Electron tunnelling across hexadecanethiolate monolayers on mercury electrodes: Reorganization energy, structure, and permeability of the alkane/water interface

Measurements of the reorganization energies (lambda) and the maximum values of the electron transfer rate constants (k(app)(max), obtained for -eta greater than or equal to 2 lambda) 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 IrCl62- and FcCH(2)N(CH3)(3)(2+) 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 k(app)(max) values. The former show that both probes reside initially in an aqueous environment at the alkane/solution interface. A larger value of k(app)(max) for the ferrocene probe was interpreted to indicate its closer approach to the interface. Access of the more strongly hydrated IrCl62- 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 IrCl62-, proving that the iridium probe is located in the aqueous environment outside the alkane monolayer film. In contrast, a more hydrophobic ferrocene probe permeates re alkanethiolate monolayer immediately when even a small expansion of the Hg drop of ca. 2% is attempted.