The ac-voltammetric study of fast charge transfer processes in benzene solutions

Oxidation of ferrocene (Fc) and reduction of p-dicyanobenzene (DCB) were examined by ac-voltammetry in benzene solutions containing tetra-n-hexylammonium perchlorate at a hemispherical (Hg) and a disk (Au) μm-electrode, respectively. The specific conductivity of benzene solutions indicates that the degree of dissociation of the supporting electrolyte is less than 1%. The low concentration of free ions combined with the low dielectric permittivity of the solvent cause large double layer effects evident from the values of the charge transfer coefficients (0.35 and 0.69 for Fc and DCB, respectively). In addition, the low concentration of ions causes problems in measurements of charge transfer kinetics if the concentration of the depolarizer exceeds 0.2 mM. The standard rate constants obtained in benzene solutions (0.22 and 0.15 cm s−1 for Fc and DCB, respectively) are not much different from values observed in common polar solvents. Apparently, in benzene solutions, very weak electrical coupling of the activated state with solvent molecules is compensated by very strong interactions with counter ions and large double layer effects. The experimental results indicate the necessity of taking into account ionic relaxation processes in a theoretical description of interfacial charge transfer dynamics.