Surfactants –– classification,
classification,
Surfactants
features and
and applications
applications
features
N. D. Denkov and S. Tcholakova
Department of Chemical Engineering,
Faculty of Chemistry, Sofia University, Sofia, Bulgaria
Lecture at COST P21 Training School
“Physics of droplets: Basic and advanced topics”
Borovets, Bulgaria, 12–13 July, 2010
Contents
Contents
1. Classification of surfactants.
2. Particles as foam and emulsion stabilizers.
3. Role of surfactants for various foam and
emulsion properties:
(a) Thin film drainage and stability
(b) Foam rheology
(c) Foam drainage
(d) Ostwald ripening
4. Current directions.
1. Classification of surfactants
•
Low molecular mass surfactants
9 Nonionic
9 Ionic
9 Amphoteric
•
Polymeric surfactants
9 Synthetic
9 Natural
•
Particles as surfactant species
9 Spherical vs. non-spherical
9 Hydrophilic vs. hydrophobic
Low-molecular mass surfactants
1. Nonionic surfactants
Alkylpolyoxyethylenes
CnEOm
Spans
Tweens
2. Ionic surfactants
(a) Anionic
Diffuse
electric
layer
n=12 ⇒ sodium dodecyl sulfate, SDS
(b) Cationic
n=12 ⇒ dodecyl trimethyl ammonium chloride, DTAC
3. Amphoteric surfactants
(a) Natural soaps (alkylcarboxylates), Lipids
(b) Betaines
Stabilization of foam films by surfactants
Ionic surfactants
Electrostatic stabilization
by ionic surfactants
Nonionic surfactants
Steric stabilization by
nonionic surfactants
Both can be explained as a result of higher
osmotic pressure in the foam film
Role of surfactant micelles
Structural forces in foam films:
Solution rheology (shampoos, dish-washing gels)
higher solution viscosity
Control of micelle shape: mixtures (SLES+CAPB),
counterions (Ca2+), temperature (for EO surfactants)
Comparison
of the low-molecular mass surfactants
Sensitivity*
Nonionic
Ionic
Amphoteric
Electrolytes
NO
YES
Depends on pH
Temperature
YES
NO
NO
pH
NO
NO
YES
*Adsorption, surface tension, CMC, micelle size and
shape, foam and emulsion stability
Mixtures are usually used in applications
(main surfactant + cosurfactant ± polymers)
Hydrophile Lipophile Balance
(HLB)
HLB =
20*M(hydrophilic) / M(surfactant)
Example
Brij 58 = Polyoxyethylene-20 hexadecyl ether
C16H33(C2H4O)20OH
M(hydrophylic) =20*44=880
M(surfactant) = 1120
⇒ HLB = (880*20)/1120 = 15.7
Relation between HLB
and surfactant applications
• Mixing unlike oils together
– surfactants with HLB’s of 1 to 3
HLB < 8
• Preparing water-in-oil emulsions
– surfactants with HLB’s of 4 to 6
• Preparing self emulsifying oils
– surfactants with HLB’s of 7 to 10
• Preparing oil-in-water emulsions
– surfactant blends with HLB’s of 8 to16
• Detergent solutions
– surfactants with HLB of 13 to 15
HLB > 10
• Solubilization of oil into water (microemulsion)
– surfactant blends with HLB of 13 to 18
Polymeric surfactants
1. Synthetic polymers
(a) Homopolymers
Polyvinyl alcohol, PVA
(b) Block-copolymers
Synperonics, EOnPOmEOn
Modified polysacharides
2. Natural polymers (proteins)
(a) Globular
Bovine serum albumin, BSA
β-lactoglobulin, BLG
(b) Fibrilar
β-casein
κ-casein
Very often mixtures of proteins + polysaccharides are
used in applications
Modes of foam stabilization
by polymeric surfactants
Synthetic polymers
Natural polymers
Usually: combination of steric + electrostatic stabilization
2. Particles as surfactant species
1. Types of Solid particles
(a) Mineral - SiO2, Ore particles
(b) Polymeric - latex
2. Particle monolayers
Particle adsorption energy = πR2σ(1-cosθ)2 >> kBT
Particle stabilized emulsions and foams
particles
Oil
Particle layer on drop surface
Main factors:
• Particle hydrophobicity
• Particle size
• Particle shape
Dinsmore et al., Science, 2002
Stabilization of films
by capillary forces
Relatively thick foam films that are very stable, if the
particle layers are complete
Problems with particle stabilized foams
Velikov et al., Langmuir, 1998
Strong capillary attraction between particles
⇒ Creation of “weak” spots (free of particles) in the films!
Lateral capillary forces
12
F
qσQ1Q2 10
8
6
4
2
0
0
1
2
qL
3
4
Role of particle shape
“Foam super-stabilization by polymer microrods”
Alargova et al., Langmuir 20 (2004) 10371.
