Translational Diffusion of Ion Radicals Created by Electron Transfer in Charged Micellar Solutions Probed by the Transient Grating Method and the Taylor Dispersion Method

Abstract

Diffusion processes of intermediate ion radicals created by the photoexcited electron-transfer reaction of benzoquinone (BQ) and aniline (AN) in sodium dodecyl sulfate (SDS; anionic micelle) and cetyltrimethylammonium bromide (CTAB; cationic micelle) solutions were studied by using the transient grating (TG) and Taylar dispersion (TD) methods. The diffusion coefficients (D) of the anion radical of BQ (BQ•-) in SDS and the cation radical of AN (AN•+) in CTAB are similar to D of those radicals in neat water, while D of AN•+ in SDS and D of BQ•- in CTAB are smaller than D in neat water and more close to D of the self-diffusion of the micelles of SDS and CTAB. This fact suggests that most of the ion radicals exist in the bulk phase by the electric repulsion when the ion radicals have the same charge as the micellar surface. On the other hand, when the ion radicals have an opposite charge to that of the micellar surface, they are trapped on the surface and diffuse with the micelles. In any case, the parent molecules (BQ and AN) predominantly exist in the micelles. A diffusion model, which takes into account of the equilibrium between the micellar surface and bulk phase, can reproduce the observed micellar concentration dependence of D and the equilibrium constants of the transient radicals are determined.