Isobaric heat capacities of eleven ionic liquids containing imidazolium, pyrrolidinium, pyridinium, sulfonium, and ammonium cations with cyano-based and trifluoromethanesulfonyl-based type anions. Experimental and prediction

Abstract

The isobaric heat capacities were determined using a differential scanning calorimeter with the Tian-Calvet type heat flux detector for selected eleven ionic liquids, i.e., butyltriethylammonium bis(trifluoromethylsulfonyl)imide [N2224][NTf2], diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide [S122][NTf2], 1-(2-methoxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C1C2(OC1)im][NTf2], 1-hexyl-3-methylimidazolium tricyanomethanide [C1C6im][C(CN)3], 1-methyl-3-octylimidazolium tricyanomethanide [C1C8im][C(CN)3], 1-methyl-3-propylimidazolium trifluoromethanesulfonate [C1C3im][CF3SO3] 1-butyl-1-methylpyrrolidinium dicyanamide [C1C4pyr][N(CN)2], 1-butyl-1-methylpyrrolidinium tricyanomethanide [C1C4pyr][C(CN)3], 1-butyl-2-methylpyridinium bis(trifluoromethylsulfonyl)imide [C12 C4py][NTf2], 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide [C13 C4py][NTf2], and 1-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide [C14 C4py][NTf2]. The influence of the position (2, 3, or 4) of the methyl substituent on the value of the heat capacity in the last three structural isomers is observed in the case of position 2 only; isomers with methyl substituent in positions 3 and 4 are indistinguishable. The experimental data determined at atmospheric pressure 98.0 kPa in the temperature range 293.15 – 323.15 K were also compared with values estimated by two predictive models based on the group contribution method and model based on the contribution of the individual ions. The predictions are in relatively good accordance with experimental data, regardless of ionic liquids with [N(CN)2]– and [C(CN)3]– anions and [S122]+ cation. Furthermore, the simplicity of these models does not regard arene substitution patterns, and only a single generalized result has been obtained for pyridinium ionic liquids.