Kinetics of Triglyceride Solubilization by Micellar Solutions of Nonionic Surfactant and Triblock Copolymer. 2. Theoretical Model

Abstract

A theoretical model of oil solubilization in micellar surfactant solutions is developed. We consider oils that are practically insoluble in pure water, like triolein and other triglycerides. The nonionic micelles, which are capable to solubilize such oils, are usually rodlike aggregates, composed of surfactant molecules and poly(oxyethylene)-poly(oxypropylene) triblock copolymers, like Synperonic L61 (SL61). The swollen micelles, formed after solubilization, are smaller than the empty ones. The model describes the elementary act of solubilization as a sequence of three steps: (a) adsorption of an empty micelle at the oil-water interface; (b) uptake of oil by the adsorbed empty micelle which then splits into several swollen micelles; (c) desorption of the swollen micelles. Theoretical expressions are derived, which describe the diminishing of an oil drop in the course of solubilization. The parameter values, determined from the best fit of experimental data, imply that the rate-controlling step is the step of micellar adsorption. From the determined rate constant of adsorption we estimate the respective kinetic barrier to micelle adsorption, taking into account the action of surface forces and the hydrodynamic resistance. Our analysis indicates that the triblock copolymer SL61 promotes the solubilization of triglycerides by decreasing the length of the mixed surfactant-copolymer rodlike micelles, which leads to a lowering of the kinetic barrier to their adsorption at the oil-water interface.