title: 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(f)H°(m){[Hmim][O(2)CCH(3)], 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(f)H°(m)(mim, g) = 126.5 ± 1.1 kJ mol(-1), and Δ(f)H°(m)(CH(3)COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O(2)CCH(3)](l) → mim(g) + CH(3)COOH(g) as Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 119.4 ± 3.0 kJ mol(-1). The agreement between this value and Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 117.3 ± 0.5 kJ mol(-1), obtained for the direct vaporisation of [Hmim][O(2)CCH(3)], 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][O(2)CCH(3)] essentially involves a proton transfer mechanism with formation of the two volatile neutral precursor molecules (mim and CH(3)COOH). Although being a low ionicity protic ionic liquid, [Hmim][O(2)CCH(3)] was chosen to validate the methodology proposed here, since its vaporisation mechanism has been unequivocally demonstrated by different methods and for different pressure ranges.
authors.affiliation: Centro de Química e Bioquímica e Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Portugal.
authors.forename: Joana
authors.initials: J
authors.lastname: Vitorino
authors.forename: Carlos E S
authors.initials: CE
authors.lastname: Bernardes
authors.forename: Manuel E
authors.initials: ME
authors.lastname: Minas da Piedade
mesh_terms: D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D011522:Protons; D013816:Thermodynamics; D014835:Volatilization
chemical_list: D007093:Imidazoles; D052578:Ionic Liquids; D011522:Protons; C018100:1-methylimidazole