Pressure-Induced Spectral Changes of Room-Temperature Ionic

Sep 12, 2011 - Yusuke Imai and Hiroshi Abe. Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, ...
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Pressure-Induced Spectral Changes of Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium Bis(trifluoromethylsulfonyl)imide, [DEME][TFSI] Yukihiro Yoshimura* and Takahiro Takekiyo Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

Yusuke Imai and Hiroshi Abe Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan ABSTRACT: We have investigated pressure-induced Raman spectral changes of ionic liquid, N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) up to 5.5 GPa. We found that the CH stretching spectrum of [DEME] cation changes significantly above 4 GPa together with the large shift of conformational equilibrium between C1 and C2 conformers of [TFSI] anion toward C2 one. The results along with the visual observations show that no crystallization occurs even at 5.5 GPa. We suppose that structural organization in [DEME][TFSI] might significantly change at 4 to 5 GPa region.

1. INTRODUCTION Room-temperature ionic liquids (RTILs) consisting of organic cations and inorganic anions remain in the liquid state at room temperature.1,2 A bulky, asymmetric organic cation prevents ions from packing and the solidification. Therefore, it is most probable that the liquid structure of RTILs results from a balance between long-range Coulomb electrostatic forces among the constituent ions and local geometric factors. The merit of use of pressure as a variable function is to change, in a controlled way, the intermolecular interactions without encountering the major perturbations such as produced by changes in temperature and chemical composition.3 Specifically, pressure can work mainly density changes and the contribution to activation energy thorough the activation volume might be small, whereas temperature varies both activation energy and density. Under high-pressure conditions, both attractive and repulsive sides of the intermolecular potential should be explored depending on the applied pressure. In this situation, an intriguing question is raised: if we compress the RTIL using pressure as an external factor, then what happens to the phase behavior? Does crystallization occur or does it hold the liquid state up to very high pressure? Recently, there has been growing interest in the works under high pressure,4 10 but most of the studies so far concern the results of binary systems such as RTIL + water mixed solutions up to at most 2.5 to 3 GPa. The aim of the present Article is to show the phase stability of pure RTIL at high pressures over 5 GPa. r 2011 American Chemical Society

In our preceding studies, we reported the phase transition behavior of two typical (prototype) imidazolium-based ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4])11 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]), 12 at room temperature. We showed that [bmim][PF6] easily crystallizes upon compression, but [bmim][BF4] maintains the liquid state over 1 GPa. The results indicate that a contribution of the anion is of importance to the phase stabilities at high pressures.13 An aliphatic quaternary ammonium-based ionic liquid, which has a flexible methoxyethyl group on the nitrogen atom combined with the bulky bis(trifluoromethylsulfonyl)imide anion, N,N-diethyl-N-methylN-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide (denoted as [DEME][TFSI]) is known to have a wide potential window and high ionic conductivity.14,15 Here we have demonstrated that [DEME][TFSI] does not crystallize over 5 GPa at room temperature, but a pressure-induced change in the structural organization might occur around this pressure range. Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: June 6, 2011 Revised: August 24, 2011 Published: September 12, 2011 2097

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Figure 2. Raman-frequency shifts of [DEME] cation against the pressure. The numbers (1∼4) correspond to the peaks of [DEME] cation in Figure 1. Figure 1. Raman CH stretching spectral changes of [DEME][TFSI] at room temperature as a function of pressure.

2. EXPERIMENTAL METHODS We used N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide, [DEME][TFSI] (Kanto Chemical), as RTIL in this study. This RTIL is hydrophobic,14 and the as-received [DEME][TFSI] sample hardly contains water, whose concentration was doubly checked to be 18.7 ppm on the basis of a Karl Fischer titration method. Raman spectroscopy is often used to explore bonding structures of liquids because it provides information on the local structure in the liquid state. The observation of the C H stretching vibration can serve as a useful probe to reflect the structural change of RTILs.4 High-pressure Raman spectra were obtained by a JASCO NR-1800 spectrophotometer with a diamond anvil cell (screw type DAC: SR-DACKYO3-3d, Kyowa). The DAC was mounted under the microscope of a JASCO NR-1800 spectrophotometer. In the DAC, few ruby chips and the sample were sealed by a stainless-steel gasket and paired diamond anvils. To estimate the pressure, we applied the relationship between the pressure and the spectral shift of the R1 fluorescence line of the ruby chip.16,17 The 514.5 nm line of argon ion laser excitation (∼350 mW) was typically used. For complementary measurements, we investigated temperature-induced Raman spectral changes at a normal pressure. The measured temperatures were controlled by LINKAM THMS-600 (Japan Hightech). The cooling rate was ∼5 K min 1. The obtained Raman spectra were fitted with Gaussian Lorentzian mixing function using the GRAMS/386 software (Galactic Ind.) for analyzing the spectral data. The detailed procedures were basically the same as those of previous studies.11 13 3. RESULTS AND DISCUSSION 3.1. Pressure-Induced Raman Spectral Changes of [DEME] [TFSI]. First, we discuss the Raman spectral changes of [DEME]

