Oxidative adsorption of n-alkanethioiates at mercury.: Dependence of adsorption free energy on chain length

The oxidative adsorption of n-alkanethiolates (CncH2nc+1S-, n(c) = 2-8) at Hg in aqueous solutions containing 0.5 M NaOH is reported. Oxidative adsorption of CncH2nc+1S- is characterized by two voltammetric waves corresponding to the formation of energetically distinct low-density and high-density surface phases of Hg-SCncH2nc+1. The high-density surface phase (9.8 x 10(-10) mol/cm(2)) corresponds to adsorption of ca. one monolayer of CncH2nc+1S-. Electrodeposition of a bulk film of a mercury thiolate complex also occurs at more positive potentials in solutions containing short-chain n-alkanethiolates, n(c) less than or equal to 5. The dependence of the adsorption half-wave potential E-1/2 on the solution concentration of CncH2nc+1S- indicates that approximately one electron is transferred per adsorbed molecule. The free energy of adsorption Delta G(ads) of CncH2nc+1S- on Hg decreases by similar to 1 kcal/mol per n(c), reflecting the influence of hydrophobic interactions and intermolecular forces on the thermodynamic stability of the monolayer.