A corresponding states principle for physisorption and deviations for quantum fluids

Abstract

The principle of corresponding states has long been observed to be valid for simple fluids in the bulk state. It has recently been proposed that fluids adsorbed in a microporous sorbent also follow a form of corresponding states [D.F. Quinn, Carbon 40, 2767 (2002)]. It was observed that adsorption isotherms for several different adsorbates follow near-universal behaviour when plotted at the reduced temperature 2.36 as a function of reduced pressures, where the critical temperature and pressure are used as the reducing parameters. Significantly, Quinn noted that hydrogen manifestly does not follow the trends of the other fluids, showing much higher adsorption than any other fluid studied; this was ascribed to hydrogen being able to adsorb in very narrow pores not accessible to other adsorbates. It is shown in the current work that the anomalous behaviour of hydrogen can be described entirely by quantum effects and the relative strength of the fluid–fluid and solid–fluid potentials. Analytical and simulation methods are used to investigate the adsorption of various gases within slit and cylindrical pores. For large pore sizes, accessible to all adsorbates, corresponding states behaviour occurs for classical gases, with deviations observed for quantum gases, in agreement with experimental observations. In contrast, size-dependent selectivity (sieving) is found in small pores.

¹Submitted to special issue of Molecular Physics in honour of Anthony Stone.

Keywords:

• adsorption
• hydrogen
• micropores
• sieving

Acknowledgements

We are grateful to Brad Bockrath for discussion of the Quinn work. This research has been supported by NSF- DMR-0505160 and the American Chemical Society Petroleum Research Fund (43431-G10). This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Notes

¹Submitted to special issue of Molecular Physics in honour of Anthony Stone.