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The Complex Dance of the Two Conformers of Bis(trifluoromethanesulfonyl)imide as a Function of Pressure and Temperature F. Capitani,†,§ S. Gatto,‡ P. Postorino,† O. Palumbo,‡ F. Trequattrini,†,‡ M. Deutsch,§ J.-B. Brubach,§ P. Roy,§ and A. Paolone*,‡ †

Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 5, 00185 Roma, Italy CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy § Synchrotron SOLEIL, 91192 Gif Sur Yvette, France ‡

S Supporting Information *

ABSTRACT: Absorbance spectra of two ionic liquids, the short alkyl chain N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA−TFSI) and the longer chain N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA−TFSI) are reported as a function of pressure and temperature. The occurrence of various phase transitions is evidenced by the changes in the relative concentration of the cisoid and transoid conformers of their common TFSI anion. The infrared spectrum of TMPA−TFSI was measured at 300 K with an applied pressure varying over the 0−5 GPa range. Above 0.2 GPa only the trans conformer is detected, suggesting the occurrence of a pressure induced crystallization. When pressure is applied to TMHA−TFSI at T = 310 K, both TFSI conformers subsist up to ∼11 GPa. However, the clear change of their intensity ratio observed around 2 GPa, suggests the onset of a glass phase as supported by measurements carried out at 4.2 GPa along a cooling/heating cycle. A careful analysis of the spectra collected along different p−T thermodynamic paths shows the occurrence of a cold crystallization at 295 K on heating from 139 K along the p = 0.5 GPa isobar. The rich phase diagrams of the two ionic liquids is the result of the competition among the anion−cation intermolecular interactions, the lower energy of trans-TFSI with respect to cis-TFSI and the smaller volume of cis-TFSI with respect to trans-TFSI.

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has a C1 symmetry,8 however being the energy separation 2.2 kJ/mol only, they are both present in the liquid state.8−10 The experimental study of ref 1 was conducted both in the liquid and in the solid phase in order to ascertain the changes of the intramolecular structure induced by the crystallization/melting process. The comparison with ab initio calculations of the infrared-active intramolecular vibrations1 has allowed the assignment of the experimental lines to the various ions composing the ionic liquids. Moreover, the two TFSI conformers present different Raman and infrared spectra.1,8−13 The liquid phases of the samples are all characterized by the presence of both TFSI conformers,1 as already reported for other TFSI based ILs.8−13 However, the solid phase shows different behavior for the two compounds: in solid N-trimethylN-propylammonium-TFSI (TMPA−TFSI), the relative concentration of the conformers is strongly shifted toward a predominance of the transoid conformer; on the contrary, the solid N-trimethyl-N-hexylammonium-TFSI (TMHA−TFSI)

INTRODUCTION Among the various ionic liquids (ILs), currently largely studied in view of their possible applications, ammonium-based ILs exhibit several interesting properties. Owing to their better chemical stability in comparison with pyridinium and imidazolium- based ILs and as they are commercially available, they find successful applications as catalysts, solvents, lubricants, gas capture agents, coating materials, or chemical sensors.1−7 We recently reported the temperature dependence of the infrared absorption spectra of two ionic liquids with bis(trifluoromethanesulfonyl)imide (TFSI) as anion and ammonium with different alkyl chains as cations.1 The shared TFSI anion, is a flexible molecule that can adopt two energetically inequivalent conformations, the transoid and the cisoid forms, whose concentration in the liquid or solid phase affect the ILs physical and chemical properties. Indeed, the molecular configurations of anions and cations is the key point to understand the dependence of the macroscopic properties of the ILs on their ionic composition and therefore to tailor them according to the requested application. The transoid conformer with a C2 symmetry is more stable than the cisoid one, which © XXXX American Chemical Society

