Highly Concentrated Water-in-Oil Emulsions: Influence of Electrolyte on Their Properties and Stability

Abstract

Concentrated water-in-oil emulsions of over 95% internal phase by volume were prepared using a variety of low HLB emulsifiers. These emulsions coarsened with time to produce a fraction of large droplets that grow at the expense of smaller droplets. This resulted in a decrease in yield stress and eventually to visible phase separation of water. There was a significant effect of emulsifier type, and the optimum HLB appears to be ∼5 for mineral oil. None of the emulsions prepared with distilled water as the internal phase could withstand a single freeze/thaw cycle. In contrast to the gradual coarsening process at room temperature, freeze/thaw instability was sudden and involved an inversion process that took place in the frozen state. The properties of these concentrated emulsions changed dramatically when electrolyte was introduced into the aqueous phase at concentrations as low as 0.02 M. The rate of coarsening decreased, and coalescence of water droplets during the freeze/thaw process was inhibited. The emulsions retained their yield stress over prolonged storage and a lower droplet size was often produced under the same conditions of emulsification. These effects seemed to be general and were observed with a variety of emulsifiers and electrolytes. The electrolytes appeared to enhance the stability of these water-in-oil emulsions by increasing the resistance of the water droplets to coalescence. It is proposed that this was achieved by a higher adsorption density of emulsifier at room temperature storage, and in the frozen state by the fractionation of a concentrated electrolyte solution that wetted the ice crystals and prevented their fusion. Analysis of emulsion rheology and processing suggested that this enhanced coalescence stability was adequate to account for the changes in yield stress and droplet sizes observed.