Thermal Decomposition, Low Temperature Phase Transitions and Vapor Pressure of Less Common Ionic Liquids Based on the Bis(trifuoromethanesulfonyl)imide Anion

Abstract

Four ionic liquids (ILs) based on the bis(trifluoromethanesulfonyl)imide (NTf2) anion were synthesized and characterized concerning their thermal stability, the occurrence of low temperature phase transitions and their volatility. All these physical quantities are highly important for possible applications. Both monocationic and dicationic ILs were considered. All ILs exhibit thermal stability exceeding 350 °C, an extremely high value, due to the presence of the NTf2 anion. Monocationic ILs can undergo crystallization, and they melt at 1 and 38 °C. On the contrary, dicationic ILs containing large positively charged ions display only a glass transition around −40 °C, without any crystallization or melting process; this fact is particularly important in view of the possibly low temperature applications of the dication ILs. The vapor pressure, pv, of the four ILs was measured by isothermal thermogravimetry in the temperature range between 250 and 325 °C; the lowest values of pv were obtained for the two dicationic liquids, suggesting that they are particularly well suited for high temperature applications. The vaporization enthalpy was calculated through the Clausius–Clapeyron equation and was found in the range between ~140 and ~180 kJ/mol depending on the specific IL.

Evaporation/Decomposition Behavior of 1-Butyl-3-Methylimidazolium Chloride (BMImCL) Investigated through Effusion and Thermal Analysis Techniques

“…The low volatility and the relatively high thermal stability are probably the properties of ionic liquids (ILs) that have aroused the greatest interest and enthusiasm for this large family of compounds in the past 20 years. For this reason, an investigation into the stability limits of the most promising classes of ILs is crucial for the assessment of their real application prospects and has been one of the subjects at the center of attention for many years [1][2][3]. The evaporation/decomposition behavior of these compounds has been the subject of a scientific debate for the past 20 years, ever since Earle et al [4] demonstrated for the first time that many ILs could be distilled without decomposition.…”