cation as a function of pressure, as shown in Figure 1. The wavenumber region from 2750 to 3150 cm 1 is the CH stretching mode (νCH) due to the alkyl chain of [DEME] cation. Although the detailed peak assignments of the νCH modes for [DEME] cation are not available at present, and thus the spectra present difficulties for quantitative interpretation, four pronounced peaks (1∼4) are observed as shown in Figure 1.

Notably a large change in the νCH spectrum was obtained over 4 GPa. At this pressure, two peaks at around 2925 and 2975 cm 1 designated as 3 and 4 merge into one broad band that becomes sharper upon further compression. The spectral resolution of each peak becomes no longer clear, and the spectra show a completely different feature from the one at 0.1 MPa. Another intriguing feature is that the complete reversibility of the pressure effects could be seen if the pressure is released to 0.1 MPa. Figure 2 illustrates the band frequency of the respective peaks in Figure 1 against pressure. Upon compression, the whole νCH band shifts to higher frequencies, which may arise from the contraction of C H bonds and the overlap repulsion effect enhanced by hydrostatic pressure.5 Notable feature is that the peak 1 becomes unclear with increasing pressure and seems to disappear near 1.8 GPa. It is most likely that these spectral features with changes in pressure arise from changes in geometrical properties of the bonding network.5 The observations shown in Figures 1 and 2 may indicate that a pressure-induced (local) structural change occurs in [DEME] cation, although the application of pressure leads to a linear shift of the alkyl C H signal to higher frequencies without any discontinuous jumping. The observed peak broadening may be not sufficient to support the structural change, but at least it is not likely to originate from the frequency shift of main C H bands (3 and 4 in Figure 1) versus pressure because both frequency shifts show a similar trend against pressure. The visual observations in the sequence of elevated pressures are shown in Figure 3. We note that apparently no boundaries indicating crystal domains were observed in the sample texture with increasing pressure up to 5.5 GPa. A conformational equilibrium of [TFSI] anion is known to exist between trans (C1) and cis (C2) conformers in the liquid state,18,19 as shown in Figure 4. The pressure-induced spectral changes in the lattice vibrational region due to the [TFSI] anion are shown in Figure 5. Although some vibrational assignments of [TFSI] anion have been previously discussed,18 20 the spectroscopic distinction between the two conformers of the anion is a somewhat difficult task. As stated by Lassegues et al.,19 the 260 360 cm 1 region is more adequate than the 380 460 or 730 760 cm 1 region for quantitative analysis because the presence of mixed cation and anion contributions in the latter regions and the broad profiles in the liquid state obscure the anion lines to which is very close, and their fitting should be highly dependent on the selected band shape. Therefore, we may 2098

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Figure 3. Photomicrographs in the sequence of elevated pressures. The p T conditions were 0.1 MPa to 5.5 GPa and 298 K.

Figure 4. Optimized structures of C1 and C2 conformers of [TFSI] anion by B3LYP/6-311G+(d) level.

Figure 6. Pressure dependence of the intensity ratio between the C1 and C2 conformers of [TFSI] anion estimated from the results of Figure 5.

