Vaporisation and thermal decomposition of dialkylimidazolium halide ion ionic liquids

Vaporisation and liquid phase thermal decomposition, TD, of two halide ion ionic liquids, 1-octyl-3-methylimidazolium chloride, [C(8)C(1)Im]Cl, and 1-octyl-3-methylimidazolium iodide, [C(8)C(1)Im]I, are investigated using temperature programmed desorption (TPD) line of sight mass spectrometry (LOSMS) at ultra-high vacuum (UHV). The ability to use MS to distinguish between vaporisation and TD allows the thermodynamics/kinetics of both vaporisation and TD to be investigated within the same experiments. Vaporisation of both halide ion ionic liquids is demonstrated. For both [C(8)C(1)Im]Cl and [C(8)C(1)Im]I the vapour is shown to be composed of neutral ion pairs (NIPs). The enthalpy of vaporisation at temperature T, Delta H-vap(T), was experimentally determined as Delta H-vap(455) = 151 +/- 10 kJ mol(-1) for [C(8)C(1)Im]Cl and Delta H-vap(480) = 149 +/- 8 kJ mol(-1) for [C(8)C(1)Im]I. Extrapolation of Delta H-vap(T) to the reference temperature, 298 K, gave Delta H-vap(298) = 166 +/- 10 kJ mol(-1) for [C(8)C(1)Im]Cl and Delta H-vap(298) = 167 +/- 8 kJ mol(-1) for [C(8)C(1)Im]I, higher than most Delta H-vap(298) values measured to date for other [C(8)C(1)Im](+)-containing ionic liquids. In addition, predictions of Delta H-vap(298) for other halide ion ionic liquids are made. Liquid phase TD is shown to proceed via nucleophilic substitution to give two sets of products: 1-octylimidazole and methylhalide, and 1-methylimidazole and 1-octylhalide. The activation energy of TD at a temperature T, E-a,E-TD,E-T, is measured for the nucleophilic substitution of [C(8)C(1)Im]I to give methyliodide; E-a,E-TD,E-480 = 136 +/- 15 kJ mol(-1). E-a,E-TD,E-T is measured for the nucleophilic substitution of [C(8)C(1)Im]Cl to give methylchloride; E-a,E-TD,E-455 = 132 +/- 10 kJ mol(-1). The fact that Delta H-vap(T) and E-a,E-TD,E-T are the same (within error) for both ionic liquids is commented upon, and conclusions are drawn as to the thermal stability of these ionic liquids.