Voltammetric Properties of the Reductive Desorption of Alkanethiol Self-Assembled Monolayers from a Metal Surface

The current peak that appears on a linear-scan voltammogram for the reductive desorption of alkanethiol self-assembled monolayers (SAMs) from a gold surface in an aqueous alkaline solution exhibits intriguing features: the narrow full width at half-maximum (fwhm) of the peak, e.g., 20 mV for dodecanethiol SAMs, the saturation of fwhm in the SAM composed of long-chain alkanethiols, an asymmetric shape, the shift of the peak potential with increasing the alkyl chain length, and the peak area that is greater than what is expected from the (√3 × √3)R30° structure of adsorbed alkanethiols on Au(111). A Padé approximant expression for the adsorption isotherm proposed by Blum and Huckaby based on the two-dimensional Ising model, in combination with the semi-infinite linear diffusion of desorbed species, well explains these salient features of the reductive desorption behavior. The double-layer charging current can amount to one-third of the charge calculated from the area of the peak of the reductive desorption, explaining the discrepancy between the adsorbed amount of an alkanethiol calculated from the peak area and that expected from the (√3 × √3)R30° structure.