The half-wave potential and homogeneous electron-transfer rate constant in sodium dodecyl sulphate micellar solution

Abstract

Cyclic voltammetric measurements were made on ML 3 3+/2+, ML 2(CN) 2 +/0, ML(CN) 4 −/2−, M(CN) 6 3−/4−, Fe(H 2O) 6 3+/2+ and methylviologen 2+/+ in aqueous solutions with and without sodium dodecyl sulphate (SDS), where M=Fe, Ru and Os and L=2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine, 1,10-phenanthroline and 5-chloro-1,10-phenanthroline. At a low scan rate every voltammogram exhibited a one-electron reversible step. In most cases an addition of SDS caused a peak current depression and a half-wave potential shift towards a positive potential, which suggested that the microenvironmental change due to micelle solubilization might be operative. On the other hand, no changes were observed in the voltammograms for Fe(CN) 6 3−/4−, Fe(bpy)(CN) 4 −/2− and Fe(H 2O) 6 3+/2+. A qualitative analysis of the results of the measurements of the rate constants k t for the electron transfer between OsL 3 3+ and Fe(H 2O) 6 2+ in micellar solutions pointed to the predominance of the reaction path in which OsL 3 3+ in the micelle reacted directly with Fe(H 2O) 6 2+ in the water. A straight line was obtained when log k t was plotted against the standard free energy of reaction calculated from the half-wave potential, including the data obtained in the absence of SDS. This shows that a linear free energy relationship is obeyed and the environmental change in micelle solubilization is reflected in the free energy change of the reactants.