Spontaneous Emulsification of Oil in Aerosol-OT/Water/Hydrocarbon Systems

Abstract

Drops containing a hydrocarbon, the anionic surfactant Aerosol-OT (AOT), and water were injected into water at 30 °C, and the resulting behavior was observed by videomicroscopy. Spontaneous emulsification of most of the injected oil yielding droplets a few microns in diameter was seen when n-octane was the hydrocarbon and when the initial drop contained at least 25 wt % AOT and no more than about 10 wt % water. Emulsification was also observed when the initial drop contained small amounts of NaCl solution instead of water and for suitable conditions when n-hexane was the hydrocarbon. However, much less emulsification occurred for n-decane and n-dodecane, probably because surfactant-rich phases in equilibrium with excess oil in these systems solubilized little hydrocarbon. The mechanism of emulsification is closely related to that of an earlier study by Rang and Miller (Prog. Colloid Polym. Sci. 1998, 109, 101) using drops containing n-hexadecane, the pure nonionic surfactant C12E6, and n-octanol. In both systems diffusion produced inversion from an oil-continuous to a water-continuous phase, leading to emulsification at locations where supersaturation in oil occurred. One difference in the present case was that inversion was not continuous but involved formation of the lamellar liquid crystalline phase. Another was that the shift to more hydrophilic conditions leading to inversion was caused by decreases in ionic strength of water in the injected drop which, in turn, was produced by diffusion of water into the drop and of AOT and, in some cases, NaCl out of the drop. Owing to this ionic strength effect, nearly complete emulsification to small oil droplets was observed in some ternary AOT/hydrocarbon/water systems in the present study but was not possible in ternary nonionic surfactant/hydrocarbon/water systems investigated previously.