Dynamic solvent effects in electrochemical kinetics: indications for a switch of the relevant solvent mode

The influence of solvent dielectric relaxation on the rate of electron transfer (ET) at an electrochemical interface is addressed using both experiment and model calculations. Water-ethylene glycol (EG) mixtures were chosen as the solvent because their optical permittivity remains practically constant over the entire composition range. This allows observation of the dynamic solvent effect with a very minor interference from the static solvent properties (being typically of opposite sign). Three groups of experimental results are presented to characterize the mixed-solvent system (dielectric spectra in the frequency range 0.1-89 GHz), the mercury/solvent interface (electrocapillary data), and the ET kinetics (dc polarography of peroxodisulphate reduction). To extract the true solvent influence on the electron transfer elementary step, the results from dc polarography are corrected for interfacial effects with the help of the electrocapillary data. An anomalous dependence of the ET rate on EG content (i.e., nonmonotonic dependence of the ET rate on macroscopic viscosity) can be inferred after all corrections. The interplay of different solvent modes is suggested to be responsible for the observed features of ET kinetics. A possible interpretation of the corrected ET rate in the framework of the Agmon-Hopfield formalism is proposed, where the dielectric spectra of the mixed solvent are modeled by a superposition of three Debye equations. The results demonstrate that the observed anomalous "viscosity effect" may be explained qualitatively by an increased contribution of the fast relaxation mode at high EG contents.