J. Phys. Chem. B 2008, 112, 12913–12919
12913
From Ionic Liquid to Electrolyte Solution: Dynamics of 1-N-Butyl-3-N-methylimidazolium Tetrafluoroborate/Dichloromethane Mixtures Johannes Hunger, Alexander Stoppa, and Richard Buchner* Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Regensburg, D-93040 Regensburg, Germany
Glenn Hefter* Chemistry Department, Murdoch UniVersity, Murdoch, W.A. 6150, Australia ReceiVed: May 23, 2008; ReVised Manuscript ReceiVed: July 21, 2008
Dielectric spectra have been measured at 25 °C for mixtures of the room temperature ionic liquid 1-N-butyl3-N-methylimidazolium tetrafluoroborate (IL) with dichloromethane (DCM) over the entire composition range at frequencies 0.2 j ν/GHz e 89. The spectra could be satisfactorily fitted by assuming only two relaxation modes: a Cole-Cole process at lower frequencies and a Debye process at higher frequencies. However, detailed analysis indicated that both spectral features contain additional modes, which could not be resolved due to overlaps. The spectra indicate that the IL appears to retain its chemical character to extraordinarily high levels of dilution (xIL J 0.5) in DCM. At even higher dilutions (xIL j 0.3), the IL behaves as a conventional but strongly associated electrolyte. 1. Introduction The growth of publications and reviews dealing with room temperature ionic liquids (RTILs) has been phenomenal over the last two decades.1,2 While most of the earlier papers were naturally focused on synthesis and the use of RTILs as reaction media,3,4 there has been increasing interest in their physical, dynamical, and structural properties.5,6 Because the information that can be gained from a physicochemical study of a single RTIL is necessarily limited, most current studies either report data on sensibly related series of salts7-9 or on the effects of other variables such as temperature.10,11 Another way of gaining insights into the nature of RTILs is to study their mixtures with cosolvents whose properties are well-known. Such investigations are particularly pertinent to potential technical applications of RTILs, as reaction media for industrial-scale syntheses or in batteries, where they would not normally be present in neat form. Of the vast range of RTILs currently available, those containing substituted imidazolium cations have been the most intensively studied,5 but relatively few of these investigations have focused on their mixtures with cosolvents. According to Dupont,12 neat imidazolium-based RTILs form an extended hydrogen-bond network that strongly resembles the corresponding solid structures.13 Upon addition of a polar cosolvent, Dupont has proposed that this network breaks up, initially forming “supramolecular” aggregates of the constituent ions of the RTIL (Figure 1). With increasing dilution, such aggregates are succeeded in turn by triple ions (TIs), contact ion pairs (CIPs) and, ultimately, solvent-separated ion pairs (S-SIPs) and free ions.12 This scheme has intuitive appeal since, except perhaps for the first step, it is essentially the mirror image of what is known to occur in conventional electrolyte solutions with increasing concentration.14,15 However, while some of the features of Dupont’s scheme have been verified for some RTIL/ cosolvent mixtures16-18 others remain speculative. * To whom correspondence should be addressed. E-mail:
[email protected] (R.B.); g.hefter@murdoch. edu.au (G.H.).
Figure 1. Scheme of Dupont12 for the dilution of an ionic liquid by a cosolvent. The arrows represent increasing cosolvent content.
