing the result.
5.2 DEGREE AND TIME OF MIXING
5.2.1 Effect on Initial Particle Size
The effect that applied shear had on the initial particle size has been so far in this study
determined by applying 9000 rpm shear for 90 seconds. It was felt that it may be
possible to achieve a specific, average droplet size of 2.5 μm with lower levels of
emulsifier so long as shear is applied for longer.
To test this, further shear was applied for varying times in an attempt to achieve an
average droplet size of 2.5 μm. The time required, keeping a constant rate of shear (9000
rpm), to achieve ≤ 2.5 μm droplet size is given in Table 5.3. The resulting stability of the
emulsions is also given. The particle size was measured as soon as possible after the
stirrer was turned off. In practice this was approximately 2 minutes after shear was
applied.
The results in Table 5.3 show that it is possible to achieve an emulsion droplet size of
2.5 μm using an emulsifier concentration as little as 0.3 %, but using considerable shear.
At lower concentrations it is not possible to achieve such a small droplet size, even when
several minutes of additional shear is applied.
- 106 -
Chapter 5
Table 5.3
Shear Time vs Emulsifier Concentration to Achieve 2.5 µm Droplet Size
Emulsifier
Concn (%)
Shear Time
(seconds)
Particle Size
(μm)
Appearance After 72 hours
0.1
Not determined
Not determined
15 mL free oil
0.2
210
4.6
10 mL free oil
300
4.2
8 mL free oil
450
4.1
5 mL free oil
150
2.8
8 mL cream + 2 mL free oil
210
2.4
5 mL cream + 0.5 mL free oil
90
2.7
4 mL cream + trace free oil
150
2.3
2 mL cream
90
2.1
1 mL cream
0.3
0.5
1.0
Although an initial droplet size of ~2.5 μm could be achieved using 0.3% emulsifier, the
resulting emulsion was not as stable as those made from a higher concentration of
emulsifier. This is despite a similar initial particle size in both cases, demonstrating the
importance of more than one technique for monitoring emulsion performance.
Furthermore, increasing the time of shear does not fully compensate for using less
emulsifier.
It was concluded that for the purposes of this study, emulsions would be prepared using
an emulsifier concentration of 0.5 % w/v and using 90 seconds of a high shear rate of
9000 rpm.
- 107 -
Chapter 6
CHAPTER 6
CHARACTERISATION OF EMULSIONS
6.1 DESIGN AND STABILITY OF TEST EMULSION SYSTEMS
At last an evaluation of the EPM technique itself can now be carried out and this is the
basis for this chapter. Using the materials detailed in Chapter 3 and the characterization
results presented in Chapter 4, two series of test emulsions were designed based on phase
matching emulsion components. These designed emulsions were themselves evaluated
for stability and also compared to the traditional HLB system.
Although it would be normal practice for emulsifiers to be selected based on the oils
chosen, in this work the range of surfactants evaluated is somewhat limited. Therefore,
for these initial tests we vary the oil to match the emulsifier. The emulsifier is varied, in
the normal way, in the next chapter.
Please note that at this stage only oil(s), water and emulsifier(s) are included in the
emulsion.
The expected emulsion stability of these systems is comparable to an
agrochemical emulsion, for example, where ~ 8 hours (once made up) good stability is
expected. Without structuring the system it is not realistic to achieve much greater
stability than this without forming a more thermally stable microemulsion.
6.1.1 ‘Ideal’ Emulsion System Design
To begin with, it was felt necessary to at least try an ‘ideal’ emulsion system in addition
to commercially representative systems which would be more thoroughly evaluated for
the EPM Technique.
- 108 -
Chapter 6
An ‘ideal’ system would be to create an emulsion where the oil and water phases are
comprised of the hydrophobic and hydrophilic moieties of the emulsifier itself.
This is
feasible for the laureth series of emulsifiers but not for the ceteareth series, where a solid
emulsion would be formed. The composition of proposed ‘ideal’ systems is shown as
Test Emulsions 1 – 3, in Table 6.1.
The emulsion systems shown are described as ‘ideal’ because the oil phase is perfectly
matched with the hydrophobic entity of the emulsifier and the aqueous phase is perfectly
matched with the hydrophilic portion of the emulsifier.
Table 6.1
Proposed ‘Ideal’ Test Emulsion Systems
Test
Lipophilic Phase
Hydrophilic Phase
Emulsifier
1
Dodecane
Diethylene Glycol
Laureth-2
2
Dodecane
Triethylene Glycol
Laureth-3
3
Dodecane
PEG 200
Laureth-4
Although these systems are feasible for the preparation of emulsions, they differ
significantly from standard commercially available emulsion systems, which use water as
the main constituent of the hydrophilic phase. Without water making up the aqueous
phase it is not possible to compare an equivalent HLB recommendation for this system.
Another more significant problem with these systems was discovered as soon as they
were prepared; the glycols and PEG 200 were each able to partially solubilise the
dodecane to give one homogenous clear solution. A fundamental requirement of forming
emulsions phases is that they be immiscible. Since this was not met in the proposed
‘ideal’ system, no useful information could be gleaned by continuing with their study.
To achieve useful and realistic emulsions for this work the glycols were replaced by
water as the aqueous phase. This also enables comparison against the HLB system.
- 109 -
Chapter 6
6.1.2 EPM vs HLB Comparison: Test Emulsion 4
Test Emulsion 3 from Table 6.1 was again selected, this time with water replacing the
glycol as the aqueous phase. This is labelled as Test 4 in Table 6.2 below.
A variant of Test 4 could also be used to test the HLB theory for this dodecane / water
system. HLB tables (Vaughan, 1988) show a required HLB value of 11.06 for dodecane.
Laureth-4 has an HLB of 9.6 and ceteareth-12 has an HLB of 13. According to the
principals of the HLB system these two emulsifiers (laureth-4 and ceteareth-12) can be
blended in the ratio that would achieve a match to the required HLB. In this case a ratio
of 55:45 laureth-4: ceteareth-12 gave the emulsifier ratio to match the HLB of 11.06.
Table 6.2 shows the composition of emulsions Test 4 (using EPM Technique) and 4HLB
(using HLB Technique) which were prepared and examined for direct comparison of the
two techniques. The interfacial tension of the dodecane / water interface was measured
as 52.8 mN m-1 which agrees with the literature value given in Table 4.9.
Table 6.2
EPM / HLB Comparison: Test Emulsion 4
Test
Oil Phase
Aqueous Phase
Emulsifier
4
Dodecane
Water
Laureth-4 (100%)
4HLB
Dodecane
Water
Laureth-4 / Ceteareth-12 (55%:45%)
Both emulsions formed spontaneously to give homogenous, white emulsions. The initial
particle size of the emulsions as well as their visual appearance after 24 hours is given in
Table 6.3.
