Sols (liosols S/L), xerosols (*/S: solid
medium), gels
István Bányai and Levente Novák
Making emulsions
Mode of action of making emulsions
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Dispersion of one of the liquid phases in the other with cavitation, turbulence, shear, etc. → energy is needed for emulsification
Emulsification proceeds in two steps
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Mixing
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Stabilization
Main methods
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Shaking
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Mechanical shear
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Sonication
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Condensation methods: solubilization of an internal phase into micelles
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Electric emulsification
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Phase inversion (“ouzo effect”)
Emulsifiers
Surface active materials
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Carbohydrates: acacia gum (gum arabic), tragacanth, agar, pectin → for o/w
emulsions.
Proteins: gelatin, egg yolk, casein → for o/w emulsions.
High molecular weight molecules: stearyl alcohol, cetyl alcohol, glyceryl
monostearate → for o/w emulsions, derivatives of cellulose, Na carboxymethyl cellulose, cholesterol → for w/o emulsion
Wetting agents (surfactants): anionic, cationic, zwitterionic, nonionic
Finely divided solids (Pickering stabilization)
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Bentonite, clays
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Silica (fumed)
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Metal hydroxides (magnesium hydroxide, aluminum hydroxide) → for o/w
emulsions
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Carbon black → for w/o emulsions
Emulsion stability
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The term “emulsion stability” can be used with reference
to three different phenomena
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creaming (or sedimentation)
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flocculation
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breaking of the emulsion due to the droplet coalescence.
Eventually the dispersed phase may become a continuous
phase, separated from the dispersion medium by a single
interface
The time taken for phase separation may be anything
from seconds to years, depending the emulsion formulation and manufacturing condition.
Emulsion stability
Factors favoring emulsion stability
1. Low interfacial tension.
2. Electrical double layer repulsions (at
lower volume fractions).
3. Steric stabilization.
4. Mechanically strong interfacial film
(proteins, surfactants, mixed emulsifiers are common). Temperature is
important.
5. Relative small volume of the dispersed phase.
6. Narrow size distribution of the
droplets (reduced Ostwald ripening).
7. High viscosity (simply retards the
rates of creaming, coalescence, etc.).
8. Reduced gravitational separation:
small density difference.
9. Reduced droplet size.
Emulsion inversion
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Emulsion inversion is the change of a given emulsion type to an other type (e.g. o/w → w/o)
Generally it proceeds by the action of
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temperature
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concentration
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change of the composition of the phase(s) → e.g dilution by a solvent of different polarity
Emulsion inversion
Increasing the concentration of
droplets (A) make them get closer
until they “pinch off” into smaller,
opposite type of emulsion (B).
Making of butter
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Cow's milk is a fairly dilute, not very stable O/W emulsion, with about 4% fat.
Creaming produces a concentrated, not very stable O/W emulsion, about 36% fat.
Gentle agitation, particularly at 10–15 °C, inverts it to make a W/O emulsion about
85% fat.
Drainage, addition of salt, then thorough mixing produces
● Butter (solid phase)
● Buttermilk (liquid phase)
1. As temperature is increased,
ethoxylated surfactants become
less water-soluble, because the
hydrogen bonding between the
oxygen of ethylene oxide and the
hydrogen of water is inhibited.
The molecules are more mobile
and cloudiness results.
Phase inversion temperature
2. Inversion o/w → w/o, oil is separated out.
The oil-in-water emulsion droplets measure just 100–
300 nm, in consequence they are of very low viscosity
and can be applied by spraying.
Scanning electron microscope can provide a visual representation of the phase
inversion: http://www.chemistrymag.org/cji/2001/03c058pe.htm
Hydrophilic-lipophilic balance (HLB)
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A practical (arbitrary) scale defining the relative balance
between hydrophilic and lipophilic character of a surfactant
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Used mainly for non-ionic detergents
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Two definitions in use
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Griffin's method: HLB=20×Mh/M (where Mh is the molar mass of the
hydrophilic part of the molecule and M the molar mass of the whole
molecule) → only valid for non-ionic surfactants
Davies' method: HLB=7+fh×nh-fl×nl (nh: number of hydrophilic groups, nl:
number of lipophilic groups, fh: weighing factor for hydrophilic groups,
fl: weighing factor for lipophilic groups)
http://www.snowdriftfarm.com/what_is_hlb.html
HLB values: applications
HLB = 7 + (number of hydrophilic groups) – (number of lipophilic groups)
Applications by HLB
Dispersibility in water by HLB
3-6
For W/O emulsions
<3
None
7-9
wetting agents
3-6
Poor
8-15
For O/W emulsions
6-8
Unstable milky dispersions
13-15
Detergents
8-10
Stable milky dispersions
15-18
Solubilizers
10-13
Translucent dispersion/solution
>13
Clear solution
Ionic detergents may have much higher HLB values:
SDS has a HLB of 40
Variation of the type and amount of residual emulsion with the
HLB value of the emulsifier
(antagonistic action)
The nature of the emulsifying agent determines the type of emulsion
Physical properties of emulsions
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Identification of “internal” and “external” phases (W/O or O/W)
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Droplet size and size distributions – generally greater than 1 µm
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Concentration of the dispersed phase – often quite high. The viscosity, conductivity, etc, of emulsions are much different than for the continuous phase.
