Origins of freeze—thaw instability in concentrated water-in-oil emulsions

Abstract

Highly concentrated emulsions of distilled water in oil are often very sensitive to storage at temperatures below the freezing point of water and can break after a single freeze—thaw cycle. Microscopic observation suggested that the instability arose from the simultaneous rupture of multiple oil films following the formation of ice domains in adjacent water droplets. This situation changed drastically when electrolytes were incorporated in the aqueous phase at concentrations as low as 0.2 M and these emulsions survived repeated freeze—thaw cycling. Note, however, that most of the water was transformed to ice during even short periods of storage irrespective of the droplet size (<1 μM to 15 μM), or the presence of electrolyte. The primary role of electrolyte was to slow down the rate of ice formation and to maintain a small amount of concentrated electrolyte solution in a liquid state at the temperature of storage employed (−20 to −25°C). This behavior was confirmed both by direct thermal (differential scanning calorimetry) analysis and by an indirect electron spin resonance probe technique. It is proposed that these two effects allow the water phase to deform gradually and the oil films to expand in area in such a way that avoids the generation of large compressive forces in the films.