Table 6.3
Results and Stability of Test Emulsion 4
Test
Emulsion
Initial Droplet
Size (µm)
Visual Appearance After 24 hours
4
3.6
1 mL cream
4 HLB
4.6
2 mL cream
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Chapter 6
Results showed that the emulsifier selected under the EPM method performed marginally
better than the emulsifier system selected under the HLB system. A smaller initial
droplet size was achieved as well as less separation in the 24 hour visual test (Section
6.1.5 tabulates further time point comparisons between HLB and EPM selected
emulsifiers). Although this indicates a weakness in the HLB system rather than proving
the strength of the EPM system, it is nevertheless, an encouraging result.
6.1.3 EPM and EPM vs HLB: Test Emulsion 5
The next series of test emulsions used oil phases that are found in commercial emulsions.
For the EPM method, using the absolute interfacial tension data previously measured, it
was clear that blends of oils would be required. None of the oils alone were an exact
match for the emulsifiers tested. The cocoglyceride oil, Myritol 331 (interfacial tension
against water of 8.3 mN m-1) was included in every oil blend, as required, to achieve an
interfacial tension match. Interfacial tension data was previously recorded in Table 4.1
for both pure oils and also one blended ratio of the glycerides. This demonstrates the
additive effect of interfacial tension in blending at least for these simple chemical
combinations of oils.
Later, in Section 6.1.6, there is a study to investigate whether a scaling factor might be
required to accurately give the EPM value based on interfacial tension. The assumption
in this section is that absolute values must be used but this is not proven at this point in
time.
In order to keep these initial systems in Test Emulsion 5 to one chemical family (and
avoid any other chemical interactions), cocoglyceride was used in combination with the
other triglyceride (capric/caprylic triglyceride, Myritol 318, which has an interfacial
tension value against water measured at 20.2 mN m-1). Using the interfacial tension data
recorded in Table 4.1 the appropriate oil blends were calculated and are detailed in Table
6.4. This Table 6.4 also outlines the corresponding emulsifier along with a summary of
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Chapter 6
the EPM value for that emulsifier and required HLB for the oil ratio derived for that
emulsifier.
Example: for test emulsion 5a, a 64:36 ratio of Myritol 331 (cocoglyceride) : Myritol 318
(capric/caprylic triglyceride) gives a calculated interfacial tension value (against water) of
12.6 mN m-1. This value matches the 12.6 mN m-1 interfacial tension value measured for
diethylene glycol against dodecane – the separate entities of the emulsifier Laureth-2.
Table 6.4
EPM Test Emulsion 5
Test
Oil Phase and Ratio
5a
HLB
Emulsifier
Myritol 331/ Myritol 318 (64:36)
13.4
5b
Myritol 331/ Myritol 318 (70:30)
5c
(Required
for Oil)
HLB
EPM
(Emulsifier)
(matching
oil &
emulsifier)
Laureth-2
7
12.6
13.4
Laureth-3
8
12.0
Myritol 331/ Myritol 318 (78:22)
13.4
Laureth-4
9.6
10.9
5d
Myritol 331/ Myritol 318 (78:22)
13.4
Ceteareth-12
13
10.9
5e
Myritol 331/ Myritol 318 (87:13)
13.4
Ceteareth-20
15
9.6
5f
Myritol 331/ Myritol 318
13.4
Ceteareth-30
17
8.6
(97:3)
Note: the aqueous phase in all Test 5 Emulsions was water
The required HLB values of the Myritol 318 and Myritol 331 (both glycerides) are almost
the same (13.3 and 13.4). This highlights one major difference between the HLB and
EPM techniques because all of the Required HLB values are the same but the EPM
values are different in the above emulsions. The summary of properties for commercial
oils was listed in Table 4.7; no relationship was observed between required HLB and
interfacial tension.
It is true to say that, for these particular sets of emulsifiers, a trend can clearly be seen in
the HLB values of the emulsifier and the EPM values achieved. This trend is increasing
from Test 5a to 5f in HLB and decreasing in the EPM values.
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Chapter 6
Comparison of the emulsions in Table 6.4 with some which were derived on the basis of
the HLB technique is again beneficial. Test emulsions 5c and 5e were selected and given
the codes 5cHLB and 5eHLB. Again, the emulsifier ratio was altered in order to obtain a
good “HLB” match. Details are given in Table 6.5.
Table 6.5
Test
Test Series 5 Emulsion Systems Matched with HLB Values
Aqueous
Oil Phase and Ratio
Emulsifier
Phase
5c HLB
Myritol 331/ Myritol 318
(78:22)
Water
Laureth-4 / Ceteareth-20 (30:70)
5e HLB
Myritol 331/ Myritol 318
(87:13)
Water
Ceteareth-12 / Ceteareth-20 (85:15)
6.1.4 Stability of Commercially Representative Emulsion Systems
The initial droplet size and resulting stability information for the test emulsion systems
are tabulated in Table 6.6.
Table 6.6
Results and Stability of Test Emulsion Series 5
Test
Initial
Particle
Size (µm)
Appearance
After 24
hours
Appearance After 72 hours
Ranking
1 = best
5a
4.3
2 mL cream
8 mL cream + 2 mL oil
7
5b
3.9
1 mL cream
6 mL cream + 2 mL oil
4
5c
2.7
trace cream
4 mL cream + trace oil
1
5cHLB
3.4
0.5 mL cream
5 mL cream + 1 mL free oil
2
5d
3.8
1 mL cream
6 mL cream + 1 mL free oil
3
5e
4.1
1 mL cream
6 mL cream + 2 mL oil
5
5eHLB
4.3
1 mL cream
6 mL cream + 3 mL oil
6
5f
4.9
3 mL cream
8 mL cream + 3 mL free oil
8
The stability of all these systems were unacceptable after 72 hours storage at 40°C,
however, cosmetic emulsion products have considerably higher viscosity than those used
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Chapter 6
here, which reduces the number of droplet collisions and markedly increases product
stability. Where emulsions of low viscosity are required for commercial products, they
are usually made as PIT (Phase Inversion Temperature) emulsions which possess greater
stability (refer section 1.5.3).
Testing standard emulsions at low viscosity however, as in Table 6.6, allows very fast
determination of emulsion stability at the early stages of formulation development and is
a very useful tool, for later development of commercial emulsions. Although the stability
of all emulsions was poor (after 72 hours), this was due to the lack of other stabilisers
(thickeners / polymers) and certainly not so poor as to abandon product development in a
“real” formulation exercise.
Moreover, the EPM emulsions were better or, at least as good as the HLB emulsions in
terms of both initial droplet size and visual stability. The EPM technique should enhance
the packing and so strength of the interfacial layer offering improved (lower) initial
particle size and improved stability (lower creaming and free oil). There is initial, albeit
limited, support of this in these early experiments.
For reference, an example of an ‘unmatched’ emulsion was made simply by emulsifying
15 mL Myritol 318 with Laureth-3. This is a poor match from both a HLB and EPM
point of view. The resulting emulsion had an initial particle size of 11 µm and showed
12 mL cream and a trace of oil after 2 hours. This example of a ‘bad’ emulsion
demonstrates the benefit of both the HLB and EPM techniques.
