Interfacial rheology and breakdown of particle-stabilized water-in-oil emulsions

Particle-stabilized water-in-oil emulsions largely derive their kinetic stability from the presence of an interfacial particle film that acts as a steric barrier against droplet-droplet coalescence. While often sought for applications that demand a long shelf-life (e.g., prolonged bioactive delivery or cosmetics), the mechanism(s) underpinning their controlled breakdown remains elusive. The purpose of this investigation was to establish a mechanistic understanding of the formation, breakdown and recovery of the interfacial film present in particle-stabilized water-in-oil emulsions subjected to different shear conditions as well as the presence of a structure-breaking surfactant. To do so, we made water-in-oil emulsions stabilized with micron-sized glycerol monostearate (GMS) crystals as well as a planar version of this interfacial film. Cryo-scanning electron microscopy (cryo-SEM) revealed an anisotropic interfacial film structure consisting of condensed platelet-like GMS particles. This gel-like structure transitioned from elastic-dominant to viscous-dominant and back when subjected to strain amplitudes above and below its critical strain. Addition of sorbitan monooleate, an oil-soluble demulsifying surfactant, led to structural breakdown and irreversible weakening of the interfacial film, which was associated with an irreversible drop in interfacial G′ and interfacial tension. Changes in the properties of the interfacial film were manifested in the emulsions, which coalesced and phase-separated upon addition of the sorbitan monooleate. This study demonstrates that, while stable under quiescent conditions, particle-stabilized water-in-oil emulsions are destabilized in the presence of structure-breaking surfactants. The origin of this breakdown lies in breakdown of the interfacial particle film.