© Copyright 2007 by the American Chemical Society
VOLUME 111, NUMBER 18, MAY 10, 2007
EDITORIAL The Physical Chemistry of Ionic Liquids Ionic liquids, or low-temperature molten salts, are becoming increasingly popular subjects of study as “neoteric” solvents for catalysis, separations, fuel cells, photoelectrochemical cells, and many other applications. As a group, they have many properties that extend beyond the ranges of normal molecular solvents; typically these include very large liquidus ranges, low volatility, high conductivity, wide electrochemical windows, the ability to solubilize a broad variety of organic and inorganic materials including macromolecules, and viscosities above those of most common solvents. All of these generalizations have exceptions; one of the joys of working with ionic liquids is that you can substitute the cations and anions to obtain the properties desired, and when that is insufficient, functional groups can be added to the ions to provide further control.1 To date, there has been a great deal of progress in ionic liquid design; however, much of that progress has been achieved through an empirical approach to property modification that would benefit greatly from a better physical understanding of the structure and interactions within ionic liquids. Ionic liquids are at once both very old and very new. Although salts that melt below 100 °C, such as ethylammonium nitrate (12 °C),2 have been known since the early 20th Century, the stepwise development of successively more tractable families of ionic liquids in recent years has created exponential growth in the field since the mid-1990’s. Excellent reviews of the history of ionic liquid development3 and their expanding applications are available.4-10 This surge in interest has fueled a number of symposia on ionic liquids at international specialist meetings and, starting in 2001, at a series of ACS National Meetings under the auspices of several ACS divisions. These symposia were packed with interesting results and revelations, but by and large, they were concerned with the development of new ionic liquids, their characterization, and their effects on chemical reactions and catalysis. The early meetings did not contain much physical chemistry beyond the material properties of the ionic liquids, such as viscosity, conductivity, thermal characterization of phase behavior, gas solubility, and so forth. The apparent imbalance between the synthetic and physical sides of the ionic liquid picture was memorably pointed out by Prof. Ken Seddon at the Fall, 2002, ACS Meeting in Boston using slides of painted
fiberglass bears. One, representing synthesis, was entirely covered with names of reactions for which data was available. The other, representing physical chemistry, was nearly blank, indicating the gaps that needed to be filled. This situation began to change as molecular dynamics simulations and solvation dynamics measurements on ionic liquids became more common. During the First International Conference on Ionic Liquids (COIL) in Salzburg, Austria in August, 2005, it was clear that a large variety of experimental and theoretical physical studies were being pursued in an attempt to discover how the unique properties of ionic liquids arise from their composition and interionic interactions. Because of this rapidly increasing level of activity, we felt the time was right to hold an ACS symposium focusing exclusively on the physical chemistry of ionic liquids, bringing ionic liquid experts and people just entering into the field to present and discuss their results in a colloquium-like environment. We were not alone in that sentiment. Prof. Frank Endres organized a unique, English-speaking international Bunsen Colloquium on “Physical Chemistry in Ionic Liquids” in Clausthal, Germany, March 2324, 2006, that was very well received. Our symposium “Physical Chemistry of Ionic Liquids” was held at the 232nd ACS National Meeting in San Francisco, September 10-13, 2006 as part of the Physical Section program. The symposium featured 48 presentations on the topics of structures and properties of ionic liquids, energy conversion and reactions in ionic liquids, interactions, dynamics, and spectroscopy in ionic liquids, ionic liquid interfaces, and (bio)macromolecules in ionic liquids. The symposium serves as the basis for this special issue of The Journal of Physical Chemistry B. In addition, we asked several leading researchers in the field who were unable to attend the meeting to make contributions to the special issue. We were very pleased with the response and we thank the authors for their outstanding contributions. During the symposium and in this special issue, one particular theme emerged from many of the presentations and papers. Experimental results are now converging with molecular dynamics simulations that indicate that ionic liquids, in some cases depending on composition, are heterogeneous on the nanoscale and may contain local domains of order or phase
10.1021/jp072262u CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
4640 J. Phys. Chem. B, Vol. 111, No. 18, 2007 segregation and that the scale of these domains decreases with increasing temperature.11,12 One can imagine many ways to exploit this property to create high-performance materials; an example is the paper in this issue13 in which an ionic liquid crystal phase is used to improve the performance characteristics of a dye-sensitized solar cell. We would also like to use the forum provided by this introduction as a soapbox to alert present and future researchers in the physical chemistry of ionic liquids to issues that have become evident as the community has gained experience working with them. First and foremost, it is vital to establish the purity of the ionic liquids being used. The presence of water and halide impurities, even in small concentrations, has been long understood to alter the properties of the liquids.14-18 It is important to quantify and report water and halide content of the ionic liquids used for publication. Second, although the price, quality, and diversity of commercially available ionic liquids have improved tremendously in recent years as the market has grown and experience has been gained, there are still a good deal of variability between commercial sources, including impurities other than water and halide that may affect experiments.19-22 Experimenters should be prepared to test the suitability of each IL batch for their experiments as if the material was prepared in their own laboratory. These and other caveats have been articulated in more detail elsewhere.23,24 It is our hope that this issue will serve the larger community of physical chemists by stimulating interest in ionic liquids as a field of study that provides true opportunities for progress in solving vital real-world problems such as sustainable energy generation and use.
