Statistical calculation of thermodynamic functions of an absorbed substance on graphite. Part 1.—Adsorption of monatomic molecules on graphite

Abstract

In accordance with the general problem of calculating the thermodynamic functions of a substance in the adsorbed state proceeding only from the properties of the adsorbent and adsorbate taken separately, the chemical potential, total energy, and entropy of argon adsorbed on graphite are calculated. The computation is based on evaluation of the potential energy of the argon molecule at the graphite surface taking into account the contribution of dispersion forces of attraction in the form of two terms, and the contribution of the repulsion forces in the exponential form. Interactions between the adsorbate molecule and the carbon atoms in the lattice were found by summing them up with respect to all the lattice atoms; the dispersion attraction constants were computed from the polarizabilities and magnetic susceptibilities of the adsorbate and adsorbent atoms. Then, using molecular statistic methods the partition function of the adsorbate in the classical approximation, and its thermodynamic functions were calculated, and corresponding approximate corrections were made for the quantum state of the adsorbate. The assumption that the adsorbate molecule vibrates harmonically at right angles to the surface is a good approximation. The theoretically calculated changes in chemical potential, total energy, and entropy of argon are close, withinmore » the limits of possible computation errors, to the measured heat of adsorption and to the corresponding values of the changes in thermodynamic functions evaluated from experimental data for graphitized carbon blacks. (auth)« less