Surfactants – classification, features and applications

Surfactants – classification, features and applications

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

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.

ContentsContents

• Low molecular mass surfactants Nonionic IonicAmphoteric

• Polymeric surfactantsSyntheticNatural

• Particles as surfactant speciesSpherical vs. non-sphericalHydrophilic vs. hydrophobic

1. Classification of surfactants

Low-molecular mass surfactants1. Nonionic surfactantsAlkylpolyoxyethylenes

CnEOm

TweensSpans

2. Ionic surfactants

n=12 ⇒ sodium dodecyl sulfate, SDS

(a) Anionic

Diffuse electric

layer

(b) Cationic

n=12 ⇒ dodecyl trimethyl ammonium chloride, DTAC

3. Amphoteric surfactants

(b) Betaines

(a) Natural soaps (alkylcarboxylates), Lipids

Stabilization of foam films by surfactants

Both can be explained as a result of higher osmotic pressure in the foam film

Electrostatic stabilization by ionic surfactants

Ionic surfactants

Steric stabilization by nonionic surfactants

Nonionic surfactants

Role of surfactant micelles

Solution rheology (shampoos, dish-washing gels)

higher solution viscosity

Structural forces in foam films:

Control of micelle shape: mixtures (SLES+CAPB), counterions (Ca2+), temperature (for EO surfactants)

Comparisonof the low-molecular mass surfactants

Mixtures are usually used in applications(main surfactant + cosurfactant ± polymers)

*Adsorption, surface tension, CMC, micelle size and shape, foam and emulsion stability

Sensitivity* Nonionic Ionic Amphoteric

Electrolytes NO YES Depends on pH

Temperature YES NO NO

pH NO NO YES

Hydrophile Lipophile Balance (HLB)

C16H33(C2H4O)20OH

HLB =20*M(hydrophilic) / M(surfactant)

ExampleBrij 58 = Polyoxyethylene-20 hexadecyl ether

M(hydrophylic) =20*44=880M(surfactant) = 1120

⇒ HLB = (880*20)/1120 = 15.7

Relation between HLB and surfactant applications

HLB < 8

HLB > 10

• Mixing unlike oils together

– surfactants with HLB’s of 1 to 3

• 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

• Solubilization of oil into water (microemulsion)

– surfactant blends with HLB of 13 to 18

Polymeric surfactants1. Synthetic polymers(a) Homopolymers

Polyvinyl alcohol, PVA

Modified polysacharides

(b) Block-copolymers

Synperonics, EOnPOmEOn

2. Natural polymers (proteins)

(b) Fibrilar

β-caseinκ-casein

(a) Globular

Bovine serum albumin, BSAβ-lactoglobulin, BLG

Very often mixtures of proteins + polysaccharides are used in applications

Modes of foam stabilizationby polymeric surfactants

Usually: combination of steric + electrostatic stabilization

Synthetic polymers Natural polymers

1. Types of Solid particles(a) Mineral - SiO2, Ore particles (b) Polymeric - latex

2. Particle monolayers

Particle adsorption energy = πR2σ(1-cosθ)2 >> kBT

2. Particles as surfactant species

Main factors:

• Particle hydrophobicity

• Particle size

• Particle shape

Particle stabilized emulsions and foams

Oil

particles

Dinsmore et al., Science, 2002

Particle layer on drop surface

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

Strong capillary attraction between particles⇒ Creation of “weak” spots (free of particles) in the films!

Velikov et al., Langmuir, 1998

Lateral capillary forces

qL0 1 2 3 4

0

2

4

6

8

10

12

1 2

Fq Q Qσ

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 effectof hydrophobic particles

Antifoam effectof hydrophobic particles

Antifoam effectTECHNOLOGY

• Pulp and paper production

• Oil industry (non-aqueous foams)

• Fermentation

• Textile colouring

• Powders for washing machines

• Paints

• Drugs

CONSUMER PRODUCTS

Composition of Typical Antifoams

2. Oil

Silicone oils (PDMS)Hydrocarbons (mineral oil, aliphatic oils)

Silica particles Emulsified oil

100 nm 30 μm

Silica (SiO2)Polymeric particles

3. CompoundOil + particles

Compoundglobule

30 μm

1. Hydrophobic solid particles

Film rupture by solid particlesbridging-dewetting mechanism

Key factors:(1) Particle contact angle(2) Particle size and shape

90> oθ 45θ > o

3. Role of surfactants forvarious foam and

emulsion properties

3. Role of surfactants forvarious foam and

emulsion properties

Foam film drainage and thickness

Anionic surfactant, SDS

Speed: ×1

Polymeric surfactant, PVA

Speed: ×4

Protein,Na caseinate

Speed: ×8

hEQ ≈ 10 nmτDR ≈ 60 sec

hEQ ≈ 120 nmτDR ≈ 300 sec

hEQ ≈ 30 nmτDR ≈ 600 sec

Large vertical films

SLES+CAPBreal time

SLES+CAPB+MAcaccelerated 5 times

Much slower film drainage in MAc-containing systemDifferent mode of film drainage - no marginal regeneration

