Ionic Liquids DOI: 10.1002/anie.200702157 Do We Understand the Volatility of Ionic Liquids?** Ralf Ludwig* and Udo Kragl* Keywords: boiling points · effusion · ionic liquids · thermodynamics · vapor pressure Ionic liquids (ILs) are organic salts with unusually low melting points, well below 100 8C. These substances have been suggested as potentially “green” replacements for traditional molecular solvents since they are nonvolatile, nonflammable, thermally stable, and recyclable. Additionally, they are open- ing the path to novel methods that are not possible using classical solvents. While remarkable progress has been made in recent years in the synthesis and application of ILs in catalytic and separation processes, as well as in electro- chemistry, their physicochemical properties have not yet been studied systematically. This holds in particular for the thermodynamic properties of pure ionic liquids. [1–3] This is somehow surprising because a key property responsible for their increasing popularity is their essentially null volatility. This feature makes it easy to manipulate ionic liquids and facilitates their use in multiple reaction and extraction cycles, and it finally led to their recognition as environmentally friendly “green” solvents. Ironically, this characteristic has impaired many major developments toward the understand- ing of their macroscopic behavior through studies at the molecular level. The vapor pressure, enthalpy of vaporization, as well as the entropy of vaporization are among the fundamental properties required for developing and testing accurate molecular models. This is in particular true for the validation of new force fields used in molecular dynamics studies where the strength and nature of interactions between ions/molecules have to be adjusted (Figure 1). Recent experiments have shed new light on the nature of the vapor, the heats of vaporization, and related vapor pressures and boiling points of ionic liquids. In practice, researchers are faced with two main problems: At room temperature the low vapor pressures of ILs are practically not measurable, whereas at high temperatures some of them may decompose through processes such as the transfer of an alkyl group or, in the case of protic ionic liquids, through deprotonation. For this reason, most of the exciting experi- ments highlighted herein are performed with 1-alkyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide [C n MIM][NTf 2 ](n = 2–8). This family of ionic liquids was chosen as a result of its thermal stability, which allowed measurements to be carried out up to 600 K. In contrast to the common view that ionic liquids are nonvolatile substances, it was recently shown by Earle et al. that they can be distilled under reduced pressure. [4,5] The researchers could evaporate and recondense some ILs in the presumably stable regime below 500 K, and could even separate two ILs by distillation. Some time before, Swiderski et al. made use of the bimolecular rate constant of a Diels– Alder reaction to estimate Hildebrand)s solubility parame- ter. [6] The reflected cohesive energy density of the liquid can be expressed in terms of the heat of evaporation D vap H. The obtained values of about 200 kJmol À1 were much higher than those of common organic solvents but lower than estimates of up to 300 kJmol À1 as sometimes reported. [7] Figure 1. A snapshot of the ionic liquid [C 8 MIM][NTf 2 ] from molecular dynamics simulations in the condensed phase. Recent experiments showed that the gas phase purely consists of ion pairs, as highlighted here. [*] Prof. Dr. R. Ludwig Institut für Chemie Abteilung Physikalische Chemie Universität Rostock Dr.-Lorenz-Weg 1, 18059 Rostock (Germany) Fax: (+ 49)381-498-6524 E-mail: ralf.ludwig@uni-rostock.de Prof. Dr. U. Kragl Institut für Chemie Abteilung Analytische, Technische und Umweltchemie Universität Rostock Albert-Einstein-Strasse 3a, 18059 Rostock (Germany) Fax: (+ 49)381-498-6452 E-mail: udo.kragl@uni-rostock.de [**] Financial support from the German Science foundation (DFG) within the program SPP1191 “Ionic Liquids” is gratefully acknowl- edged. Highlights 6582 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2007, 46, 6582–6584