Solubilization of Uncharged Molecules in Ionic Micellar Solutions: Toward an Understanding at the Molecular Level
The theoretical work done at the Chemical Center in Lund on the micellization of ionic surfactants and the solubilization of uncharged molecules 116in solutions of ionic surfactants is summarized. The aim of this research is to construct a model that may be used to give a quantitative description of the thermodynamic properties of the systems from the knowledge of the molecular properties of the individual components. The experimental observations which give the essential characteristics of ionic amphiphiles are summarized, and the various contributions to the free energy of micellization considered in the model are discussed. Examples are given of properties of ionic amphiphile-water systems for which the model can give a nearly quantitative description. The solubilization of uncharged molecules in ionic micelles has been treated at two different levels of approximation. In the first version of the model all molecules in the micelles were assumed to be anchored at the surface of the micelle. This version was used to evaluate calorimetrie measurements on the solubilization of pentanol in sodium dodecylsulfate (SDS) micelles. It was found to give an adequate description of the essential features of micelle formation of SDS in the presence of pentanol. A new model where the interior of the micelle is divided into two regions, an inner core region with no amphiphiles and an outer palisade region, was proposed to describe systems containing nonpolar solutes such as alkanes. The distribution of the solubilizate between the two regions in the micelles can be calculated and as an example the solubilization of octane in decanoate micelles is discussed. Finally, current efforts to improve the model are described.