| 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 Delta H-f(m)o\[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, Delta H-f(m)o(mim, g) = 126.5 +/- 1.1 kJ mol(-1), and Delta H-f(m)o(CH3COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O2CCH3](l) -> mim(g) + CH3COOH(g) as Delta H-vap(m)o\[Hmim][O2CCH3]\ = 119.4 +/- 3.0 kJ mol(-1). The agreement between this value and Delta H-vap(m)o\[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. |