Rod-like particles seem to be a better option for
foam stabilization
Role of particle aggregation
Surface aggregation
Bulk aggregation
Antifoam effect
effect
Antifoam
of hydrophobic
hydrophobic particles
particles
of
Antifoam effect
TECHNOLOGY
• Pulp and paper production
• Oil industry (non-aqueous foams)
• Fermentation
• Textile colouring
CONSUMER PRODUCTS
• Powders for washing machines
• Paints
• Drugs
Composition of Typical Antifoams
1. Hydrophobic solid particles
Silica particles
Emulsified oil
¾ Silica (SiO2)
¾ Polymeric particles
2. Oil
¾ Silicone oils (PDMS)
¾ Hydrocarbons (mineral oil,
100 nm
30 μm
aliphatic oils)
3. Compound
Compound
globule
¾ Oil + particles
30 μm
Film rupture by solid particles
bridging-dewetting mechanism
Key factors:
(1) Particle contact angle
(2) Particle size and shape
θ > 90
o
θ > 45o
3. Role
Role of
of surfactants
surfactants for
for
3.
various foam
foam and
and
various
emulsion properties
properties
emulsion
Foam film drainage and thickness
Anionic
surfactant, SDS
Speed: ×1
Polymeric
surfactant, PVA
Speed: ×4
Protein,
Na caseinate
Speed: ×8
hEQ ≈ 10 nm
hEQ ≈ 120 nm
hEQ ≈ 30 nm
τDR ≈ 60 sec
τDR ≈ 300 sec
τDR ≈ 600 sec
Large vertical films
SLES+CAPB
real time
SLES+CAPB+MAc
accelerated 5 times
Much slower film drainage in MAc-containing system
Different mode of film drainage - no marginal regeneration
Foam rheology
rheology
Foam
τV
V
γ&
1. Viscous friction in flowing foams.
2. Role of surface modulus (surfactant
dependent).
3. Bubble breakup.
Dimensionless viscous stress
Effect of surfactant type
100
10-1
Anionic+CTAC
+LOH
+LAc
+MAc
n
+PAc
+MAc/PAc
Soap
≈ 0.2
Anionic
+ 6 other
cosurfactants
10-2
10-3
10-7
n ≈ 0.5
Theoretical
curve
10-6
10-5
10-4
Capillary number, Ca
⎛ τV R32 ⎞ Dimensionless
τ%V = ⎜
⎟ viscous stress
σ
⎝
⎠
10-3
10-2
Capillary number
⎛ μγ& R32 ⎞
Ca = ⎜
⎟
σ
⎝
⎠
Role of surface modulus
Oscillating drop
S ( t ) − S0 = δS sin ( ωt )
σ ( t ) − σ0 = δσ sin ( ϕ + ωt )
Storage modulus
S0 + δS0 ⇒ Γ < Γ0
σ > σ0
GST =
δσ
cos ϕ
δS0
Loss modulus
GLS =
δσ
sin ϕ
δS0
Experimental results for surface modulus
103
Total modulus
2
LS
)
12
102
GD, mN/m
(
GD = G + G
2
ST
ν = 0.2 Hz
T = 25 °C
Soap
101
Anionic
100
10-1
100
δS/S0, %
Soap solutions: GD > 100 mN/m
CAPB, SDS, SLES, ... GD < 5 mN/m
101
Mechanisms of energy dissipation in
sheared foam
S0
Friction in foam films
0.5
%
τ V ∝ Ca
S0 + δS
S0
Surface dissipation
0.2
%
τ V ∝ Ca
Denkov et al., PRL, 2008; Soft Matter 2009; Tcholakova et al., PRE, 2008
Dimensionless viscous stress
Comparison with experimental data
for emulsions
Hexadecane, ηD = 3 mPa.s
10-1
Light oil, ηD = 30 mPa.s
Heavy oil, ηD = 150 mPa.s
10-2
n = 0.47
10-3
Φ = 0.8
EO8, EO20
10-4
10-7
10-6
10-5
10-4
10-3
10-2
Capillary number, Ca
Very good description without any adjustable parameter
Foam-wall friction
10-1
τWR32/σ
Soap
Soap
10-2
n = 1/2
Synthetic
CAPB
surfactants
0 to 70 %
Glycerol
V0
n = 2/3
10-3
10-6
10-5
τW = kV0 n
10-4
10-3
Capillary number, Ca
Dimensionless
wall stress
Capillary number
τ W R32
τ% =
σ
μV0
Ca =
σ
Lower friction for CAPB
compared to soap
surfactants
Bubble (drop) breakup in sheared foams
(emulsions)
Bubble breakup in a rheometer
Final foam
Initial foam
R32 = 730 μm
γ& = const
R32 = 230 μm
Shear stress as a function of time:
At γ& = const
1
τ~ n
R
Bubble breakup in steadily sheared foam
40
γ& = 150 s
Anionic+MAc
−1
R32 = 220 μm
Shear stress, Pa
35
Anionic+0.05 wt % MAc
30
25
Anionic
20
Anionic
R32 = 480 μm
15
Φ = 0.95
10
0
30
60
90
120
150
180
Time, sec
Smaller and less polydisperse bubbles are formed at high
surface modulus
Foam drainage
