Effect of the type of the oil phase on stability of highly concentrated water-in-oil emulsions

The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19–24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.