Interactions between Molecules Adsorbed on a Surface

Abstract

The intermolecular potential energy between two inert gas molecules is considerably altered when these molecules are next to a solid surface as in physical adsorption. The change in the interaction is evidenced by the additional long‐range repulsion that is often observed between the molecules of a monolayer and also by the additional attractions that must play a role in multilayer formation. In this article, the two‐molecule‐surface potential is derived from quantum mechanical third‐order perturbation theory. It is shown that this potential consists of two parts just as the energy giving the van der Waals attraction of a single molecule to a surface does. The first part exists only when the surface has a net electrostatic field and this is equivalent to the classical polarization effect. The second part arises from the fluctuations of the surface fields and is of the same origin as the dispersion forces. The third‐order energy, i.e., the new intermolecular interaction caused by the surface, is directly related to the zero‐coverage heat of adsorption and except for this experimental quantity, the results do not require specific assumptions about the surface. Thus, the theory is applicable to either metal or insulator surfaces. When both the two‐molecule‐surface and the one‐molecule‐surface interactions are available experimentally (for example, from the application of virial coefficients treatment in physical adsorption) the electrostatic field of the surface can be estimated. The fluctuation or dispersion part of the third‐order energy is shown to yield a repulsion between two molecules in a monolayer that amounts to 20–40% of the gas phase Lennard‐Jones potential minimum ε₀. The same energy yields an additional attraction of about 10–20% of ε₀ when the two molecules are on top of one another as in multilayer formation. The theory is applicable also when more than two molecules at a time need be considered on the surface.