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Unraveling the Stepwise Melting of an Ionic Liquid Thamires A. Lima, Vitor Hugo Paschoal, Luiz Felipe de Oliveira Faria, and Mauro Carlos Costa Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03178 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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The Journal of Physical Chemistry

Unraveling the Stepwise Melting of an Ionic Liquid Thamires A. Lima, Vitor H. Paschoal, Luiz F. O. Faria, 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-mail: [email protected]

Abstract

Differential scanning calorimetry, X-ray diffraction, and Raman spectroscopy were used to reveal the premelting events precursors of melting of the ionic liquid triethylsulfonium bis(trifluoromethanesufonyl)imide, [S222][NTf2]. On heating the crystalline phase of [S222][NTf2], melting occurs along a sequence of at least three-steps. First, the crystalline long-range order breaks-down, but local order is retained. The second step is characterized by conformational freedom of the ethyl chains of cations related to premelting of nonpolar domains, and the complete melting finally occurs when anions acquire conformational freedom. This work provides a microscopic view on the mechanism of melting of [S222][NTf2] in line with the picture of melting taking place as a sequence of structural changes. The results of this work shed light on the understanding of the complex melting process of ionic liquids.

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Introduction Even though the melting process is one of the most important phenomenon in nature, there are still many unanswered questions concerning the mechanism of melting from a molecular point of view. Microscopic models focusing on lattice vibrations, dislocations, grain boundaries, dimensionality, etc. have been developed.1-4 Melting is a first-order transition, but many authors argued that its mechanism is better described as a continuous process of gradual change of physical properties, for instance, heat capacity, density, compressibility, electrical conductivity, etc.1 The events of premonitory melting act as precursors of the melting, and the entropy of fusion can be thought of as the sum of different contributions related to positional, orientational, vibrational, and configurational disorder.4-5 The term premelting has been used within the context of surface melting models when a quasi-liquid layer appears on a crystal surface below the melting temperature.1-2,6 In this work, premelting will be used in the general sense concerning structural events happening prior complete melting of the solid phase. Ionic liquids exhibit complex thermal behavior besides low melting points in comparison to traditional inorganic salts. The low melting point itself results from both low lattice energy and entropic reasons because the typical large and asymmetric ions with shielded or delocalized charge minimize anion–cation interaction and provide significant freedom for molecular conformations. The crystallization and melting processes have been investigated mainly in imidazolium-based ionic liquids, whose calorimetric measurements usually indicate several premelting events. Polymorphism in 1-butyl-3-methylimidazolium chloride, [C4mim]Cl, with different conformations of the butyl chain hampers the easy crystallization of [C4mim]Cl.7 It has been suggested that


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supercooling and premelting of [C4mim]Cl and [C4mim]Br result from the dynamics of conformational changes of the [C4mim]+ cation.8 Okajima and Hamaguchi9 investigated the melting process of [C4C1im]Cl by considering the intramolecular Raman bands that characterize [C4C1im]+ conformers, and also the low frequency range of the Raman spectrum (< 200 cm-1) carrying information from intermolecular vibrational modes. Vibrational modes of crystal lattice gradually disappear during the melting process, whereas the quasi-elastic scattering intensity increases as expected for the normal liquid phase. However, there was a time lag between the disappearance of crystal lattice modes and the equilibration of conformers population in the liquid phase.9 Thus, cation conformers remain in local structures and only after the slow collective conversion of those arrangements is the melting process completed.9-10 Nishikawa et al.11-12 concluded that the signature of premelting given by a large tail in the melting peak of DSC scans comes from reversible phase transitions of domains in the ionic liquid structure. The authors called such sequential crystallization and melting as rhythmic melting, and they assigned it to changes in locally melted domains composed of different conformers.11-12 The ionic liquids are good model systems to investigate general questions related to melting process, e.g. how premelting evolves and its relationship with structural changes. Despite the previous studies mentioned above, a complete microscopic understanding of the events prior the melting of crystalline phases of ionic liquids is still lacking. In this work, we combine different techniques (DSC, X-ray diffraction, and Raman spectroscopy) to investigate the melting process of triethylsulfonium bis(trifluoromethanesulfonyl)imide, [S222][NTf2] (see Chart 1). In the authors’ knowledge, it is the first structural study of a sulfonium-based ionic liquid and the first study using X-ray diffraction to follow the premelting events of an ionic liquid. The advantage of [S222][NTf2] in comparison to other ionic liquids is that its DSC scan 3 ACS Paragon Plus Environment


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exhibits peaks well-separated along the temperature axis. Each thermal event seen in the DSC heating scan defines a step on the process of melting of [S222][NTf2]. This allowed us for structural investigation by X-ray diffraction at different temperatures in between each thermal event observed in the calorimetric measurement. Furthermore, Raman spectroscopy provided insights on interactions and conformations of the ions along the phase transitions of [S222][NTf2].

