Importance of dispersion forces for prediction of thermodynamic and transport properties of some common ionic liquids

Abstract

An extensive study of interaction energies in ion pairs of pyrrolidinium and imidazolium ionic liquids is presented. The C_nmpyr and C_nmim cations with varying alkyl chains from Methyl, Ethyl, n-Propyl to n-Butyl were combined with a wide range of routinely used IL anions such as chloride, bromide, mesylate (CH₃SO₃ or Mes), tosylate (CH₃PhSO₃ or Tos), bis(trifluoromethanesulfonyl)amide (NTf₂), dicyanamide (N(CN)₂ or dca), tetrafluoroborate (BF₄) and hexafluorophosphate (PF₆). A number of energetically favourable conformations were studied for each cation–anion combination. The interaction energy and its dispersion component of the single ion pairs were calculated using a sophisticated state-of-the-art approach: a second-order of Symmetry Adapted Perturbation Theory (SAPT). A comparison of energetics depending on the cation–anion type, as well as the mode of interaction was performed. Dispersion forces were confirmed to be of importance for the overall stabilisation of ionic liquids contributing from 28 kJ mol⁻¹ in pyrrolidinium ion pairs to 59 kJ mol⁻¹ in imidazolium ion pairs. The previously proposed ratio of total interaction energy to dispersion components and melting points was assessed for this set of ionic liquids and was found to correlate with their melting points for the anionic series, producing separate trends for the C_nmim and C_mpyr series of cations. Chlorides, bromides and tetrafluoroborates formed close-to-ideal correlations when both types of cations, C_nmim and C_nmpyr, were combined in the same trend. Correlation of the dispersion component of the interaction energy with transport properties such as conductivity and viscosity was also considered. For imidazolium-based ionic liquids strong linear correlations were obtained, whereas pyrrolidinium ionic liquids appeared to be insensitive to this correlation.