Isothermogravimetric determination of the enthalpies of vaporization of 1-alkyl-3-methylimidazolium ionic liquids

Vaporization enthalpies for two series of ionic liquids (ILs) composed of 1-n-alkyl-3-methylimidazolium cations, [Im_m1⁺] (m = 2, 3, 4, 6, 8, or 10), paired with either the bis(trifluoromethanesulfonyl)amide, [Tf₂N^-], or the bis(perfluoroethylsulfonyl)amide anion, [beti^-], were determined using a simple, convenient, and highly reproducible thermogravimetric approach, and from these values, Hildebrand solubility parameters were estimated. Our results reveal two interesting and unanticipated outcomes: (i) methylation at the C2 position of [In _m1⁺] a significantly higher vaporization enthalpy; (ii) in all cases, the [beti^-] anion served to lower the enthalpy of vaporization relative to [Tf₂N^-]. The widespread availability of the apparatus required for these measurements coupled with the ease of automation suggests the broad potential of this methodology for determining this critical parameter in a multitude of ILs.