POLARITY OF AOT MICELLAR INTERFACES - USE OF THE PREFERENTIAL SOLVATION CONCEPTS IN THE EVALUATION OF THE EFFECTIVE DIELECTRIC-CONSTANTS

Fluorescence lifetimes (τF) of 2-[p-(dimethylamino)phenyl]-3,3-dimethyl-3H-indole (1) have been extensively studied in a two-component mixture of p-dioxane/water and also in a three-component mixture of n-heptane/AOT (sodium bis(2-ethylhexyl) sulfosuccinate)/water. We have applied the concept of "dielectric enrichment" to solute/mixed solvents systems in order to evaluate the effective dielectric constants at the AOT interfaces in n-heptane. We have shown that the effective dielectric constant (D) at the interface increases from 2.3 (water molar ratio W = O) to approximately 9.0 before reaching a plateau at W ≈ 12. This was explained by the solvation of the polar head groups of the surfactant. Equations have been given for the variation of D vs W and for the variation of D vs [AOT] at various AOT and water concentrations, respectively. The validity of these results has been discussed by taking into account the role played by the probe molecule in the stabilization of the solvation energy (dielectric enrichment effect). © 1990 American Chemical Society.