Flammability, thermal stability, and phase change characteristics of several trialkylimidazolium saltsThe authors wish to thank the scientists at the Occupational Safety and Health Administration ? Salt Lake Technical Center for their measurement of the imidazolium flashpoints.The policy of the National Institute of Standards and Technology (NIST) is to use metric units of measurement in all its publications, and to provide statements of uncertainty for all original measurements. In this document however, data from organizations outside NIST are shown, which may include measurements in non-metric units or measurements without uncertainty statements. The identification of any commercial product or trade name does not imply endorsement or recommendation by NIST or the United States Air Force (USAF). Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the USAF or NIST.

Abstract

Room temperature ionic liquids (RTILs) have emerged as tunable and potentially “greener” solvents for a multitude of applications. To investigate the solvent properties and potential use as a thermal fluid, a study was initiated to determine the effects of anion type, C-2 hydrogen substitution, and alkyl chain length on the flammability, thermal stability, and phase change characteristics of 1,2,3-trialkylimidazolium room temperature ionic liquids. A Setaflash flashpoint apparatus was used to determine the flammabilities of the RTILs. No flashpoints were detected for any of the imidazolium based RTILs below 200 °C, the maximum temperature of the instrument. The thermal stabilities of the RTILs were measured using the technique of thermogravimetric analysis. The 1,2,3-trialkylimidazolium compounds exhibit slightly higher thermal stabilities than the comparable 1,3-dialkylimidazolium compounds; RTILs with nucleophilic anions decompose about 150 °C lower than RTILs with bulky fluoride containing anions; the alkyl chain length does not have a large effect on the thermal stability of the RTILs; and the pyrolysis decomposition exhibits higher thermal stabilities via a different mechanism than the oxidative decomposition. In addition, it was found that although the calculated onset temperatures were above 350 °C, significant decomposition does occur 100 °C or more below these temperatures. The phase change behaviors of several imidazolium based RTILs were characterized by differential scanning calorimetry. The melting points of the RTILs increased with increasing alkyl chain length. Most of the salts studied exhibited significant undercooling, which decreased as the length of the alkyl chain was increased. The hexafluorophosphate and bromide RTILs exhibited polymorphic and liquid crystalline behaviors as the alkyl chain length was increased above C10. The clearing point temperatures increased more rapidly with alkyl chain length than the melting point temperatures.