The cohesive properties and pyrolysis mechanism of an aprotic ionic liquid tetrabutylammonium bis(trifluoromethanesulfonyl)imide

Abstract

As the cohesive properties (such as the enthalpy of sublimation) of solid organic salts (or ionic liquids, ILs) are unmeasurable, a method of their indirect determination is proposed in this paper. For this purpose, the thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) were carried out over a wide range of temperatures. In this study, the mathematical relationship of the thermodynamic properties between the liquid and solid phases of ILs is established using the Born–Fajans–Haber cycle, in which the sum of the vaporization enthalpy of ILs, melting enthalpy and the enthalpy of solid–solid phase transition is regarded as the sublimation enthalpy of solid organic salts. With this method, the cohesive properties of tetrabutylammonium bis(trifluoromethanesulfonyl)imide ([N₄₄₄₄][NTf₂]), which is an aprotic IL, were successfully obtained. Additionally, the difference between the lattice energy and the cohesive energy was employed to quantitatively calculate the charge separation distance of single ion pair (r₁₂) in the gas phase of ionic liquids for the first time, which can serve as a standard methodology to measure the closeness in distance between the anion and the cation in a gas phase ion pair. The pyrolysis mechanism of [N₄₄₄₄][NTf₂] was also explored.