INTERMOLECULAR POTENTIAL FUNCTION FOR THE PHYSICAL ADSORPTION OF RARE-GASES IN SILICALITE

We present a simple method for the derivation of two-body and three-bo dy dispersion coefficients for incrystal atoms from the knowledge of t heir dipole polarizability and effective number of electrons. This met hod is checked by comparison with results from quantum mechanical calc ulations and used to derive two- and three-body coefficients for the d ispersion interaction of argon, krypton, and xenon adsorbed in the sil iceous zeolite silicalite-l. Repulsive parameters for the argon/silica lite system are obtained by fitting experimental data over a wide rang e of temperature and the full scale potential constructed in this way (referred to as PN1) is shown to perform well in predicting other argo n data. The parameters for the repulsive energy for the krypton/silica lite and xenon/silicalite systems obtained using combination rules and the PN1 potential for these adsorbates (without any parameter adjustm ent) are also found to be successful in predicting low coverage proper ties. We compare the performance of the PN1 function with the Kiselev adsorption potential, widely used in the field of modeling adsorption in zeolite cavities, and show that the latter tends to overestimate th ermodynamic properties and also predicts a wider pore than the new pot ential. The energetics of adsorption are discussed in terms of site lo cation and shape and the effective size of zeolite oxygen atoms.