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

The enthalpies of vaporisation, Delta H-vap(298), 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, [C(2)C(1)Im][PO2(C2F5)(2)]; 1-butyl-3-methylimidazolium octylsulfate, [C(4)C(1)Im][C8OSO3]; 1-butyl-3-methylimidazolium tetrafluoroborate, [C(4)C(1)Im][BF4]; 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C(6)C(1)Im][FAP]; 1-butylpyridinium methylsulfate, [C4Py][C1OSO3]; trihexyl(tetradecyl)phosphonium tetrafluoroborate, [P-6,P-6,P-6,P-14][BF4] and O-ethyl-N,N,N',N'-tetramethylisouronium trifluoromethanesulfonate, [C-2(C-1)(4)iU][TfO]. The values were found to be consistent with a previously proposed, predictive, model in which Delta H-vap(298) 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 Delta H-vap(298) 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 Delta H-vap(298) for 13 ILs with as yet unmeasured Delta H-vap(298) values (using experimental molar volumes), and for a further 44 ILs using estimated molar volumes.