Solubilizate mobility

In an early work, Kotlarchyk et al. [67] studied AOT reverse micelles where purified AOT still had a small quantity (w = 0.7 0.2) of water. These authors showed that the polar core contained water bound to the sulfonate head-groups. Jain et al. [138] suggested at least three different solubilizate environments and hence, three types of molecular water in AOT/isooctane/water reverse microemulsions. They are (i) the interface water, (ii) the bound water layer and (iii) the water pool. Interfacially trapped water molecules were indicated to be located in between polar head-groups of AOT as monomers or dimers. Bound water molecules were bonded to the head-groups of AOT through the counterions (Na ) present at the interface. Bulk water deeper in the pool had greater mobility. Water molecules, of course, also hydrated the counterions. Another possibility was presence of water, albeit in negligible amounts, in the oil phase. 

A molecule solubilized by an ionic surfactant may be in the core of the micelle, between the hydrophobic chains, in the layer formed by the counterions, or on the surface of the micelle. Because the micelle is in a dynamic state, the loci of the solubilizate in a micelle cannot be sharply defined. Like surfactant molecules or ions in the micelle, molecules of the solubilizate are mobile and a rapid interchange between different sites may be possible. It is believed, however, that the preferred location of the solubilizate in a given micelle depends on the structure of the solubilizate as well. 

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