Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica

TLDR

A short review of experimental findings on particle‑stabilized emulsions is presented, with comparisons to surfactant‑stabilized systems. The study aims to describe the preparation and properties of water‑in‑oil emulsions stabilized solely by nanometer‑sized hydrophobic silica particles. The authors prepared w/o emulsions using only hydrophobic silica particles and examined their interfacial adsorption and particle network formation. The silica‑stabilized w/o emulsions are fully coalescence‑stable, with sedimentation resistance rising with particle concentration, but undergo a hysteresis‑free phase inversion to o/w at ≈70 % water volume, producing larger cream‑forming droplets that remain coalescence‑stable; the same inversion volume applies to hydrophilic silica, indicating that the particle wettability sets the system’s hydrophile–lipophile balance.

Abstract

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 μm 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 μm) 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.