A corresponding states principle for physisorption and deviations for quantum fluids1

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.