Proton transfer impedance of electrodes modified with acid thiol monolayers

Abstract

Analytical expressions describing the electrochemical impedance spectrum of a metallic electrode modified with an acid thiol monolayer, that undergoes a potential-induced proton transfer, have been derived from an electrostatic model of the interface. The frequency dispersion of the electrode impedance is described by an equivalent circuit that bears some similarities with that associated with a surface redox process. Good agreement is found between the theoretical predictions and the impedance of a Au(111) electrode modified with a 11-mercaptoundecanoic monolayer recorded as a function of ac frequency, dc potential and surface concentration of electrochemically active carboxylic groups. The same thermodynamic and kinetic parameter values are obtained from independent analysis of the voltammetric and impedimetric responses, except in the case of the reorganization energy for proton transfer, which seems to be affected by a slow potential-induced reorientation and dissociation of the external acid groups of the monolayer.