Solution Viscosity Effects on the Heterogeneous Electron Transfer Kinetics of Ferrocenemethanol in Dimethyl Sulfoxide−Water Mixtures

The electrochemical kinetics for the oxidation of ferrocenemethanol (FcCH2OH) over the whole composition range of dimethyl sulfoxide (DMSO)−water solutions of different viscosities (η) containing 50.0 mM (CH3)4NClO4 (TMAP) at a Pt microelectrode was studied using scanning electrochemical microscopy (SECM). The measured diffusion coefficient, DFcCH2OH, as well as the standard rate constant of the heterogeneous electron transfer, k0, as a function of solution composition, showed a minimum at about a DMSO molar fraction (xDMSO) of 0.33, corresponding to the mixture with the maximum solution viscosity. The largest k0 value found, 2.06 ± 0.31 cm s-1 in pure water (electrolyte) medium, was about 15 times larger than that obtained in the solution of xDMSO = 0.33 (0.14 ± 0.02 cm s-1). A good linear correlation between ln k0 and ln η was observed within the solution composition range of 0.10 ≤ xDMSO ≤ 0.60. An excellent linear correlation between ln k0 and ln τL, the longitudinal relaxation time, was also obtained with a slope equal to 1.0 when xDMSO = 0−0.60. Unusually small rate constants found in the solutions of xDMSO ≥ 0.70 were attributed to adsorption effects at the tip and the substrate electrode. The k0 obtained for the present system was generally found to be inversely proportional to the viscosity of the solution and directly proportional to the diffusion coefficient of the electroactive species.