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J. Phys. Chem. B 2003, 107, 6170-6178
Ionic Liquids: Ion Mobilities, Glass Temperatures, and Fragilities Wu Xu,† Emanuel I. Cooper,‡,§ and C. Austen Angell*,†,‡ Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604, and Department of Chemistry, Purdue UniVersity, Lafayette, Indiana 87907 ReceiVed: December 1, 2002; In Final Form: March 28, 2003
We combine old, unpublished data on ionic liquids containing quaternary ammonium cations with new data on salts of aromatic cations containing a variety of anions, to demonstrate the existence for ionic liquids of an unexpectedly wide range of liquid fragilities. The pattern is one now familiar for other liquids. Here, the pattern is important in determining the relative fluid properties at ambient temperatures. We find that the optimization of ionic liquids for ambient temperature applications requiring low-vapor-pressure fluid phases involves the proper interplay of both cohesive energy and fragility factors. The cohesive energy is discussed in terms of the coulomb and van der Waals contributions to the attractive part of the pair potential. On the basis of the relation between the glass-transition temperature and the molar volume for salts with less-polarizable anions, we find evidence for a broad minimum in the ionic liquid cohesive energy at an internuclear separation of ca. 0.6 nm. This minimum lies between those of the BF4 and TFSI anions for the small quaternary ammonium cations of this study. The minimum is expected to be narrower and less well-defined for salts with polarizable anions. The relation of fluidity to conductance is considered in terms of a Walden plot that is shown to provide a useful basis for organizing the observations on ionic liquids and solutions. Low vapor pressure and ideal Walden products are intimately related.
Introduction At the present time, there is much interest in ambienttemperature liquids that consist only of cations and anions. These are known variously as “room temperature molten salts” (RTMSs), “ambient temperature melts” (ATMs), or, alternatively and increasingly, simply as “ionic liquids” (ILs). Relative to common ambient-temperature liquids and solvent mixtures, these liquids are distinguished by the fact that they do not evaporate. This renders them particularly suitable for several applications in which loss of liquid to the vapor phase constitutes a performance limitation and, often, a health hazard. The physics of ionic liquids to this point has been developed in a very empirical fashion. The first ionic liquid was reported almost a century ago by Walden,1 who protonated ethylamine with nitric acid (HNO3) to yield ethylammonium nitrate, which has a melting point of Tm ) 14 °C. Since that time, a succession of (i) viscous, waterstable, ionic liquids,2-4 (ii) highly fluid, but water-unstable (hepta- and tetra-chloroaluminate, and chloroferrate) liquids,5-8 and, finally, (iii) both fluid and water-stable (mostly fluorinated anion) liquids have been discovered. It is the latter cases that are of special interest for applications. The first of the latter cases to be reported appears to have been the tetrafluoroborate (BF4 ) of a quaternary ammonium cation, which was discovered in 1984.9 This salt (methoxyethyl dimethyl ethylammonium tetrafluoroborate, MOENM2E-BF4) melts at 13 °C and, on cooling, only loses its fluidity at a glasstransition temperature of Tg ) -98 °C. However, although the * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Arizona State University. ‡ Purdue University. § Now at IBM T. J. Watson Research Center, Yorktown Heights, NY.
