Phase Transitions of the Ionic Liquid [C2C1im][NTf2] under High

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Phase transitions of the ionic liquid [C2C1im][NTf2] under high pressure: a synchrotron X-ray diffraction and Raman microscopy study Luiz F. O. Faria, Thamires A. Lima, and Mauro C. C. Ribeiro Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00865 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Crystal Growth & Design

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Phase transitions of the ionic liquid [C2C1im][NTf2] under high pressure: a synchrotron X-ray diffraction and Raman microscopy study Luiz F. O. Faria*, Thamires A. Lima, Mauro C. C. Ribeiro* Laboratório de Espectroscopia Molecular, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000, Brazil * e-mails: [email protected], [email protected]

Abstract The interplay between crystallization and glass transition in the archetypal ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2C1im][NTf2], has been studied as a function of pressure up to ca. 12 GPa. Besides heterogeneous crystal nucleation, homogeneous nucleation in the sample inside the diamond anvil cell (DAC)

was

also

observed

depending

on

compression/decompression

rate.

Amorphization of the crystal and glass formation under pressure has been followed by synchrotron X-ray diffraction. The characteristic Raman bands of the [NTf2]- anion provide microscopic probe for the different phases. The crystalline phase is composed by [NTf2]- cisoid conformer, but moisture implies formation of crystal with the transoid conformer. Raman spectra show that crystalline phases might become microscopically heterogeneous because of [NTf2]- conformational disorder. Raman mapping reveals the order–disorder evolution from crystal to glass. Crystal of [C2C1im][NTf2] formed under high pressure and room temperature is similar to previously reported low temperature and atmospheric pressure crystal. Thus, it is concluded that density is the main factor 1 ACS Paragon Plus Environment


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controlling crystallization and glass formation under high pressure of [NTf2]- based ionic liquids due to hindrance of efficient ion packing. The results highlight that ionic liquids are good models to understand fundamental questions related to mechanism of crystallization and glass transition.

Introduction Liquids might be supercooled to temperatures well below the melting point becoming a glass on further cooling to the glass transition temperature. In contrast, if crystal nucleation is triggered in the supercooled liquid then the crystal growing process takes place until the conclusion of thermodynamic liquid-solid phase transition. From this point of view, crystallization and glass transition are competitive processes. Some theories indeed describes the glass transition in terms of avoidance of crystal nucleation and growth, for example, theories based on bond orientational ordering1,2 or geometric frustration.3 Even though crystal nucleation and growth are better described than glass transition phenomenon, e.g. by classical nucleation theory (CNT), many fundamental aspects still remain to be clarified.4-6 For example, the interplay between homogeneous and heterogeneous nucleation, crystal polymorphism, anomalies in crystal nucleation and growth kinetics, deviations from CNT, and so on. As a counterpart of changing temperature, pressure can be applied in order to induce phase transitions under isothermal condition. The comparison between variation of pressure or temperature allows discriminating between the role played by density and thermal effects on dynamics, structure, and phase transitions.7-9 Compression results in a superpressed liquid leading to crystallization or glass transition in analogy to cooling a

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liquid. However, superpressing has been less exploited in comparison to cooling probably because of experimental difficulties. Ionic liquids are molten salts in temperatures below 100 °C and many of them are good glass formers. A glassy or crystalline state of an ionic liquid is achieved depending on cooling rate and thermal history. Pressure effect on structure and phase transitions of ionic liquids have been investigated in the last years.10-24 The effect of pressure on the well-known segregation of polar and nonpolar domains in ionic liquids was studied with X-ray scattering experiments14 and classical molecular dynamics simulations.23 These works suggested reduction of nonpolar domains with increase of pressure due to folding of the alkyl chain of the organic cations.14,23 Phase transitions under high pressure (crystallization, solid-solid transition, and glass transition) have been reported for different combinations of cation and anion forming ionic liquids.1022,25

