Adsorption of asymmetric rigid rods or heteronuclear diatomic molecules on homogeneous surfaces

Abstract

We treat the adsorption on homogeneous surfaces of asymmetric rigid rods (like for instance heteronuclear diatomic molecules). We show that the $n\ensuremath{\rightarrow}0$ vector spin formalism is well suited to describe such a problem. We establish an isomorphism between the coupling constants of the magnetic Hamiltonian and the adsorption parameters of the rigid rods. By solving this Hamiltonian within a mean-field approximation, we obtain analytical expressions for the densities of the different rod's configurations, both isotherm and isobar adsorptions curves. The most probable configurations of the molecules (normal or parallel to the surface) which depends on temperature and energy parameters are summarized in a diagram. We derive that the variation of ${Q}_{v}$, the heat of adsorption at constant volume, with the temperature is a direct signature of the adsorbed molecules configuration change. We show that this formalism can be generalized to more complicated problems such as for instance the adsorption of symmetric and asymmetric rigid rods mixtures in the presence or not of interactions.