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

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