CH3 CH2

H3C

H2 C

O O

N S

S CH2 CH3

F3C

[S222]+

S

CF3

O

O

[NTf2]-

Chart 1. Structures of triethylsulfonium and bis(trifluoromethanesulfonyl)imide.

Methods The

ionic

liquid

triethylsulfonium

bis(trifluoromethanesufonyl)imide,

[S222][NTf2], was purchased from Iolitec and used without further purification, except by drying process under high vacuum for 48 h before analyses.

Differential scanning calorimetry Thermophysical characterization of [S222][NTf2] was performed with a differential scanning calorimeter model Q500 (TA Instruments) under a dynamic N2 atmosphere (50 mL min−1). Approximately 10 mg of samples was hermetically sealed in an aluminum pan. The samples were heated to 313 K to remove crystal nuclei eventually present in the liquid phase. The heating and cooling rates were 1, 10, 20, and 4 ACS Paragon Plus Environment

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30 K min−1. Uncertainty in temperature was ± 0.1 K. Duplicate measurements were performed to confirm the results.

X-ray diffraction Temperature dependent X-ray diffraction (XRD) measurements were performed at LNLS - Brazilian Synchrotron Light Laboratory using a closed-cycle Helium Joule– Thomson Cryostat (AS Scientific). The beam energy was set to 20 keV for a wavelength of λ = 0.6199 Å using a sagittal Si(111) monochromator and the diameter of the beam size was 350 mm. The CCD detector used was a RAYONYX SXS165. Twodimensional images obtained were integrated to provide intensity as a function of 2Θ using the software FIT2D.13 The scattering angle (Θ) in degrees was converted to wavevector (Q) in reciprocal angstrons (Å-1) using Q = (4π /λ).senΘ.

Raman spectroscopy Raman spectra were recorded with a Jobin-Yvon T64000 triple monochromator spectrometer equipped with CCD. Raman spectra were excited with the 647.1 nm line of a mixed argon-krypton Coherent 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. Temperature control (± 0.1 K) at atmospheric pressure was achieved with an OptistatDN cryostat (Oxford Instruments) in which the sample is contained in a small glass tube. After a relatively fast cooling (~17 K min−1), Raman spectra were obtained along heating from 190 K to room temperature in 10 K steps waiting ~ 30 min for equilibration at each temperature.



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Quantum chemistry calculations Quantum chemistry calculations were performed using the Gaussian0914 package to obtain optimized structures and vibrational frequencies for three conformers of the [S222]+ cation. Density functional theory (DFT) using the Becke’s three-parameter hybrid exchange functional and Lee−Yang−Parr correlation functional (B3LYP) with 6311++G(d,p) basis set was used. The important role of including dispersion effects has been shown in calculations of structure and interaction energy of ionic pairs,15 in ab initio molecular dynamics,16 and in the calculation of vibrational frequencies of cluster of ions of protic ionic liquids.17 In this work, however, we are concerned with vibrational frequencies of an isolated ion, so that we used the common functional B3LYP.18-21 No imaginary vibrational frequencies were obtained indicating that the vacuum geometries of [S222]+ were at the minimum of the potential surface except for one of the conformers. Anharmonicity and condensed phase effects were not included in the DFT calculation of an isolated [S222]+ cation, so that a scaling factor of 0.98 was used to minimize the difference between calculated and experimental values of all of the vibrational frequencies assigned to the [S222]+ cation. It is worth noting that the scaling factor used in this work for the vibrational frequencies of [S222]+ is close to the benchmark database of scaling factors (0.97 for the level of theory considered in this work).22 Table S1 of Supporting Information gives calculated frequencies and Raman

activities for the three [S222]+ conformers.

