A simple physical model for the simultaneous rationalisation of melting points and heat capacities of ionic liquids

Abstract

The analysis of potential energy surfaces of ion pairs within the framework of an anharmonic oscillator model allows rationalization and prediction of melting points (T_mp) and heat capacities (C_p) of ionic liquids (ILs) comprising di- and trialkylimidazolium or tetraalkylphosphonium cations and weakly coordinating BF₄, PF₆, or Tf₂N anions. Multiple short contacts between the counterions are demonstrated to be typical for the imidazolium based ILs. Differences in the types of contacts result in moderate changes of melting points of the ILs, comparable with differences in T_mp experimentally determined for the same crystal. The theoretical evaluation of IL heat capacities additionally requires a consideration of conformational behaviour of the corresponding cations. A similar conformational composition of 1-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate at ambient temperature is demonstrated by the combined DFT-vibrational spectroscopy approach. A rough proportionality of C_p to 1/T_mp of ionic liquids is suggested, provided that the conformational composition of the ILs does not change on crystal-to-liquid transition.