Effects of Osmotic Pressure on Water Transport in W1/O/W2 Emulsions

Abstract

In a W1/O/W2 double-emulsion globule, when the W1 phase was made of pure water while salt (NaCl) was present only in the W2 phase, water was transported from W1 to W2 at a constant transport rate, −dR/dt. In the case of hydrated-surfactant transport, rates rose linearly with increasing salt concentration in W2 through acceleration of the dehydration process of the hydrated surfactants at the O/W2 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 W2. When salt was present in both the W1 and W2 phases—though at a higher concentration in W2—water transport stopped when the salt concentrations in W1 and W2 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 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 at visual noncontact, 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.