A general strategy for the experimental study of the thermochemistry of protic ionic liquids: enthalpy of formation and vaporisation of 1-methylimidazolium ethanoate

A general strategy to determine enthalpies of formation of protic ionic liquids, based solely on enthalpy of solution measurements, was conceived and tested for 1-methylimidazolium ethanoate, leading to ΔfHom{[Hmim][O2CCH3], 1} = −(425.7 ± 1.2) kJ mol−1. This result in conjunction with the enthalpy of formation of gaseous 1-methylimidazole (mim) proposed in this work, ΔfHom(mim, g) = 126.5 ± 1.1 kJ mol−1, and ΔfHom(CH3COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O2CCH3](l) → mim(g) + CH3COOH(g) as ΔvapHom{[Hmim][O2CCH3]} = 119.4 ± 3.0 kJ mol−1. The agreement between this value and ΔvapHom{[Hmim][O2CCH3]} = 117.3 ± 0.5 kJ mol−1, obtained for the direct vaporisation of [Hmim][O2CCH3], by Calvet-drop microcalorimetry, gives a good indication that, as previously suggested by Fourier transform ion cyclotron resonance mass spectrometry, Raman spectroscopy, and GC-MS experiments, the vaporisation of [Hmim][O2CCH3] essentially involves a proton transfer mechanism with formation of the two volatile neutral precursor molecules (mim and CH3COOH). Although being a low ionicity protic ionic liquid, [Hmim][O2CCH3] was chosen to validate the methodology proposed here, since its vaporisation mechanism has been unequivocally demonstrated by different methods and for different pressure ranges.