We investigate the selective adsorption of xenon, argon, and methane in zeolite NaA by applying the grand canonical ensemble Monte Carlo simulation technique to an adsorbed binary mixture and to two reference systems: 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 adsorbed system and these reference systems. We observe that xenon selectively adsorbs in both xenon-argon and xenon-methane mixtures at low chemical potential (low pressure) due to its greater energetic interaction with the zeolite. However, a reversal in selectivity occurs at higher chemical potential in both of these mixtures. This is due in large part to the greater efficiency in which the smaller component "packs" 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.
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We wish to thank M. C. Mitchell and D. Keffer for helpful discussions and for assistance in data retrieval, the Minnesota Supercomputer Institute for use of a Silicon Graphics IRIS workstation and the Minnesota Supercomputer Center, Inc. for a grant on a Cray X-MP EA supercomputer. P. R. V. T. additionally thanks P. Viot, G. Tarjus, and P. A. Monson for helpful discussions and acknowledges support from a NATO-NSF Postdoctoral Fellowship.
- Binary mixture
- Grand canonical
- Monte Carlo