Foam rheologyFoam rheology

1. Viscous friction in flowing foams.

2. Role of surface modulus (surfactant dependent).

3. Bubble breakup.

VτV

&γ

Effect of surfactant type

Capillary number, Ca10-7 10-6 10-5 10-4 10-3 10-2

Dim

ensi

onle

ss v

isco

us s

tres

s

10-3

10-2

10-1

100

Soap

Anionic+CTAC+LOH+LAc+MAc+PAc+MAc/PAc

Anionic+ 6 othercosurfactants

Theoreticalcurve

n ≈ 0.5

n ≈ 0.2

32μγ σRCa ⎛ ⎞= ⎜ ⎟

⎝ ⎠

&

Capillary number

V 32 σVRτ⎛ ⎞τ = ⎜ ⎟

⎝ ⎠% Dimensionless

viscous stress

( ) ( )0 sinS t S S t− = δ ω

( ) ( )0 sint tσ − σ = δσ ϕ + ω

Oscillating drop

0cosSTG

Sδσ

= ϕδ

Storage modulus

0sinLSG

Sδσ

= ϕδ

Loss modulus

S0 + δS0 ⇒ Γ < Γ0 σ > σ0

Role of surface modulus

δS/S0, %

10-1 100 101

GD, m

N/m

100

101

102

103

ν = 0.2 HzT = 25 °C

Experimental results for surface modulus

( )1 22 2D ST LSG G G= +

Soap solutions: GD > 100 mN/mCAPB, SDS, SLES, ... GD < 5 mN/m

Anionic

SoapTotal modulus

Mechanisms of energy dissipation in sheared foam

Friction in foam films

S0 S0 + δS S0

Surface dissipation

0.2V Ca∝%τ

Denkov et al., PRL, 2008; Soft Matter 2009; Tcholakova et al., PRE, 2008

0.5V Ca∝%τ

Φ = 0.8

Capillary number, Ca10-7 10-6 10-5 10-4 10-3 10-2

Dim

ensi

onle

ss v

isco

us s

tres

s

10-4

10-3

10-2

10-1

Hexadecane, ηD = 3 mPa.sLight oil, ηD = 30 mPa.sHeavy oil, ηD = 150 mPa.s

EO8, EO20

n = 0.47

Very good description without any adjustable parameter

Comparison with experimental datafor emulsions

Capillary number, Ca10-6 10-5 10-4 10-3

τ WR

32/ σ

10-3

10-2

10-1

Syntheticsurfactants

Soap

0n

W kVτ =n = 2/3

n = 1/2

Soap

CAPB0 to 70 % Glycerol

0VCa μ=

σ32W Rτ

τ =σ

%

Capillary numberDimensionless

wall stress Lower friction for CAPB compared to soap

surfactants

Foam-wall friction

V0

Initial foam

R32 = 730 μm

Final foam

R32 = 230 μmconstγ =&

Bubble breakup in a rheometer

Shear stress as a function of time:

γ =& const1~ nR

τAt

Bubble (drop) breakup in sheared foams (emulsions)

Time, sec0 30 60 90 120 150 180

Shea

r str

ess,

Pa

10

15

20

25

30

35

40

AnionicR32 = 480 μm

R32 = 220 μmAnionic+0.05 wt % MAc

Smaller and less polydisperse bubbles are formed at high surface modulus

Φ = 0.95

1150 s−γ =&

Anionic

Anionic+MAc

Bubble breakup in steadily sheared foam

Foam drainage

Surface stress in foam drainage

High surface modulus Low surface modulus

Longitudinal section of the Plateau channel

( )= = =0 0S ZV V x ( )τ = τ = =0 0S V x

Time, sec0 500 1000 1500 2000 2500 3000

(R(t)

/R0)

2

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Anionic

Anionic+LAc

Anionic+MAc

Much slower bubble coarsening in FAc-containing system

p1, V1 p2, V2

Foam

Ostwald ripening in foams, R32(t)

Effect of surfactants on film permeability

(Princen & Mason, 1965)

C2(p2)

C1(p1)C1

I

C2I

h

2 ML

DHkh D k

=+

D – diffusion coefficienth – thickness of the filmH – Henry constantkML – monolayer permeability

( )1 22

FML

dn DHA C Cdt h D k

= −+

Arrest of Ostwald ripening by solid particles

P1 > P2

Xu et al., Langmuir, 2005

P1 = P2

Applications:Ice-cream, whipped cream, chocolate mousse, …

P2P1

P1 > P2

Summary

Surfactants with high surface modulus:Fatty acids, fatty alcohols and acids as cosurfactants, …

Surfactants with low surface modulus(anionic, cationic, nonionic; HLB concept; …):SDS, SLES, CAPB, CTAC, EO7, ...

Particles (in combination) give new options:Arrest of Ostwald ripening, different rheology, …

Mixtures are often preferred in applications:SDS+LaOH, DTAB+LaOH, SLES+CAPB, CAPB+FAc, LAS+EO7, ...Surfactant + Polymer, Surfactant + Particles

Current activityCurrent activity

• Biosurfactants:

• Polymers-surfactant mixtures.

• Other dynamic phenomena – acoustic response, foam extrusion, ...

• Role of surfactants and polymers in emulsions.

Relevant References

Basic LiteratureK. 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 LiteratureS. 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 oil-solid compounds in aqueous foams”, in Colloidal Particles at Liquid Interfaces, B. P. Binks and T. S. Horozov Eds., Cambridge University Press, 2006.