(211e) A Relative Volatility Scale for Ionic Liquids

An earlier collaborative work [1] demonstrated laboratory distillation of ionic liquids and established the feasibility of separative distillation of ionic liquid mixtures. As a continuation of the earlier work, relative volatilities of a variety of common, aprotic ionic liquids were determined for the first time [2]. Equimolar mixtures of ionic liquids were vacuum distilled in a glass sublimation apparatus at approximately 473 K. The composition of the initial distillate, determined by NMR spectroscopy, was used to establish the relative volatility of each ionic liquid in the mixture. The effect of alkyl chain length was studied by distilling mixtures of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, or mixtures of N-alkyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids, shown in Fig. 1, with different alkyl chain lengths. For both classes of salts the volatility is highest when the alkyl side chain is a butyl group. The effect of cation structure on volatility has been determined by distilling mixtures containing different types of cations. Generally speaking, ionic liquids based on imidazolium and pyridinium cations are more volatile than ionic liquids based on ammonium and pyrrolidinium cations, regardless of the types of counter ions present. Similarly, ionic liquids based on the anions [(C2F5SO2)2N]-, [(C4F9SO2)(CF3SO2)N]- and [(CF3SO2)2N]-; are more volatile than ionic liquids based on [(CF3SO2)3C]- and [CF3SO3]-, and are much more volatile than ionic liquids based on [PF6]-. The laboratory demonstration of RTIL volatility has opened a new branch of scientific research on ionic vapors and unlocked doors leading to potential applications, including new routes to purify ionic liquids to unprecedented levels, and to regenerate spent ionic liquids from industrial processes. References [1] Earle, M. J., Esperança, J. M. S. S., Gilea, M. A., Canongia Lopes, J. N., Rebelo, L. P. N., Magee, J. W., Seddon, K. R., and Widegren, J A., "The Distillation and Volatility of Ionic Liquids" Nature 439: 831-834 (2006). [2] Widegren, J. A., Wang, Y.-M., Henderson, W. A., and Magee, J. W., "Relative Volatilities of Ionic Liquids by Vacuum Distillation of Mixtures" J. Phys. Chem. B 111: 8959-8964 (2007).