Solubilization rates of oils in surfactant solutions and their relationship to mass transport in emulsions

Information on solubilization rates of oils in aqueous micellar solutions is reviewed. For ionic surfactants electrostatic repulsion prevents close approach of micelles to the oil–water interface, so that solubilization results from oil molecules dissolving individually in the solution and being taken up by micelles during and/or after transport across a diffusion boundary layer to the bulk solution. Experiments with SDS solutions and single oil drops having low (but not negligible) solubility indicate that mass transfer is often not rate-controlling. Instead phenomena near the oil–water interface including, but not limited to, the rates of micellar uptake of oil from the aqueous solution seem to control the solubilization rate. In contrast, Ostwald ripening experiments involving multiple oil drops in SDS solutions are often interpreted in terms of molecular dissolution and diffusion alone since ripening rates are typically only slightly different from those observed in the absence of surfactant micelles, where this mechanism is considered to hold.For many nonionic surfactant systems and oils of low or negligible solubility the principal mechanism of solubilization is incorporation of surfactant at the oil–water interface from micelles, which coalesce or “adsorb” at the interface or else dissociate nearby, permitting individual surfactant molecules to be adsorbed. Subsequently the excess surfactant is emitted as oil-containing micelles. Most experiments have indicated that this process, which appears in the analyses as an interfacial resistance, is rate controlling. New results are presented here supporting this model and showing that resistance to mass transfer is often quite low because natural convection can arise near an oil drop owing to the density change produced by solubilized oil in micelles near the drop surface. Provided that polydispersity of drop sizes is properly accounted for, experiments on solubilization and compositional ripening in emulsions stabilized with nonionic surfactants can be interpreted using the interfacial resistance model with values of resistance obtained from single-drop experiments. However, it is unclear whether mass transfer, interfacial resistance or perhaps some combined mechanism controls the rate of Ostwald ripening. One uncertainty limiting predictions of the interfacial resistance model is the lack of information on the oil-to-surfactant ratio in micelles when the concentration of individually dissolved oil molecules slightly exceeds the equilibrium value for a plane oil–water interface, the situation during Ostwald ripening.