James F. Wishart BrookhaVen National Laboratory Edward W. Castner, Jr. Rutgers, The State UniVersity of New Jersey
References and Notes (1) Davis, J. H. Chem. Lett. 2004, 33, 1072-1077. (2) Walden, P. Bull. Acad. Imper. Sci. (St. Petersburg) 1914, 1800. (3) Wilkes, J. S. Green Chem. 2002, 4, 73-80. (4) Welton, T. Chem. ReV. 1999, 99, 2071-2083. (5) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 13911398. (6) Brennecke, J. F.; Maginn, E. J. AIChE J. 2001, 47, 2384-2389. (7) Olivier-Bourbigou, H.; Magna, L. J. Mol. Catal. A-Chem. 2002, 182, 419-437. (8) Davis, J. H.; Fox, P. A. Chem. Commun. 2003, 1209-1212. (9) Welton, T. Coord. Chem. ReV. 2004, 248, 2459-2477. (10) Forsyth, S. A.; Pringle, J. M.; MacFarlane, D. R. Aust. J. Chem. 2004, 57, 113-119. (11) Triolo, A.; Russina, O.; Bleif, H.-J.; DiCola, E. J. Phys. Chem. B 2007, 111, 4641-4644. (12) Xiao, D.; Rajian, J. R.; Cady, A.; Li, S.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2007, 111, 4669-4677. (13) Yamanaka, N.; Kawano, R.; Kubo, W.; Masaki, N.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. J. Phys. Chem. B 2007, 111, 47634769. (14) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275-2287. (15) Baker, S. N.; Baker, G. A.; Bright, F. V. Green Chem. 2002, 4, 165-169. (16) Widegren, J. A.; Laesecke, A.; Magee, J. W. Chem. Commun. 2005, 1610-1612. (17) Saha, S.; Hamaguchi, H. O. J. Phys. Chem. B 2006, 110, 27772781. (18) Silvester, D. S.; Compton, R. G. Z. Phys. Chem.-Int. J. Res. Phys. Chem. Chem. Phys. 2006, 220, 1247-1274. (19) Endres, F.; El Abedin, S. Z.; Borissenko, N. Z. Phys. Chem.-Int. J. Res. Phys. Chem. Chem. Phys. 2006, 220, 1377-1394. (20) Paul, A.; Mandal, P. K.; Samanta, A. Chem. Phys. Lett. (Netherlands) 2005, 402, 375-379. (21) Paul, A.; Mandal, P. K.; Samanta, A. J. Phys. Chem. B 2005, 109, 9148-9153. (22) Burrell, A. K.; Del Sestoa, R. E.; Baker, S. N.; McCleskey, T. M.; Baker, G. A. Green Chem. 2007, 9, 10.1039/b615950h. (23) Deetlefs, M.; Seddon, K. R. Chim. Oggi 2006, 24, 16-23. (24) Scammells, P. J.; Scott, J. L.; Singer, R. D. Aust. J. Chem. 2005, 58, 155-169.