Concerning ionic liquids based on the anion bis(trifluoromethylsulfonyl)imide,

[NTf2]-, all of these previous studies reported formation of glass and no crystallization was observed during compression or decompression of the samples.13,15,18,22,26 In this work we investigated the high pressure phase behavior of an archetypical ionic

liquid,

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide,

[C2C1im][NTf2]. In the authors’ knowledge, it is the first report of the crystallization under high pressure of a [NTf2]- based ionic liquid. Using Raman spectroscopy and synchrotron X-ray diffraction experiments under high pressure with a diamond anvil cell (DAC), we showed that [C2C1im][NTf2] can crystallize or not depending on the compression rate. The unusual finding of homogeneous crystal nucleation in the sample inside the DAC has been observed depending on the history of pressure variation. Furthermore, the sample might become microscopically heterogeneous, and moisture also plays a role on the resulting crystalline phase. Figure 1 presents a scheme



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summarizing the complex phenomenology of phase transitions of [C2C1im][NTf2] discussed in this work. Proper to the slow dynamics of the crystallization process, it was also possible to perform a Raman microscopy imaging of the high pressure experiment and to evaluate the order-disorder in the borderline from crystal to glass. The results of this high pressure study are compared to the low temperature behavior of [C2C1im][NTf2]27,28 allowing for fundamental insights on the interplay between crystallization and glass transition.

Stepwise increase of pressure Crystal I Crystal amorphization

~ 0.4 – 1.2 GPa

*heterogeneous nucleation **non-dried sample: Crystal II

~ 12 GPa

Stepwise decompression Melting

Crystal I

~ 0.1 MPa

~ 1.2 GPa

Sudden increase of pressure Glass transition

~ 1.8 GPa

Sudden decompression Crystallization

Melting

~ 0.1 MPa

~ 0.1 MPa

*homogeneous nucleation

Stepwise decompression Melting

~ 0.1 MPa

Crystal I ~ 0.4 – 1.2 GPa

*heterogeneous nucleation **non-dried sample: Crystal II

Figure 1. Scheme summarizing the complex phenomenology of phase transitions of [C2C1im][NTf2].


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Experimental The ionic liquid [C2C1im][NTf2] was purchased from Iolitec and used without further purification, except by drying process under high vacuum (10-5 mbar) for 48 h before analyses. Samples without any drying treatment were used for some experiments. The water content determined by Karl-Fischer titration was 863 ppm for the sample from the flask and always less than 350 ppm after drying.

Pressure dependent X-ray diffraction (XRD) measurements were performed at Brazilian Synchrotron Light Laboratory (LNLS) using a homemade DAC having a diamond culet size of 600 µm. The Micro Electrical Discharge Machine driller (Hylozoic Products) was used to drill a 260 µm hole in a stainless steel gasket (7 mm diameter, 250 µm thick) preindented to ∼ 95 µm. The beam energy was set to 20 keV for a wavelength of λ = 0.6199 Å using a sagittal Si(111) monochromator and the beam size on horizontal and vertical directions was 200 and 150 µm, respectively. The CCD detector used was a RAYONYX SXS165. Two-dimensional images obtained were integrated to provide intensity as a function of 2θ using the software FIT2D.29 The scattering angle (θ ) in degrees was converted to wavevector (Q) in reciprocal angstroms (Å-1) using Q = (4π /λ).senθ. Raman spectra were recorded with a Jobin-Yvon T64000 triple monochromator spectrometer having a coupled Olympus BX41 microscope equipped with CCD. Raman spectra were excited with the 647.1 nm line of a mixed argon-krypton laser. The spectra were obtained in the 180◦ scattering geometry with no polarization selection of the scattered radiation. Spectral resolution was kept at 2.0 cm-1. High pressure at room temperature was achieved with a DAC from EasyLab Technologies Ltd., model Diacell LeverDAC Maxi, having a diamond culet size of 500 µm. The Boehler microDriller 5 ACS Paragon Plus Environment


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(EasyLab) was used to drill a hole in a stainless steel gasket (10 mm diameter, 250 µm thick) preindented to ∼150 µm. Different hole diameters were used: 200 µm, 150 µm, and 100 µm. Pressure calibration has been done by the usual method of measuring the shift of the fluorescence line of ruby.30

