Solubilization kinetics of oils by ionic and nonionic micelles: a semiquantitative model

Abstract Experimental data on solubilization kinetics found in literature were analyzed by using the model proposed earlier (A. Kabalnov and J. Weers, “Kinetics of mass transfer in micellar systems: Surfactant adsorption, solubilization kinetics, and ripening,” Langmuir , vol. 12, no. 14, pp. 3442–3448, 1996). The rates of oil molecular exchange between the micellar core and the surrounding aqueous solution were determined. It was concluded that the solubilization of hydrocarbon molecules by nonionic micelles of ethylene oxide type is essentially barrier-free, that is, is diffusion controlled. It is quite different for ionic surfactants, where the rate is one-two orders of magnitude slower, indicating the existence of a potential barrier for hydrocarbons to get inside the micelles. A Fickean diffusion model of solubilization has been proposed to explain these trends, specifically, the micellar sink-source term of the model was analyzed. For ionic micelles, the diffusing hydrocarbon molecules are predicted to be excluded from the micellar double layer region because of their low molecular polarizability. The Poisson–Boltzmann framework was used to model this effect; the diffusion retardation factors were compared with the experiment and a fair agreement was seen. For nonionic groups, such as oligoethylene oxide, on the other hand, no such barrier was predicted to exist. The analysis of this paper is performed only for the case of the ‘slow’ solubilization; it does not cover the case of the rapid, ‘catastrophic’ solubilization observed in the other group of experiments; the distinction between the slow and fast mechanisms is discussed and a possible explanation is suggested.