DEPENDENCE OF ELECTRON-TRANSFER KINETICS OF THE FERROCENE/FERROCENIUM COUPLE ON THE VISCOSITY IN AMBIENT-TEMPERATURE CHLOROALUMINATE IONIC LIQUIDS

The heterogeneous electron transfer kinetics of the ferrocenium/ferrocene couple have been measured throughout the basic range of composition of the ambient temperature chloroaluminate ionic liquid composed of 1-ethyl-3-methylimidazolium chloride (ImCl) and aluminum chloride. Normal and reverse pulse voltammetries with pulse times as short as 5 mu s were employed. After subtraction of the background currents, the experimental voltammograms were directly compared with those calculated from the COOL algorithm using a model for reversible and quasi-reversible electron transfer reactions. The standard rate constants, uncorrected for double layer effects, were found to decrease from 0.45+/-0.35 cm s(-1) at 0.95:1 to 0.027+/-0.004 cm s(-1) at 0.45:1 mole ratio of aluminum chloride to ImCl. Possible factors contributing to the observed dependence are discussed. The most likely reason for the retardation of the electron transfer rate is the slow relaxation of the increasingly associated basic ionic liquid.