Received: December 22, 2015 Revised: January 28, 2016

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DOI: 10.1021/acs.jpcb.5b12537 J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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retains only the cis-TFSI, which is the less thermodinamically stable rotamer. A possible explanation for the retention of the less stable conformer in solid TMHA−TFSI could be found in the different anion−cation interaction occurring in the two compounds with different length of the alkyl chain. The concentration of different conformers of TFSI is strictly related to the possible interactions experienced by the ion, and indeed it is also affected by the interaction with polymer membranes, as shown by recent IR measurements combined with ab initio calculations on systems composed by PYR14− TFSI based IL swelling a polyvinylidenefluoride membrane.13 In the pure IL, both conformers of TFSI are detected in the liquid, supercooled and glass phases, while only the transconformer is retained in both solid phases. In contrast, when the ionic liquid swells the membrane, both conformers are retained in all the physical phases of the system showing that the interaction between the polymer and the ionic liquid inhibits the complete transformation of TFSI into the transconformer in the solid phases. Indeed, calculations suggested that the interaction with the PVdF chain makes the cis-TFSI energetically favored in the ILs layers in close contact with the membrane.13 A fine and clean tuning of intermolecular interactions can be attained by applying an external pressure.14−17 As a matter of fact, volume compression shorten the average distances among the ionic charges thus enhancing the direct Coulomb interaction, with a possible competition with the steric hindrance of both cation and anion.17 Indeed, it has been reported that the pressure variation can induce liquid-crystal and crystal phase transitions.14−19 In the case of ionic liquids based on the N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium [DEME] cation, the crystallization was observed at 1.2 GPa for the case of tetrafluoroborate (BF4−) anion,18 whereas no crystallization was detected for the TFSI anion up to 5.5 GPa,14 but a change in the conformational equilibrium of the TFSI anion above 4 GPa suggested the possible occurrence of a glass state.14 Similarly, it was reported that at room temperature 1-butyl-3methylimidazolium hexafluorophosphate ([bmim-PF6]) easily crystallizes upon compression, whereas changing the anion to tetrafluoroborate, bmim-BF4 maintains its liquid state over 1 GPa,19 indicating that the anion contribution is of importance to the phase stabilities at high pressures. On the other hand, in analogy with the cold crystallization observed when supercooled ionic liquids are reheated, high pressure Raman experiments showed that DEME-BF4 can be overpressurized and enters a superpressed liquid state at ∼3.3 GPa. An unusual crystallization could then be induced by releasing the pressure of the superpressed liquid.15 In this framework, the present work is aimed at exploring the effects induced by the variation of pressure and temperature conditions on the physical state and the microscopic conformations in ILs. In particular, we report a detailed study of the infrared spectrum of TMPA−TFSI and TMHA−TFSI as a function of pressure and temperature, showing that the difference in the alkyl chains length induces different behaviors of the ILs physical state as well as of the relative concentrations of the TFSI conformers. The measurements of ILs infrared absorbance as a function of both p and T are scarce; we will demonstrate here that they can provide extremely reliable and valuable information about the interplay among intermolecular interactions and steric hindrance effect in ILs.

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EXPERIMENTAL SECTION

TMPA−TFSI, with a declared melting temperature Tm = 15 °C (288 K), and TMHA−TFSI with Tm = 270 °C (300 K) were purchased from Solvionic. The purity of those samples was higher than 99.9%, and therefore, no further purification was used before measurements. The samples here investigated are from the same batch already studied as a function of temperature in ref.1 The structure of the ions composing the ILs is reported in Figure 1.

Figure 1. Schematic view of the ions composing the ionic liquids.

Infrared spectroscopy measurements were performed by means of a Bruker 125 HR spectrometer at the AILES beamline of SOLEIL exploiting synchrotron radiation.20,21 For these measurements at a spectral resolution of 0.5 cm−1, the interferometer was equipped with a 6 μm Mylar beamsplitter and a wide band bolometer from Infrared Lab. Absorbance measurements through a diamond anvil cell (DAC) from BETSA were obtained thanks to a new developed setup for reflection/transmission experiments as a function of both pressure and temperature. The pressure was detected by in situ measurements of the fluorescence of ruby balls placed inside the DAC, while temperature was measured by means of a Si diode placed in contact with the cell, close to the sample.

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RESULTS AND DISCUSSION TMPA−TFSI vs p. The dependence of the infrared (400− 1000 cm−1) spectrum of TMPA−TFSI on pressure in the range 0.0−5.1 GPa is reported in Figure 2. The temperature is fixed at 300 K. In the following all pressures are overpressures with respect to ambient p. At p = 0.0 GPa, one can observe intense

