SIMPLE-MODELS FOR ADSORPTION ON ELECTRODES .1. ADSORPTION OF MONOMERIC NEUTRAL ORGANIC-MOLECULES

Simple molecular models are developed to describe the single adsorption of monomer neutral organic compounds and the reorientation and coadsorption processes of these substances on ideally polarized electrodes. The equilibrium properties are determined within the framework of lattice statistics using the mean field approximation under the assumption of a mean electrical field acting at each site. Neglecting short-range interactions, the first model for single adsorption predicts langmuirian adsorption isotherms, which are congruent only with respect to potential, and a quadratic dependence of the Gibbs energy of adsorption on the potential. The model was tested by comparison with a variety of experimental data and satisfactory agreement was found over a wide range of adsorbate concentrations and polarizations, even in cases where the experimental data exhibit deviations from langmuirian behaviour. Generalizations of this model to include short-range interactions, variations of the adsorbed layer thickness and changes in the state of the solvent molecules are examined. The model for reorientation and coadsorption processes is also an extension of the first model. Comparisons of its predictions with experimental data show that it describes almost quantitatively all the experimental features observed during a reorientation process of an organic adsorbate when it exhibits two distinct polarization states on a mercury electrode. Moreover, it can provide a satisfactory description of the coadsorption of two organic adsorbates, without using additional adjustable parameters, but from the known properties of their single adsorption. Thus, the models developed in this paper combine both simplicity and applicability and therefore they can be used to analyse a variety of experimental adsorption data.