A study of water transport in W(1)/O/W(2) emulsions in microcapillary
Description
Water art in W1/O/W2 emulsions under osmotic pressure was studied and quantified visually using capillary video-microscopy. Under certain conditions, a new mechanism was observed directly, in which water droplets emulsified spontaneously from the pure-water phase and migrated to the saline aqueous phase. The oil layer thickness determines which transport mechanism will be predominant. Specifically, when W1 (the internal pure-water droplets) and W2 (the suspending saline-water medium were in visual contact, water fort occurred mainly through the hydrated surfactant mechanism. In the case of a visible oil layer between W1 and W2, measuring from a few to over one hundred microns, the water transport rate, -dR/dt, was found to be significantly lower than the rate at visual contact and water migration occurred via spontaneously emulsified droplets and reverse micelles. Under the experimental conditions used, the water transport was controlled by interfacial processes, rather than being diffusion controlled as has been suggested by previous work The effects of oil-soluble and water-soluble surfactants were also studied. Water transport rates were found to rise linearly with increasing oil-soluble surfactant concentration in the oil phase over a significant range. In contrast to the system-stabilizing effect of the oil-soluble surfactants, water-soluble surfactants in the aqueous phases always weakened the emulsion stability. Although the eventual tendency was that transport rates increased with increasing water-soluble surfactant concentration in W1, a small amount of the surfactants in W1 retarded transport for all transport mechanisms. Water-soluble surfactants in W2 always accelerated the water transport mess of their concentrations Water transport exhibited significant dependence on the osmotic pressure gradients between W1 and W2. When W1 was made of pure water while salt (NaCl) was present only in W2, water was transported from W1 to W2 at a constant transport rate. In the case of hydrated-surfactant art, rates rose linearly with increasing salt concentration in W2 through acceleration of the dehydration process of the hydrated surfactants at the O/W2 interface. For spontaneous emulsification and reverse micellization water transport, rates were independent of the osmotic pressure a over a significant range of salt concentration in W2. What salt was in both W1 and W 2---though at a higher concentration in W2---water transport stopped when the salt concentrations in W1 and W 2 equalized, indicating that only water may art through the oil phase while salt stays trapped in the W compartments. When transport was controlled by the hydrated-surfactant mechanism, water transport rates were initially constant to then decrease asymptotically to zero, showing that, as salt concentration in W1 increased with time, the controlling process shifted from surfactant dehydration at the O/W2 interface to hydration at the W1/O interface. For the spontaneous emulsification and reverse-micellar mechanisms in visual non-contact, water transport rates remained constant during a given experiment and decreased with increasing initial salt concentration in W1, indicating that the formation process of emulsified water droplets and reverse micelles at the W1/O interface was the rate-controlling step