authors of the report recognized their finding as “an important development in ambient-temperature molten-saltsupporting electrolytes”, and reported three years later10 a case (methoxymethyl dimethyl ethylammonium tetrafluoroborate, MOMNM2E-BF4) of even lower Tm (-16 °C) and Tg (-115 °C), they did not develop the subject nor report all their findings. For fundamental reasons,9 these liquids did not support the decoupled Li+ transport phenomenon that was in focus at Purdue University at that time. Indeed, there was no follow-up until Cooper and O’Sullivan11 and Wilkes and co-workers,12 in close succession in 1992, reported systematic studies of the fluorinated anion class of ionic liquids and set the stage for the current flowering of interest in this field.13-22 One of the reasons for the high fluidity of these liquids is the weak van der Waals interaction between the F atoms of the anions and their neighboring cations.23 This is manifested in the low Tg values that are reported.9,10,15,17 However, although the Tg value of the second Cooper ionic liquid (MOMNM2EBF4)10 remains the lowest on record, the fluidity at ambient temperature (which is now reported here for the first time) has been surpassed by several other formulations. Also, the ambienttemperature ionic conductivity of this salt, although five times higher than the 1.7 mS cm-1 value of the initial case9 (in which the cation contained one extra CH2 group), is also well below those of recently reported ionic liquids.13-18 Clearly, there are other factors besides the starting point of the liquid state (Tg) involved in determining the ambient-temperature values of these important properties. The rates at which the fluidities of different formulations change with temperature must be very different, implying that there is much to understand about the dynamics of particle motion in this class of fluid. Because one of the attractive features of the ionic liquid class of fluids is the extended temperature range of liquid-state behavior they offer, it is important to understand the effect of
10.1021/jp0275894 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/31/2003
Ionic Liquids: Ion Mobilities, Tg, and Fragilities temperature on the various liquid properties. It is the purpose of the present paper to elucidate this problem and thereby bring the phenomenological description of ionic liquids in line with that which is common for other fluids. A point of interest in relation to fundamental understanding of liquids is that ionic liquids are single-component systems in which the cations and anions may possibly play independent roles in determining the liquid behavior. In addition to data on salts containing familiar anions such as I-, BF4 , triflate (CF3SO3 ), ClO4 , and bis-trifluoromethanesulfonyl imide (TFSI ) anions, we will present some data for salts of large anions. These include the enlarged version of TFSI-, bis(perfluoroethanesulfonyl) imide (known as BETI-), and the new BOB- anion (bis-oxalato-orthoborate),24 whose behavior in nonaqueous solutions and Li+ ion cells is currently proving to be of great interest.25,26 More-detailed studies of the orthoborate anion class of ionic liquid will be reported subsequently.27 Finally, some alternative tetrahalide anions, FeCl4 and GaCl4 , have been included, because it was determined long ago5 that the lowest glass temperatures of ILs were obtained with FeCl4 anions. Experimental Section Materials. The chemicals used in this work were obtained from Aldrich, except for LiTFSI and LiBETI, which were received as gratis from 3M, and all were used as received. Syntheses and Characterization. The synthetic procedures used for making the two Cooper quaternary ammonium tetrafluoroborate salts (i.e., MOMNM2E-BF4 and MOENM2E-BF4) were described in earlier reports9 in which they were applied to prepare the iodide salts. The advantages of introducing ether groups into the side chains were also outlined in that work. The tetrafluoroborates characterized previously were obtained using AgBF4 solutions in acetone to precipitate AgI from solutions of the iodides in acetone or acetonitrile, followed by removal of solvent by rotary evaporation and vacuum oven drying. In the present work, the methoxyethyl dimethyl ethylammonium salts of tetrachlorogallate and tetrachloroferrate were prepared in a nitrogen-filled drybox by mixing stoichoimetric methoxyethyl dimethyl ethylammonium chloride and gallium(III) chloride or iron(III) chloride at room temperature. The other quaternary ammonium salts and the 1-n-butyl-3methyl imidazolium salts were produced by refluxing the onium chlorides with lithium or sodium salts of different anions in acetonitrile solutions. This