SOLUBILIZATION OF UNCHARGED MOLECULES IN IONIC SURFACTANT AGGREGATES .1. THE MICELLAR PHASE

This is the first paper of two dealing with solubilization of different types of nonionic molecules into ionic surfactant aggregates. A theoretical model is presented which is an extension of an earlier model dealing with three-component systems: ionic surfactant-water-long-chain alcohol (Jonsson, B.; Wennerstrom, H.J. Phys. Chem. 1987, 91, 338). The solubilization of uncharged molecules into normal micellar aggregates is treated and discussed in this first paper. The extension of the micellar phase is discussed in paper two, where the phase equilibria between the micellar phase and the lamellar and reversed micellar phases are calculated. In the new model the description of the micellar phase is a refinement of the earlier model in which the micelle is divided into two regions; the surface region (palisade layer) and the core region. Depending on the polarity of the solubilizate it is preferentially solubilized in either of the two regions. This preference is described by a difference in standard chemical potential of the solubilizate in the two regions. The difference in standard chemical potential is in the model calculated from the surface tension between pure solubilizate and water. The distribution of solubilizate between the two regions has been determined. The general findings are as follows: (i) Hydrophobic, nonpolar molecules are solubilized mainly in the core region, while polar molecules such as alcohols are dissolved mainly in the surface layer. (ii) Even for polar molecules the first molecule is often solubilized in the core region due to the large decrease in surface area per molecule in the aggregate. (iii) A low charge density of the micelle or a high salt content favors solubilization in the core region.