Results and Discussion A. X-ray diffraction Crystallization vs. glass transition Figure 2 shows the distinct XRD patterns obtained for different phases of [C2C1im][NTf2] formed inside the DAC depending on compression rate and water content. The XRD pattern for the normal liquid phase exhibits two amorphous peaks at ~ 0.9 Å-1 and ~ 1.4 Å-1. These peaks are characteristic of XRD patterns of ionic liquids being assigned, respectively, to charge-charge correlation (Qcc), i.e. cation-cation and anion-anion distances, and adjacency correlations (Qa), i.e. cation-anion distances.31,32 There is no other low-Q peak related to polar-nonpolar segregation because of the short ethyl chain in [C2C1im][NTf2].33-36 Peak positions and relative intensities of Qcc and Qa agree with previous experimental results and molecular dynamics simulations of [C2C1im][NTf2].33-37 We found that [C2C1im][NTf2] can crystallize or not depending on compression rate. The same finding has been reported for two triflate based ionic liquids with [C2C1im]+ or [C4C1im]+ cation.16,17 Crystallization may occur in the range 0.4 – 1.2 GPa if pressure is increased stepwise (crystal I in Figure 2). On the contrary, a glassy phase is obtained when sudden compression is applied. Evidence for formation of crystal or glass is the XRD pattern exhibiting sharp Bragg peaks or keeping amorphous peaks alike the liquid, respectively. The photographs of the sample inside 6 ACS Paragon Plus Environment

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the DAC chamber also indicate whether crystalline or glassy phase is presented (see insets in Figure 2). Moreover, moisture plays a role in the kind of crystal formed. If the liquid is used without any prior drying process or it is kept in contact with room moisture, a different crystal II phase is formed as indicated by a distinct XRD pattern. Nevertheless, dependence on the compression rate for the non-dried sample was found similar to the dried ionic liquid. In both the systems (dried or not) the crystallization process may happen from the glassy phase depending on the decompression rate. If the pressure is released stepwise, then the sample crystallizes in the same pressure range (0.4 – 1.2 GPa) achieving a crystal structure similar to the one formed during compression.

Figure 2. XRD patterns of [C2C1im][NTf2] obtained at different pressures and room temperature. Crystal I is obtained from the dried sample and crystal II is obtained from the sample without prior drying or exposed to moisture. The insets show photographs of DAC sample chamber containing the sample and the ruby chip for pressure calibration. 7 ACS Paragon Plus Environment


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Heterogeneous vs. homogeneous crystallization The crystallization process is also dependent on the size of DAC sample chamber. The roughness of the chamber walls formed during the gasket drill process by electron corrosion acts as nucleation sites and larger sample chamber increases the probability of crystal nucleation. The top panels in Figure 3 show photographs taken along crystallization when [C2C1im][NTf2] was compressed to ~ 1.0 GPa. Crystal nucleation starts on the walls of the sample chamber and crystals growth until crystallization of the whole sample. A movie of the heterogeneous crystallization of [C2C1im][NTf2] under compression at 1.2 GPa occurring during ~ 3.5 minutes has been provided in Ref. [10]. Heterogeneous crystal nucleation also occurs from the glassy phase when the pressure is released stepwise. In the author’s knowledge, homogeneous crystal nucleation has never been reported in high pressure experiments using a DAC. We observed, however, that if sudden decompression is done to pressure close to atmospheric pressure, then homogeneous nucleation can happen. The bottom photographs in Figure 3 show many crystal nuclei appearing at the same time in distinct regions, so that they are not due to some impurity or irregular bounder. Unfortunately, we were not able to perform XRD and Raman spectroscopy analysis of the homogeneously formed crystals because they melted soon after formed. Most probably, the homogeneous nucleation happened because the abrupt increase of free volume after decompression reduces the activation barrier to crystal nucleation. The homogeneous crystallization takes place in few seconds, whereas heterogeneous crystallization time ranges from few seconds to minutes depending on pressure, sample chamber size, and the history of pressure variation.


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Figure 3. Photographs of DAC sample chamber during crystallization of [C2C1im][NTf2] starting in the gasket wall (heterogeneous nucleation along compression at ~ 1.0 GPa; top photographs) and in the bulk (homogeneous nucleation after sudden decompression of the glass to pressure close to atmospheric pressure, bottom photographs).

