Particles as surfactants and antifoams N. D. Denkov and S. Tcholakova Department of Chemical Engineering, Faculty of Chemistry, Sofia University, Sofia, Bulgaria

Why particles ? Cost – surfactants are expensive! Environment – surfactants are polluting! Specific applications – metal foams, mineral foams … Health – dietary particles (globular proteins, cellulose fibers, CaCO 3, …)

Aims of the presentation Brief overview of the main mechanisms of foam and emulsion stabilization by particles. Comparison of particles and surfactants as stabilizers.

Contents 1.Introduction – surfactants as foam and emulsion stabilizers. 2.Foams and emulsions stabilized by particles (mechanisms, factors). 3.Examples of specific applications: Metal foams Mineral foams and porous materials Particles as antifoams 4.Conclusions.

1. Surfactants as foam and emulsion stabilizes (a)Emulsification and foaming (b)Structure of foams and emulsions (c)Modes of destabilization (d)Role of surfactants in foam and emulsion stabilization

(a) Emulsification Basic processes during emulsification

Foaming Bubble breakup and coalescence Air entrapment

Films nm thickness Plateau channels  10  m for foams  1  m for emulsions Nodes Similar to Plateau channels (b) Structural elements of foams and emulsions

(c) Modes of foam and emulsion destabilization Creaming Flocculation Flocculation – aggregate formation due to interdroplet attraction Creaming – floating of drops due to buoyancy force

Coalescence Particle stabilization or particle destabilization?

Ostwald ripening p1,V1p1,V1 p 2, V 2 C2(p2)C2(p2) C1(p1)C1(p1) C1IC1I C2IC2I h From Fick’s law (after Princen & Mason, 1965)

(d) Role of surfactants

Applied stress Capillary pressure = Pressure balance (Kolmogorov) Drop breakup Interfacial tension, , depends strongly on surfactant type and concentration shear flow

Typical dependence of equilibrium interfacial tension on surfactant concentration Gibbs adsorption isotherm: CMC

Drop-drop coalescence Film stability is governed by the surfactants adsorbed

Stabilization of foam films by surfactants Electrostatic stabilization Ionic surfactants Steric stabilization Nonionic surfactants

2. Particle stabilization of emulsions and foams Introduction Specific features of particles as stabilizers: (a) Adsorption and desorption energies (b) Surface coverage and surface tension (c) Film stabilization - role of capillary effects (d) Role of particle aggregation (e) Slow kinetics of adsorption Illustrations of the effect of these features

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

Mineral flotation is probably the most voluminous industrial process bubble Mineral froth flotation

Food products Whipped cream Ice-cream Chocolate mousse …

Types of solid particles Mineral - SiO 2, Al 2 O 3, CaCO 3, … Polymeric – latex, … Particle stabilized films Particle monolayers

Specific features Particle adsorption energy = -  a 2  (1-cos  ) 2 >> k B T (a) High energies: Adsorption, Desorption, Barrier to adsorption

Barrier to particle adsorption Surface forces (Derjaguin approximation) a, nmE b /kT Surfactant0.3  1 1 Particle20  60 Irreversible adsorption, but high barrier !

(b) High interfacial tension Ideal two-dimensional gas For typical surfactants: A  = 0.25 nm 2 ;   6  mol/m 2  k B T   15 mN/m For particles with 20 nm radius: A  = 315 nm 2 ;   5  mol/m 2  k B T   0.01 mN/m For dilute adsorption layers:  0 = 30 to 70 mN/m

Required surface coverage for  S = 40 mN/m Surfactant   0.7 Particles (20 nm)   Volmer equation of state Surface coverage For particle adsorption layers    0 (unless complete particle adsorption layer is formed) For dense adsorption layers:

(c) Capillary stabilization of liquid films Stabilization by particle monolayer Capillary component of disjoining pressure

Stabilization by dense particle bilayer Maximum in capillary pressure Mason & Morrow, 1994 Very high stabilizing pressures are predicted

Problems with particle stabilized foams Strong capillary forces push particles away from the film  Creation of “weak” spots (free of particles) in the films! Velikov et al., Langmuir, 1998

Lateral capillary forces Kralchevsky et al.

“Foam super-stabilization by polymer microrods” Alargova et al., Langmuir 20 (2004) Rod-like particles aggregate on the surface and form very stable foams (can be dried) (d) Role of particle aggregation

Types of aggregation Surface aggregation Bulk aggregation

(e) Kinetics of adsorption Two consecutive stages Stage 1 - adsorption from the "subsurface layer" onto surface. Stage 2 - diffusion from the bulk to the subsurface layer Possible additional stages Molecule rearrangement Formation of intermolecular bonds

Estimate of the adsorption time (barrier-less adsorption)  =  (C S ) t = t 1 (t1)(t1) t = t 2 (t2)(t2) Diffusion time Surfactant needed Adsorption time Much longer time for particles

Monolayer adsorption  M stabilizes the drops Coalescence  <  M  Drop size: No coalescence    M Example 1 - Emulsification

Solid latex particles mM NaCl Golemanov et al, Langmuir, 2006, 22, Particle

1/(concentration), (wt %) -1  M = 1.9 mg/m 2 Globular protein mM NaCl Tcholakova et al, Langmuir, 2003, 19, 5640; Langmuir, 2004, 20, Protein

Concentration needed for complete monolayer In continuous phaseIn dispersed phase Required initial concentration for obtaining 1  m drops in 50 vol. % emulsion A , nm 2  , nmol/m 2 C ini wt % Surfactant Particle (20 nm)

Applications: Ice-cream, whipped cream, chocolate mousse, … P 1 > P 2 Xu et al., Langmuir, 2005 P 1 = P 2 Bubble shrinking is stopped by particle armor Air particles Example 2 – Arrest of Ostwald ripening

Metal foams Low mass density at high mechanical strength Automotive & airplane industries Banhart et al., 2000 Particles in metal foams

h =  m Observation of foam films made of liquid aluminum Kumar et al., 2007 No surfactant could survive liquid metal temperatures – particle stabilized foam films!

Particle-stabilized foams as precursors of porous materials Solid foam-based materials with different pore sizes and porosities Juillerat et al., 2011

Antifoam effect of hydrophobic particles Antifoam effect of hydrophobic particles Antifoam effect TECHNOLOGY 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  m  Silica (SiO 2 )  Polymeric particles 3. Compound  Oil + particles Compound globule 30  m 1. Hydrophobic solid particles

Foam film rupture by antifoam particles Foam film fromed on glass frame (high speed camera, 500 fps) The antifoam particles may rupture the foam films immediately after their formation

Film rupture by solid particles bridging-dewetting mechanism Key factors: (1) Particle contact angle (2) Particle size and shape Garrett et al., Aveyard et al.

Conclusions Particles demonstrate a large number of specific features They are used in several very important applications The combination of particles with surfactants and polymers often shows strong synergistic effect B. P. Binks and T. S. Horozov Eds., Colloidal Particles at Liquid Interfaces, Cambridge University Press, P.A. Kralchevsky, K. Nagayama, Particles at Fluid Interfaces and Membranes, Elsevier, Amsterdam, 2001; S. Tcholakova et al., Phys. Chem. Chem. Phys. 10 (2008) N. Denkov, K. Marinova, Antifoam effect, Ch. 10 in the book by Binks & Horozov