Solid Particles at Liquid Interfaces, Including Their Effects on Emulsion and Foam Stability

ROBERT AVEYARD and JOHN H. CLINT HuU University, Hull, United Kingdom 

Both hydrophilic and hydrophobic particles are surface active at air/water (a/w) and oil/water (o/w) interfaces. The free energy of attachment to an interface is simply related to the contact angle, 6, that the interface makes with the particle surface. Particles are most strongly anchored to a liquid surface when d = 90°. 

Isolated hydrophobic particles resting at one surface of a thin aqueous film can cause the film to rupture (depending on 6) when the second surface engages the particle, causing the particle to bridge the film. For this reason, small hydrophobic particles can act as antifoams or foam breakers. Commercially, antifoams are often formulations in which hydrophobic particles are dispersed in a mineral or a silicone oil. Particles adsorbed in close-packed layers on droplet interfaces in an emulsion can stabilize the emulsion even in the absence of surfactant. Indeed, in some respects monolayers of particles behave similarly to adsorbed smfactants. Thus, hydrophobic particles [which are akin to surfactants with low hydrophile-Upophile balance (HLB)] can stabilize water-in-oil emulsions, whereas hydrophilic particles can be expected to stabilize oil-in-water emulsions (just as high-HLB surfactants do). In the first part of the chapter aspects of the current state of imderstanding of the effects that adsorbed particles have on foam and emulsion stabihty are reviewed. 

The second part of the chapter describes some of our recent work on (1) possible effects of line tension on the wettability of particles at fluid interfaces and (2) lateral interactions between (charged) polystyrene particles in monolayers at a/w and o/w interfaces and the consequences for monolayer 

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Solid Particles at Liquid Interfaces, Including Their Effects on Emulsion and Foam Stability... 

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