Crystal amorphization under high pressure Figure 4 shows XRD patterns for the crystal at 1.2 GPa compressed until 11.5 GPa, and then decompressed back to 1.2 GPa. The Bragg peaks broaden, shift to higher Q values, and the intensity decreases with compression. In contrast to crystals of other ionic liquids which undergo solid-solid transitions with further compression after crystallization,12,16,38 the results of Figure 4 indicate instead amorphization with loss of long-range order. Pressure induced amorphization of crystalline materials have been reported in several studies.39,40 It has been found that many of these amorphous materials recovered the crystalline phase with the same initial crystallographic orientation when pressure was released, prompting for the proposition that the amorphous phase retains a “memory” of the original crystalline phase.31,32 In line with these studies, Figure 4 shows that the XRD pattern obtained after decompression of 9 ACS Paragon Plus Environment


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[C2C1im][NTf2] is indeed the same of the original crystalline phase at 1.2 GPa. It is worth noting that this induced amorphous state was considered “a kinetically preferred state arising due to frustration in reaching the high pressure equilibrium crystalline state owing to lack of enough thermal energy”.40 Thus, experiments changing simultaneously pressure and temperature would be interesting for searching other [C2C1im][NTf2] crystalline phases.

1.2 GPa 3.2 GPa 4.2 GPa 5.7 GPa

Intensity

7.2 GPa 8.3 GPa 9.2 GPa 10.2 GPa

Pressure variation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.5 GPa 8.0 GPa 3.6 GPa 1.2 GPa

0.6

0.9

1.2 1.5 Q / Å-1

1.8

2.1

Figure 4: XRD patterns of [C2C1im][NTf2] obtained from the crystal I at 1.2 GPa under compression up to 11.5 GPa, and then decompression back to 1.2 GPa.

The pressure of glass transition The pressure of glass transition, Pg, of [C2C1im][NTf2] was obtained by the pressure dependence of the bandwidth of the ruby fluorescence spectrum, Γ(P).41,42 10 ACS Paragon Plus Environment

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Figure 5 shows that Γ(P) is discontinuous at 1.8 GPa. It is worth to noting that the bandwidth becomes sharper after Pg and the changes are tiny compared with results to other ionic liquids.18 This opposite behavior might be due the distinct structural organization of [C2C1im][NTf2] that also promotes the interplay between crystallization and glass transition processes. However, this value for Pg is very close to 1.6 and 1.7 GPa reported for [C4C1im][NTf2] and [C6C1im][NTf2], respectively.18 Figure 5 also shows the peak shift of the Qcc and Qa peaks from the liquid phase at atmospheric pressure to the glassy phase at 3.4 GPa. The peaks shift to higher wavevectors reaching plateau values after Pg. The peak positions shift continuously with increasing pressure with no indication of discontinuity at 1.8 GPa. The pressure dependence of Qcc and Qa seems to manifest the pressure dependence of density as suggested by other

Γ(P) - Γ(0) / nm

imidazolium based ionic liquid.43

0.1 0.0

Pg = 1.8 GPa

-0.1

Qcc / Å-1

1.00 0.95 0.90 0.85

Qa / Å-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.6 1.5 1.4

0

1

2 P / GPa

3



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Figure 5: The upper panel shows the pressure dependence of the bandwidth of the ruby fluorescence spectrum, Γ(P), with respect to the atmospheric pressure value, Γ(0). Middle and lower panels show the shift of charge-charge correlation peak (Qcc) and adjacency correlations peak (Qa), respectively, in the XRD pattern of amorphous phases.

B. Raman spectroscopy The [NTf2]- conformations Raman spectroscopy is a powerful tool to reveal changes of molecular conformations along phase transitions of ionic liquids.10,21,44-50 One of the most investigated anion is [NTf2]-, whose the most intense Raman band at ~ 741 cm-1 and the vibrations within the spectral range 250–450 cm-1 have been used to characterize the cisoid and transoid conformations.10,51 Figure 6 shows the Raman band assigned to the whole deformation of the [NTf2]− anion in different phases of [C2C1im][NTf2] under high pressure. This band shifts to higher frequency and splits in two components at 743 and 749 cm-1 in the crystal I formed at 0.4 GPa. The frequencies and relative intensities of this doublet of bands change gradually by further compression. The Raman band shape does not exhibit significant changes for pressures above ~2.0 GPa except for the spectral shift to higher frequencies. Martinelli et al.51 fitted this Raman band in the normal liquid phase spectrum with two functions assigned to transoid and cisoid conformers. However, such split of this [NTf2]- Raman band has never been observed on crystalline phases of pure [NTf2]- ionic liquids, but only for mixtures of these ionic liquids with salts containing strongly coordinating cations such as Li+ or Mg2+.52-55 These previous works assigned the lower frequency band to uncoordinated (“free”) anion and the higher frequency band to coordinated (“bounded”) anion.52-55 On the other 12 ACS Paragon Plus Environment

