Effects of Osmotic Pressure on Water Transport in W(1)/O/W(2) Emulsions.

In a W(1)/O/W(2) double-emulsion globule, when the W(1) phase was made of pure water while salt (NaCl) was present only in the W(2) phase, water was transported from W(1) to W(2) at a constant transport rate, -dR/dt. In the case of hydrated-surfactant transport, rates rose linearly with increasing salt concentration in W(2) through acceleration of the dehydration process of the hydrated surfactants at the O/W(2) interface. When the water was transported through spontaneous emulsification and reverse micellization, the water transport rates were independent of the osmotic pressure over a significant range of salt concentration in W(2). When salt was present in both the W(1) and W(2) phases-though at a higher concentration in W(2)-water transport stopped when the salt concentrations in W(1) and W(2) equalized, indicating that only water may transport through the oil phase while salt stays trapped in the W compartments. In visual-contact experiments, where transport was controlled by the hydrated-surfactant mechanism, the water transport rates were initially constant to then decreased asymptotically to zero. This showed that, as salt concentration in W(1) increased with time, the controlling process shifted from surfactant dehydration at the O/W(2) interface to hydration at the W(1)/O interface. For the spontaneous emulsification and reverse-micellar mechanisms at visual noncontact, water transport rates remained constant during a given experiment and decreased with increasing initial salt concentration in W(1), indicating that the formation process of emulsified water droplets and reverse micelles at the W(1)/O interface was the rate-controlling step. Copyright 2001 Academic Press.