The range of techniques available to study the structure and dynamics of RTILs and their mixtures is limited. Thermodynamic (solubility, excess volume, etc.) and transport (conductivity, viscosity, etc.) measurements are useful but provide only indirect insights into the nature of solutions at the molecular level. The powerful spectroscopic techniques (NMR, Raman, etc.) on the other hand, generally provide information only about
10.1021/jp8045627 CCC: $40.75 2008 American Chemical Society Published on Web 09/19/2008
12914 J. Phys. Chem. B, Vol. 112, No. 41, 2008 short-range (bonding) interactions and have a specific weakness with regard to the detection of S-SIPs.19 Dielectric relaxation spectroscopy (DRS) is particularly suited to the investigation of the long- and medium-range ordering20 that is implied, at least in RTIL-rich solutions, by Dupont’s scheme.12 DRS also has unique abilities to detect and quantify the formation of all ion pair types in solution.14,15 A modest number of papers have reported the dielectric properties of neat RTILs,16,21-25 including some containing imidazolium cations.23-25 However, few such studies have been made on RTIL/cosolvent mixtures and these have been limited with respect to the range of frequencies and/or compositions investigated.16 This paper reports dielectric spectra of mixtures of 1-N-butyl3-N-methylimidazolium tetrafluoroborate ([bmim][BF4]) with dichloromethane (DCM) over a broad range of frequencies (0.2 e ν/GHz e 89) and at closely spaced intervals over the whole composition range. The imidazolium salt was chosen as a representative RTIL because some of its other properties (both neat and in mixtures with DCM) relevant to the present investigation have been reported26 and because it is stable (in the absence of water) and readily prepared in high purity. DCM was selected as the cosolvent because it is fully miscible with [bmim][BF4] at 25 °C26 yet sufficiently polar to support the formation of ion pairs and (to some extent) free ions.16 In addition, the dynamics of DCM are fast on the DRS timescale and it has a low static permittivity, which means its contribution over the frequency range of interest should be relatively small. 2. Experimental Section Materials. The room temperature ionic liquid [bmim][BF4] (hereafter IL), was prepared27,28 from purified reactants and dried under vacuum (p < 10-8 bar) at ∼40 °C for 7 days. Coulometric Karl Fischer titration indicated a water content of 1 THz.44,47 However, these make no significant contribution to the present spectra recorded at ν e 89 GHz and so have been omitted from the discussion. (43) Vij, K. J.; Hufnagel, F.; Grochulski, T. J. Mol. Liq. 1991, 49, 1. (44) Hunger, J.; Stoppa, A.; Thoman, A.; Walther, M.; Helm, H.; Buchner, R. in preparation. (45) A previous study of this compound23 described this mode in terms of a CD process. More recent measurements over a wider frequency range (up to 9 THz)47 indicate that a CC model is more appropriate. (46) Note, that in the light of recent measurements up to 9 THz,47 the mode at ∼400 GHz previously reported23 for [bmim][BF4] is clearly a weighted average of the contributions at ∼80 and ∼760 GHz. (47) Hunger, J.; Stoppa, A.; Thoman, A.; Walther, M.; Turton, D. A.; Wynne, K.; Helm, H.; Hefter, G.; Buchner, R. unpublished results. (48) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, C. J. Chem. Phys. 2003, 119, 464. (49) Schro¨der, C.; Haberler, M.; Steinhauser, O. J. Chem. Phys. 2008, 128, 134501. (50) Sangoro, J.; Iacob, C.; Serghei, A.; Naumov, S.; Galvosas, P.; Ka¨rger, J.; Wespe, C.; Bordusa, F.; Stoppa, A.; Hunger, J.; Buchner, R.; Kremer, F. J. Chem. Phys. 2008, 128, 214509. (51) The value of µ was calculated52 for the [bmim]+BF4- CIP using MP2-COSMO and assuming ε ) εDCM to produce a value relevant to the present mixtures. For [bmim]+, the value refers to the gas phase. (52) Zahn, S.; Uhlig, F.; Hunger, J.; Stoppa, A.; Buchner, R.; Kirchner, B. unpublished results. (53) Zahn, S.; Uhlig, F.; Thar, J.; Spickermann, C.; Kirchner, B. Angew. Chem., Int. Ed. 2008, 47, 3639. (54) Wulf, A.; Ludwig, R.; Sasisanker, P.; Weinga¨rtner, H. Chem. Phys. Lett. 2007, 439, 323. (55) Katsuta, S.; Imai, K.; Kudo, Y.; Takeda, Y.; Seki, H.; Nakakoshi, M. J. Chem. Eng. Data 2008, 53, 1528. (56) Solvent-shared ion pairs (with one intervening solvent molecule between the ions) have an approximate dipole moment of ∼30 D and are unlikely on the basis of the relatively weakly coordinating nature of DCM. Double-solvent-separated ion pairs are even less plausible. (57) Buchner, R. J. Mol. Liq. 1995, 63, 55. (58) Buchner, R. Pure Appl. Chem. 2008, 80, 1239. (59) Gutman, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum: New York, 1978.
JP8045627