Figure 5. Raman spectral changes due to the [TFSI] anion as a function of pressure at room temperature. The spectra can be represented by several mixed Gaussian Lorentzian curves. Representative band component analyses of Raman bands are shown as dashed curves.

safely use the 260 380 cm 1 region for the band analysis of the conformational equilibrium. The C1 conformer is well-characterized by a doublet at 320 and 326 cm 1 without any significant

interference from the C2 conformer. The C2 conformer can be well-identified by a separated band at 295 cm 1 and another band at 334 cm 1. Using this criterion, in an effort to gain further insight into the local structures, we investigated changes in the conformational equilibrium of [TFSI] anion. The values of the ratio {(I295 + I334)/(I320 + I326) = C2/C1)} are plotted in Figure 6. At a normal pressure and room temperature, the population of C2 conformer is reported to be more favorable than the C1 one,18 21 which is in agreement with the present results. Basically, the C2/C1 increases monotonically with increasing pressure. At ∼1.8 GPa, the slope of the C2/C1 against pressure looks to begin changing. Then, when the pressure was elevated at ∼4 GPa, we can see a clear inflection point. The dependence against pressure becomes even more constant upon further compression. Additionally, the change in the peak frequency of anion in contrast with the case of the cation is useful. Basically, all frequencies of [TFSI] anion shift to higher frequencies with increasing pressure without any discontinuous jump, as in the case of νCH of [DEME] cation. These results indicate that the [DEME] cation and [TFSI] anion show a 2099

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Figure 7. Comparison of the Raman spectra of [DEME][TFSI] at various p-T values. (a) The lattice vibrational region and (b) the CH stretching region.

cooperative change against pressure. An external vibrational mode at lower frequency (50 200 cm 1) in the Raman spectra might provide further useful information, but it is unfortunately outside of the limit accessible to the spectrophotometer used in this study. These changes could indicate that a significant structural change might occur under the conditions somewhere around 4 to 5 GPa region. Here we have to mention about the nature of this state. Application of high pressure is an ideal tool to tune the bonding properties of the materials. If a system is compressed, the reaction will adjust to favor the components with smaller volume. As already mentioned, the higher-frequency shift may originate from the overlap repulsion effect enhanced by hydrostatic pressure. The pressure-enhanced interactions such as C H 3 3 3 N, C H 3 3 3 O, C H 3 3 3 F are a compensatory mechanism to provide the stability. Aggregation formation may be favored with increasing pressure instead of the isolated form.6 It is interesting to refer that imidazolium-based RTILs show a nanoscale spatial heterogeneity (nanophase separation) at a normal pressure where an anion and a positively charged imidazolium ring pair up, and the pairs aggregate and form polar domains.22,23 Alkyl side chains of imidazolium cations also aggregate and form nonpolar domains. This domain structure due to the separation of each polar/nonpolar domain is a cause of the heterogeneity. The nanophase separations exist when an imidazolium cation has over 4 (i.e., longer than butyl chain) on the alkyl side chain.10,11 The “nanoheterogeneity” may have an influence on the nonequilibrium phase behavior. We do not know exactly that the same phenomenon can be applied to the case of an aliphatic quaternary ammonium-based ionic liquid, but we suspect that the broad band features at 4 to 5 region in Figures 1 and 5 correlate with any changes in the nanoscale spatial heterogenity upon compression, although this is just a speculation. Unfortunately, the detailed liquid structure of [DEME][TFSI] has apparently not been reported as yet. Therefore, it is difficult to say more about what the referred local structure is at present, and this is beyond the present study. 3.2. Comparison with Temperature-Induced Raman Spectral Changes. A complementary insight concerning the responses in the temperature changes of the spectra might be beneficial. Figure 7 shows the (a) lattice vibrational and (b) νCH stretching Raman spectra of [DEME][TFSI] at 125 K and 0.1 MPa. For a comparison, the spectra of 0.1 MPa and 5.5 GPa at 298 K are also shown in the same Figure. [DEME][TFSI] is easy to form the glassy state at low temperatures upon cooling.

