Systematic estimation and interpretation of fractional free volume in 1-alkyl-3-methylimidazolium-based ionic liquids

Abstract

With the aim of providing a plausible explanation for fractional free volume (FFV) behavior in ionic liquids (ILs) such as 1-alkyl-3-methylimidazolium-based cations paired with common anions like [BF4]−, [PF6]−, and bis(trifluoromethane)sulfonamide ([NTf2]−), we comprehensively estimated the van der Waals volume of the ions and subsequently determined the FFV behavior present within these liquids with the aid of quantum chemical calculations. Unlike the values seen in previous studies, our results showed an increase in the FFV character of the IL under investigation when either the cation's alkyl chain was lengthened and/or the size of the anion was expanded. The sizes of cations and anions present in the IL were shown to affect the FFV values in different ways: whereas the effect of the anion's size could be accounted for by looking at changes in the Coulomb interaction between cations and anions in the polar (charge localized) domains, the effect exerted by the alkyl chain was found to be related to the alkyl chain's dependence on the FFV obtained for alkanes due to the introduction of non-polar (alkyl group aggregated) domains to ILs with increasing alkyl chain lengths. An empirical equation was proposed in order to calculate FFV values for alkyl-imidazolium-based ILs.