Figure 2. Absorbance of TMPA−TFSI at 300 K as a function of pressure. B


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However, a change of the relative contributions of the two rotamers is evident, with the bands of the trans conformer becoming less intense as p increases. To quantify the relative concentration of the two conformers, we considered the relative ratio of the intensity of bands due to the two conformers as a function of pressure: I + I656 I r = 602 = cis I620 Itrans (1)

bands centered around 407, 517, 572, 602, 618, 655, 741, 763, 790, 892, 945, 967 cm−1. The last three were attributed to the vibrations of the cation,11 while all the others are due to the anion movements.11 A detailed attribution of the lines is reported in Table S1 of the Supporting Information. In particular, the bands around 602 and 655 cm−1 are ascribable to the cis conformer of TFSI, while the line around 618 cm−1 is due to the trans rotamer of TFSI.1,11 In the liquid state both conformer of TFSI are present,1,11 consistently with the spectrum collected at p = 0 GPa (ambient pressure). On increasing p, all bands shift toward higher frequencies, as expected. At 0.1 GPa the bands at ∼602 and 655 cm−1 due to the cis conformer are still present, but, on further increase of the pressure they become undetectable, while all other lines are retained. Both TFSI conformers have been usually observed in the liquid and in the glass state of many TFSI-based ILs.1,8,10−13,22 The lack of the cis conformer in the high pressure spectra, or at least the strong decrease of their band intensities, thus suggests the transition toward a solid phase where the lower energy trans conformer is retained, as already observed in the solid state of ILs containing TFSI.1,13,22 As a confirmation, the bands in the frequency range between 550 and 700 cm−1, at high pressure are the same reported for the low temperature crystalline phase of TMPA−TFSI.1 However, in the previous study conducted at p = 0 GPa at variable T, the persistence of a small concentration of cis-TFSI in the crystalline phase was inferred from the behavior of the bands centered around 200 cm−1, which are not accessible in the present study. These findings coherently suggest that the solid phase obtained at p = 0 GPa below 270 K1 and the one here observed at T = 300 K and p ≥ 0.2 GPa possess the same crystalline structure. TMHA−TFSI vs p. A remarkably different pressure behavior is shown by TMHA−TFSI. For this compound, measurements were conducted at 310 K in order to start from the liquid state (Tm = 300 K). At ambient pressure, well-defined bands centered around 514, 571, 602, 620, 656, 740, 762, 789, 893, 909, 927, and 959 cm−1 (see Figure 3) can be observed. The last four contributions were ascribed to the TMHA cation, while all others bands are due to the TFSI anion.1,11 A detailed attribution of the lines is reported in Table S1 of the Supporting Information. The lines due to the cis (at ∼602 and ∼656 cm−1) and trans (∼620 cm−1) conformers of TFSI are visible in all spectra collected between 0 and 11 GPa.

When pressure is applied, the conformer with the smaller volume becomes energetically favored and the difference of volume between the two conformer is given by23 ⎧ ln(r ) ⎫ ⎬ Vcis − Vtrans = −RT ⎨ ⎩ p ⎭T

where R is the gas constant, T and p are the temperature and pressure of the sample. The pressure dependence of ln(r) is shown in Figure 4. To avoid misinterpretation of the data, we limited our analysis to

Figure 4. Pressure dependence of the logarithm of the ratio of the intensities of the bands due to the two TFSI conformers and best fit lines. In the inset an example of the fit of the absorbance spectrum and deconvolution into the contributions of the two conformers is reported.

pressures lower than 4.1 GPa, where the single contributions of trans and cis conformers are easily discernible in the spectra. Two linear regimes below and above 2 GPa, are observed. For the lower pressures, the configuration with the cis conformer has a smaller volume by 0.41 ± 0.05 cm3/mol, while at higher pressures Vtrans − Vcis = 0.7 ± 0.1 cm3/mol. The apparent change of the slope of ln(r) vs p at ∼2 GPa and the presence of both conformers up to the maximum pressure of 11 GPa can be interpreted as the onset of a glass phase above 2 GPa. Further evidence will be given in the next section where the effect of temperature will also be considered. TMHA−TFSI vs p and T. In order to discover suitable pressure and temperature conditions for the occurrence of a crystalline phase in TMHA−TFSI, the p and T of the TMHA− TFSI sample were changed following a pressure−temperature cycle as shown in Figure 5. In particular, from the condition T = 310 K and p = 11 GPa, the pressure was released down to 4.2 GPa in subsequent steps (red path in Figure 5). The spectra collected decreasing p practically superimposed on the corresponding spectra collected pressurizing the sample (measurements not shown). At 4.2 GPa, the sample was cooled down from 310 to 200 K (dark green path in Figure 5).

Figure 3. Absorbance of TMHA−TFSI at 310 K as a function of pressure. C


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GPa and T = 310 K) a series of spectra (see Figure 7) was collected on cooling down to 137 K at constant pressure

Figure 5. Schematic representation of the pressure and temperature cycles along which the absorbance spectra of TMHA−TFSI were collected and sketch of the derived phase diagram.