6.1.5 Stability Comparison of HLB and EPM Derived Emulsion System
Three emulsion systems have so far been prepared where a direct comparison of the HLB
and EPM selected emulsifiers was possible. However, stability has only been assessed at
the initial time points.
Table 6.7 provides a summary for the extension of the stability trial. It shows stability
results for test emulsions 4 and 4HLB, 5c and 5cHLB, 5e and 5eHLB from 1 week to 3
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Chapter 6
months.
Initial particle size is also included for reference.
Particle size was not
measured once visible signs of destabilization were apparent because the emulsion was
no longer homogenous enough to be able to obtain a representative sample. Re-mixing
of the emulsion to obtain a representative sample compromises the integrity of that
sample in further time points.
Table 6.7
HLB and EPM Emulsion Comparison
Test
Emulsion
Initial
Particle
Size (µm)
Appearance After 1
week
Appearance After 1
month
Appearance After 3
months
4
3.6
4 mL cream
6 mL cream + trace oil
8 mL cream + 1 mL oil
4HLB
4.6
7 mL cream
9 mL cream + 2 mL oil
9 mL cream + 3 mL oil
5c
2.7
6 mL cream + tr oil
8 mL cream + 2 mL oil
9 mL cream + 3 mL oil
5cHLB
3.4
8 mL cream + 2 mL oil
9 mL cream + 5 mL oil
9 mL cream + 6 mL oil
5e
4.1
9 mL cream + 4 mL oil
9 mL cream + 5 mL oil
9 mL cream + 6 mL oil
5eHLB
4.3
9 mL cream + 5 mL oil
9 mL cream + 6 mL oil
9 mL cream + 7 mL oil
A difference in terms of levels of creaming and free oil can be observed between test
emulsions 4 and 4HLB and also between 5c and 5cHLB; each displaying improved
stability with the EPM selected emulsifiers. The amount of free oil is the most important
data to consider because this comparison indicates the relative degrees that each emulsion
has broken.
Test emulsions 5e and 5eHLB both performed quite poorly with a high percentage of free
oil occuring after 72 hours (Table 6.6) and 1 week. There is little observable difference
between the two emulsions; they both would be removed from stability at an early date as
failed formulations.
Ceteareth-20, the emulsifier used solely in test emulsion 5e, is a very common emulsifier
used in cosmetic and pharmaceutical formulations and so the fact that it performed poorly
was something of a surprise.
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Chapter 6
Because ceteareth-20 is usually emulsified at 75 – 80 degrees C (along with ceteareth-12
in test emulsion 5cHLB and 5eHLB) it had to be confirmed whether it was the
emulsification temperature that was affecting the results. Test emulsions 5cHLB, 5e and
5eHLB were repeated emulsifying at 75 degrees C. Test emulsion 7 could not be
prepared at this temperature because this temperature is above the cloud point of this
emulsifier.
Initial particle size and visual stability results for these emulsions are tabulated in Table
6.8. Rate of duration of shear remained as previously defined. The results obtained for
test emulsion 5c emulsified at 20 degrees C are included in the table for reference only.
Table 6.8
Ceteareth Emulsifier Systems Emulsified at 75ºC
Test
Emulsion
Initial
Particle
Size (µm)
Appearance
After 24
Hours
Appearance
After 1 week
Appearance
After 1 month
Appearance
After 3
months
5c (at 20°C)
2.7
trace cream
6 mL cream +
8 mL cream +
9 mL cream +
0.5 mL oil
2 mL oil
3 mL oil
6 mL cream +
8 mL cream +
9 mL cream +
1 mL oil
3 mL oil
4 mL free oil
5 mL cream +
6 mL cream +
8 mL cream +
1 mL oil
2 mL oil
3 mL oil
7 mL cream +
8 mL cream +
9 mL cream +
2 mL oil
4 mL oil
5 mL oil
5cHLB
3.1
0.5 mL cream
(at 75°C)
5e
(at 75°C)
5eHLB
2.9
3.6
trace cream
1 mL cream
(at 75°C)
Emulsifying at the higher temperature has produced improved initial droplet size and
emulsion stability. This would be expected and is the reason why the vast majority of
commercial emulsions are prepared at high temperature. However, the trend that the
EPM selected emulsifiers have given emulsions with improved stability as compared to
HLB selected emulsifiers, remains.
Emulsification will continue to be carried out at 20º C for the reminder of the work for
consistency with all emulsifiers under evaluation. However, some selected emulsion
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Chapter 6
systems, where the ceteareth emulsifiers are the major emulsifier, may be re-checked
with emulsification carried out at 75º C.
All EPM emulsion tests have been carried out using a single emulsifier only whereas the
HLB matched systems have used emulsifier combinations. As explained in Chapter 2,
surfactant combinations do tend to show better stability than single systems so, if tested,
may show a further advantage over the HLB matched systems. However, the commercial
EO emulsifiers, as covered in Chapters 1 &2, are actually mixtures of different EO
number surfactants and not pure surfactants.
For this reason and considering only EO emulsifiers are being tested, it was not deemed
necessary, at this stage, to repeat any of the tests using multiple EO emulsifier
combinations as little, if any, improvement would be expected.
6.1.6 Confirmation of Optimal Oil Blends for EPM Technique
Even when screening for optimal emulsifiers using the HLB technique, it is common to
run a simple visual emulsion test exactly as outlined in Section 3.2.3.1 using different oil
ratios, a range of emulsifiers or to test emulsifier concentration. The HLB technique will
narrow the range down for the formulator but testing is still carried out to confirm the
prediction.
A similar screening test can easily be carried out to demonstrate the EPM technique is
predicting the correct emulsifier for different ratios of oil and would be useful to confirm
a value in this technique. An aim of the EPM technique would be to provide sufficient
accuracy of prediction of the emulsifiers that a screening process would not be necessary.
However, at this stage the method is not yet proven and the screening exercise can help to
show the effectiveness of the EPM technique.
Table 6.9 and 6.10 provides a summary of the oil ratios screened with the respective
results. All emulsions were prepared in the same way and with Laureth-4 again as the
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Chapter 6
emulsifier at 0.5% to provide a tough test. All emulsions formed were initially white and
homogenous. A difference could be clearly seen after 24 hours in the first screening but
the test had to be continued for 72 hours in the second (narrower) screening.
Table 6.9
EPM Emulsion Screening 1
Myritol 318
composition
(%)
Myritol 331
composition
(%)
EPM
Value
Appearance
After 2 Hours
Appearance After 24
hours
100
0
20.2
1 mL cream
8 mL cream
80
20
17.8
1 mL cream
8 mL cream
60
40
15.4
1 mL cream
7 mL cream
40
60
13.1
0.5 mL cream
4 mL cream
20
80
10.68
trace cream
1.5 mL cream
0
100
8.3
0.5 mL cream
5 ml cream
The optimum oil ratio from screening 1 is close to the 80:20 ratio of Myritol 331:Myritol
318.
