Thermal Stability and Phase Transitions of Ionic Liquids

Ionic liquids (ILs) are organic salts with low melting points (below 100 C). In general, ILs have many promising properties, such as good solvation qualities for both polar and nonpolar compounds, wide temperature range for ILs to be liquid, high thermal stability and very low vapor pressure. These properties are dependent on choosing suitable organic cations and organic or inorganic anions, which enable the fine-designed ILs for specific processes. ILs are studied in a variety of applications, including solvents for reactions and separations, high temperature heat transfer fluids, geothermally driven absorption refrigeration systems and working fluid in a variety of electrochemical applications. The basic thermophysical properties, like decomposition temperature, phase transition and heat capacities, are important for design and evaluation for these applications. Combination of a variety of cations and anions can make ionic liquids virtually endless. The classes of organic cations include imidazolium, pyrdinium, quaternary ammonium and tetra alkylphosphonium. Possible organic/inorganic anions include hexafluorophosphate [PF6], tetrafluoroborate [BF4], bis(trifluoromethylsulfonyl) imide [(CF3SO2)2N], acetate [CH3CO2], nitrate, chloride and amino acids.

Vapor pressures of thermally stable imidazolium and pyridinium ILs with the bis(trifluoromethylsulfonyl)amide anion (Tf2N) were studied by a rapid method using isothermal thermogravimetric analysis under an inert environment. Methyl paraben was used as a calibration standard to determine the TGA vaporization constant, and then vapor pressures of several low volatility organic compounds, as well as ILs, were calculated by the Langmuir and Guckel equations. The calculated vapor pressures and temperatures were correlated with the Clausius-Clapeyron model. In addition, vaporization/decomposition products of several ILs were first studied by chromatoprobe gas chromatography/mass spectrometry (GC/MS) under a helium environment in both EI and NCI modes. GC/MS chromatogram and their spectra showed that ILs in this study could be vaporized without significant degradation (less than 5 %) at temperature below 320 C.