Predicting second gas–solid virial coefficients using calculated molecular properties on various carbon surfaces

Abstract

Gas–solid chromatography was used to obtain values of the second gas–solid virial coefficient, B 2 s , in the temperature range from 343 to 493 K for seven adsorbate gases: methane, ethane, propane, chloromethane, chlorodifluoromethane, dimethyl ether, and sulfur hexafluoride. Carboxen-1000, a 1200 m 2/g carbon molecular sieve (Supelco Inc.), was used as the adsorbent. These data were combined with earlier work to make a combined data set of 36 different adsorbate gases variously interacting with from one to four different carbon surfaces. All B 2 s values were extrapolated to 403 K to create a set of 65 different gas–solid B 2 s values at a fixed temperature. The B 2 s value for a given gas–solid system can be converted to a chromatographic retention time at any desired flow rate and can be converted to the amount of gas adsorbed at any pressure in the low-coverage, Henry's law region. Beginning with a theoretical equation for the second gas–solid virial coefficient, various quantitative structure retention relations (QSRR) were developed and used to correlate the ln B 2 s values for different gas adsorbates with different carbon surfaces. Two calculated adsorbate molecular parameters (molar refractivity and connectivity index), when combined with two adsorbent parameters (surface area and a surface energy contribution to the gas–solid interaction), provided an effective correlation ( r 2=0.952) of the 65 different ln B 2 s values. The two surface parameters provided a simple yet useful representation of the structure and energy of the carbon surfaces and thus our correlations considered variation in both the adsorbate gas and the adsorbent solid.