This can then be further screened as desired to pinpoint the exact ratio. Table 6.10 shows
the next possible screening ratios.
Table 6.10
EPM Emulsion Screening 2
Myritol 318
composition
(%)
Myritol 331
composition
(%)
Appearance
After 2 Hours
Appearance
After 24
hours
Appearance After 72
hours
25
75
0.5 mL cream
2 mL cream
3 mL cream + tr oil
24
76
trace cream
1 mL cream
2 mL cream
23
77
trace cream
1 mL cream
2 mL cream
22
78
trace cream
1 mL cream
2 mL cream
21
79
trace cream
1 mL cream
2 mL cream
20
80
trace cream
1.5 mL cream
2.5 mL cream + tr oil
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Chapter 6
As the oil ratios tested become narrower it becomes harder to distinguish visually, the
optimum emulsion.
A more sensitive technique (like Turbiscan™ - transmission and
light backscattering measuring equipment) might be required to pick up smaller
indications of instaiblity.
However, it can clearly be seen that the optimum ratio
calculated from the interfacial tension blends of 78% Myritol 331 and 22% Myritol 318
(EPM value of 10.9) correlates with the practical optimum ratio as shown in Table 6.10.
It is not possible to distinguish the best level of Myritol 331 between 76 and 79% from
the practical test results with only small differences between the results of all of
Screening 2. These results also indicate that when the EPM values are determined using
interfacial tension, that a scaling factor is not required.
All of the emulsions produced were white initially and the cream layer was,
unfortunately, not dense enough, during the period of the test to be easily distinguishable
using photography. It is true to say that with almost all of the screened emulsions,
useable cosmetic creams could be achieved with sufficient viscosity modifiers but that
there is a ratio range that does offer optimal stability for that particular system.
Of course, these screening tests are time consuming and quite tedious test to carry out so
if proven to not be required using the EPM Technique it would give greater impetus for
the new technique to be taken on board.
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Chapter 7
CHAPTER 7
PREDICTING OPTIMAL EMULSIFIERS
7.1
USING THE EPM TECHNIQUE FOR EMULSIFIER PREDICTION
A major driving force to begin this work was the need for a simple technique for
emulsion development which eliminates, or at least reduces, the trial and error approach
and consistently achieves reliable results. The EPM technique has taken the first steps to
achieving this goal.
The following steps again outline the procedure required to
effectively utilise the EPM technique:
1.
The formulator establishes the materials that will make up the emulsion oil and
water phases.
Marketing, sensory, viscosity, compatibility, and preservation
requirements should all be considered at this stage.
2.
Interfacial tension is measured for the water phase versus the oil phase at room
temperature (20ºC). For simple cases (where no surface active components are
included and the oil and water phases are single components) this could be
determined from the EPM raw material database. Only limited materials have
been evaluated so far and these were summarised in Table 4.4. Otherwise, a
single measurement of interfacial tension is all that is required.
3.
From the resulting interfacial tension value an emulsifier is selected by matching
with the EPM emulsifier database. Again only limited emulsifiers have so far
been characterised for the EPM technique and these materials were summarised in
Table 4.1. Until an extensive database is available, further measurements of
interfacial tension are required.
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Chapter 7
7.2
MODEL EMULSION DEVELOPMENT
This section provides an example, step by step guide, to a formulation development
example and demonstrates how the EPM technique is applied. It begins with a fairly
typical marketing brief which the formulator needs to develop a product to meet.
Marketing personnel are well known in the personal care industry for changing their
minds and putting pressure on the development chemist to modify the formula at short
notice (and consequent reduced stability) on the basis of a promise of improved sales.
The EPM technique could also be used as a basis of deciding whether the changes are
allowed. If the change to the formula changes the EPM value then development and full
stability should be re-started completely from the beginning. If the change has not
resulted in a different EPM value then the change may be possible with a few
confirmatory tests. If the chemist has a non-subjective method of making this decision
then a lot of arguments will be avoided.
7.2.1 Product Requirements
The marketing brief provided for a new product as given in Figure 7.1.
Figure 7.1 Marketing Brief for a New Product Development
‘The latest cosmetic magazines have focused on the damaging effects caused by long
wavelength sun radiation and how this can contribute to skin damage and premature
ageing. There is an opportunity for our company to launch a high quality skin protection
cream, which offers protection for the active skin cells. The product should be light,
elegant and be rapidly absorbed into the skin. The following claims are to be made:
•
Cyto-immuno-photo-protector
•
Helps to prevent skin ageing caused by visible light’
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Chapter 7
Once the formulator has received the marketing brief the next step is the selection of the
materials that will meet the requirements.
The active ingredient around which the claims can be made is called Photonly LS, based
on natural cellular components (full composition is listed in Chapter 3, Table 3.5). A
usage level of 2 % is required to use the efficacy data generated by the manufacturer.
The light feel of the product is mainly attributed to the nature of the oil phase materials
that are incorporated. It is common to use at least one, fast spreading oil, at a low enough
level to give an initial feeling of smoothness, together with one or two medium spreading
oils to give a longer lasting smooth feeling on the skin. For this product, hexyl laurate
(Cetiol A) will be used as the fast spreading oil and a cocoglyceride (Myritol 331) as the
medium spreading oil.
The requirement for the product to rapidly absorb into the skin can be achieved by using
a gel-structured emulsion.
High viscosities can still be reached but the product
immediately absorbs into the skin on application. A carbomer based gel can be used to
obtain this effect.
Other auxiliary materials include sodium hydroxide (to neutralise the carbomer), Kathon
CG as the preservative and Sandalwood oil to provide the fragrance.
7.2.2 Application of the EPM Technique
The ingredients outlined so far make up the separate emulsion phases as shown in Table
7.1 (step 1 of EPM process).
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Chapter 7
Table 7.1 Model Formulation Ingredients
Emulsion Phase
Raw Material
% w/w
Oil
Cetiol A
3.0
Myritol 331
12.0
Sandalwood Oil (fragrance)
0.1
Water
82.45 (including emulsifier)
Photonyl LS
2.0
Carbopol ETD 2001
0.3
Kathon CG (preservative)
0.05
Sodium Hydroxide
0.1
Aqueous
This is now the complete formulation excluding the emulsifier. The interfacial tension of
the two phases is next determined (step 2 of the EPM process).
The two phases were prepared in the lab simply by adding all the ingredients of each
phase into separate beakers at room temperature and mixing until homogenous. The
interfacial tension was then measured as described in Section 3.2.1.2. The average of
three independent measurements was 10.9 mN m-1.
From the data presented in Table 4.5 it can be seen that both Laureth-4 and Ceteareth-12
possess an EPM interfacial tension value of 10.9 mN m-1 and match the proposed
emulsion phase interfacial tension value (step 3 of the EPM process). Either (or both)
emulsifier(s) could be used. The advantage of gel-based emulsions is that no heating is
required to form them. On this basis laureth-4 was selected for this emulsion system.
