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

Abstract

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 Δ_fH^o_m{[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, Δ_fH^o_m(mim, g) = 126.5 ± 1.1 kJ mol⁻¹, and Δ_fH^o_m(CH₃COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O₂CCH₃](l) → mim(g) + CH₃COOH(g) as Δ_vapH^o_m{[Hmim][O₂CCH₃]} = 119.4 ± 3.0 kJ mol⁻¹. The agreement between this value and Δ_vapH^o_m{[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.