Relative Volatilities of Ionic Liquids by Vacuum Distillation of Mixtures Jason A. Widegren,* ,² Yi-Ming Wang, ‡ Wesley A. Henderson, § and Joseph W. Magee ² Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Boulder, Colorado 80305, and Department of Chemistry, United States NaVal Academy, Annapolis, Maryland 21402 ReceiVed: April 16, 2007; In Final Form: June 1, 2007 The relative volatilities of a variety of common ionic liquids have been determined for the first time. 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, 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 counterions present. Similarly, ionic liquids based on the anions [(C 2 F 5 SO 2 ) 2 N] - , [(C 4 F 9 SO 2 )(CF 3 SO 2 )N] - , and [(CF 3 SO 2 ) 2 N] - are more volatile than ionic liquids based on [(CF 3 SO 2 ) 3 C] - and [CF 3 SO 3 ] - , and are much more volatile than ionic liquids based on [PF 6 ] - . Introduction Ionic liquids (ILs) are molten salts with melting temperatures of <373 K. 1 Because of their ionic nature, ILs are uniquely suited for certain applications in electrochemistry, separations, synthesis, and engineering. 2 The conventional wisdom about ILs has been that they have no observable vapor pressure. Primarily for this reason, ILs are often referred to as “green” solvents because they do not create atmospheric pollution in the way that volatile molecular solvents do. Indeed, there is excellent evidence that the vapor pressures of ILs are extremely low near room temperature. For example, the ideal gas vapor pressure of 1-butyl-3-methylimidazolium hexafluorophosphate has been estimated to be 10 -10 Pa at 298 K. 3 Other ILs have been studied by X-ray photoelectron spectroscopy in an ultra- high-vacuum chamber (p ≈ 10 -7 Pa) at room temperature. 4 By simulation 5 and experiment, 6 the cohesive energy densities for a variety of ILs are found to be very large near room temperature, which explains the low volatility of these liquids. 5a Recently, however, there have been reports indicating that the vapor pressures of some ILs become significant at higher temperatures. In 2005 Rebelo et al. 7 used surface tension and density data to predict the normal boiling temperatures of salts of 1-alkyl-3-methylimidazolium (abbreviated [C n mim] + , where n is the number of carbons in the alkyl group) with the anions [BF 4 ] - , [PF 6 ] - , and [(CF 3 SO 2 ) 2 N] - (abbreviated as [Tf 2 N] - ). They predicted that salts of [Tf 2 N] - would have the lowest boiling temperatures (around 525 K for [C 10 mim][Tf 2 N]). 7 Soon after this prediction was made, vapor pressure measurements by Knudsen effusion on [C 2 mim][Tf 2 N], [C 4 mim][Tf 2 N], [C 6 - mim][Tf 2 N], and [C 8 mim][Tf 2 N] were reported. 8 Vapor pres- sures for [C 4 mim][Tf 2 N] were determined over the widest temperature range, and reported to be 0.0036 Pa at 437.84 K (the lowest temperature) and 0.515 Pa at 517.45 K (the highest temperature). 8 It is worth noting that the Knudsen effusion apparatus could not detect a vapor pressure for [C 4 mim][PF 6 ] at temperatures up to 473 K. 3 In early 2006 Earle et al. 9 reported vacuum distillations of ILs using both a Kugelrohr apparatus (at 573 K and 600 Pa) and a sublimation apparatus (at 473 K and 0.1 Pa). Several classes of pure ILs were evaporated and condensed without decomposition. Additionally, distillations of a few binary mixtures of ILs showed that the initial distillate was enriched in one of the ILs, as would be expected for the separative distillation of compounds with different vapor pressures. 9 We realized that a relative volatility series for ILs could be established by analyzing the initial distillate from mixtures to see which salts distill preferentially. 10 This straightforward approach avoids the difficulties and pitfalls of absolute vapor pressure measurements in the low-pressure regime. Herein we report the results of more than 30 distillations of IL mixtures. In this way we determined the relative volatilities of ILs containing the following ions: 1-alkyl-3-methylimidazolium, [C n mim] + ; 1-alkyl-2,3-dimethylimidazolium, [C n mmim] + ; N- alkyl-N-methylpyrrolidinium, [C n mpyrr] + ; N-propyl-3-methyl- pyridinium, [C 3 mpy] + ; tetraalkylammonium, [R 4 N] + ; tetra- fluoroborate, [BF 4 ] - ; hexafluorophosphate, [PF 6 ] - ; trifluoro- methanesulfonate, [CF 3 SO 3 ] - (abbreviated as [TfO] - ); bis(trifluoro- methylsulfonyl)imide, [(CF 3 SO 2 ) 2 N] - (abbreviated as [Tf 2 N] - ); bis(pentafluoroethylsulfonyl)imide, [(C 2 F 5 SO 2 ) 2 N] - ; (nonafluo- robutylsulfonyl)(trifluoromethylsulfonyl)imide, [(C 4 F 9 SO 2 )- (CF 3 SO 2 )N] - (abbreviated as [(C 4 F 9 SO 2 )(Tf)N] - ); and tris- (trifluoromethylsulfonyl)methanide, [(CF 3 SO 2 ) 3 C] - (abbreviated * To whom correspondence should be addressed. Tel: +1 303 497 5207. Fax: +1 303 497 5224. E-mail: jason.widegren@nist.gov. ² National Institute of Standards and Technology. ‡ Current address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138. § United States Naval Academy. 8959 J. Phys. Chem. B 2007, 111, 8959-8964 10.1021/jp072964j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007