inexpensive procedure, which will be described in more detail elsewhere,27 yielded products without NMR-detectable Na+ impurities when sodium salts were used in the preparation. No NMR resonances characteristic of residual water or solvents could be detected by NMR spectroscopy. The water-stable ionic liquids (both hydrophilic and hydrophobic) synthesized in this manner were checked for residual chloride using aqueous AgNO3 solutions, and no AgCl precipitate could be detected in hydrophobic ionic liquids and hydrophilic ones that were prepared using sodium salt intermediates. When lithium salt intermediates were used, very weak Li NMR signals could be seen and traces of residual chloride were detected (with the exception of LiTFSI and LiPF6, because of water washing in the procedure). However, these impurities in the ILs were insufficient to affect the measured physical properties. Notation. In this paper, we will use the notation indicated at the start of the previous section for the quaternary ammonium salts of this study. However, we take note of an alternative notation that has some advantages, namely, MOMNM2E ) N(1-O-1)112, MOENM2E ) N(1-O-2)112, etc. This latter notation
J. Phys. Chem. B, Vol. 107, No. 25, 2003 6171 would be more consistent with previous usage19,23 and is more adaptable to the cations with longer alkyl groups. It will be used in one of the summary diagrams, where economy of space becomes an issue. For imidazole-based cations, we will use B and M to denote the n-butyl and methyl side groups, respectively; thus, BMI+ represents the 1-n-butyl-3-methyl imidazolium cation. For pyridine-based cations, we will use B to represent n-butyl-capping; thus, BPy+ represents the n-butylpyridinium cation. Glass-Transition Temperatures. The Tg values of most of the ionic liquids of this study were determined using a differential scanning calorimeter (model DSC-7, Perkin-Elmer) in the temperature range from -150 °C to 80 °C. The instrument temperature scale was calibrated at the crystal-crystal transition of cyclopentane (-151.16 °C) and the melting point of indium (+156.60 °C). Samples were sealed in aluminum pans and scanned at a rate of 20 K‚min-1 in a helium atmosphere. For the quaternary ammonium salts of TFSI-, except for MOENM2E-TFSI, the values of Tg were determined using a simple differential thermal analysis (DTA) instrument in the temperature range from -150 °C to 80 °C. The instrument was calibrated in an ice-water mixture (T ) 0 °C), at the solidsolid transition temperature of anhydrous cyclohexane (99.5%, TS1fS2 ) -86.6 °C) and at the melting point of anhydrous ethylbenzene (99.8%, Tm ) -95.0 °C). Samples were sealed in Pyrex glass tubes in a drybox filled with argon, quenched in liquid nitrogen, and then scanned during heating at ca. 20 K‚min-1. The Tg values reported are the onset values for upscans. Conductivities. The conductivities of the ionic liquids were measured by the standard complex impedance method, using a Hewlett-Packard model HP 4192A LF impedance analyzer in the frequency range from 5 Hz to 1 MHz. We used sealed diptype cells containing two parallel platinum disks. The cell constants were 0.5-0.7, calibrated with a 0.1 m KCl aqueous solution to a precision of 0.2%. Measurements were made during controlled slow cooling from 120 °C to -50 °C. The cooling rate was low enough that the data points differed negligibly from those obtained in static isothermal measurements. Viscosities. The dynamic viscosities of the quaternary ammonium salts of BF4 , GaCl4 , and I were obtained using a Wells-Brookfield cone-plate digital viscometer, which has the advantage that only small samples are required and the disadvantage that it is not a high-precision device. The optimum volume of 1.0 mL was determined using o-terphenyl as the calibrating fluid, and values that agreed with the literature data to better than 5% at high temperatures (considerably better at T < 100 °C) were obtained using a model CP-40 conic spindle (Wells-Brookfield). The kinematical viscosities of other ionic liquids were measured between ambient temperature and 150 °C, using Cannon-Ubbelohde viscometers. CaCl2 drying tubes were used to protect the samples from moisture in the air. The viscometers were placed within a tall, aluminum, temperature-smoothing block, and the temperature of the sample was maintained for half an hour before measurement. The precision of measurement with Cannon-Ubbelohde viscometers is controlled by the reproducibility of flow times, and the accuracy is controlled by the accuracy of the calibration constants and by the temperature measurement. Precision was limited at the highest temperatures (at >100 °C) by the short flow times (
