INTERFACIAL SOLVATION AND DOUBLE-LAYER EFFECTS ON REDOX REACTIONS IN ORGANIZED ASSEMBLIES

Formal potentials for ferrocene in self-assembled monolayers of N-(7-m ercaptoheptyl)ferrocenecarboxamide coadsorbed with n-alkanethiol deriv atives of variable chain length and terminal functionality are substan tially more positive than the corresponding potentials for the same fe rrocene compound in bulk solution. The differences in formal potential are a strong function of the chain length and terminal functionality of the alkanethiol coadsorbate, the nature and concentration of suppor ting electrolyte, the coverage of ferrocene on the electrode, and the solvent. Two physical models for the electrode/monolayer/solution inte rface are invoked to explain these differences. One model is based on ion solvation energetics in the interfacial microenvironment relative to that in bulk solution and describes essentially a solvent effect on the formal potential for the immobilized redox-active moieties. The o ther is based on the spatial distribution of ions in the interfacial r egion and describes essentially a double-layer effect on the apparent formal potential for the immobilized redox-active moieties. Quantitati ve predictions are developed from these models that specifically addre ss the effects of electrolyte type and concentration, solvent, ferroce ne surface coverage, and coadsorbate chain length. It is concluded tha t both interfacial solvation and ion spatial distribution effects must be considered to adequately explain the data.