Role of ion pairing in double-layer effects at self-assembled monolayers containing a simple redox couple

The influence of ion pairing on the potential distribution at an electrode modified with a redox-active self-assembled monolayer is considered. The voltammetric response of the film is modeled as a function of its dielectric properties and the extent of association between the redox centers and counterions in solution. On the basis of the scheme of squares, a general treatment for any number of association steps is introduced. In addition, the effect of the counterion selective permeation on the voltammetric features is presented. It is shown that ion pairing and double-layer effects are strongly coupled, and may lead in some situations to the same variation of the peak potential with electrolyte concentration. The possibility of obtaining association parameters from voltammetric experiments is discussed. Comparison is made with Rowe and Creager's data on the oxidation of mixed monolayers of ferrocene-n-hexanethiol and n-alkanethiols.