Latex-particle-stabilized emulsions of anti-Bancroft type.

Here, we investigate water-in-oil (W/O) emulsions that are stabilized by polystyrene latex particles with sulfate surface groups. The particles, which play the role of emulsifier, are initially contained in the disperse (water) phase. The existence of such emulsions formally contradicts the empirical Bancroft rule. Theoretical considerations predict that the drop diameter has to be inversely proportional to the particle concentration, but should be independent of the volume fraction of water. In addition, there should be a second emulsification regime, in which the drop diameter is determined by the input mechanical energy during the homogenization. The existence of these two regimes has been experimentally confirmed, and the obtained data agree well with the theoretical model. Stable W/O emulsions have been produced with hexadecane and tetradecane, while, in the case of more viscous and polar oils (soybean and silicone oil), the particles enter into the oily phase, and Pickering emulsions cannot be obtained. The formation of stable emulsions demands the presence of a relatively high concentration of electrolyte that lowers the electrostatic barrier to particle adsorption at the oil-water interface. Because the attachment of particles at the drop surfaces represents a kind of coagulation, it turns out that the Schulze-Hardy rule for the critical concentration of coagulation is applicable also to emulsification, which has been confirmed with suspensions containing Na(+), Mg(2+), and Al(3+) counterions. The increase of the particle and electrolyte concentrations and the decrease of the volume fraction of water are other factors that facilitate emulsification in the investigated system. To quantify the combined action of these factors, an experimental stability-instability diagram has been obtained.