Results and Discussion Figure 1 shows DSC scan of [S222][NTf2] obtained by slow cooling (–1 Kmin-1, upper curve) followed by heating (+10 Kmin-1, bottom curve). We also performed DSC scans of [S222][NTf2] at other different rates as shown in Figure S1 of Supporting 6 ACS Paragon Plus Environment

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Information. The sample was cooled under the faster rate of -10 Kmin-1 followed by the same heating rate of +10 Kmin-1 (top panel of Figure S1), and also faster cooling/heating rates of ±20 and ±30 Kmin-1 (middle and bottom panels of Figure S1). All of the DSC curves exhibit similar behavior and only slight differences in the temperatures of transitions. The finding that DSC patterns of ionic liquids might depend on cooling/heating rate is not unusual because of the sluggish dynamics and changes on molecular conformations. Actually, much more evident thermal history effects have been found in DSC patterns of other systems, for instance, in ionic liquids based on tetraalkylammonium cations and the [NTf2]- anion.23,24 Figures 1 and S1 together indicate that crystallization of [S222][NTf2] under cooling is observed at 239 K and a solid-solid transition follows at 208 K irrespective of the cooling rate. Heating the crystal does not lead to a single step of fusion as the DSC heating curve exhibits premelting events. After other solid-solid transition at 216 K (more clearly seen in the DSC curves of Figure S1 of Supporting Information), there are three endothermic peaks: 240 K, 260 K, which is seen as a shoulder in Fig. 1, and 262 K.



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Figure 1. Left: DSC scan of [S222][NTf2] obtained by slow cooling (–1 K min-1, upper curve) followed by heating (+10 K min-1, bottom curve). The asterisk indicates that the peak at 239 K is very sharp and intense. Right: Photographs of the sample inside the glass tube used for Raman measurements of the liquid at 297 K, crystal at 220 K, and along heating the crystal.

Figure 1 shows photographs of the sample inside the glass tube used in Raman measurements for the liquid at 297 K, the crystal at 220 K, and along heating up to 267 K. Melting starts at 240 K, then the aspect of the sample becomes different at 260 K and less opaque jelly-like at 264 K until melting at 267 K. The 260 K peak seen as a shoulder on the 262 K peak can be discernable in the DSC scans for all of the cooling/heating rates shown in Figures 1 and S1. The X-ray measurement to be discussed in the following takes relatively long time of about half an hour waiting for thermal equilibrium plus an hour for data acquisition at each temperature. Thus, it is worth mentioning that the sample keeps the appearance for long time if temperature is maintained constant. Figure S2 of Supporting Information shows photographs of the sample at 260 K for a period of ten hours. The total enthalpy of the endothermic events indicated in Fig. 1 is similar to enthalpies of fusion previously reported for other ionic liquids.23,25-26 The X-ray data discussed below show that positional disorder is triggered at 240 K. The enthalpy of this process, 25.2 kJ mol-1, is indeed compatible with such a process in the case of an ionic crystal. Akdeniz and Tosi27 have done a detailed comparison of melting parameters of halide salts of monovalent, divalent and trivalent metal cations. For instance, taking a representative alkali halide salt, KBr, its enthalpy of fusion is 25.5 kJ mol-1.27 The enthalpy of the 260 and 262 K peaks in the DSC curve of [S222][NTf2], respectively, 7.6 and 11.9 kJ mol-1, have been obtained from the areas


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of two log-normal functions fitted to the data in this temperature range. The lower enthalpies involved in these thermal events are compatible with changes of molecular conformations as will be confirmed below by Raman spectroscopy. Rennie et al.28 reported similar DSC heating curve for [S222][NTf2] with slight differences in position of peaks. However, these authors claimed that [S222][NTf2] is a supercooled liquid at 173 K, undergoes cold crystallization at 216 K, and they assigned the two endothermic peaks at lower temperatures as solid−solid transitions before melting at 262 K.28 We found instead that crystallization of [S222][NTf2] gives a very sharp and intense exothermic peak at 239 K. In fact, the three endothermic peaks actually belong to the melting process of [S222][NTf2] as it will be confirmed below by the X-ray and Raman data. Figure 2 shows X-ray diffraction (XRD) data of [S222][NTf2] for the supercooled liquid at 240 K and the crystal at 190 K recorded upon cooling, then along heating up to 275 K. There are three amorphous halos in the XRD pattern of the liquid as previously reported for many ionic liquids.29-30 The halo at Q ~ 1.4 Å-1, usually called adjacency peak, is due to cation-anion intermolecular distances and some intramolecular correlations. The halo at Q ~ 0.9 Å-1 is due to cation-cation and anion-anion correlations, i.e. charge ordering. The lowest wavevector peak at Q ~ 0.7 Å-1 is due to segregation of polar and nonpolar domains in ionic liquids. In the case of [S222][NTf2], the nonpolar domains are composed by the ethyl chains of [S222]+ cations. Comparison between XRD patterns for the liquid and the crystal indicates that the Bragg peaks of the crystalline phase appear within the same Q-range of the amorphous halos of the liquid phase. Therefore, it is evident that correlation distances in the liquid resemble the crystalline phase. We provide in Figure 2 a schematic model for the structure of the



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[S222][NTf2] crystal based on distances d = 2π/Q corresponding to Q values of the amorphous halos.

