Electrochemical and second harmonic generation study of bromide adsorption at the Au(111) electrode surface

A thermodynamic analysis of charge density data has been performed to describe bromide adsorption at the Au(111) electrode surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed bromide ion at a constant potential (electrosorption valency) and at a constant bromide concentration (reciprocal of the Esin-Markov coefficient) and the surface dipole formed by the adsorbed ion and its image charge in the metal were determined. Bromide ion forms a chemisorption bond with the gold surface whose polarity is a strong function of the charge on the metal. The bond is quite polar at the negatively charged surface; however, its polarity drops significantly at positive charges. The adsorption behaviour of this anion is quite asymmetric with respect to the charge on the metal. At low charge densities and coverages, the change of the bond polarity is determined by the ability of the free electrons in the metal and surface solvent molecules to screen the dipole formed by adsorbed Br- and its image charge in the metal. At high charge densities and coverages, the chemisorption bond has a predominately covalent character as a result of charge transfer between the adsorbed bromide ion and the metal surface. The covalent character of Br- adsorption at the Au(111) is consistent with the results of independent SHG experiments. The SHG data indicate that adsorption of bromide strongly affects the electronic structure of the metal surface.