A usage level of 1.0 % laureth-4 was selected. This level has been shown to produce an
average droplet size of 2.5 μm when 90 s of 9000 rpm shear is applied (from Table 5.4).
Too high an emulsifier level may affect both the solubility and interfacial tension of the
system under investigation, and hence this should be avoided for these early tests of the
EPM technique.
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Chapter 7
1.0 % laureth-4 was added to the oil phase and the water phase (minus the sodium
hydroxide) added to this mixture. An emulsion formed spontaneously but, consistent
with the work in this project, 90 seconds of 9000 rpm shear was applied to ensure
adequate mixing and to promote a smaller initial droplet size. The sodium hydroxide was
added after homogenisation. This neutralises the carbomer and causes it to form a gel.
7.2.3 Emulsion Stability
The emulsified product had a viscosity of 2,000 cps, which was thicker than the previous,
basic emulsion systems tested. It would be expected, therefore, that phase separation
would be observed after a longer storage period and/or higher storage temperatures. The
storage tests on this product needed to be continued for longer with a focus on higher
temperature storage tests. It should be noted, however, that 2,000 cps is at the lower end
of a viscosity specification for a lotion (usually 2,000 – 8,000 cps). This viscosity was
specifically selected to help pick up initial signs of instability more quickly and to be a
‘tough test’ for the EPM technique.
A summary of droplet size and emulsion stability results is given in Table 7.1. Due to the
high stability of this formulation there was very little to report for the early storage test
conditions. Consequently, only major time points are tabulated.
The test sample after 6 months at 40°C (TGA, 1994) still displayed no signs of phase
separation. This stability condition is used by the Therapeutic Goods Administration
(TGA) to indicate stability of the product after two years at room temperature, which is
the standard shelf-life for commercial products. This product would be considered to
have passed its stability trial and be released for commercialisation. The 20ºC test
would usually be continued for the two years to complete the stability trial.
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Chapter 7
Table 7.2
Model Emulsion Stability Results at 20ºC and 40ºC
Test Condition
Visual Appearance
Particle Size (μm)
Initial
Homogenous, viscous emulsion
2.7
Homogenous, viscous emulsion
2.7
Homogenous, viscous emulsion
2.7
After 24 hours
@ 20ºC & 40 ºC
After 1 week @
20ºC & 40 ºC
After 1 month
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 1 month
Homogenous, viscous emulsion.
@ 40 ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
2.7
2.8
2.8
2.8
2.8
2.8
For comparison the same emulsion was prepared using laureth-2 as the emulsifier instead
of laureth-4. A homogenous, viscous emulsion was initially formed but with a particle
size of 11 µm. It is not possible to see ‘cream’ in such a thick product but free oil was
clearly evident after 24 hours. Free oil so soon in the stability test is an immediate failure
and so testing was abandoned.
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Chapter 7
7.2.4 Model Emulsion System Using Alternative EPM Emulsifier
The EPM technique identified two possible emulsifiers as being suitable for the test
emulsion detailed in Table 7.1.
These emulsifiers were laureth-4 and ceteareth-12.
Laureth-4 was used to form the emulsion and produced a product with very good
stability. According to the HLB system laureth-4 and ceteareth-12 have very different
values (9.6 for laureth-4 and 13 for ceteareth-12) and would not be selected to emulsify
the same oil component. It would, therefore, be a useful exercise to test the stability of
the emulsion described in Table 7.1 using ceteareth-12 as the emulsifier. This would
further test the EPM technique and also highlight the differences between the EPM and
HLB techniques.
As before, 1.0 % of the emulsifer - this time ceteareth-12, was added to the oil phase and
the water phase (minus the sodium hydroxide) was added to this mixture. Again an
emulsion formed spontaneously and for consistency 90 seconds of 9000 rpm shear was
applied. The sodium hydroxide was added after homogenisation to form the gel. The
viscosity of the emulsion formed was this time slightly higher at 2300 cps. The stability
results for this emulsion are summarised below in Table 7.3.
The emulsion droplets initially formed using ceteareth-12 were marginally larger than
those formed in the emulsion produced using laureth-4 as the emulsifier. This may be
due to the smaller size of the laureth-4, allowing better packing at the interface but may
also be due to the low temperature processing of the ceteareth-12. However, the overall
stability of the emulsions were very similar.
Both formulas provided very stable
products.
For the sake of completion and to follow the lead set in Chapter 6, this last emulsion was
repeated with the emulsification carried out at 75ºC. The initial droplet size was slightly
improved at 2.8 μm with again excellent stability throughout the 6 month life of the
stability tests (final emulsion droplet size was 3.0 μm after the 6 month period).
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Chapter 7
Table 7.3 Model Emulsion (Using Ceteareth-12) Stability Results at 20ºC and 40ºC
Test Condition
Initial
After 24 hours
@ 20ºC & 40 ºC
After 1 week @
20ºC & 40 ºC
Visual Appearance
Particle Size (μm)
Homogenous, viscous emulsion
3.0
Homogenous, viscous emulsion
3.0
Homogenous, viscous emulsion
3.0
After 1 month
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 1 month
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
3.0
3.1
3.1
3.2
3.2
3.2
7.2.5 Second Model Emulsion System Using Alternative EPM Emulsifier
The first model emulsion system was an example of the newer type of emulsion systems
which are cold processable with their viscosity controlled by a polymer. It was also
thought useful to give an example of the more traditional emulsion type which is
processed at 75 – 80ºC and thickened with fatty alcohol.
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Chapter 7
The product brief was for a traditional moisturising handcream with Vitamin E and
glycerine as the actives. Because both actives are quite heavy and sticky on the skin,
only a low quantity and lighter oils (IPM and Myritol 331) were required to achieve a
nice skin feel. The fragrance and preservative were kept the same as the previous system.
The individual components of each emulsion phase are detailed in Table 7.4.
Table 7.4 Second Model Formulation Ingredients
Emulsion Phase
Raw Material
% w/w
Oil
IPM
1.0
Myritol 331
12.0
Vitamin E
2.0
Lanette MY
2.0
Sandalwood Oil (fragrance)
0.1
Water
79.85 (including emulsifier)
Glycerine
3.0
Kathon CG (preservative)
0.05
Aqueous
The interfacial tension between the two phases to be emulsified was measured (at 20ºC)
to be 9.4 mN m-1.
Refering to Table 4.5, this interfacial tension value equates to an emulsifier requirement
of 0.8 % Ceteareth-20 (0.8 x 9.6 = 7.68) and 0.2% Ceteareth-30 (0.2 x 8.6 = 1.72) to
make up the 1% emulsifier level selected to match the 9.4 mN m-1 value of the two
phases to be emulsified. These two emulsifiers were the ones closest (immediately above
and below) the figure to be matched; at a later stage it could be determined if the
emulsifiers closest to the value to be matched should preferentially be used to make up
the emulsifier concentration used, or if any combination that matches the required figure
will suffice.
