Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromatography

Abstract

The surface properties of 1-alkyl-3-vinylimidazolium hexafluorophosphate ([CnVIM][PF6], n = 2,4,6,8) ionic liquids and 1-allyl-3-vinylimidazolium ([AVIM]+), ionic-based liquids with different anions (Cl−, Br−, [BF4]−, [Tf2N]−) were investigated at temperatures between 303.15 and 343.15 K on the basis of inverse gas chromatography (IGC). The Schultz method and Dorris-Gray methods were used to estimate the dispersive free energy based on the net retention volumes of homogeneous n-alkanes, and it was found that the discrepancy between the two methods increases with increasing temperature, with the Dorris-Gray method providing slightly higher values than the Schultz method at the same test temperature. The Lewis acid-base parameters, KA and KB, were likewise assessed by the method proposed by Schultz, Flour and Papirer, Brookman and Sawyer, respectively. Although the difference between the values of acid-base parameters were observed, the ratios of KB/KA obtained by three methods presented the same tendency and implied that the surface of all the tested ionic liquids exhibited the Lewis amphoteric and stronger basic character. In addition, the values of the enthalpies of adsorption between the probes and ionic liquids were also determined.