Phase Inversion of Silica Particle-Stabilized Water-in-Water Emulsions

An aqueous two-phase system (ATPS) is of great value in low calorie foods or oil-free cosmetics and pharmaceuticals. In contrast to the recent work on polymer/polymer ATPSs, a simple polymer/salt ATPS (polyethylene glycol/Na₂SO₄) was chosen to study water-in-water (w/w) emulsions stabilized by solid particles. The binodal curve and the tie lines were first determined for the mixture at room temperature. Above the binodal curve, two water-based phases coexist; the upper phase is rich in polymer, whereas the lower phase is rich in salt. Within the two-phase region, we attempted to prepare w/w emulsions with or without the addition of common emulsifiers. Ionic and nonionic surfactants, a polymer, and various solid particles (hydrophilic calcium carbonate particles of different sizes and shapes, wax microspheres) were selected, but no stable emulsion was possible. However, stable w/w emulsions of both types (polymer-in-salt and salt-in-polymer) were formed using dichlorodimethylsilane-modified nanosilica particles. Using partially hydrophobic fumed silica as the emulsifier, emulsions remained fully emulsified for over 1 year and we link the extent of hydrophobization of particles to the properties of the emulsions via contact angle measurements. Furthermore, systematic emulsion studies were conducted at different overall compositions such that changes in emulsion type and stability were mapped onto the phase diagram. Catastrophic phase inversion of emulsion type and evolution of emulsion stability were monitored along the tie lines. Importantly, stability to coalescence was found to decrease approaching conditions of phase inversion.