We notice that the spectra collected at the initial and the final temperatures, shown in Figure 6, are substantially coincident. In

Figure 7. Temperature dependence of the absorbance of TMHA− TFSI at a constant pressure of 0.7 GPa and an example of the fit quality (dashed line). In the inset the dependence of the logarithm of the relative concentration of the bands attributed to the two TFSI conformer versus 1/T is reported, together with the best fit curve.

(orange path in Figure 5). In all the spectra the markers of the two TFSI conformers (lines centered at 602, 620, and 655 cm−1) are observed and the conformer population evolves without anomalies on varying the temperature (Figure 7). Differently from what observed in TMHA−TFSI at ambient pressure where, below 300 K, a solid phase characterized by the presence of cis conformer only (602 and 650 cm−1) is found,1 no evidence of a phase transition is here observed. A quantitative analysis of the relative concentration of the two conformers as a function of temperature can be performed considering the quantity ln(r), where r is defined by eq 1. For liquids, a Boltzmann distribution of the conformers is expected and therefore

Figure 6. Absorbance of TMHA−TFSI measured at T = 300 and 200 K, with an applied pressure of 4.2 GPa.

1 ΔH ΔS + +c T R R where ΔH and ΔS are the enthalpy and entropy difference between the rotamers, in our case the two conformers of TFSI.8−10,12,13 The inset of Figure 7 reports a fit of the dependence of ln(r) vs 1/T. From the slope of the linear regression, one obtains ΔH = 4.7 ± 0.6 kJ/mol, a value compatible with the ones reported for other ILs containing the TFSI anion, which range between 3 and 7 kJ/mol.10,12,13 The occurrence of a distribution of the conformers between the two energy levels following the Boltzmann law supports the idea that for an applied pressure of 0.7 GPa the IL remains liquid even down to low temperatures. A different behavior would be indeed observed if the IL was either in a crystalline state, as only one conformer would be retained (or at least strongly predominant), or in a glass state, where the relative concentration of the rotamers would be constant as a function of T.12 Finally starting from T = 137 K and p = 0.7 GPa, the pressure was decreased to 0.5 GPa (pink path in Figure 5) and a series of spectra (see Figure 8) was collected at constant p increasing the temperature from 150 K up to 314 K (see gray line in Figure 5). In the temperature range between 150 and 290 K, the vibrational bands of both conformers are clearly visible in the spectra. However, on further heating, at 295 K the relative intensity of the lines of the cis conformer decreases and above ln(r ) = −

particular, the peak intensities of the bands centered at 602, 620, and 650 cm−1, which are the spectroscopic markers of cisand trans-conformers of TFSI, are almost the same in the two spectra. Therefore, the relative concentration of the two conformers does not change with temperature over the above range. To the best of our knowledge, all previous studies on conformers of TFSI in the liquid state indicated that both rotamers are present and that their temperature dependence is governed by the Boltzmann distribution.1,8,10−13 In the present case, the absence of a temperature dependence of the relative concentration of the TFSI conformers indicates the occurrence of a frozen-in solid phase at 4.2 GPa. It is worth to notice that a constant ratio of the conformer concentration as a function of the temperature was observed in N,N-dimethyl-N-ethyl-Nbenzylammonium bis(trifluoromethanesulfonyl)imide below 200 K, where DSC measurements identify the occurrence of a glass state.12 Combining our results on the pressure and temperature dependence (Figure 4 and Figure 5 respectively), the picture that emerges more consistently is that TMHA− TFSI enters a glass phase above 2 GPa at 310 K and that this phase is retained up to 11 GPa (at 310 K) and down to 200 K (at 4.2 GPa). The sample was at first heated back at 310 K keeping the pressure at 4.2 GPa and then depressurized at constant T down to 0.7 GPa (light green path in Figure 5). From this point (0.7 D


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ammonium-based ionic liquids compared to phosphoniumbased ILs.35 These authors reported that hydrogen bonding cannot account alone for the larger charge transfer in ammonium ILs, but Coulombic interactions between anion and cation should be considered.35 Moreover, the existence of mesoscale structures have been reported for ammonium based ILs.36,37 Within this scenario, when changing the external variables p and T, one must consider that the increase of p increases the concentration of the smaller volume conformer of TFSI, the cisoid one; on the other hand the trans conformer is the lower energy configuration, at least in the initial liquid state at ambient conditions. In this framework, the rich phase diagrams of the two ionic liquids is the results of the competition of various factors: the interactions between anion and cation, the lower energy of trans-TFSI with respect to cisTFSI and the smaller volume of cis-TFSI with respect to transTFSI.

