Adsorption on metal electrodes

Abstract

Whenever the concentration of a species at the interface is greater than can be accounted for by electrostatic interactions, we speak of specific adsorption. It is usually caused by chemical interactions between the adsorbate and the electrode, and is then denoted as chemisorption. In some cases adsorption is caused by weaker interactions such as van der Waals forces; we then speak of physisorption. Of course, the solvent is always present at the interface; so the interaction of a species with the electrode has to be greater than that of the solvent if it is to be adsorbed on the electrode surface. Adsorption involves a partial desolvation. Cations tend to have a firmer solvation sheath than anions, and are therefore less likely to be adsorbed. The amount of adsorbed species is usually given in terms of the coverage θ, which is the fraction of the electrode surface covered with the adsorbate. When the adsorbate can form a complete monolayer, θ equals the ratio of the amount of adsorbate present to the maximum amount that can be adsorbed. In a few systems the area covered by a single adsorbed molecule changes with coverage; for example, some organic molecules lie flat at low coverage and stand up at higher coverages. In this case one must specify to which situation the coverage refers. Another definition of the coverage, often used in surface science, is the following: θ is the ratio of the number of adsorbed species to the number of surface atoms of the substrate. Fortunately, most authors state which definition they use. The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams.