Dynamic Solvent Effects in Electrochemical Kinetics: Indications for a Switch of the Relevant Solvent Mode

Abstract

The influence of solvent dielec. relaxation on the rate of electron transfer (ET) at an electrochem. interface is addressed using both expt. and model calcns. H2O-ethylene glycol (EG) mixts. were chosen as the solvent because their optical permittivity remains practically const. over the entire compn. 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 exptl. results are presented to characterize the mixed-solvent system (dielec. spectra in the frequency range 0.1-89 GHz), the Hg/solvent interface (electrocapillary data), and the ET kinetics (d.c. polarog. of peroxodisulfate redn.). To ext. the true solvent influence on the electron transfer elementary step, the results from d.c. polarog. are cor. 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 probably is responsible for the obsd. features of ET kinetics. A possible interpretation of the cor. ET rate in the framework of the Agmon-Hopfield formalism is proposed, where the dielec. spectra of the mixed solvent are modeled by a superposition of 3 Debye equations. The obsd. anomalous viscosity effect may be explained qual. by an increased contribution of the fast relaxation mode at high EG contents.