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Chapter 7
Although it has been stressed that a full database is traditionally required by formulators
before any new technique would be accepted, the above example demonstrates that it
may not be necessary to have all materials tabulated in an EPM database. This example
shows how simple it is to take the measurement for the interfacial tension of each phase
and so individual measurements in a database are not necessarily informative. It is the
value for the system in it’s entirety that is the only relevant data.
0.8 % ceteareth-20 and 0.2% ceteareth-30 were added to the oil phase (minus fragrance)
which was heated to 75ºC and the water phase (also at 75ºC) added to this mixture. An
emulsion formed spontaneously but, consistent with the work in this project, 90 seconds
of 9000 rpm shear was applied to ensure adequate mixing and to promote a smaller initial
droplet size. The product was cooled with slow stirring (magnetic stirrer on a hotplate).
The Sandalwood oil was added to the product below 40ºC to avoid evaporation.
Once the product had cooled to 20ºC it had a viscosity of 2,500 cps, which is again a
lower viscosity than a standard commercial emulsion, but serves to help test the EPM
selected emulsifier system. Although an attempt was made to achieve a similar viscosity
to the first model emulsion it was not possible to achieve this exactly. Cetearyl alcohol
controlled viscosities do tend to be a little bit harder to accurately control than the
polymer systems due to influences of stirring and rate of cooling. A summary of droplet
size and emulsion stability results (major time points only) for this emulsion is given in
Table 7.5.
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Chapter 7
Table 7.5
Second Model Emulsion Stability Results at 20ºC and 40ºC
Test Condition
Visual Appearance
Particle Size (μm)
Initial
Homogenous, viscous emulsion
2.8
Homogenous, viscous emulsion
2.8
Homogenous, viscous emulsion
2.8
After 24 hours
@ 20ºC & 40 ºC
After 1 week @
20ºC & 40 ºC
After 1 month
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 1 month
Homogenous, viscous emulsion.
@ 40 ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 3 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 20ºC
No sign of separation
After 6 months
Homogenous, viscous emulsion.
@ 40ºC
No sign of separation
2.8
2.8
2.8
2.9
2.9
2.9
Once again this product would be considered to have passed its stability trial and be
released for commercialisation.
Two examples (the first with 2 emulsifier types) of different model emulsion
formulations have shown how the EPM technique would be used in practice by the
formulating chemist and that very stable commercial style emulsions can be achieved.
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Chapter 8
CHAPTER 8
CONCLUSIONS AND RECOMMENDATIONS FOR
ADVANCEMENT OF EPM TECHNIQUE
The research and practical studies outlined in this thesis has attempted to examine the
many proposed methods for optimal emulsifier selection and to suggest a new technique
with a different approach.
The HLB technique is by far the most widely used method of those studied but even since
this work was begun new, alternate methods have been put forward. The fact that new
methods are continually being offered reflects the amount of trial and error that is still
required to find an optimum emulsifier system for each new formulation developed.
There is no definitive method currently in the public forum and this was the initial driver
to carry out this study. New methods proposed in the last 8 years or so include using the
PIT parameter or Emulsion Inversion Point (EIP), both proposed by Gasic et al. in 1998,
or using colour difference (Koga et al., 2002). One common theme of most of the
emulsifier selection methods proposed post HLB system is that they try to link their
method to the HLB system. This is where the current work takes a different approach
with the author strongly believing that all elements of the emulsion must be considered
prior to selecting an emulsifier system and both moieties of the emulsifier must also be
considered.
8.1
OUTLINE SUMMARY OF EXPERIMENTATION
Following the detailed literature search and background research relevant for this work it
is in Chapter 4, that the first ‘splitting’ of the selected emulsifiers took place to yield
alkanes (from the hydrophobic chain of the emulsifier) and different molecular weight
glycols (from the hydrophile). From here the whole evaluation process could begin.
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Chapter 8
The interfacial tension data was initially measured between the range of alkanes and
glycols that made up the selected emulsifiers. These values would derive a so-called
EPM value for a particular emulsifier and be used to match oil and aqueous phases to be
emulsified. Due to the physical nature of the higher molecular weight materials (PEG
1000, PEG 1500 and C16:C18 alkane) some extrapolation from the data measured for the
lower molecular weight materials was required but finally EPM values, expressed and
measured as interfacial tension, were determined for the selected emulsifiers. Collated,
or measured interfacial tension data for the selected oils was also complied in this chapter
giving the full set of initial data that could then be used to test out the EPM technique.
Chapter 5 selected and standardized the optimal emulsifier concentration and the degree
and time of mixing that would be used throughout the work. This was very important to
ensure that the effect of the emulsifier only was evaluated throughout the work.
In Chapter 6 we could, at last, begin to test out the EPM technique by making a range of
emulsions. Although the number of materials that could be used was limited, eleven
emulsions were prepared with six allowing direct comparison between the HLB and EPM
selected emulsifiers (3 emulsions of each). The initial particle size and visual stability of
the so formed emulsions were monitored for up to 72 hours to assess stability. The visual
method to measure stability although basic, remains the method of choice throughout the
formulation industry due to its proven reliability. The intial results achieved in Chapter 6
were encouraging with, in all cases the EPM emulsions showing better or at least as good
particle size and stability than the HLB emulsions to emulsify the same oil which will be
discussed later in this chapter.
In Chapter 7, the final step to produce commercially acceptable cosmetic emulsions was
carried out using the EPM technique to fully demonstrate how it would be used. Stable
emulsion systems, suitable for commercial cosmetic products were produced.
The work so far has shown the technique to be simple to use and the initial results
obtained in this study have been very promising. The remainder of this chapter
summarises the positive and negative findings of the EPM technique as a result of this
work and outlines the ways in which the technique can be further advanced.
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Chapter 8
8.2
POSITIVE ATTRIBUTES OF THE EPM TECHNIQUE
The positive attributes of the EPM technique can be summarised as follows (with further
explanation given in Section 8.2.1):
•
All ingredients to be included in the emulsion can be taken into consideration before
the emulsifier selection is made.
•
Only one simple measurement is required to be carried out by the formulator.
•
Where materials are changed or added as the formula develops, a repeat of the same
simple measurement is all that is required to see if modification to the emulsifier
system is required.
•
Interfacial tension has been shown to be a useful method to characterise the raw
materials and emulsion phases for the emulsifiers and emulsions tested.
•
Emulsions with good stability have been able to be produced using the EPM
technique.
•
With additional tests required by the formulator, the EPM technique can offer
guidance on the optimal emulsifier concentration to be used.
8.2.1 Discussion of Positive Attributes
The EPM technique has addressed the issue of complex auxiliary ingredients that may
effect the initial mixing and formation of the emulsion as well as effecting prolonged
emulsion stability. The technique purposely utilises all formulation ingredients in its
selection process.
