Thermal properties of imidazolium ionic liquids
Introduction
Salts based on the imidazolium cation have low melting points, with many being liquids at room temperature. The solvating properties of these ionic liquids, their high conductivity (ca. 10 mS/cm), and their wide window of electrochemical stability (ca. 4 V) make these materials excellent candidates for a range of applications, including catalysis, supercapacitors, and photovoltaics [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
The dependence of the thermal properties on imidazolium and anion structure was investigated. The chemical abbreviations and structures of the investigated salts are detailed in Scheme 1. We also compare the imidazolium salts to tetraethyl ammonium (TEA) and tetrabutyl ammonium (TBA) salts. Previous work has looked at the pyrolysis of 1,3-dialkyl-imidazolium halides at 220–260°C [11]. Chan et al. found the dealkylation of imidazole quaternary salts consistent with a SN2 mechanism, where the halide attacked the primary alkyl groups preferential to secondary positions. We have determined that the imidazolium salts have lower melting points and are thermally more stable than the lithium ion analogs [12]. We were interested in comparing the thermal properties of the salts containing the conjugate base of the superacids bis(trifluoromethylsulfonyl)imide (Im), bis(perfluoroethylsulfonyl)imide (Beti), and tris(trifluoromethylsulfonyl)methide (Me) [13].
Section snippets
Experimental
The imidazolium salts were synthesized according to standard procedures and dried before use [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [12]. For example, EMIBeti was synthesized from the reaction of EMICl with LiBeti in acetonitrile and used after drying at 100°C under vacuum. A Mettler DL18 Karl Fischer titrator was used to determine water content of the samples (typically <20 ppm).
Thermal analysis was performed on a Mettler DSC821e and TGA850 using the STAR analysis software.
Results and discussion
We first describe our investigation of the phase transition dependence (DSC data) on salt structure followed by a discussion of thermal stability (TGA/SDTA data).
Conclusions
Many of the imidazolium salts investigated are liquids at room temperature and below, displaying substantial supercooling characteristics. These ionic liquids have little vapor pressure up to their decomposition, providing a wide liquid range with no vapor pressure. The thermal stability of the imidazolium salts increases with increased alkyl substitution, as long as linear alkyl groups are used. The presence of nitrogen substituted secondary (and likely tertiary) alkyl groups decreases the
Acknowledgements
We thank Richard Laura for his assistance. This work was financially supported by DOE contract DE F602-96ER82149 and NASA contract NAS3-98083.
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