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hand, Figure 6 shows that this band does not split for the glassy phase at 2.8 GPa and the crystal II at 0.3 GPa. It has been found in a Raman study for other [NTf2]- based ionic liquids that this band exhibits continuous high frequency shift with increase of pressure.22 The low frequency shift in the Raman spectrum of crystal II of non-dried sample strongly suggests the dominance of attractive interaction on the probe oscillator. We proposed that water molecules are coordinated to [NTf2]- in crystal II structure as it is well-known that water molecule interacts preferentially with the anion in ionic liquids.56 The presence of water indeed affects the [NTf2]- conformation in the crystals of [C2C1im][NTf2] as discussed in the following.

liquid 0.1 MPa

Raman Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

crystal I 0.4 GPa 1.7 GPa 2.5 GPa glass 2.8 GPa

crystal II 0.3 GPa

720

735 750 765 780 wavenumber / cm-1

Figure 6. Raman spectra of [C2C1im][NTf2] in the 720–790 cm-1 range at different pressures and room temperature. The dashed line is a reference for the peak position of the liquid phase Raman spectrum.



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The spectral range 250 – 450 cm-1 of the Raman spectra shown in Figure 7 is a better fingerprint for the [NTf2]- conformers.10 Raman spectra reveal that liquid and glass are composed by a mixture of [NTf2]- conformers as reported for other ionic liquids.10,26,50 The glass spectrum resembles the one obtained by Yoshimura et al.26 in a similar pressure for a [NTf2]- based ionic liquid with an ammonium cation indicating that pressure increases the population of transoid conformer. The comparison between experiment and the spectrum calculated by Quantum Chemistry method (DFT) for cisoid conformer (blue bars in Figure 7) indicates that crystal I is composed mainly by [NTf2]- cisoid conformer. In this spectral range, the Raman spectrum of crystal I of [C2C1im][NTf2] is analogous to the low temperature crystal of [N4111][NTf2] with [NTf2]-

in

cisoid

conformation,

where

[N4111]+

is

the

cation

N-butyl-N-

triethylammonium (see Figure 4 in Ref. [49]). Thus, the split of [NTf2]- band in crystal I spectrum showed in Figure 6 should be interpreted as due to the occurrence of cisoid conformers with different degrees of coordination. On the contrary, crystal II formed from the non-dried sample is composed mainly by [NTf2]- transoid conformer on the basis of the comparison between crystal II spectrum with calculated normal modes for transoid conformer (red bars in Figure 7) and also the experimental spectrum for the [N4111][NTf2] crystalline phase containing [NTf2]- transoid conformer (see Figure 4 in Ref. [49]). The agreement between calculated and experimental spectra is fair, but the relative intensities and bandwidth in the crystal II spectrum shown in Figure 7 suggest distorted [NTf2]- conformations.


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Figure 7. Raman spectra of [C2C1im][NTf2] in 250 - 450 cm-1 range at different pressures and room temperature. The bars indicate vibrational frequencies and relative intensity of Raman modes calculated in Ref. [49] by the DFT level of theory for isolated [NTf2]- in

cisoid (blue bars) and transoid (red bars) conformations. Theoretical

vibrational frequencies were multiplied by a scaling factor of 1.097 to take into account pressure induced frequency shift.

Raman imaging of the crystal The XRD patterns with broad Bragg peaks (Figures 2 and 4) are evidences of distorted [NTf2]- conformers in both high pressure crystals I and II of [C2C1im][NTf2]. Further evidence is obtained by Raman spectra from different parts of the sample in the DAC chamber. Figure 8 shows that Raman spectra of both the crystalline phases exhibit differences in relative intensities of bands for different parts of the sample. However, the vibrational frequencies that characterize each conformer are not changed in different 15 ACS Paragon Plus Environment


