The preparation and properties of emulsions, stabilised solely by the adsorption of solid particles at the oil-water interface, are reviewed especially in the light of our own work with particles of well-controlled surface properties. Where appropriate, comparison is made with the behaviour of surfactant-stabilised emulsions. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions. Many of the properties can be attributed to the very large free energy of adsorption for particles of intermediate wettability (contact angle at the oil-water interface, say, between 50 and 130degrees). This effectively irreversible adsorption leads to extreme stability for certain emulsions and is in contrast to the behaviour of surfactant molecules which are usually in rapid dynamic equilibrium between the oil-water interface and the bulk phases. There is. evidence that, in some systems, weak flocculation of the particles improves the emulsion stability. Phase inversion from w/o to o/w can be brought about by increasing the volume fraction of water. Emulsions close to this inversion point tend to be the most stable, again in contrast to surfactant systems. The volume fraction needed for inversion depends on the particle wettability and the nature of the oil and these effects have been rationalised in terms of surface energy components. Stable multiple emulsions (w/o/w and o/w/o) can be made using two types of particles with slightly different wettability. Similar multiple emulsions prepared with two types of surfactant tend to be much less stable. The possibility of preparing novel solid materials by evaporating solid-stabilised emulsions is also proposed. Finally we report on some extensions to the work of Levine et al. who obtained expressions for the free energy of formation of emulsion drops covered with close-packed monolayers of monodisperse spherical particles. In particular in the light of the observations that nanoparticles can act as excellent emulsion stabilisers, we have considered potential effects on the free energy of emulsion formation of the action of small (physically realistic) positive and negative line tensions in the 3-phase contact lines skirting particles adsorbed at the droplet interfaces. We also explore the possibility that curvature properties of close-packed particle monolayers can affect emulsion properties in much the same way that surfactant monolayer properties influence emulsion type and stability. (C) 2002 Elsevier Science B.V. All rights reserved.
