Electrosorption valency and partial charge transfer in halide and sulfide adsorption on Ag(111)

The electrosorption valency l(B) of an adsorbed species is usually obtained from the slope of plots of the charge density on the metal against the surface concentration of the given species, at constant applied potential. Herein, two alternative procedures for the estimate of l(B) are proposed and applied to the formation of ordered overlayers of chloride, bromide, iodide, and sulfide on Ag(111). One of these procedures applies to strongly adsorbing anions, whose incipient adsorption turns out to be diffusion controlled under limiting conditions when stepping from a potential negative enough to exclude their specific adsorption. This procedure allows l(B) to be estimated as a function of the applied potential. Partial charge-transfer coefficients lambda estimated from l(B) values on the basis of some modelistic assumptions decrease in the order sulfide approximate to iodide > bromide > chloride, namely in the order of increasing Pauling's electronegativity. Some direct procedures for the estimate of lambda, which avoid the intermediate estimate of l(B), are shown to lead to erroneous results.