Actually, in a separate measurement, we observed that [DEME] [TFSI] has a glass-transition temperature at 182 K. Therefore, the Raman spectrum at 125 K should correspond to the glassy state of [DEME][TFSI]. We can see that the glassy spectrum at 0.1 MPa basically shows a similar feature to that of the liquid state at 298 K and 0.1 MPa. However, the spectrum at 5.5 GPa is totally different from that at 125 K (and also that at 298 K and 0.1 MPa). Clearly the structure at 5.5 GPa and 298 K is different from that of the glassy state at 0.1 MPa and 125 K, where we suppose that the ions consisting of this RTIL may begin to interact with each other and largely organize themselves by enhanced molecular packing. In this way, there are large distinctions in the phase behavior in response to the external functions, that is, pressure and temperature. It is, however, to be noted that the values of C2/C1 are comparable between the two p-T values; C2/C1 values are 2.4 ( 0.05 at 5.5 GPa and 298 K and 2.6 ( 0.05 at 0.1 MPa and 125 K. The results mean that cis (C2) conformers are favorable to trans (C1) ones under such extreme conditions and suggest the similarity of the molecular conformation of [TFSI] anion under both conditions. Looking into the results, we find the differences in the Raman spectra between 298 K (C2/C1: 1.3 ( 0.05) and 125 K under a normal pressure in the range of 250 350 cm 1 in Figure 7a, which are also ascribed to the conformational change between C1 and C2 conformers. We point out that under ambient pressure condition the overall spectral shape of [DEME] cation does not change on going from the liquid state at 298 K to the glassy sate at 125 K. This is in contrast with the fact that the environment around the alkyl chains of [DEME] cation is largely perturbed with applying pressure as well as the anion. Next, we calculated the volume change between C1 and C2 conformers (ΔVC1fC2). From the pressure dependence of the relative Raman intensities of the conformers (C2/C1) in Figure 6 (curve-fitted by a quadratic function), the partial molar volume (PMV) change of ΔVC1fC2 is determined to be 0.69 ( 1.46  10 5 cm3/mol at 0.1 MPa. Interestingly, this value is the same order with the PMV changes associated with the transformation from trans to gauche conformers of other ionic liquids ([emim] [BF4] and [bmim][BF4])24 and small organic compounds such as dihaloethane25 and tetraethylammonium salts (Et4NBr).26 Concordant with the results of a conformational equilibrium of [TFSI] anion in Figure 6, the ΔVC1fC2 changes from a negative value to a positive one near 4 GPa. This result may show that the molecular orientation in [DEME][TFSI] significantly changes above 4 GPa. 2100

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The Journal of Physical Chemistry C Because the spectra at high pressures of 4 to 5 GPa are quite different from those at ambient pressure, [DEME][TFSI] may take another glass-like structure induced by pressure. Glassy states generally show a hysteresis depending on external fields and the exerting the rate of the field. We found that there was a pressure hysteresis when applying and releasing pressure processes. On the other hand, as other supplementary information, no “cold crystallization”27 occurred upon heating from the glassy state to a room temperature at 0.1 MPa. The identification of the absolute nature of the possible phase requires further continuous studies.

4. CONCLUSIONS We have demonstrated large pressure-induced spectral changes of [DEME][TFSI] at room temperature by a Raman spectroscopy. We found that [DEME][TFSI] withstands forming the crystalline state by applying pressure of 5.5 GPa accompanied by large spectral changes of both the constituent ions. Under high pressure, intermolecular interactions in the RTIL that provide its structure might be largely modified by a change in the bonding properties along with the molecular packing. We believe that the present study gives useful insight into aggregation behavior in the RTIL at severe conditions. The questions of what kind of clusters are responsible and how the intermolecular bonding looks in detail may be settled by molecular dynamics simulation study. A detailed understanding of the phase behavior/diagram of ionic liquids is of high importance to further extend the range of applications for these important materials.

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(16) Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. Appl. Phys. 1978, 49, 3276. (17) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res. 1986, 91, 4673. (18) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayasi, Y.; Ishiguro, S. J. Phys. Chem. B 2006, 110, 8179. (19) Lassegues, J. C.; Grondin, J.; Holomb, R.; Johansson, P. J. Raman Spectrosc. 2007, 38, 551. (20) Herstedt, M.; Smirnov, M.; Johansson, P.; Chami, M.; Grondlin, J.; Servant, L.; Lassegues, J. C. J. Raman Spectrosc. 2005, 36, 762. (21) Herstedt, M.; Henderson, W. A.; Smirnov, M.; Ducasse, L.; Servant, L.; Talaga, D.; Lassegues, J. C. J. Mol. Struct. 2006, 783, 145. (22) Wang, Y. T.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (23) Lopes, J. N. A. C.; Pauda, A. A. H. J. Phys. Chem. B 2006, 110, 3330. (24) Takekiyo, T.; Imai, Y.; Hatano, N.; Abe, H.; Yoshimura, Y. Chem. Phys. Lett. 2011, 511, 241. (25) Taniguchi, Y.; Takaya, H.; Wong, P. T. T.; Whalley, E. J. Chem. Phys. 1981, 75, 4815. (26) Takekiyo, T.; Yoshimura, Y. J. Phys. Chem. A 2006, 110, 10829. (27) Yoshimura, Y.; Goto, T.; Abe, H.; Imai, Y. J. Phys. Chem. B 2009, 113, 8091.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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