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Chapter 8
The choice of interfacial tension to characterise the raw materials for the EPM database
has been proven to have some credence. The preliminary work, in Chapter 5, as well as
the emulsifiers used in the test emulsions have all given a smaller initial droplet size
(relative to alternative emulsifiers) and more importantly, have been shown to give
systems with greater stability than those formed with a slightly larger, initial droplet size.
Interfacial tension is, in addition, quite a quick and easy test to perform.
As already discussed, it is known that lowering the interfacial tension makes it easier to
form an emulsion but also makes it easier to break that emulsion. Nevertheless, it was
hoped to use the expected correlation between low interfacial tension and high surfactant
adsorption to promote emusion stability. Matching exactly the ‘difference’ between the
two phases to be matched and the ‘difference’ between the two moieties of the emulsifier
may have achieved conditions that are the most energetically favourable for a surfactant
to be at the interface between the oil and water droplets. This would then result in the
highest possible concentration of emulsifier present at the interface. A strong interfacial
film should be achieved and good stability would result.
The emulsions tested have shown excellent long-term stability and with only 1%
emulsifier being utilised. This is very unusual for commercial products, which often
have 3 – 5 % emulsifier. It is possible, though not proven, that the favourable energetical
conditions for higher surfactant concentration at the interface has been achieved.
One further advantage the EPM technique has over the HLB system is that there is scope
within the EPM method to measure optimal surfactant concentration. For example, this
could be done by measurement of interfacial tension at varying emulsifier concentration
to pinpoint the optimal concentration.
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Chapter 8
8.3 NEGATIVE ATTRIBUTES OF THE EPM TECHNIQUE
The negative attributes of the EPM technique can be summarised as follows:
•
The use of interfacial tension as the method to characterise the system has shown
some ‘limitations’ with the nonionic emulsifiers that can be selected when using
standard oil selection. Refer 8.2.1 below.
•
Interfacial tension of some high melting point emulsifiers cannot be carried out on
the pure material. Extrapolation can be done here but is not ideal.
•
An EPM database has been started but considerable work will be required to build
up data for the wide range of emulsifiers that would be required to make this a
viable technique for everyday use.
8.3.1 Discussion of Negative Attributes
Although only a limited number of emulsifiers have been tested it appears that the EPM
technique is somewhat limited in terms of the oils that will be suitable with nonionic
emulsifier systems or else that a blend of oils will always be required. From Table 4.6 in
Chapter 4, it is clear that the majority of oils possess interfacial tension values greater
than 20 whereas the EPM values of the emulsifiers tested where all well below 20. With
the use of anionic surfactants it is expected that EPM values of over 20 will be achieved,
however, nonionics are by far the most commonly used surfactants used in emulsifiers so
it is important that all nonionic surfactants can be used. More highly polar oils can be
used to bring down the interfacial tension value to make the EPM technique viable but it
should be confirmed that this blending to match interfacial tension is beneficial.
Alternatively, or additionally, some time could be devoted to evaluate the possible
benefits in using a scaling factor.
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Chapter 8
8.4 FUTURE REQUIREMENTS TO VALIDATE EPM THEORY
The results so far have been very promising. However, there is still a lot of work
required and questions to be answered before the technique can be either properly
validated or effectively used. The major points are detailed below:
1.
Initial results have shown smaller initial droplet size and improved emulsion
stability (greater time or higher temperature) before separation has occurred. This
is believed to be due to optimal emulsifier packing of the interface resulting in
higher emulsifier concentrations than ‘usual’. This needs to be proven so that the
mechanism can be understood. Further studies could also investigate the effect
the emulsifier concentration has on surface viscosity, viscoelastic effects and
steric hindrance.
2.
An EPM database has been started but needs to be built up to contain a
comprehensive set of data. Without sufficient data in the database no new system
would be fully utilised. Additional data needs to be added for many of the
emulsifiers currently in commercial use. It may be found that relationships can be
developed to predict the interfacial tension values rather than measuring every
one.
3.
Polarity of the oil may need to be also taken into account. The interfacial tension
between an oil and water phase could equally apply to a high or a low HLB
surfactant.
4.
Intensive research done on microemulsions in connection with enhancing the
recovery of petroleum from oil rock reservoir continues which has yielded
equations to determine the conditions under which a microemulsion was formed.
This work is detailed in Chapter 8 of Rosen (2004) and whilst this particular
application and the use of microemulsions has little relevance to this work the
HLD Method (hydrophilic- lipophilic deviation) has been adapted (Salager et al.,
1983, 2000) to macroemulsion formation and this may now make this more
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Chapter 8
relevant to the current work. The HLD method does take into consideration other
components of the system (salinity (not relevant here), cosurfactant, alkane chain
length, temperature and hydrophilic and hydrophobic groups of the surfactant (all
relevant)) and so does have some similarities to the EPM technique. The current
status and overview of the research of this technique should be evaluated at the
time of further development because there may be some links here to help
validate the EPM technique which should be considered.
8.5
FUTURE DIRECTION OF COSMETIC EMULSIONS
The EPM technique was originally devised for cosmetic emulsion because this was the
area of interest for the author.
There is, of course, no reason why this could not be
applied to other emulsion areas i.e., agrochemicals, food etc. However, because the aim
of this work was cosmetic systems, the future direction of cosmetic emulsions must be
considered when planning future work for this technique.
The first model emulsion system covered in Chapter 7 was a cold processable emulsion
that was stabilised by a polymer. These polymer systems are becoming increasingly
popular because gel type emulsions as well as cold processed systems have increased in
commerical popularity.
As a further development of polymers, they are also being used to promote an emulsifierfree concept where the polymer is ‘self-emulsifying’ and no other surfactant is required.
Emulsions formed using the polymers are very stable and offer a light product that is
rapidly absorbed into the skin. In actual fact, as shown in Figure 8.1 (Cognis GmbH,
2002) the oil becomes trapped in the voids of the polymer matrix rather than actually
being emulsified into the water. This system can form creams/lotions for simple oilwater systems but can struggle with a high oil i.e., in sunscreens where a standard
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Chapter 8
emulsifier is also required. These polymers are quite expensive which will slow their
progress to replace standard emulsifiers but they will certainly play a significant part in
cosmetic emulsions of the future.
This area is being advanced also by the trend towards EO-free emulsions for ‘apparent’
safety reasons. EO emulsifiers, like those studied in this thesis, remain by far the most
common emulsifiers used in skincare due to the large range available and the consequent
flexibility offered ot the formulator. However, to answer this trend and in addition to
polymer alternatives, a number of new emulsifiers are appearing.
