Mechanisms of oil transport by micelles in oil-in-water emulsions.

摘要

Solubilization kinetics of different chain length alkanes, n-hexadecane, n-tetradecane, and n-dodecane, in ionic SDS and nonionic Tween 20 micellar solutions, was studied by measuring changes in emulsion turbidity (UV-VIS spectrophotometer) and droplet size (light scattering). The progressive decrease in average droplet size with time was obtained by using nearly monodisperse emulsions, in contrast with previous studies with polydisperse emulsions. Employing monodisperse emulsions allowed us for the first time to track solubilization from the decrease in size. The independence of the rate on initial droplet size eliminated a pure bulk diffusion control mechanism.;During Ostwald ripening in bimodal emulsions, it was clearly observed that oil transferred from the small to the large-sized mode. The effect of SDS micelles was studied in both uni- and bimodal emulsions. SDS micelles enhanced the ripening rate, but the rates were not affected by the increase in micelle concentration.;Two mechanisms are generally proposed for oil solubilization by micelles: one controlled by diffusion of the oil molecule through the aqueous solvent, and the second an interfacially dominated process. The strong dependence of the solubilization rate on aqueous viscosity for the shorter alkanes supported a mechanism in which the oil undergoes molecular diffusion, suggesting that reaction-enhanced diffusion occurred. For the longer n-hexadecane, especially in nonionic Tween 20 micellar solutions, an interfacial mechanism appeared to substantially contribute to the overall rate. Addition of salt only slightly increased the solubilization rate in SDS solutions, and thus indicated a weak role of electrostatic repulsion for ionic surfactants. We studied Ostwald ripening of n-decane and n-dodecane emulsions in aqueous solutions of various nonionic surfactants with different ethylene oxide head group chain lengths: C12E7, C12E8, C12E 10, Brij 35, and Tween 20. Ostwald ripening rates were found to increase with micelle concentration for all nonionic surfactants, with the exception of C12E10. Strikingly, Brij 35 micelles were found to enhance substantially the Ostwald ripening rate, with an effect by an order magnitude larger than all other surfactants studied.