Inversion of silica-stabilized emulsions induced by particle concentration.

Emulsions of equal volumes of a cyclic silicone oil and water stabilized by fumed silica nanoparticles alone can be inverted from oil-in-water (o/w) to water-in-oil (w/o) by simply increasing the concentration of particles. The phenomenon is found to be crucially dependent both on the inherent hydrophobicity of the particles and on their initial location. Inversion only occurs in systems with particles of intermediate hydrophobicity when dispersed in oil; emulsions prepared from the same particles but initially dispersed in water remain o/w at all particle concentrations. The stability and drop size distributions in the different emulsions are compared. Various hypotheses are put forward and argued to explain this novel inversion route including adsorption of oil onto particle surfaces, hysteresis of contact angle affecting particle wettability in situ, and the structure of particle dispersions in oil or water prior to emulsification inferred from rheology and light scattering measurements. We propose that the tendency for particles to behave more hydrophobically at higher concentrations in oil is due to the reduction in the effective silanol content at their surfaces as a result of gel formation via silanol-silanol hydrogen bonds. In water, solvation of particle surfaces prevents this from occurring and particles behave as hydrophilic ones at all concentrations. A concentration-induced change in particle wettability is thus advanced.