Measuring and predicting Delta H-vap(298) values of ionic liquids

We report the enthalpies of vaporisation ( measured using temperature programmed desorption by mass spectrometry) of twelve ionic liquids (ILs), covering four imidazolium, [C(m)C(n)Im](+),. five pyrrolidinium, [C(n)C(m)Pyrr](+), two pyridinium, [CnPy](+), and a dication, [C-3(C(1)Im)(2)](2+) based IL. These cations were paired with a range of anions: [BF4](-), [FeCl4](-), [N(CN)(2)](-), [PF3(C2F5)(3)](-) ([FAP](-)), [(CF3SO2)(2)N](-) ([Tf2N](-)) and [SCN](-). Using these results, plus those for a further eight imidazolium based ILs published earlier ( which include the anions [CF3SO3](-) ([TfO](-)), [PF6](-) and [EtSO4](-)), we show that the enthalpies of vaporisation can be decomposed into three components. The first component is the Coulombic interaction between the ions, Delta U-Cou,(R), which is a function of the IL molar volume, V-m, and a parameter R-r which quantifies the relative change in anion - cation distance on evaporation from the liquid phase to the ion pair in the gas phase. The second and third components are the van der Waals contributions from the anion, Delta H-vdw,H-A, and the cation, Delta H-vdw,H-C. We derive a universal value for R-r, and individual values of Delta H-vdw,H-A and Delta H-vdw,H-C for each of the anions and cations considered in this study. Given the molar volume, it is possible to estimate the enthalpies of vaporisation of ILs composed of any combination of the ions considered here; values for fourteen ILs which have not yet been studied experimentally are given.