Surface stress in foam drainage
Longitudinal section of
the Plateau channel
High surface modulus
VS = VZ ( x = 0 ) = 0
Low surface modulus
τS = τV ( x = 0 ) = 0
Ostwald ripening in foams, R32(t)
2.6
Foam
2.4
Anionic
(R(t)/R0)
2
2.2
2.0
Anionic+LAc
1.8
1.6
1.4
p1, V1
p2, V2
Anionic+MAc
1.2
1.0
0
500
1000
1500 2000
Time, sec
2500
3000
Much slower bubble coarsening in FAc-containing system
Effect of surfactants on film permeability
(Princen & Mason, 1965)
dn
DH
= AF
( C1 − C2 )
dt
h + 2 D kML
C1(p1)
C1 I
k=
C2 I
C2(p2)
h
DH
h + 2 D kML
D – diffusion coefficient
h – thickness of the film
H – Henry constant
kML – monolayer permeability
Arrest of Ostwald ripening by solid particles
Xu et al., Langmuir, 2005
P1 > P2
P2
P1
P1 > P2
P1 = P2
Applications:
Ice-cream, whipped cream,
chocolate mousse, …
Summary
Surfactants with low surface modulus
(anionic, cationic, nonionic; HLB concept; …):
SDS, SLES, CAPB, CTAC, EO7, ...
Surfactants with high surface modulus:
Fatty acids, fatty alcohols and acids as cosurfactants, …
Mixtures are often preferred in applications:
SDS+LaOH, DTAB+LaOH, SLES+CAPB, CAPB+FAc, LAS+EO7, ...
Surfactant + Polymer, Surfactant + Particles
Particles (in combination) give new options:
Arrest of Ostwald ripening, different rheology, …
Current activity
activity
Current
•
Biosurfactants:
•
Polymers-surfactant mixtures.
•
Other dynamic phenomena – acoustic response,
foam extrusion, ...
•
Role of surfactants and polymers in emulsions.
Relevant References
Basic Literature
K. Tsujii, “Surface Activity: Principles, Phenomena And Applications”,
Academic Press, 1998.
D. J. McClements, “Food Emulsions: Principles, Practices, and
Techniques”, CRC Press, 2nd ed., 2005.
K. Robert Lange, “Surfactants: A Practical Handbook”, Hanser, 1999.
J. Goodwin, “Colloids and interfaces with surfactants and polymers”, Wiley
2nd ed., 2009.
B. P. Binks,” Particles as surfactants – similarities and differences”,
Current Opinion Colloid Interface Sci. 7 (2002) 21 (review).
S. Tcholakova et al., “Comparison of solid particles, globular proteins, and
surfactants as emulsifiers”, Phys. Chem. Chem. Phys. 12 (2008) 1608
(review).
N. D. Denkov et al., “Role of surfactant type and bubble surface mobility in
foam rheology”, Soft Matter 5 (2009) 3389 (review).
Additional Literature
S. Tcholakova et al., “Role of surfactant type and concentration for the mean drop
size during emulsification in turbulent flow”, Langmuir 20 (2004) 7444.
S. Tcholakova et al., “Coalescence stability of emulsions containing globular milk
proteins”, Adv. Colloid Interface Sci. 123-126 (2006) 259.
K. Golemanov et al., “Selection of surfactants for stable paraffin-in-water
dispersions, undergoing solid-liquid transition of the dispersed particles”
Langmuir 22 (2006) 3560.
N. D. Denkov et al., "Wall slip and viscous dissipation in sheared foams: effect of
surface mobility", Colloids Surfaces A 263 (2005) 129.
S. Tcholakova et al., “Theoretical model of viscous friction inside steadily sheared
foams and concentrated emulsions”, Phys. Rev. E 78 (2008) 011405.
K. Golemanov et al., “Surfactant mixtures for control of bubble surface mobility in
foam studies”, Langmuir 24 (2008) 9956.
K. Golemanov et al., “Breakup of bubbles and drops in steadily sheared foams and
concentrated emulsions”, Phys. Rev. E 78 (2008) 051405.
N. D. Denkov, K. G. Marinova, “Antifoam effects of solid particles, oil drops and oilsolid compounds in aqueous foams”, in Colloidal Particles at Liquid Interfaces,
B. P. Binks and T. S. Horozov Eds., Cambridge University Press, 2006.