Thermal stability of low temperature ionic liquids revisited

The range of thermal stability of low temperature ionic liquids published in the literature (often >400degreesC) is severely overrated. The decomposition temperature calculated from fast TGA scans in a protective atmosphere does not imply a long-term thermal stability below that temperature. Even at temperatures as low at 200degreesC, 1-alkyl-3-methylimidazolium phosphates (alkyl = C4-C10) and 1-decyl-3-methylimidazolium triflate showed a slow, but appreciable mass loss. On the other hand, 1-butyl-3-methylimidazolium triflate was stable at 200degreesC. The carbonization occurred in most studies salts irrespectively of the nature of the anion (hexafluorophosphate, triflate), but the salts with a shorter side chain (C4) did not show changes in their color after conditioning for 10h at 200degreesC in air. 1-Butyl-3-methylimidazolium triflate shows extremely good wettability against aluminum oxide and silver at elevated temperatures. Addition of silica (amorphous or quartz) accelerates the thermal decomposition of 1-alkyl-3-methylirnidazolium phosphates and triflates at 200degreesC, while the effect of other ceramic powders (titania, alumina) is less significant. (C) 2003 Elsevier B.V. All rights reserved.