SOLVENT AND DOUBLE-LAYER EFFECTS ON REDOX REACTIONS IN SELF-ASSEMBLEDMONOLAYERS OF FERROCENYL-ALKANETHIOLATES ON GOLD

Formal potentials for ferrocene/ferricenium in mixed monolayers of fer rocenyl-hexanethiol with several n-alkanethiol derivatives on gold are apparently shifted positive relative to the formal potential for an a lkylferrocene derivative in bulk aqueous solution. The magnitude of th e shift varies systematically with the nature of the alkanethiol coads orbate, reaching a maximum value of 490mV for ferrocenyl-hexanethiol c oadsorbed with n-decanethiol. The shift is smaller in monolayers of fe rrocenyl-hexanethiol coadsorbed with short-chain alkanethiols and/or a lkanethiols substituted with polar functional groups. Two effects, one involving ion solvation energetics and another the ion spatial distri bution in the interfacial region, can be invoked to account for this b ehavior. The first is essentially a solvent effect and the second a do uble-layer effect on the interfacial redox reaction. Predictions from a Born solvation model and from two recent models describing double-la yer 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 m icroenvironment experienced by immobilized ferrocene groups.