Figure 8.1
Example of a Polymer Displaying ‘Self-Emulsifying’ Properties
Initially some of these alternate emulsifiers were quite inefficient and expensive i.w.,
cetearyl glucoside and could not significantly threaten the common and inexpensive
nonionic emulsifiers. But since 2004 some quite efficient alternate emulsifiers have
become available. Examples of these are sodium stearoyl glutamate and several sucrose
esters which also meet the trend towards vegetable derived emulsifiers. These offer
superior sensorial properties compared with EO emulsifiers, which can be a little tacky.
In years to come these are likely to become more widespread in the cosmetic industry and
so should be included in future work for the EPM technique.
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Chapter 8
8.6 RECOMMENDATIONS FOR FURTHER WORK
Requirements to validate the EPM technique were listed in Section 8.4.
A mechanism for the EPM technique is the highest priority to understand and explain
how this process can offer benefits. The importance of factors including oil polarity
would also then be realised and a decision can be made if future work in this direction is
also required.
Even without a full, known mechanism work could be continued building up the
emulsifier database.
However, to limit the quantity of work (prior to a validated
mechanism) the emulsifiers should be limited to those that can be used in combination
with polymers in the newer, cosmetic emulsion types. If this is further limited to coldprocessable emulsions then this limits the emulsifier options further and makes the level
of work required quite possible to achieve relatively quickly. Because this area of cold
processable emulsions with polymers is still developing, this is a good opportunity to
encourage the use of a new, improved emulsifier selection technique.
The proposed EPM technique has given some initial good results and also raised some
interesting questions regarding a higher surfactant concentration at the interface and a
resulting, stronger interfacial film. There is much work still required but I hope that this
current study has offered a different approach to emulsifier selection that will be worthy
of future consideration.
- 139 -
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- 151 -
APPENDIX 1
- 152 -
- 153 -
- 154 -
APPENDIX 2
Mastersizer Standard Analysis Results (Chapter 3.2.3.2.1)
Figure 3.4
Analysis Output - 0.9 µm Standard
Result: Histogram Table
ID: 0.9 micron std
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 11.6 %
Analy sis: Poly disperse
Residual: 1.511 %
Conc. = 0.0021 %Vol
Distribution: Volume
D(v , 0.1) = 0.68 um
Span = 4.010E-01
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.30
16
4
Measured: 27/8/1999 08:59
Analy sed: 27/8/1999 08:59
Source: Analy sed
Density = 0.960 g/cm^3
D[4, 3] = 0.85 um
D(v , 0.5) = 0.85 um
Unif ormity = 1.222E-01
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
2.88
11.17
35.04
70.58
94.47
99.55
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 7.5734 m^2/g
D[3, 2] = 0.83 um
D(v , 0.9) = 1.02 um
Volume
Under %
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
Particle Diameter (µm.)
- 155 -
100.0
1000.0
0
Figure 3.5
Analysis Output – 5.0 µm Standard
Result: Histogram Table
ID: 5 micron std
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 14.2 %
Analy sis: Poly disperse
Residual: 0.776 %
Conc. = 0.0104 %Vol
Distribution: Volume
D(v , 0.1) = 3.07 um
Span = 8.930E-01
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
8
1
Measured: 27/8/1999 08:34
Analy sed: 27/8/1999 08:34
Source: Analy sed
Density = 0.960 g/cm^3
D[4, 3] = 5.08 um
D(v , 0.5) = 4.88 um
Unif ormity = 2.753E-01
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.16
0.63
1.79
4.13
8.32
15.45
26.70
41.82
59.34
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 1.3826 m^2/g
D[3, 2] = 4.52 um
D(v , 0.9) = 7.43 um
Volume
Under %
75.28
86.82
94.58
98.98
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
Particle Diameter (µm.)
- 156 -
100.0
1000.0
0
Figure 3.6
Analysis Output – 45 - 62 µm Beads
Result: Histogram Table
ID: Glass Beads 45-62
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 12.5 %
Analy sis: Poly disperse
Residual: 0.761 %
Conc. = 0.0540 %Vol
Distribution: Volume
D(v , 0.1) = 41.22 um
Span = 5.342E-01
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.09
0.19
0.33
0.51
7
6
Measured: 30/8/1999 14:10
Analy sed: 30/8/1999 14:10
Source: Analy sed
Density = 0.960 g/cm^3
D[4, 3] = 59.82 um
D(v , 0.5) = 51.87 um
Unif ormity = 3.095E-01
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
0.74
0.97
1.22
1.45
1.68
1.91
2.14
2.32
2.41
2.46
2.48
2.48
2.48
2.48
2.48
2.48
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 0.3344 m^2/g
D[3, 2] = 18.69 um
D(v , 0.9) = 68.93 um
Volume
Under %
2.48
2.48
2.48
2.48
2.48
2.48
2.48
2.48
2.48
2.49
2.55
2.76
3.86
8.58
26.82
60.09
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
83.17
92.13
95.40
96.84
97.12
97.14
97.14
97.14
97.14
97.21
97.71
98.51
99.24
99.73
99.95
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
Particle Diameter (µm.)
- 157 -
100.0
1000.0
0
Figure 3.7
Analysis Output – 0.9 µm and 5.0 µm Mixed Standards
Result: Histogram Table
ID: 0.9/5 micron stds
File: STDS
Path: A:\
Run No:
Rec. No:
Sampler: Internal
Presentation: 5OHD
Modif ications: None
Measured Beam Obscuration: 13.2 %
Analy sis: Poly disperse
Residual: 1.373 %
Conc. = 0.0033 %Vol
Distribution: Volume
D(v , 0.1) = 0.61 um
Span = 5.231E+00
Size
(um)
0.058
0.067
0.077
0.090
0.104
0.120
0.139
0.160
0.185
0.214
0.248
0.287
0.332
0.384
0.444
0.513
Volume
Under %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.52
3.09
17
5
Measured: 27/8/1999 09:03
Analy sed: 27/8/1999 09:04
Source: Analy sed
Density = 0.960 g/cm^3
D[4, 3] = 2.23 um
D(v , 0.5) = 0.97 um
Unif ormity = 1.546E+00
Size
(um)
0.594
0.687
0.794
0.919
1.06
1.23
1.42
1.65
1.90
2.20
2.55
2.94
3.41
3.94
4.56
5.27
Volume
Under %
8.40
18.23
31.94
45.43
56.45
63.01
66.08
67.43
68.11
68.18
68.33
68.81
69.93
73.05
78.90
86.31
Size
(um)
6.10
7.05
8.16
9.44
10.91
12.62
14.60
16.89
19.54
22.60
26.14
30.24
34.97
40.45
46.79
54.12
S.S.A.= 5.8146 m^2/g
D[3, 2] = 1.07 um
D(v , 0.9) = 5.68 um
Volume
Under %
92.99
97.28
99.35
99.93
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Size
(um)
62.60
72.41
83.76
96.88
112.1
129.6
149.9
173.4
200.6
232.0
268.4
310.4
359.1
415.4
480.4
555.7
Volume
Under %
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
Volume (%)
10
100
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1.0
10.0
Particle Diameter (µm.)
- 158 -
100.0
1000.0
0