Vaporisation and thermal decomposition of dialkylimidazolium halide ion ionic liquids

Abstract

Vaporisation and liquid phase thermal decomposition, TD, of two halide ion ionic liquids, 1-octyl-3-methylimidazolium chloride, [C₈C₁Im]Cl, and 1-octyl-3-methylimidazolium iodide, [C₈C₁Im]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 [C₈C₁Im]Cl and [C₈C₁Im]I the vapour is shown to be composed of neutral ion pairs (NIPs). The enthalpy of vaporisation at temperature T, Δ_vapH_T, was experimentally determined as Δ_vapH₄₅₅ = 151 ± 10 kJ mol⁻¹ for [C₈C₁Im]Cl and Δ_vapH₄₈₀ = 149 ± 8 kJ mol⁻¹ for [C₈C₁Im]I. Extrapolation of Δ_vapH_T to the reference temperature, 298 K, gave Δ_vapH₂₉₈ = 166 ± 10 kJ mol⁻¹ for [C₈C₁Im]Cl and Δ_vapH₂₉₈ = 167 ± 8 kJ mol⁻¹ for [C₈C₁Im]I, higher than most Δ_vapH₂₉₈ values measured to date for other [C₈C₁Im]⁺-containing ionic liquids. In addition, predictions of Δ_vapH₂₉₈ 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,TD,T, is measured for the nucleophilic substitution of [C₈C₁Im]I to give methyliodide; E_a,TD,480 = 136 ± 15 kJ mol⁻¹. E_a,TD,T is measured for the nucleophilic substitution of [C₈C₁Im]Cl to give methylchloride; E_a,TD,455 = 132 ± 10 kJ mol⁻¹. The fact that Δ_vapH_T and E_a,TD,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.