Influence of intermolecular interactions in the redox kinetics performance of surface confined probes by Square Wave Voltammetry

Analysis of the Square Wave Voltammetry (SWV) responses of surface confined electroactive probes under finite kinetics conditions, by considering the influence of intermolecular interactions on the current, is presented. This non-ideality gives rise to very significant deviations in the most relevant features of the current potential response as compared with predictions of ideal models. The theoretical model does not require that the dependence of the rate constant with the potential (i. e., Butler-Volmer or Marcus-Hush kinetic formalisms) is explicitly established a priori. Both location and height of the Quasi-Reversible Maximum and symmetry of the SWV current-potential curves are strongly affected by the interaction parameters G = a(OO) + a(RR) - 2a(OR) and S = a(RR) - a(OO) (with a(ij) being the interaction coefficient for species i and j). Quantitative methods for obtaining kinetic and interaction parameters of the redox probes are presented, based on the measurement of the peak currents and potential under quasi-reversible conditions where the current exhibits two asymmetric peaks. A discussion is done concerning the deviations found in the estimation of the rate constants with SWV when intermolecular interactions are not considered. The above methods have been applied to the characterization of mixed ferrocenylundecanethiol/decanethiol monolayers on gold electrodes at an ethanolic medium for which the SWV is clearly non-ideal.