Solubilization in AOT-water reverse micelles. Effect of the external solvent

The partition of 1-pyrenemethanol (PyM) between the external medium and AOT/water reverse micelles formed in several organic solvents has been measured and compared with its partition between bulk solvents. The organic solvents employed were n-heptane, cyclohexane, isooctane, dodecane, benzene, chlorobenzene, toluene, and tetrachloroethylene. The partition of PyM in AOT/water/nonpolar solvent reverse micelles is dominated by the free energy of the probe in the external solvent. To evaluate the stability of the probe in micelles dispersed in different solvents, the micellar data were referred to a common (water) solvent. A weakly positive correlation was obtained for the partition of the probe between the different micelles and water. Furthermore, the polarity sensed by PyM is higher in AOT/alkane/water that in AOT/aromatic solvent/water reverse micelles. These results can be explained in terms of two driving forces whose relative importance is determined by the characteristics of the solute and the external solvent. One of them, solvation of the surfactant head by the polar group of PyM, will be disfavored by polar solvents. The second one, interaction of the aromatic part of the micelle associated PyM molecules with the external solvent, would be determined by the solvent/probe affinity, measured by the solvent/water partition constant.