Discreteness of charge and ion association effects on electroactive self-assembled monolayers

The voltammetric response of an electrode covered with an electroactive self-assembled monolayer is modeled including discreteness of charge effects and interfacial ion association. Discreteness of charge potentials are estimated according to the hexagonal array model of Macdonald and Barlow, and results are compared with those obtained in previous work with the cutoff disk model. As a consequence of the slower variation of the discreteness of charge potential on the applied electrode potential, voltammetric waves are predicted to be asymmetrical and wider than those computed from the cutoff disk model. For relatively high redox coverage and/or small values of the integral capacities of the inner and outer part of the monolayer, a negative differential capacity is predicted. This is a consequence of the additional stabilization provided by the discreteness of charge effect, which allows the charge density at the redox plane to increase faster than the charge density on the electrode surface when the monolayer is being oxidized. Comparison with experimental results shows that inclusion of the discreteness of charge effects results in a variation of the absolute value of the interfacial parameters, while their qualitative trends are the same as those obtained on the basis of an average potential model. Therefore, only the physical significance of the fitting parameters may help to discriminate among the different models.