Some features of breakdown in water-in-oil-in-water multiple emulsions

Abstract

W/O/W multiple emulsions break down by several possible routes, for example by coalescence of the internal aqueous droplets or of the oil drops, by expulsion of the internal droplets following rupture of the oily lamella which surrounds them, or by passage of water through the oil phase by diffusion. This paper describes an investigation of the breakdown pathways associated with three different W/O/W systems using cinemicrography. Three types of multiple (oil) drops were identified in the systems prepared-these being identified by the number and size of the internal aqueous droplets. Type “A” drops contained one large internal droplet, type “B” contained several small internal droplets, and type “C” globules entrapped large numbers of internal droplets. One type predominates in a given multiple emulsion although all three types and simple oil drops can coexist. Hence, in general, we can refer to three main types of multiple emulsion systems, A, B, and C. The W/O/W emulsions were filmed over periods ranging from a few seconds after formation to 30 hr using time-lapse, continuous, and high-speed cinemicrography. The emulsions were also studied by conventional photomicrography. Attempts were made to predict which breakdown pathways would be followed taking into account surface area changes on coalescence and expulsion and, in general, experimental evidence was in agreement with these predictions. The results indicate that all the proposed mechanisms of breakdown occur in W/O/W systems but that some are more favored than others, their predominance depending on the type of multiple emulsion formed. A preliminary analysis of the attractive forces between multiple droplets has also been made.