THE ROLE OF 3-BODY INTERACTIONS IN THE ADSORPTION OF ARGON IN SILICALITE-1

The significance of three-body interactions for argon adsorption in si licalite al 77 K is considered, for triplets involving two or more arg on atoms. Configurations from previous simulations, obtained using a n ew potential function for single argon atoms interacting with the zeol ite: and a conventional 12-6 argon-argon effective pair potential were used as a starting point. The total energy of these configurations wa s recalculated with the Barker-Fisher-Watts potential for argon pair i nteractions, and three-body dispersion terms up to the triple quadrupo le term. The overall contribution from the higher order nonadditive te rms is always negative, and decreases with adsorbate loading. A substa ntial part (similar to 20%) of the total interaction was found to come from the non-additive contributions, in contrast to other calculation s on zeolitic systems; and largely from Ar-Ar-O triplets. The cancella tion between fourth-order terms and third-order quadrupole terms which occurs in isotropic homogeneous systems does not occur here. An itera tive scheme is described by which heats of adsorption can be obtained. A first-order calculation of this type shows that the effect of the n on-additive contributions is initially to reduce the adsorbate-adsorba te contribution. This has the effect of producing an isosteric heat cu rve which is nearly invariant with loading for low adsorbate loading. Thus it appears that energetic homogeneity need not be invoked as an e xplanation of experimental observations showing this type of behaviour .