The enthalpies of vaporisation of ionic liquids: new measurements and predictions

The enthalpies of vaporisation, ΔvapH298, of seven ionic liquids (ILs) (four imidazoliums, a pyridinium, a phosphonium and an isouronium) have been determined by temperature programmed desorption using line of sight mass spectrometry. They were: 1-ethyl-3-methylimidazolium bis(pentafluoroethyl)phosphinate, [C2C1Im][PO2(C2F5)2]; 1-butyl-3-methylimidazolium octylsulfate, [C4C1Im][C8OSO3]; 1-butyl-3-methylimidazolium tetrafluoroborate, [C4C1Im][BF4]; 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C6C1Im][FAP]; 1-butylpyridinium methylsulfate, [C4Py][C1OSO3]; trihexyl(tetradecyl)phosphonium tetrafluoroborate, [P6,6,6,14][BF4] and O-ethyl-N,N,N′,N′-tetramethylisouronium trifluoromethanesulfonate, [C2(C1)4iU][TfO]. The values were found to be consistent with a previously proposed, predictive, model in which ΔvapH298 is decomposed into a Coulombic component (computable from the IL density) and van der Waals components from the anion and cation. Two previously predicted values of ΔvapH298 were found to be within 6 kJ mol−1 of the measured experimental values. Values for the van der Waals components are tabulated for eleven cations and twelve anions. Predictions are made for ΔvapH298 for 13 ILs with as yet unmeasured ΔvapH298 values (using experimental molar volumes), and for a further 44 ILs using estimated molar volumes.