Solvent and double-layer effects on redox reactions in self-assembled monolayers of ferrocenyl-alkanethiolates on gold

Formal potentials for ferrocene/ferricenium in mixed monolayers of ferrocenyl-hexanethiol with several n-alkanethiol derivatives on gold are apparently shifted positive relative to the formal potential for an alkylferrocene derivative in bulk aqueous solution. The magnitude of the shift varies systematically with the nature of the alkanethiol coadsorbate, reaching a maximum value of 490mV for ferrocenyl-hexanethiol coadsorbed with n-decanethiol. The shift is smaller in monolayers of ferrocenyl-hexanethiol coadsorbed with short-chain alkanethiols and/or alkanethiols substituted with polar functional groups. Two effects, one involving ion solvation energetics and another the ion spatial distribution in the interfacial region, can be invoked to account for this behavior. The first is essentially a solvent effect and the second a double-layer effect on the interfacial redox reaction. Predictions from a Born solvation model and from two recent models describing double-layer effects at monolayer-coated electrodes are presented and compared with the experimental data. A full deconvolution of solvent and double-layer effects on apparent formal potentials was not achieved; however, it still proved possible to draw conclusions about the interfacial microenvironment experienced by immobilized ferrocene groups.