Equilibrium conformation and surface motion of hydrocarbon molecules physisorbed on graphit

Abstract

The equilibrium configurations of methane, ethane, propane, ethylene, benzene, cyclohexene and cyclohexane adsorbed on the basal plane of a graphite crystal have been determined in the additive pairwise approximation with Lennard-Jones potentials and using Steele's method of potential expansion. The normal modes for the motion of the rigid molecules at the graphite surface and the corresponding zero point energies have been studied in the harmonic approximation. The evaluation of the adsorption energies has been improved through a perturbative method using the harmonic functions as zero approximation.The influence of the surface topography of the adsorbing solid and of the structure of the adsorbed molecule on physisorption energies, was discussed in a previous paper with reference to various hydrocarbons adsorbed on graphite, by assuming additive atom-pair interactions as expressed by Lennard-Jones potentials and by considering harmonic residual motions of the rigid molecules adsorbed at the surface.The adsorption of methane, ethylene, ethane, cyclopropane and benzene was considered and it was found that the model of an ideal bidimensional gas of structureless particles, is inadequate in these cases. In particular it was shown that the zero point energies of some molecules may make up as much as 20 % of the total adsorption energy. Even if the single highest contribution to the zero point energy was in most cases associated with a vibration normal to the surface, the largest part of it was found to be due to the hindered rotations at the surface.Such conclusions were, however, open to criticism mostly on two grounds; the choice of the single pair interaction potentials suffers from a high degree of arbitrariness, and furthermore, the harmonic approximation may be inadequate to describe the motion of the molecule at the surFace. The present work, while extending the study to other hydrocarbons (propane, cyclohexene, cyclohexane), aims at partially overcoming these limitations. Concerning the first point, we have examined the differences introduced in the results by the use of different potentials; the potentials previously used and largely based on a priori assumptions, are here compared with potentials adjusted against thermodynamic adsorption data. On the second point, since it was difficult to bypass the harmonic approximation (which is essential to the analysis of the normal modes of the adsorbed molecule), a perturbation technique has been adopted in order to evaluate the overall effect of anharmonicity on the zero point energy.Furthermore, the computer program has been completely restructured by substituting the method introduced by Steele and based on the Fourier expansion of the potential, to the direct summation of the interactions over the atoms of the solid. This change is justified both on grounds of elegance and because it makes it possible t

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