Molecular models for adsorbed monolayers at charged interfaces

Abstract

A statistical–mechanical treatment of various models concerning a monolayer of solvent molecules in different states and adsorbate molecules in one state on a charged plane surface is presented. The study is concentrated on the evaluation of the adsorption isotherm, the equilibrium conditions between the various solvent states and the dielectric properties at the charge of maximum adsorption of the following models: a three-state model with site parity for the two solvent states; a four-state model of three solvent states also with site parity; a four-state model in which the solvent molecules are present in two monomer states and one dimer state; and a five-state model with two monomer and two cluster states for the solvent. In all these models a site disparity is assumed for the adsorbate state. The general case of the N-state model is also studied.

Results suggest that the site parity models for the solvent states predict a concentration independent adsorption maximum. They all give the same adsorption isotherm at the charge of maximum adsorption which shows serious weakness when applied to real systems. The site disparity models for the solvent states predict a concentration dependent adsorption maximum in disagreement with the majority of the experimental data. The variation of the adsorption maximum with the concentration of the adsorbate is attributed to entropy effects due to the difference in the molecular sizes of the solvent states and to a lesser extent to short-range interactions between adsorbate and solvent molecules.