Influence of adsorption/diffusion coupling on surface voltammetric waves.: First stages of 2-mercaptoethanesulfonate oxidative adsorption on gold

The influence of reactant adsorption/diffusion coupling on the voltammetric behavior of redox processes that lead to the formation of a passivating product monolayer was investigated. The orthogonal collocation technique was used to compute cyclic voltammetric waves under reversible Langmuirian conditions. Systematic analysis of the peak parameter dependence on reactant adsorption, concentration, and scan rate allows the characterization of five distinct adsorption/diffusion regimes, which can be rationalized in terms of a zone diagram. Convoluted voltammograms provide directly the reactant concentration in the vicinity of the electrode surface in the absence of reactant adsorption. An approximate method is proposed to extend the semiintegral approach in the presence of moderate reactant adsorption. Far scan rates in the range 0.5-5 V/s and low enough thiol concentrations (less than or equal to 0.25 mM), it is shown that oxidative adsorption of 2-mercaptoethanesulfonate on gold can be described by a fast adsorption/electron-transfer step in equilibrium. Under these conditions, the maximum surface concentration becomes smaller than expected for a hexagonal closed-packed monolayer of adsorbed product. For higher concentrations and/or lower sweep rates, anodic waves are narrower and higher than predicted for a simple adsorption/electron-transfer mechanism. These results suggest-that high-order kinetics are involved in a following surface rearrangement process leading to the formation of a more compact product monolayer.