Displacement of interfacially-bound monoglyceride crystals in water-in-oil emulsions by a non-ionic surfactant.

Hypothesis: Micron and nano-scale particles are increasingly used to stabilize water-in-oil (W/O) emulsions. Though remarkably stable, the resulting emulsions can be broken by adding low molecular weight surfactants that modify the wettability of the interfacially-adsorbed particles.

Experiments: W/O emulsions were prepared using lipophilic crystals of the monoglyceride glycerol monostearate (GMS), followed by addition of sorbitan monooleate (SMO) at concentrations below and above its critical micelle concentration (CMC). Systematic measurements of interfacial tension and three-phase contact angles, as well as characterization of emulsion sedimentation and microstructure, were used to assess GMS crystal wettability and emulsion destabilization.

Findings: GMS crystals formed shells around the dispersed droplets, resulting in emulsions stable against breakdown under quiescent conditions. With SMO concentrations added below CMC, emulsion stability was not significantly affected. At SMO concentrations above CMC, the integrity of the crystalline shell was markedly affected. Notably, the GMS crystals transitioned from preferential oil-wet to water-wet behavior, eventually leading to their diffusion into the droplets. Therefore, in-situ modification of particle wettability at the oil-water interface was responsible for emulsion breakdown. Findings from this study may provide a pathway for the design of particle-stabilized W/O emulsions with controllable breakdown properties for applications such as tailored release of aqueous bioactive compounds.