Figure 8. Temperature dependence of the absorbance of TMHA− TFSI at a constant applied pressure of 0.5 GPa.

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CONCLUSIONS The investigation of the absorbance spectra of N-trimethyl-Npropylammonium-bis(trifluoromethanesulfonyl)imide (TMPA−TFSI) and N-trimethyl-N-hexylammonium- bis(trifluoromethanesulfonyl)imide (TMHA−TFSI) provides new information about the physical state of the two ionic liquids over a wide pressure−temperature region. Shorter chain TMPA−TFSI displays a rather simple behavior, with the occurrence of a crystalline phase at 300 K for p ≥ 0.2 GPa characterized by a predominant population of trans conformer. The present investigation obviously does not allow for a structural characterization; therefore, properly tailored high pressure diffraction studies are mandatory to characterize the new phase. A much more complex phase diagram is found for the longer alkyl-chain TMHA−TFSI, thanks to the detailed study of the relative concentration of the two conformers of TFSI. The comparative study of the pressure and temperature dependence of the measured spectra shows the onset of a transition to a frozen-in solid phase above 2 GPa at 310 K. The present findings, obtained by exploiting different thermodynamic paths, combined with literature results allow us to propose this as a glass phase. Lowering the pressure down to 0.7 GPa, the sample remains liquid down to the lowest temperature here investigated (139 K). Decreasing further the pressure to 0.5 GPa, a cold crystallization is observed on heating from 290 to 300 K. It is worth to stress that this is a new crystalline phase characterized by the presence of transTFSI, which is quite different from the one observed in the same compound at ambient pressure and low temperature. In conclusion, the detailed study of the relative concentration of the TFSI conformers obtained by means of infrared spectroscopy measurements seems to be a precious tool to unveil the complex phase diagrams of TFSI based ionic liquids. This new series of data provides a new set of parameters to tailor ILs with specific phase transitions: indeed, the IL state is governed by the competition among the interactions between anion and cation, the relative energy of the conformers and the volume size of anions.

300 K only the trans conformer is retained. This behavior indicates the onset of a solid phase on increasing the temperature. This phenomenon, called cold crystallization, is not unusual in ionic liquids; for example, it has been reported for PYR14−TFSI.13 As previously described, decompression induced crystallization was reported in DEME-BF4, where crystal polymorphism was also observed upon decreasing p.14 Remarkably, the solid phase obtained after the pressure− temperature cycle is different from that observed on cooling at ambient pressure where only the cis conformer of TFSI can be detected.1 Based on the previous measurements on TMHA−TFSI as a function of pressure and temperature a tentative phase diagram has been reported in Figure 5. One can note that it is extremely reach and complex, with the occurrence of various phases: crystalline, liquid, glass and a region of cold crystallization. The great variety and complexity of the phase diagrams of ionic liquids is witnessed by the cases reported in the introduction, concerning mainly the behavior of ILs as a function of temperature or pressure.1,8−19 Moreover, further complexity of the phase diagrams has been reported for mixtures of ILs, mixtures of ILs with water or alcohols, or IL microemulsions.24−33 Some final remarks about the differences in the phase diagram of the two ionic liquids can be drawn from our results. It can be argued that the length of the alkyl chain is mainly responsible for the remarkable differences observed between the two ILs over the wide pressure−temperature region here investigated. Indeed, the shorter chain TMPA−TFSI, is crystalline at p = 0 GPa and low T where the concentration of lower energy trans-conformer of TFSI is extremely high, while the crystalline state of long chains TMHA obtained in the same external conditions (i.e., p = 0 GPa and T < Tamb) contains cis-TFSI only. This result has been previously attributed to the different interactions of anion and cation in the two ILs. Moreover, starting from the liquid state, as a function of pressure, TMPA−TFSI easily transforms into a crystalline state, while TMHA−TFSI undergoes a glass transition. Previous studies about the cation−anion interactions in ammonium-based ionic liquids pointed out that they possess a larger steric effect compared to phosphonium-based ILs34 and that reducing the length of the alkyl chain allows for a more open structure of the cation which permits the anion to more strongly interact with the cation.35 Moreover a larger degree of charge transfer between anion and cation was evidenced in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b12537. Assignment of experimental vibration lines (PDF) E


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AUTHOR INFORMATION

Corresponding Author

*(A.P.) [email protected]. Fax: +39-0649694323. Telephone: +39-06-49914400. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank the European Commission for financial support for beamtime #20140817 at Synchrotron Soleil in the framework of the FP7 I3 CALIPSO Project.

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REFERENCES

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