Figure 2. Left: XRD data of [S222][NTf2] for the supercooled liquid at 240K and the crystal at 190 K obtained on cooling, and along heating up to 275 K. Right: Schematic two-dimensional model proposed for the crystalline phase of [S222][NTf2]. The intermolecular distances are based on d = 2π/Q corresponding to the three-peaked pattern of the XRD data of the supercooled liquid. The ion conformers were obtained by Quantum Chemistry DFT calculations of isolated ions.

The sequence of thermal events seen in the DSC scan of Figure 1 has a structural counterpart in the XRD data shown in Figure 2 along heating of the crystal. The solidsolid transition is indicated by the difference between XRD patterns at 190 K and 230 K. The first step of melting at 250 K results in the amorphous pattern under the Bragg peaks. It must be noted that the Bragg peaks vanish gradually from lower to higher wavevectors as temperature increases. The Bragg peaks above the charge ordering peak Q ~ 0.9 Å-1 are still seen at 260 K. This trend continues until the sharp Bragg peaks are 10 ACS Paragon Plus Environment

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washed out within the broad halo of short-range correlations when the process of melting is finished. Raman spectroscopy and quantum chemistry calculations have been used to reveal changes of molecular conformations along phase transitions of ionic liquids, the [NTf2]- anion being one of the most investigated species.23-24,31-37 Figure 3 shows Raman spectra within the 260–360 cm-1 range, which is useful to characterize [NTf2]- in cisoid or transoid conformation, for [S222][NTf2] in the normal liquid phase and the crystal at 180 K. The comparison provided in Figure 3 between experimental and calculated vibrational modes indicates that the Raman bands observed at 277, 296, 314, and 338 cm-1 characterize the transoid [NTf2]- conformer, whereas Raman bands observed at 287 and 326 cm-1 characterize the cisoid [NTf2]- conformer. In line with previous studies for other ionic liquids,23-24, 37 there is mixture of [NTf2]- conformers in the liquid phase of [S222][NTf2]. On the other hand, the Raman spectrum of the crystalline phase of [S222][NTf2] exhibits bands only of transoid [NTf2]- conformer.

180 K 297 K

Intensity

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transoid

cisoid 260

280

300

320

340 -1

wavenumber/ cm



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Figure 3. Raman spectra of [S222][NTf2] in the normal liquid and crystal phases, respectively, red and black lines, in the spectral range useful to characterize [NTf2]conformers. The bars indicate vibrational frequencies and relative intensities of Raman bands calculated by DFT method for transoid [NTf2]- (277, 299, 314, 315 and 347 cm-1, black bars) and cisoid [NTf2]- (278, 286, 312, 331, and 337 cm-1, red bars). The vibrational frequencies and Raman activities calculated for [NTf2]- conformers are the same reported in a previous work.23

In the authors knowledge, there is no previous Raman study concerned with the conformation of [S222]+, so that we performed quantum chemistry calculations in order to identify the more suitable spectral range to characterize the [S222]+ conformers. Figure 4 shows three optimized [S222]+ conformers obtained by the DFT level of theory. Taking as reference the pyramidal geometry of the three carbon atoms next to the sulfur atom, each methyl group may adopt either an axial (ax) or an equatorial (eq) position. In Figure 4, the conformer called all-ax has the higher energy, and the conformer called all-eq has the lower energy. The energy difference between the conformers all-ax and all-eq, 35 KJ mol-1, is larger than energy differences previously reported for conformers of other typical cations and anions of ionic liquids.37 The energy of the conformer with one eq group and two ax groups, 1-eq-2-ax, is 12 KJ mol-1 larger than all-eq conformer. The calculations showed that the most suitable range of the Raman spectrum to distinguish [S222]+ conformers is 580–630 cm-1. The experimental Raman bands at 600 and 614 cm-1 for the liquid phase (red line in Figure 4) are assigned to all-ax and all-eq conformations, respectively. In contrast, the Raman spectrum of crystal at 180 K (black line in Figure 4) is composed mostly by conformer all-ax and conformer 1-eq-2-ax. It is worth remembering that the calculations of vibrational frequencies and relative energies were performed for an isolated [S222]+ cation. Condensed phase effects imply that 12 ACS Paragon Plus Environment

