Electroanalytical kinetic and diffusion studies in surfactant media

Abstract

Voltammetry yields kinetic data for electron transfer in surfactant media. An electrocatalytic cycle is set up so that reactants exchange electrons in a rate-determining step following production of one reactant (the activated catalyst) at an electrode. Microdisk electrodes of radii < 10 μm are used to measure a ratio of catalytic limiting current (catalyst and substrate present) to that for catalyst alone. This “catalytic efficiency” can be used to find the apparent rate constant for reaction of activated catalyst with substrate. Microelectrodes can be used in highly resistive microemulsions. Electron transfer from the Co (I) form of vitamin B 12 to alkyl vicinal dihalides in AOT water-in-oil microemulsions is discussed. Apparent diffusion coefficients ( D′) from cyclic voltammetry decrease with increasing concentration of electroactive probe ( C X) in many ionic micellar systems. Analysis of D′ versus C X data with approximate models provided micelle diffusion coefficients for systems with one or two types of micelles. Similar studies by steady state voltammetry at microdisk electrodes of radii < 13 μm gave D′ values with no C x dependence in aqueous micellar systems. The latter experiments appear to yield average diffusion coefficients.