(28e) Volatility of Room-Temperature Ionic Liquids: Distilling the Undistillable

Room-temperature ionic liquids (RTILs) are salts that melt at or below room temperature. They are typically composed of asymmetric, bulky, organic cations and weakly coordinating anions. It has long been believed that coulombic forces cause RTILs to have negligible vapor pressure, making distillation impossible. However, recent experiments at NIST show that RTILs do evaporate and can be distilled at low pressure and high temperature. In a seminal experiment, a well-characterized RTIL was evaporated at reduced pressure in a vacuum-tight sublimation apparatus at a temperature of 473 K, and the distillate was captured on a cold finger. Analysis of the distillate by nuclear magnetic resonance spectroscopy ruled out thermal decomposition as a mechanism of evaporation. Further experiments with mixtures established the feasibility of separative distillation. In these experiments, equimolar mixtures of two RTILs were distilled, and the distillate was enriched in the more volatile RTIL, again with no detectable decomposition. This work was corroborated by the work of a research team based in Portugal and the United Kingdom, which independently made the same discovery using a different experimental method. The combined studies were published together in Nature 439, 831-834 (2006). The laboratory demonstration of RTIL volatility has opened a new branch of scientific research on ionic vapors and unlocked the door leading to potential applications, including new routes to purify ionic liquids to unprecedented levels, and to regenerate spent ionic liquids from industrial processes.