Stability mechanisms of liquid water-in-oil emulsions

Abstract

Although the stability of emulsions is widely discussed in the literature, most of them dealt with oil-in-water emulsions or water-in-oil systems with solid and semi-solid structures, which are easier to stabilize. In this study, the stability mechanism of liquid water-in-oil emulsions was investigated in different systems. The combination of two different oils (soybean oil and hexadecane) and three emulsifiers (PGPR, Span 80 and lecithin) at two water:oil ratios was investigated. Emulsions with higher kinetic stability – water and soybean oil emulsion stabilized with PGPR and water and hexadecane with Span 80 – presented an interface with low initial interfacial tension and practically constant complex viscoelastic modulus with time. Therefore, small droplets were formed and their coalescence was hindered by a stable elastic interface. The molecular structure of both oil and emulsifier were important to define the emulsion stability. Better chemical affinity of the hydrophobic moieties of the emulsifier and the oil led to more stable interface. Steric stability was obtained in more viscous systems, such as those at higher water volume fraction content. However, the water incorporation capacity into the emulsion depended on the molecular structure of hydrophilic portion of the emulsifier. Moreover, the water – soybean oil systems with Span 80 or lecithin emulsifiers did not form a macroemulsion, but a gelled structure. This study discussed the many factors that affect the emulsion formation and stability, which can contribute to the development of new water-in-oil emulsion based products with higher stability.