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

Abstract

The enthalpies of vaporisation, Δ_vapH₂₉₈, 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, [P_6,6,6,14][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 Δ_vapH₂₉₈ 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 Δ_vapH₂₉₈ 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 Δ_vapH₂₉₈ for 13 ILs with as yet unmeasured Δ_vapH₂₉₈ values (using experimental molar volumes), and for a further 44 ILs using estimated molar volumes.