Symmetry versus asymmetry game in vaporization enthalpies of imidazolium-based ionic liquids

Abstract

The vaporization enthalpy as a function of alkyl chain length was studied using both experimental techniques and molecular dynamics simulations for imidazolium based ionic liquids (ILs) with symmetric and asymmetric imidazolium substitution. It was found that the vaporization enthalpy increases with increasing alkyl chain length due to the increased van der Waals (vdW) interaction. In addition, the vaporization enthalpies of the symmetric ILs ([CnCnIm][X], X = NTf2-, Cl-, Br-, I-) series were found to be systematically lower (by 5–10 kJ mol−1) in comparison with the corresponding asymmetric imidazolium-based ILs ([CmC1Im][X]). This energetic difference was attributed to the intramolecular dispersion interactions between the two alkyl side chains existing in the symmetric cation series.