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relative energies between conformers and vibrational frequencies may not be reproduced accurately for all of the conformers from calculations performed at a given level of theory for an isolated ion. Thus, in the normal liquid phase, one expects mixture of [S222]+ conformers as commonly found in ionic liquids. Figure S4 of the Supporting Information shows the displacement vectors of the characteristic normal modes calculated for the three [S222]+ conformers.

180 K 297 K

all-ax

all-eq

Intensity

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* *

1-eq-2-ax

585

600

615

630

645

-1

wavenumber/ cm

Figure 4. Raman spectra obtained along heating of [S222][NTf2] at 180K (crystal, black line) and 297 K (liquid, red line). The bars indicate vibrational frequencies and Raman activities calculated by DFT method for three conformers of [S222]+: all-ax (600 cm-1, black bar), all-eq (624 and 625 cm-1, red bars), and one eq group and two ax groups, 1eq-2-ax (604 and 630 cm-1, green bars). The asterisk at 591 cm-1 indicates a Raman band assigned to the [NTf2]- anion.



Once we know the fingerprint regions of the Raman spectrum that characterizes anion and cation conformers, the conformational evolution during the melting process can be followed. Figure 5 shows Raman spectra obtained along heating of [S222][NTf2]. Starting from the crystal at 180 K, one sees the lattice modes in the low frequency range, the [NTf2]- Raman bands at 277, 296, 314, and 338 cm-1 that characterize the transoid conformation, and the [S222]+ Raman band at 600 cm-1 that characterizes the allax conformation. The solid-solid transition is indicated in Figure 5 by the changes in the pattern of the low-frequency Raman spectrum and in the cation characteristic bands. Raman spectra of Figure 5 show that the new crystalline phase formed after solid-solid transition is composed mainly by the conformer 1-eq-2-ax because the intensity of the 600 cm-1 band decreases, whereas the intensity of the 603 cm-1 band, which is seen only as a shoulder in the previous crystal spectrum, increases. Spectral changes observed in the 660–710 cm-1 region, which includes other cation Raman bands, also indicate the solid-solid transition.

T/K 290 280

Raman Intensity

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270 260

Melting step 3 Melting step 2

240

Melting step 1

220 200

Solid-solid transition

180 50

100 150 200 250 300 350 570 -1

wavenumber / cm

600

630

660

690

720

-1

wavenumber / cm


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Figure 5. Raman spectra of [S222][NTf2] obtained along heating from the crystalline phase at 180 K until the liquid phase at 290 K.

In line with DSC and XRD results, the Raman spectra of Figure 5 indicate that melting starts at 240 K because of the disappearance of low frequency crystal lattice vibrational bands. Furthermore, there is significant enhancement of the quasi-elastic scattering in the Raman spectrum of [S222][NTf2] at 240 K. Anharmonicity and fast relaxation processes contribute to the quasi-elastic scattering in low-frequency Raman spectra of viscous liquids. It is usually fit by a Lorentzian function centered at zero frequency with a bandwidth of ∼5 cm-1, whose intensity decreases as the liquid is supercooled, being only a small contribution to the spectrum of the solid phase, either crystal or glass.37 Therefore, the quasi-elastic scattering intensity in the Raman spectra of Figure 5 means that dynamical precursors of the melting already started in [S222][NTf2] at 240 K. Figure 6 highlights the temperature dependence along the steps of melting for the spectral ranges useful to characterize anion and cation conformers, respectively, 260 – 360 cm-1 and 580 – 640 cm-1. Raman bands related to transoid [NTf2]- conformer and 1-eq-2-ax [S222]+ conformer remain the same, although broader, in the Raman spectrum of [S222][NTf2] at 240 K. Therefore, the long-range structure supporting lattice vibrational modes is broken, but local organization is retained with ions in selected conformation. The spectral pattern remains the same for at least 3 h at 240 K (see Figure S3 in Supporting Information). Figure 6 shows that, as the heating process continues, the intensity of the 614 cm-1 band increases already resembling the Raman spectrum of the liquid at 260 K in which there is mixture of [S222]+ conformers. However, the spectral range 260–360 cm-1 indicates that the [NTf2]- anion still remains in transoid



conformation at 260 K, and the full spectral pattern of the normal liquid phase is recovered only at 270 K.

