Catastrophic phase inversion of water-in-oil emulsions stabilized by hydrophobic silica

A short review of the experimental findings concerning the stabilization of emulsions by solid particles is given. We then describe the preparation and properties of water-in-oil (w/o) emulsions stabilized by nanometer-sized hydrophobic silica particles' alone. Emulsions of median diameter equal to 0.6 mu m are completely stable to coalescence as a result of an adsorbed layer of particles at the oil-water interface. Their stability to sedimentation increases with particle concentration due to network formation of the particles in the continuous oil phase. The w/o emulsions catastrophically invert, without hysteresis, to oil-in-water (o/w) at volume fractions of water around 0.7, i.e., as soon as the drops begin to deform. The drops in o/w emulsions are larger (100 mu m) and cream rapidly but remain stable to coalescence. We demonstrate that for emulsions stabilized by hydrophilic silica particles, phase inversion from o/w to w/o occurs at the same dispersed phase volume fraction as above. It is therefore suggested that the system hydrophile-lipophile balance is determined by the particle wettability. Comparisons with the behavior of surfactant-stabilized emulsions are given throughout.