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Volatilization and Thermal Decomposition Mechanisms of Room-Temperature Ionic Liquids (PRE-PRINT)
AIR FORCE RESEARCH LAB EDWARDS AFB CA EDWARDS AFB United States
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Recent interest in room temperature ionic liquids RTILs was based initially on the replacement of volatile organic compounds VOCs used as industrial solvents with involatile liquids. RTILs also show promise for applications in fuel cells, batteries, solar cells and many other potential applications. RTILs are organic salts with low melting temperatures T100 C, and initially were thought to have no vapor pressure. However, some types of RTILs have been shown to distill in vacuum without decomposition. Lately, the design and choice of many ionic liquids is focused on physical properties such as miscibility, conductivity, viscosity, solubility and melting points. The details of how the chemical structure of the ionic liquid affects these various physical characteristics are not well understood. Similarly, due to the ionic nature of these liquids, their mechanism for vaporization may be dramatically different from molecular liquids, and predicting thermodynamic properties such as the heats of vaporization and heats of formation in the gas phase presents a challenge. Photoionization studies on the thermally stable 1-ethyl-3-methylimidazolium bistrifluoromethylsulfonylimide where the anion is also known as the bistriflamide anion and related imidazolium bistriflamides have indicated that volatilization of these species occurs as a single ion pair. Recent ALS studies on the photoionization of other 1- alkyl-3-methylimidazolium based RTILs, show these systems can be more complex, with possible dissociative photoionization occurring at even the lowest detectable reservoir temperatures. At high temperatures, photoionization of the thermal decomposition products of the imidazolium RTILs indicates possible anion-cation reactions and possible polymerization to higher mass species. Note the increase in fragmentation in the spectra as a result of increased internal energy imparted by increased photon in energy, 10.0 eV versus 8.0 eV.
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