#

*

#

#

*

//

#

+ 270 K

Raman intensity

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260 K

240 K

220 K

260 280 300 320 340 360

580

600

620

640

-1

wavenumber / cm

Figure 6. Raman spectra of [S222][NTf2] along the three steps of melting (220 – 240 K, 240 – 260 K, 260 – 270 K) in the spectral ranges that characterize [NTf2]- conformers (left) and [S222]+ conformers (right). The bands that characterize anion and cation conformers are marked in the Raman spectrum of [S222][NTf2] at 270 K. In the left, bands marked

#

are characteristic of transoid [NTf2]-, and bands marked

*

are

characteristic of cisoid [NTf2]-. In the right, the band marked // is characteristic of all-ax [S222]+, and the band marked + is characteristic of all-eq [S222]+.

The XRD and Raman results allow for the proposition of a microscopic picture of the stepwise melting of [S222][NTf2]. First, the long range order is broken as indicated by the amorphous halos under the Bragg peaks in the XRD pattern. This step corresponds to the endothermic peak at 240 K with the highest enthalpy change in the DSC curve. The resulting mesophase is characterized by more freedom for molecular 16 ACS Paragon Plus Environment

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rotation giving the quasi-elastic scattering in the low-frequency range of the Raman spectrum. We propose that this phase is not a plastic crystal or a liquid crystal because it does not have long-range order of the centers of mass since no sharp Bragg peaks at low wavevectors are seen in the XRD pattern of the mesophase. It would be interesting for future works an analysis of this mesophase mechanically in order to characterize the state of the sample. Local order in this mesophase is indicated by the Raman bands of specific ionic conformers and the high wavevector Bragg peaks in the XRD pattern. The second step is characterized by the jelly-like aspect of the sample seen in Figure 1. In this second step, melting in nonpolar domains takes place with the mixture of [S222]+ conformers, while the [NTf2]- anions keep the transoid conformation. The last step occurs with the melting of polar domains and the mixture of [NTf2]- conformers, so that the XRD and Raman patterns of the liquid phase is recovered. We can understand the last step of melting of polar domains proper to strong electrostatic interactions in comparison to van der Waals interactions in nonpolar domains.

Conclusions The results of this work reinforce the mechanisms of melting proposed by Ubbelohde4-5 of distinct contributions to the total entropy of fusion. The melting of [S222][NTf2] is in line with the picture of a mechanism of melting following steps of structural changes until the normal liquid phase being recovered. Melting occurring by multi-step events most probably happens also for other ionic liquids, although a clear separation of distinct events might be blurred within a narrow temperature range. It is worth stressing that this work was based on the analysis of the bulk, but it does not rule out the surface melting mechanism.1-2,6 It has been found a typical relation between the surface melting (Ts) and the bulk melting (Tb) temperatures, Ts ≤ 0.9Tb.2 Interestingly, 17 ACS Paragon Plus Environment


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the first and last melting steps of [S222][NTf2], respectively, 240 and 262 K, are in the ratio of 0.9. The interplay between bulk and surface melting in ionic liquids deserves future studies.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: …

The Supporting Information presents DSC scans of [S222][NTf2] obtained during the cooling and heating processes with different rates of temperature (10, 20 and 30 K min-1), photographs of [S222][NTf2] in a glass tube at 260 K taken during a period of 10 h, and Raman spectra and photographs of [S222][NTf2] at 240 K after 10 min and 3 h. Also available as Supporting Information are the displacement vectors of some characteristic normal modes of [S222]+ conformers, and a table with all calculated frequencies and relative Raman intensities for each conformer.

Acknowledgment

The authors are indebted to FAPESP (Grant nos. 2015/07516-8, 2015/05803-0, 2014/15049-8, and 2012/13119-3) and CNPq for financial support. They also thank the CEM – UFABC for DSC analyses and LNLS - Brazilian Synchrotron Light Laboratory (proposal XDS-20160098).


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AUTHOR INFORMATION Corresponding author *E-mail adress: [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.

References (1)

Dash, J. History of the search for continuous melting. Rev. Mod. Phys.

