Particles as surfactants and antifoams

1. Particles as surfactants and antifoams N. D. Denkov and S. Tcholakova Department of Chemical Engineering, Faculty of Chemistry, Sofia University, Sofia, Bulgaria Lecture at COST D43 Training School “Fluids and Solid Interfaces”Sofia, Bulgaria, 12–15 April,2011

2. Why particles ? • Cost – surfactants are expensive! • Environment – surfactants are polluting! • Specific applications – metal foams, mineral foams … • Health – dietary particles (globular proteins, cellulose fibers, CaCO3, …)

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.

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

5. 1. Surfactants as foam and emulsion stabilizes • Emulsification and foaming • Structure of foams and emulsions • Modes of destabilization • Role of surfactants in foam and emulsion stabilization

6. (a) Emulsification Basic processes during emulsification

7. Foaming Air entrapment Bubble breakup and coalescence

8. Films 10-30 nm thickness Plateau channels  10 m for foams  1 m for emulsions Nodes Similar to Plateau channels (b) Structural elements of foams and emulsions

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

10. Coalescence Particle stabilization or particle destabilization?

11. p2, V2 p1,V1 C1(p1) C1I C2I C2(p2) h Ostwald ripening From Fick’s law (after Princen & Mason, 1965)

12. (d) Role of surfactants

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

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

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

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

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

18. 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

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

20. Food products Whipped cream Ice-cream Chocolate mousse …

21. Types of solid particles • Mineral - SiO2, Al2O3, CaCO3, … • Polymeric – latex, … • Particle monolayers • Particle stabilized films

22. Specific features (a) High energies: Adsorption, Desorption, Barrier to adsorption • Particle adsorption energy = - a2(1-cos)2 >> kBT

23. Barrier to particle adsorption Surface forces (Derjaguin approximation) Irreversible adsorption, but high barrier !

24. (b) High interfacial tension For dilute adsorption layers: Ideal two-dimensional gas 0 = 30 to 70 mN/m For typical surfactants: A = 0.25 nm2;  610-6 mol/m2  kBT 15 mN/m For particles with 20 nm radius: A = 315 nm2;   510-9 mol/m2  kBT 0.01 mN/m

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

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

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

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

29. Lateral capillary forces Kralchevsky et al.

30. (d) Role of particle aggregation “Foam super-stabilization by polymer microrods” Alargova et al., Langmuir 20 (2004) 10371. Rod-like particles aggregate on the surface and form very stable foams (can be dried)

31. Types of aggregation Surface aggregation Bulk aggregation

32. Possible additional stages • Molecule rearrangement • Formation of intermolecular bonds (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

33. t = t1 t = t2 (t1) (t2) Estimate of the adsorption time (barrier-less adsorption)  = (CS) Surfactant needed Diffusion time Adsorption time Much longer time for particles

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

35. Solid latex particles + 500 mM NaCl Particle Golemanov et al, Langmuir, 2006, 22, 4698.

36. Globular protein + 150 mM NaCl Protein M = 1.9 mg/m2 1/(concentration), (wt %)-1 Tcholakova et al, Langmuir, 2003, 19, 5640;Langmuir, 2004, 20, 7444.

37. Concentration needed for complete monolayer In continuous phase In dispersed phase Required initial concentration for obtaining 1 m drops in 50 vol. % emulsion

38. Example 2 – Arrest of Ostwald ripening P1 > P2 particles Air Bubble shrinking is stopped by particle armor P1 = P2 Applications: Ice-cream, whipped cream, chocolate mousse, … Xu et al., Langmuir, 2005

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

40. Observation of foam films made of liquid aluminum Kumar et al., 2007 No surfactant could survive liquid metal temperatures – particle stabilized foam films! h = 10-20 m

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

43. TECHNOLOGY • Pulp and paper production • Oil industry (non-aqueous foams) • Fermentation • Textile colouring CONSUMER PRODUCTS • Powders for washing machines • Paints • Drugs Antifoam effect of hydrophobic particles Antifoam effect

44. 3. Compound • Oil + particles Compound globule 30 mm Composition of Typical Antifoams Silica particles Emulsified oil 1. Hydrophobic solid particles • Silica (SiO2) • Polymeric particles 2. Oil • Silicone oils (PDMS) • Hydrocarbons (mineral oil, aliphatic oils) 100 nm 30 mm

45. 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

46. 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.

47. 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, 2006. P.A. Kralchevsky, K. Nagayama, Particles at Fluid Interfaces and Membranes, Elsevier, Amsterdam, 2001; S. Tcholakova et al., Phys. Chem. Chem. Phys.10(2008) 1608. N. Denkov, K. Marinova, Antifoam effect, Ch. 10 in the book by Binks & Horozov