DISCRETENESS OF CHARGE AND ION ASSOCIATION EFFECTS ON ELECTROACTIVE SELF-ASSEMBLED MONOLAYERS

The voltammetric response of an electrode covered with an electroactiv e self-assembled monolayer is modeled including discreteness of charge effects and interfacial ion association. Discreteness of charge poten tials are estimated according to the hexagonal array model of Macdonal d and Barlow, and results are compared with those obtained in previous work with the cutoff disk model. As a consequence of the slower varia tion of the discreteness of charge potential on the applied electrode potential, voltammetric waves are predicted to be asymmetrical and wid er 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 capacit y 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. Compar ison with experimental results shows that inclusion of the discretenes s of charge effects results in a variation of the absolute value of th e interfacial parameters, while their qualitative trends are the same as those obtained on the basis of an average potential model. Therefor e, only the physical significance of the fitting parameters may help t o discriminate among the different models.