Redox and mass transport characteristics of domain-free mixed ferrocenyloctanethiol/alkanethiol monolayers on gold

Mixed monolayers of ferrocenyloctanethiol (FcC8SH) and alkanethiols of various chain lengths were constructed after predetermining the surface composition by the initial coverage of Pb underpotentially deposited on a bare gold electrode during the first step of the preparation. Single waves observed in the reductive desorption voltammograms show no formation of single-component domains. These mixed monolayers showed the following characteristics. First, a considerable amount of the ferrocene moieties were not electroactive. Second, the mass change during the redox reaction of the ferrocene was greater than that expected from the simple association of the ferrocenium cation with the electrolyte anion, and that for the mixed monolayers prepared from mixed solutions of the precursor thiols. This large mass change was interpreted by the incorporation of the anion and water upon oxidation into the pocket surrounded by alkanethiol and FcC8SH at the bottom. These results are consistent with those expected for the FcC8SH molecules in a domain-free environment, or surrounded by more alkanethiol molecules than FcC8SH molecules.