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Rheology – complex combinations of viscous, elastic and viscoelastic
properties.
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Electrical properties – useful to characterize the structure.
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Multiple phase emulsions – drops in drops in drops in drops, …
Emulsification by particles (Pickering
emulsions)
W/O/W
double emulsion
O/W/O
double emulsion
Each interface needs a different HLB value.
The curvature of each interface is different.
Almost all particles are only partially
wetted by either phase.
When particles are “adsorbed” at the
surface, they are hard to remove – the
emulsion stability is high.
Crude oil is a W/O emulsion and is very
old (several millions of years)!
(Pickering stabilization)
Bentonite clays tend to give O/W, whereas carbon black tends to give W/O emulsions
Multiple phase emulsions
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“Drops in drops”
More and more studied and used
Great potential in drug-delivery
http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?
doi=b501972a&JournalCode=SM
Methods of breaking emulsions
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First, determine the type (o/w or w/o). Continuous phase will
mix with water or oil.
Chemical demulsification, i.e. change the HLB
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Add an emulsifier of opposite type (antagonistic action).
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Add agent of opposite charge.
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Freeze-thaw cycles.
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Add electrolyte. Change the pH. Ion exchange.
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Raise temperature (HLB depends on the temperature)
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Apply electric field.
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Filter through fritted glass or fibers.
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Centrifugation.
Type of colloids
on the basis of structure (appearance)
colloids
Coherent (solid-like) gel
Incoherent (fluid-like)
Colloidal
Dispersions
sols
Macromolecular
solutions
Association
Colloids
Porodin
Reticular
Spongoid
(porous)
Colloidal solutions
corpuscular
diszpersion macromolecular association
lyophobic
lyophilic
lyophilic
(IUPAC proposal)
fibrillar
lamellar
Types of sols (incoherent)
categorized by inner / outer phases
• aerosols
L/G liquid in air: fog,
mists, spray
S/G solid aerosol, solid
in gas: smoke, colloidal
powder
Complex, smog
• lyosols
G/L gas phase in liquid (sparkling
water, foam, whipped cream)
L/L emulsion, liquid in liquid, milk
S/L colloid suspension (gold sol,
toothpaste, paint, ink)
xerosols, xerogels
G/S solid foam: polystyrene foam
L/S solid emulsion: opals, pearls
S/S solid suspensions: pigmented
plastics
Definitions
• Sol stability: property of a lyophobic sol to remain
unaggregated → only kinetic (see DLVO theory, steric
stabilization), lyophobic sols are thermodynamically
unstable
• Sol: incoherent, dispersion colloidal system
• Xerosol: solidified sol, no aggregation, no skeleton
structure → not a gel!
• Gel: coherent colloidal system, has a skeleton structure
• Cream: concentrated emulsion (L/L), o/w type
• Grease: high viscosity gel, with shear-thinning properties
Preparation of sols
Importance of monodispersity
It is important to make sols of well controlled particle size
and size distribution for most uses.
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Top to bottom technique: it is almost impossible to achieve this by
dispersion.
Bottom to top technique: precipitation with chemical synthesis works
often
AgI sol (AgNO3+ KI → KNO3 + AgI)
Gold sol (H[AuCl4] + Na3-citrate → ruby-colored Au sol)
Sulfur sol (Na2S2O3 + 2 HCl → 2 NaCl + S + SO2 + H2O)
Iron(III) hydroxide sol (FeCl3 in water → Fe(OH)3 with hydrolysis)
LaMer diagram (1950): precipitation
Example: ceria nanoparticles
LaMer diagram (1950): precipitation
Yugang Sun, Chem. Soc. Rev. 42: 2497—2511 (2013)
Phenomena after preparation (changes in
size)
Ageing of colloids; Ostwald-ripening (moving slowly to
equilibrium)
lyophobic colloid systems are thermodynamically unstable → ageing
(spontaneous slow, irreversible change) → coarsening
Kelvin equation
Ostwald equation
pr 2 γ V m
ln ( )=
p
RT r
Lr 2 γ V m
ln( )=
L
RT r
pr: vapor pressure over surface of radius r (N)
p: saturation vapor pressure in gas phase (N)
γ: surface tension (N/m)
r: radius of curvature (m)
VM: molar volume (m3/mol)
Lr: pr, cr, or μr in the droplet of radius r
L: p, c, or μ in the medium
Gels
• Definition
– Coherent colloid system, in which one of the
components forms a skeleton (network made with
primary or secondary bonds) and contains a fluid
dispersion medium
– State of transition between liquids (vapor pressure,
conductivity) and solids (shape)
• Types
– Porodin gels: consist of a skeleton of particles
– Reticular gels: skeleton of fibers, coarse fibers, bunch of
fibers
– Spongoid gels: skeleton of lamellae or films,
Definition by IUPAC (reading)
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Gel: Nonfluid colloidal network or polymer network that is expanded throughout its whole
volume by a fluid.[3]
Note 1: A gel has a finite, usually rather small, yield stress.
