ADSORPTION OF BINARY-MIXTURES IN A ZEOLITE MICROPORE

We investigate the selective adsorption of xenon, argon, and methane i n zeolite NaA by applying the grand canonical ensemble Monte Carlo sim ulation technique to an adsorbed binary mixture and to two reference s ystems: i) an adsorbed single component system and ii) a bulk mixture. We define and calculate selectivities and excess densities due to i) mixing and ii) adsorption in terms of differences between the binary a dsorbed system and these reference systems. We observe that xenon sele ctively adsorbs in both xenon-argon and xenon-methane mixtures at low chemical potential (low pressure) due to its greater energetic interac tion with the zeolite. However, a reversal in selectivity occurs at hi gher chemical potential in both of these mixtures. This is due in larg e part to the greater efficiency in which the smaller component ''pack s'' in the pore as compared to the bulk. We show that the crossover in selectivity occurs at a lower chemical potential for a mixture where one component can occupy regions of the porespace inaccessible to the other. We suggest that this crossover in selectivity may be a general feature of microporous adsorption.