Solid particles as emulsion stabilisers

Abstract

It has long been known that solid particles can act as very effective stabilisers of emulsions in the absence of surfactant. The particles are usually small in relation to the emulsion drop size (say, particle radius 0.1 times the drop radius or less). There are apparent similarities in the ways in which particles and low-molar-mass surfactants act as emulsion stabilisers. For systems with roughly equal volumes of oil and water, hydrophobic particles tend to stabilise water-in-oil emulsions whereas hydrophilic particles favour the formation of oil-in-water emulsions. Particle wettability therefore appears to parallel the hydrophile/lipophile balance of surfactants, with, for example, high hydrophile/lipophile surfactants corresponding to hydrophilic particles. In the cases of both surfactants and particles, it is probable that for significant emulsion stability, close-packed layers coating the droplets are required. Here, we give a preliminary account of work directed towards understanding more clearly the origins of the stability of solid-stabilised emulsions. Surfactant-stabilised emulsions are kinetically rather than thermodynamically stable, and we ask if the same is also true for solid-stabilised emulsions. Aspects of the stability of surfactant-stabilised emulsions can be understood in terms of the curvature properties of surfactant monolayers, and we probe the possibility that close-packed monolayers of spherical particles have curvature properties similar to those of surfactant monolayers.