1999, 71, 1737. (2)

Dash, J.; Fu, H.; Wettlaufer, J. The premelting of ice and its

environmental consequences. Rep. Prog. Phys. 1995, 58, 115. (3)

Fuoss, P.; Norton, L.; Brennan, S. X-ray scattering studies of the melting

of lead surfaces. Phys. Rev. Lett. 1988, 60, 2046. (4)

Ubbelohde, A. R. The molten state of matter: melting and crystal

structure; John Wiley & Sons, 1978. (5)

Ubbelohde, A. Melting and Crystal Structure-Some Current Problems.

Ang. Chem. Int. Ed. 1965, 4, 587-591. (6)

Frenken, J. W.; Van der Veen, J. Observation of surface melting. Phy.

Rev. Lett. 1985, 54, 134. (7)

Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnson, S.;

Seddon, K. R.; Rogers, R. D. Crystal polymorphism in 1-butyl-3-methylimidazolium halides: supporting ionic liquid formation by inhibition of crystallization. Chem. Comm.

2003, 14, 1636-1637. (8)

Nishikawa, K.; Wang, S.; Katayanagi, H.; Hayashi, S.; Hamaguchi, H. -

o.; Koga, Y.; Tozaki, K.-i. Melting and freezing behaviors of prototype Ionic Liquids,



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

Page 20 of 23

1-Butyl-3-methylimidazolium bromide and its chloride, studied by using a nano-Watt differential scanning calorimeter. J. Phys. Chem. B 2007, 111, 4894-4900. (9)

Okajima,

H.;

Hamaguchi,

H.-o.

Unusually

Long

trans/gauche

Conformational Equilibration Time during the Melting Process of BmimCl, a Prototype Ionic Liquid. Chem. Lett. 2011, 40, 1308-1309. (10)

Hamaguchi, H.; Ozawa, R. In Advances in Chemical Physics; Rice, S. A.,

Ed.; John Wiley & Sons, Inc.: Hoboken, 2005, 131, 85-104. (11)

Nishikawa, K.; Wang, S.; Tozaki, K.-i. Rhythmic melting and

crystallizing of ionic liquid 1-butyl-3-methylimidazolium bromide. Chem. Phys. Lett.

2008, 458, 88-91. (12)

Nishikawa, K.; Wang, S.; Endo, T.; Tozaki, K.-i. Melting and

Crystallization Behaviors of an Ionic Liquid, 1-Isopropyl-3-methylimidazolium Bromide, Studied by Using Nanowatt-Stabilized Differential Scanning Calorimetry.

Bull. Chem. Soc. Jpn. 2009, 82, 806-812. (13)

Hammersley, A.; Svensson, S.; Hanfland, M.; Fitch, A.; Hausermann, D.

Two-dimensional detector software: from real detector to idealised image or two-theta scan. Int. J. High Press. Res. 1996, 14, 235-248. (14)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, J. R.

Cheeseman, M. A.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian09, Inc., Wallingford, CT, USA, 2009. (15)

Grimme, S.; Hujo, W.; Kirchner, B., Performance of dispersion-corrected

density functional theory for the interactions in ionic liquids. Phys. Chem. Chem. Phys.

2012, 14, 4875-4883. (16)

Pensado, A. S.; Brehm, M.; Thar, J.; Seitsonen, A. P.; Kirchner, B.,

Effect of Dispersion on the Structure and Dynamics of the Ionic Liquid 1-Ethyl-3methylimidazolium Thiocyanate. Chemphyschem 2012, 13, 1845-1853. (17)

Fumino, K.; Fossog, V.; Stange, P.; Paschek, D.; Hempelmann, R.;

Ludwig, R., Controlling the Subtle Energy Balance in Protic Ionic Liquids: Dispersion Forces Compete with Hydrogen Bonds. Ang. Chem. Int. Ed. 2015, 54, 2792-2795. (18)

Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.;

Kanzaki, R.; Ishiguro, S., Evidence of conformational equilibrium of 1-ethyl-3methylimidazolium in its ionic liquid salts: Raman spectroscopic study and quantum chemical calculations. J. Phys. Chem. A 2005, 109, 8976-8982.


