Electron Tunneling Across Hexadecanethiolate Monolayers on Mercury Electrodes: Reorganization Energy, Structure, and Permeability of the Alkane/Water Interface

Measurements of the reorganization energies (λ) and the maximum values of the electron transfer rate constants (kapp , obtained for -η g 2λ) of two redox probes at monolayer-coated electrodes are used as diagnostic parameters of the location of a probe at the monolayer/solution interface. Kinetics of the electroreduction of IrCl6 and FcCH2N(CH3)3 at hexadecanethiolate-coated Hg drop electrodes were investigated in a broad range of overpotentials extending to values in excess of the reorganization energies of the two redox probes. Rate vs overpotential data were analyzed in terms of the Marcus-Gerischer formalism to yield the reorganization energies and kapp max values. The former show that both probes reside initially in an aqueous environment at the alkane/solution interface. A larger value of kapp max for the ferrocene probe was interpreted to indicate its closer approach to the interface. Access of the more strongly hydrated IrCl6 to the interface is more restricted by an interfacial water layer. Gradual expansion of the Hg drop, up to 20% of its initial surface area, has no effect on the magnitude of the reorganization energy obtained for IrCl6, proving that the iridium probe is located in the aqueous environment outside the alkane monolayer film. In contrast, a more hydrophobic ferrocene probe permeates the alkanethiolate monolayer immediately when even a small expansion of the Hg drop of ca. 2% is attempted.