Deformation and coalescence of particle-stabilized oil droplets in drying aqueous NaCl solutions

Abstract

The effects of ion concentration on the deformation and coalescence of particle-stabilized oil droplets dispersed in water were investigated. We emulsified a mixture of oil and an aqueous solution of silica particles with various NaCl concentrations ranging from 0 M to 3 M. Oil droplets were spontaneously compressed by evaporation of the water phase. At low ion concentrations (< 0.1 M), the droplets were tightly packed and deformed into polygonal shapes, before coalescing in the final stage of water drying. The flexible deformation of the oil droplets suggested liquid-like oil-water interfaces, even though they were stabilized by adsorbed solid particles. Conversely, tight packing of the droplets was not observed at high ion concentrations (> 0.1 M); however, they coalesced more readily and formed larger ellipsoidal droplets. The coalesced droplets did not relax to a spherical shape. We observed the formation of large aggregates of the silica particles in the water phase at high ion concentrations. This suggested that aggregates of particles adsorbed on the oil-water interfaces and this would be closely related to solid-like oil-water interfaces. In contrast, the particles were well dispersed in the water phase at low ion concentrations because of repulsive forces between the particles. Thus, the adsorbed particles on the oil-water interfaces did not form solid-like particulate shells and the interfaces remained liquid-like. We also confirmed that an increase or decrease in ion concentration, even after formation of particle-stabilized oil droplets, resulted in the same interface responses. The ion concentration affected adsorbed silica particles and was a dominant factor for the interfacial properties of particle-stabilized oil droplets in the water phase.