Page 21 of 23

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


(19)

Berg, R. W., Raman spectroscopy and ab-initio model calculations on

ionic liquids. Monatsh. Chem. 2007, 138, 1045-1075. (20)

Berg, R. W.; Riisager, A.; Van Buu, O. N.; Fehrmann, R.; Harris, P.;

Tomaszowska, A. A.; Seddon, K. R., Crystal Structure, Vibrational Spectroscopy and ab Initio Density Functional Theory Calculations on the Ionic Liquid forming 1,1,3,3Tetramethylguanidinium bis{(trifluoromethyl)sulfonyl}amide. J. Phys. Chem. B 2009,

113, 8878-8886. (21)

Grondin, J.; Lassegues, J.-C.; Cavagnat, D.; Buffeteau, T.; Johansson, P.;

Holomb, R., Revisited vibrational assignments of imidazolium-based ionic liquids. J.

Raman Spectrosc. 2011, 42, 733-743.

spectra

(22)

http://cccbdb.nist.gov/vibscalejust.asp

(23)

Faria, L. F. O.; Matos, J. R.; Ribeiro, M. C. Thermal analysis and Raman

of

different

phases

of

the

ionic

liquid

butyltrimethylammonium

bis(trifluoromethylsulfonyl) imide. J. Phys. Chem. B 2012, 116, 9238-9245. (24)

Lima, T. A.; Paschoal, V. H.; Faria, L. F. O.; Ribeiro, M. C. C.; Ferreira,

F. F.; Costa, F. N.; Giles, C. Comparing two tetraalkylammonium ionic liquids. II. Phase transitions. J. Chem. Phys. 2016, 144, 224505. (25)

Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. Ionic

liquid crystals: hexafluorophosphate salts. J. Mat. Chem. 1998, 8, 2627-2636. (26)

Troncoso, J.; Cerdeiriña, C. A.; Sanmamed, Y. A.; Romaní, L.; Rebelo,

L. P. N. Thermodynamic properties of imidazolium-based ionic liquids: densities, heat capacities, and enthalpies of fusion of [bmim][PF6] and [bmim][NTf2]. J. Chem. Eng.

Data 2006, 51, 1856-1859. (27)

Akdeniz, Z.; Tosi, M. Correlations between entropy and volume of

melting in halide salts. Procs. Roy. Soc. London A 1992, 437, 85-96. (28)

Rennie, A. J.; Martins, V. L.; Torresi, R. M.; Hall, P. J. Ionic Liquids

Containing Sulfonium Cations as Electrolytes for Electrochemical Double Layer Capacitors. J. Phys. Chem. C 2015, 119, 23865-23874. (29)

Araque, J. C.; Hettige, J. J.; Margulis, C. J. Modern room temperature

ionic liquids, a simple guide to understanding their structure and how it may relate to dynamics. J. Phys. Chem. B 2015, 119, 12727-12740. (30)

Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R. Mesoscopic structural

heterogeneities in room-temperature ionic liquids. J. Phys. Chem. Lett. 2012, 3, 27-33.



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

(31)

Page 22 of 23

Endo, T.; Kato, T.; Tozaki, K.-i.; Nishikawa, K. Phase behaviors of room

temperature ionic liquid linked with cation conformational changes: 1-butyl-3methylimidazolium hexafluorophosphate. J. Phys. Chem. B 2009, 114, 407-411. (32)

Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.;

Ishiguro, S.-i. Conformational equilibrium of bis (trifluoromethanesulfonyl) imide anion of a room-temperature ionic liquid: Raman spectroscopic study and DFT calculations. J.

Phys. Chem. B 2006, 110, 8179-8183. (33)

Hamaguchi, H.; Ozawa, Structure of ionic liquids and ionic liquid

compounds: Are ionic liquids genuine liquids in the conventional sense. R. Adv. Chem.

Phys 2005, 131, 85-104. (34)

Katayanagi, H.; Hayashi, S.; Hamaguchi, H.-o.; Nishikawa, K. Structure

of an ionic liquid, 1-n-butyl-3-methylimidazolium iodide, studied by wide-angle X-ray scattering and Raman spectroscopy. Chem. Phys. Lett. 2004, 392, 460-464. (35)

Okajima,

H.;

Hamaguchi,

H.-o.

Unusually

Long

trans/gauche

Conformational Equilibration Time during the Melting Process of BmimCl, a Prototype Ionic Liquid. Chem. Lett. 2011, 40, 1308-1309. (36)

Faria, L. F. O.; Penna, T. C.; Ribeiro, M. C. C. Raman spectroscopic

study of temperature and pressure effects on the ionic liquid propylammonium nitrate.

J. Phys. Chem. B 2013, 117, 10905-10912. (37)

Paschoal, V. H.; Faria, L. F. O.; Ribeiro, M. C. C. Vibrational

Spectroscopy of Ionic Liquids. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00461.


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