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spectra of the sample. The occurrence of cisoid conformers with different degree of coordination in crystal I is also supported by the matching to Raman spectrum of a crystalline phase at -100 °C of a mixture of [NTf2]- ionic liquid with Li+ (see panel d of Fig. 4 in Ref [52]). Further compression promotes more disorder until a certain pressure when the spectrum does not change any longer in line with the previous discussion concerning Figure 6. The conformational disorder implies heterogeneous microscopic crystallization of [C2C1im][NTf2]. The Raman mapping of the sample in the DAC chamber shown in Figure 8 indicates regions of the crystal I with different degrees of conformational disorder. At this point it is interesting a comparison with heterogeneous microscopic crystallization of the protic ionic liquid propylammonium nitrate, [C3H7NH3][NO3].21,24 The distorted network of hydrogen bonds in [C3H7NH3][NO3] promotes a distribution of local environments that might originate crystalline regions with distinct structures.21,24 In the case of [C2C1im][NTf2], it is the pressure induced conformational disorder that promotes microscopic heterogeneity of the crystallization.


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Figure 8: Raman spectra of [C2C1im][NTf2] crystal I and II in the 250–450 cm-1 range obtained in different regions of the sample in the DAC chamber. The inset shows the photograph of DAC sample chamber with crystal I at 0.4 GPa and the resulting microRaman imaging using the spectral region indicated by the square. Raman spectra with different colors correspond to different regions of the mapping.

Raman imaging of the glass/crystal border The slow dynamics of crystallization of [C2C1im][NTf2] under high pressure allowed to perform a sudden compression in order to stop the crystallization process. The crystallization started at 1.2 GPa, then the pressure was increased to 4.5 GPa in a single step. The left panel of Figure 9 shows the photograph of the DAC chamber in which the sample partially crystallized is in contact with the amorphous glassy phase after fast compression. The right panel of Figure 9 shows that the Raman bands are broad at such high pressure, so that bands overlap precludes proper analysis of conformers. Nevertheless, the most intense [NTf2]- band was used to probe different 17 ACS Paragon Plus Environment


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sample regions by the Raman mapping shown as inset in the right panel of Figure 9. The Raman mapping provides a picture of the order-disorder evolution from crystal to glassy phase. It is evident the boundary between the ordered crystalline phase and the amorphous glassy phase. Interestingly, heterogeneity is revealed also in the crystalline phase. The blue areas in the Raman mapping indicate regions of more ordered structure whereas the green areas indicate an intermediate degree of order. It is worth noting that the [NTf2]- vibrational mode has higher frequency (green spectrum in Figure 9) in regions of intermediate organization between crystal and glass. This shift of the Raman band arises from [NTf2]- anions probing an intermediate local environment along the crystallization process and pressure favoring the repulsive contribution of molecular interactions. This kind of experiment allows following the order-disorder evolution from crystal to glass and opens interesting possibilities for studying intermediate steps during crystallization.

Figure 9. Left: photograph of DAC chamber obtained after a sudden compression to 4.5 GPa stopping the crystallization of [C2C1im][NTf2]. The black square is the region used to perform a Raman mapping analysis using the spectral region between 740 - 800 cm-1. Right: Raman spectra with colors corresponding to different regions of the mapping shown in the inset.


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C. Comparison between high pressure and low temperature crystals It is interesting to compare the high pressure results of this work with the low temperature phase behavior of [C2C1im][NTf2]. Lassègues et al.27 showed that formation of glass or crystal strongly depends on cooling rate. These authors showed by Raman spectroscopy that the low temperature crystal has the [NTf2]- anion in cisoid conformation, whereas a mixture of conformers occurred in the glass.27 Choudhury et al.28 determined the low temperature crystal structure of [C2C1im][NTf2] as the noncentrosymmetric space group Pca21 with two [NTf2]- cisoid conformers in the asymmetric unit. These low temperature results are analogous behavior of [C2C1im][NTf2] under high pressure found in this work. Figure 10 compares XRD patterns for crystal I at 1.2 GPa and the low temperature single crystal at -130 oC. The XRD pattern of the low temperature crystal was simulated from the unit cell coordinates provided in Ref. [28]. This comparison indeed indicates similar structure for the highpressure and the low-temperature crystals. Furthermore, the Raman spectrum of crystal I corresponds to the one obtained by Lassègues et al,27 despite of the absence of split of the [NTf2]- Raman band at ~741 cm-1 in the spectrum of the low temperature crystal (see Fig. 7 in Ref. [27]). The finding that the 741 cm-1 Raman band split under high pressure can be understood considering that pressure has distinct effect on the shift of vibrational frequency for each of the two cisoid conformers in the asymmetric unit cell. Summing up, in spite of significant difference in thermodynamic states, the structure of [C2C1im][NTf2] crystal formed under high pressure is similar to the low temperature crystal. Thus, density seems the main factor, rather than thermal contribution, controlling the crystallization of [C2C1im][NTf2]. This finding is in line with the study of Reichert et al.57 who investigated the crystalline structure of a series of imidazolium based ionic liquids with the [PF6]- anion and concluded that the crystal 19 ACS Paragon Plus Environment


