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

The electrochemical kinetics for the oxidation of ferrocenemethanol (FcCH 2 OH) over the whole composition range of dimethyl sulfoxide (DMSO)−water solutions of different viscosities (η) containing 50.0 mM (CH 3 ) 4 NClO 4 (TMAP) at a Pt microelectrode was studied using scanning electrochemical microscopy (SECM). The measured diffusion coefficient, D FcCH 2 OH, as well as the standard rate constant of the heterogeneous electron transfer, k 0, as a function of solution composition, showed a minimum at about a DMSO molar fraction ( x DMSO ) of 0.33, corresponding to the mixture with the maximum solution viscosity. The largest k 0 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 x DMSO = 0.33 (0.14 ± 0.02 cm s -1 ). A good linear correlation between ln k 0 and ln η was observed within the solution composition range of 0.10 ≤ x DMSO ≤ 0.60. An excellent linear correlation between ln k 0 and ln τ L, the longitudinal relaxation time, was also obtained with a slope equal to 1.0 when x DMSO = 0−0.60. Unusually small rate constants found in the solutions of x DMSO ≥ 0.70 were attributed to adsorption effects at the tip and the substrate electrode. The k 0 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.