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. Oxidat ive adsorption of CncH2nc+1S- is characterized by two voltammetric wav es corresponding to the formation of energetically distinct low-densit y and high-density surface phases of Hg-SCncH2nc+1. The high-density s urface 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 so lutions containing short-chain n-alkanethiolates, n(c) less than or eq ual to 5. The dependence of the adsorption half-wave potential E-1/2 o n the solution concentration of CncH2nc+1S- indicates that approximate ly one electron is transferred per adsorbed molecule. The free energy of adsorption Delta G(ads) of CncH2nc+1S- on Hg decreases by similar t o 1 kcal/mol per n(c), reflecting the influence of hydrophobic interac tions and intermolecular forces on the thermodynamic stability of the monolayer.