Electrochemical studies of the benzoate adsorption on Au (111) electrode

Cyclic voltammetry (CV), differential capacity (DC), and charge density mea surements have been employed to study the benzoate (BZ) adsorption at the A u(lll) electrode surface. Thermodynamic analysis of charge density (sigma ( M)) data has been performed to describe the properties of the adsorbed benz oate ion. The Gibbs excess Gamma, Gibbs energy of adsorption DeltaG, and th e number of electrons flowing to the interface per adsorbed benzoate ion at a constant potential (electrosorption valency) and at a constant bulk conc entration of the benzoate (reciprocal of the Esin-Markov coefficient) have been determined. The results demonstrate that although benzoate adsorption starts at negative charge densities, it takes place predominantly at a posi tively charged surface. At the most positive potentials, the surface concen tration of benzoate attains a limiting value of about 7.3 x 10(-10) mol-cm( -2), which is independent of the bulk benzoate concentration. This value is consistent with packing density corresponding to a closed-packed monolayer of vertically adsorbed benzoate molecules. At negative charge densities, b enzoate assumes a flat (pi -bonded) surface coordination. The surface coord ination of benzoate changes, by moving from a negatively to positively char ged surface. At the negatively charged surface, the electrosorption bond is quite polar. The polarity of the chemisorption bond is significantly reduc ed due either to a charge transfer or a screening of the charge on the anio n by the charge on the metal.