Note 2: A gel can contain:
(i) a covalent polymer network, e.g., a network formed by crosslinking polymer chains or by nonlinear
polymerization;
(ii) a polymer network formed through the physical aggregation of polymer chains, caused by
hydrogen bonds, crystallization, helix formation, complexation, etc., that results in regions of local
order acting as the network junction points. The resulting swollen network may be termed a
“thermoreversible gel” if the regions of local order are thermally reversible;
(iii) a polymer network formed through glassy junction points, e.g., one based on block copolymers. If
the junction points are thermally reversible glassy domains, the resulting swollen network may also be
termed a thermoreversible gel;
(iv) lamellar structures including mesophases, e.g., soap gels, phospholipids, and clays;
(v) particulate disordered structures, e.g., a flocculent precipitate usually consisting of particles with
large geometrical anisotropy, such as in V2O5 gels and globular or fibrillar protein gels.
(above) rather than of the structural characteristics that describe a gel.
Hydrogel: Gel in which the swelling agent is water.
Note 1: The network component of a hydrogel is usually a polymer network.
Note 2: A hydrogel in which the network component is a colloidal network may be referred
to as an aquagel.
Typical gels
a) reversible polymer gel
b) reversible porodin gel
c) irreversible polymer gel
d) irreversible solid-gas xerogel
pregel
apolar
solvent
gel
a) Ionic, b) hydrophobic, c) H-bridge, d) van der Waals, e) hairy micelles, f-g)
coordination bond
Porodin gel (e.g. silica)
Silica gel (SiO2 · n H2O)
The size of silica gel particles is determined by the pH.
In acidic medium the hydrolysis is faster
The condensation is slow: small particles form.
In alkaline medium: bigger particles, loose structure
TEM pictures
Porodin gel (e.g. clay)
Example: (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n (montmorillonite)
Drilling mud:
1. Viscosity is high: takes up solids
2. Cools and lubricates
3. Increases pressure to keep away
liquids (density)
4. Cover the pores of the wall
5. Keeps the stability of the wall
Takes 4-5 times its weight of water
Composition: water + clay +
baryte (for its weight) + xanthan
or carboxymethyl cellulose (for
their viscosity)
Sol-gel technology
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Aerogel („frozen smoke”)
Aerogels are the lightest solid materials. They are very good insulators.
Silica based aerogel was the first to make, but today Al, Cr, Zn or carbon are
also used for synthesizing aerogels.
http://www.youtube.com/watch?v=mAJWyRIDDVQ
http://www.youtube.com/watch?v=HoCAxS4vqwQ
Structure of aerogel
http://stardust.jpl.nasa.gov/photo/aerogel.html
Preparation of silica aerogels
Exchange the liquid to gas!
http://www.resonancepub.com/aerogel.htm
http://en.wikipedia.org/wiki/Aerogel
Si or Al are biocompatible
In addition, there is no surface tension in a supercritical fluid, as there is
no liquid/gas phase boundary. By changing the pressure and
temperature of the fluid, the properties can be “tuned” to be more
liquid- or more gas-like.
Lyogels (solvent in the skeleton)
Polymer gels (e.g. “intelligent” gels) → reversible transformations
(as a function of T, pH, salt content, etc.)
gel
solvent
Example:
drug delivery
syneresis
swelling
Disposable diapers
Hydrogels
Poly (sodium propenoate): poly acrylic acid.
The monomer:
Randomly coiled molecules, swelling in water
Examples of hydrogels: gelled foods, fruit jellies, etc.
http://www.gcsescience.com/o69.htm
Disposable diapers
By addition of salt water flows out.
Solidification of liquid waste
• Easier to handle
• Storage
• Destruction is easier
Intelligent gels
Magnetic nanoparticles
PDMS: poly(dimethyl-siloxane) elastomers
Polyaspartic acid gel: artifical muscle
Non-ionized in acidic medium: shrinks
Temperature-sensitive gels (e.g. NIPA)
N-isopropylacrylamide gel:
transition at 34 oC
PEM (proton exchange mebrane)
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Xerogel coating
Xerogel coating: applications,
modern artificial opal
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Light interference (e.g.
anti-reflection coatings
for the areas of UV, VIS
and NIR).
Applications: From
architectural application
to UV protection
1992, Prinz Optics (Sol-Gel
Dip Coating Process).
http://www.prinzoptics.de/en/home/index.php
http://www.variotrans-glas.de/htdocs_en/home/index.html
http://www.molecularexpressions.com/primer/lightandcolor/
interferenceintro.html