Visualization of water transport in W 1/O/W 2 emulsions

Abstract

Water transport between two aqueous phases and through an intervening oil phase under osmotic pressure was observed and quantified visually using capillary video-microscopy. Under certain conditions, a new mechanism was observed directly, according to which the pure-water phase emulsifies spontaneously and the resulting emulsified droplets migrate to the saline aqueous phase. Another finding was the importance of the thickness of the oil phase, O, that separates two aqueous bodies, W 1 and W 2, in that it determines which transport mechanism of water between W 1 and W 2 will be predominant. Specifically, in a W 1/O/W 2 emulsion globule where W 1 represents the internal pure-water droplets and W 2 the suspending saline-water medium, when W 1 and W 2 were at visual contact, water transport occurred mainly through the hydrated surfactant mechanism. In the case of a visible minimum distance of separation between W 1 and W 2, measuring from a few to over 100 μm, the water transport rate was found to be significantly lower than the rate at visual contact and water migration occurred via spontaneously emulsified droplets and reverse micelles. In all cases, the transport rate was independent of the size of the water droplets and the oil globule, and in the case of no visual contact, it was also independent of the minimum separation distance between W 1 and W 2. This result implies that, under the experimental conditions used, the water transport rate in W 1/O/W 2 emulsions is controlled by interfacial processes, rather than being diffusion controlled as has been suggested by previous work.