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structures are dominated by efficient packing of the ions. The fact that [NTf2]- based ionic liquids having as counter ion a cation with long alkyl chain are difficult to crystallize under high pressure is due to hindrance of efficient ion packing, so that these ionic liquids are good glass formers. Moreover, amorphization of the [C2C1im][NTf2] crystal by compression is understood as a continuous ionic packing without intervening solid-solid transition. It is to be verified in further studies whether microscopic heterogeneity is also present in low temperature crystal as has been found in this work for the high pressure crystal of [C2C1im][NTf2]. Crystal I 1.2 GPa

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Single crystal - 130 oC

0.8

1.2

1.6

2.0

2.4

2.8

-1

Q/Å

Figure 10. XRD patterns of [C2C1im][NTf2] crystal obtained in this work at 1.2 GPa and room temperature, and at -130 °C and atmospheric pressure simulated using the single crystal structure reported by Choudhury et al.28 The low temperature XRD pattern was simulated considering linewidth of 0.02 Å-1 and shifted by +0.05 Å-1 to take into account the pressure effect. Inset shows the unit cell of a non-centrosymmetric space group Pca21 along b-axis composed by two distinct [NTf2]- cisoid conformers in the asymmetric unit.


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Conclusions The ionic liquid [C2C1im][NTf2] has a complex pressure dependent phase behavior as compression/decompression rates determines crystallization or glass transition. Homogeneous and heterogeneous crystal nucleations have been observed inside the DAC. The crystalline phase formed at high pressure does not show solid-solid transition on further compression and amorphization was revealed by synchrotron XRD experiments. Moreover, the glass formation process was also followed by XRD illustrating the reduction of interionic distances as the density increases. Raman spectra were used to determine the [NTf2]- conformation in different phases formed under high pressure. Crystalline phases containing cisoid or transoid conformers are obtained depending on the water content in the sample, the latter been favored with high water amount. The [NTf2]- conformers might become distorted under high pressure. Raman microscopy

allowed

to

probe

the

microscopic

crystalline

heterogeneity

in

[C2C1im][NTf2] promoted by [NTf2]- conformational disorder. The slow dynamics of crystallization under high pressure allowed stopping the crystal growth by sudden compression of the system. Raman imaging showed order-disorder in crystal and glass highlighting intermediate structures. The structure of previously reported low temperature crystal and the high pressure crystal of [C2C1im][NTf2] are similar, suggesting that ion packing is the main factor controlling the crystallization process. The usual glass formation under high pressure of [NTf2]- based ionic liquids is then understood as hindrance of efficient ion packing. Future experiments demand simultaneous variations of temperature and pressure in order to disentangle other density and thermal effects in phase behavior of [C2C1im][NTf2]. The results of this work highlight that ionic liquids are good model systems to shed light on fundamental questions related to crystallization and glass transition. 21 ACS Paragon Plus Environment


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Acknowledgment

The authors are indebted to FAPESP (Grant nos. 2015/05803-0, 2014/15049-8, and 2012/13119-3) and CNPq for financial support and Brazilian Synchrotron Light Laboratory (LNLS) for X-ray diffraction experiments (proposal XDS 201518685).

Author information

Corresponding authors *E-mail address: [email protected] (L.F.O.F.), [email protected] (M.C.C.R.)

Notes The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. No competing financial interests have been declared.

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For Table of Contents Use Only

Phase transitions of the ionic liquid [C2C1im][NTf2] under high pressure: a synchrotron X-ray diffraction and Raman microscopy study Luiz F. O. Faria, Thamires A. Lima, Mauro C. C. Ribeiro

This work presents synchrotron X-ray diffraction and Raman spectroscopy experiments under high pressure for the archetypal ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. It sheds light in fundamental questions related to the mechanism of crystallization and glass transition of ionic liquids.


Phase Transitions of the Ionic Liquid [C2C1im][NTf2] under High

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