Electrochemical quartz crystal microbalance investigation of the reductivedesorption of self-assembled monolayers of alkanethiols and mercaptoalkanoic acids on Au

The reductive desorption of self-assembled monolayers (SAMs) of alkanethiol s and mercaptoalkanoic acids from gold has been examined in various alkalin e solutions and supporting electrolytes using an electrochemical quartz cry stal microbalance (EQCM). The desorption exhibits two voltammetric waves: a large dominant wave and another smaller wave at more negative potentials. The appearance of two waves is due to the heterogeneity in the substrate cr ystallinity and/or in the packing state of the thiol molecules. The desorpt ion charge is calculated after consideration of the charge required to esta blish the double layer of the uncoated electrode. The mass change per mole of electrons (mpe) for desorption is determined by comparing the frequency change with the total charge passed upon desorption. The mpe increases line arly with the chain length and has a slope close to the mass of CH2, as is expected for a one-electron process. In the case of alkanethiol desorption, however, the mpe is much smaller than the molar mass of the desorbed alkan ethiol. This difference is attributed to the simultaneous adsorption of cat ion species from solution, which is supported by the observed cation depend ence of mpe. We estimated the molar mass of the desorbed mercaptoalkanoic a cid by summing the mass of this cation species and the mpe for the mercapto alkanoic acid SAM. From the value thus calculated, we conclude that the ter minal carboxylate of the SAM is associated with the solvated cation. The so lvation numbers are determined to be 1.6, 2.4-3.0, and 0.2 for Li+, K+, and Cs+, respectively.