The kinetics of solubilization of nonpolar oils by nonionic surfactant solutions

Abstract

A quantitative study of the rate of solubilization of nonpolar oils in aqueous surfactant solutions has been made using a new technique. The dissolution of a single microscopic oil droplet adhering to a thin fiber suspended in an aqueous surfactant solution is recorded photomicrographically and the solubilization rate (rate of volume change per unit interfacial area) is calculated from the shape of the droplet using previously published theory (B. J. Carroll, J. Colloid Interface Sci. 57, 488 (1976)), which in this case reduces to a simple approximate form. The new technique has been used to study solubilization kinetics in some well-characterized systems containing a high-purity nonionic surfactant. The effects of surfactant concentration, temperature, and oil type were studied. The rate of solubilization is proportional to the surfactant concentration above the CMC and depends on the oil polarity and molecular weight. The rate is strongly temperature dependent in the region of the nonionic cloud point: 15°K below the cloud point, the rate is extremely small relative to that at the cloud point. It is shown that in the case of highly insoluble oils the solubilization mechanism must involve the adsorption-desorption of micelles at the oil/water interface, rather than the diffusion of molecules of oil into the micelle via the aqueous phase. The experimental data are shown to be consistent with the slow stage of the process being the adsorption, not desorption, of the micelle at the interface. The magnitude of the measured rate constant is consistent with theory if dissociation of the micelle occurs prior to the adsorption step and an estimate of the relaxation time for demicellization is obtained. The temperature dependence of the rate in the cloud point region is discussed in terms of changes in the properties of the nonionic solution in this region.