Ultraslow Phase Transitions in an AnionâAnion Hydrogen-Bonded

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Ultraslow Phase Transitions in an Anion-Anion Hydrogen-Bonded Ionic Liquid Luiz F. O. Faria, Thamires A. Lima, Fabio Furlan Ferreira, and Mauro C. C. Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09497 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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Ultraslow Phase Transitions in an Anion-Anion Hydrogen-Bonded Ionic Liquid Luiz F. O. Faria1, Thamires A. Lima1, Fabio F. Ferreira2, Mauro C. C. Ribeiro1* 1

Laboratório de Espectroscopia Molecular, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508000, Brazil 2 Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, SP, Brazil * e-mail: [email protected]

ABSTRACT A Raman spectroscopy study of 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], as a function of temperature has been performed in order to reveal the role played by anion–anion hydrogen bond on the phase transitions of this ionic liquid. Anion–anion hydrogen bonding implies high viscosity, good glass-forming ability, and also moderate fragility of [C2C1im][HSO4] in comparison to other ionic liquids. Heating [C2C1im][HSO4] from the glassy phase results in cold crystallization at ∼245 K. A solidsolid transition (crystal I → crystal II) is barely discernible in calorimetric measurements at typical heating rates, but it is clearly revealed by Raman spectroscopy and X-ray diffraction. Raman spectroscopy indicates that crystal I has extended ([HSO4]-)n chains of hydrogen-bonded anions, but crystal II has not. Raman spectra recorded at isothermal condition show the ultraslow dynamics of cold crystallization, solid-solid transition, and continuous melting of [C2C1im][HSO4]. A brief comparison is also provided between [C2C1im][HSO4] and [C4C1im][HSO4], as Raman spectroscopy shows that the latter does not form the crystalline phase with extended anion-anion chains. 1 ACS Paragon Plus Environment


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I. INTRODUCTION The plethora of potential applications of ionic liquids has stimulated a side-byside development of new technological possibilities and deeper understanding on scientific issues related to these interesting systems. Applications of ionic liquids as alternative solvents in chemical and pharmaceutical industries,1,2 electrolytes for electrochemical devices,3 media for biomass extraction,4 lubricants5 etc. demand extensive physical-chemistry characterization. There is also a need for knowing chemical stability6,7 and phase transitions8 since those applications may submit ionic liquids under extreme temperature and pressure conditions. Concerning lowtemperature phase transitions of ionic liquids, differential scanning calorimetric (DSC) measurements have revealed some recurrent patterns of thermal behavior of ionic liquids.8,9 Some ionic liquids crystallize when cooled, but many others become supercooled and then experience glass transition. The so-called cold-crystallization, i.e. heating the glassy phase leads to crystallization, is a common finding in DSC scans of ionic liquids. It is also found that solid-solid transitions and complex pre-melting processes take place before melting of the crystal to the normal liquid phase. The capability of going from the knowledge of molecular structures and intermolecular forces to understand, and even to predict, the pattern of phase transitions remains a major fundamental issue in the physical-chemistry of ionic liquids. This work concerns the phase transitions of the ionic liquid 1-ethyl-3methylimidazolium hydrogen sulfate, [C2C1im][HSO4] (see molecular structures in Figure 1). [C2C1im]+ and [HSO4]- are relatively small ions and their molecular structures are not as asymmetric as other ionic liquid forming ions, such as derivatives of imidazolium, tetraalkylammonium, pyrrolidinium etc. containing longer alkyl chains or more complex flexible anions, e.g. bis(trifluoromethylsulfonyl)imide, [NTf2]-. One 2 ACS Paragon Plus Environment

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expects that ionic liquids containing [C2C1im]+ would crystallize upon cooling, whereas supercooling and glass transition being favored in derivatives with longer, more asymmetric and flexible alkyl side chains, such as [C4C1im]+ or [C6C1im]+. Nevertheless, [C2C1im][HSO4] is a very viscous liquid and consequently it is a good glass former. The high viscosity of [C2C1im][HSO4] comes from anion–anion hydrogen bonding, which adds another interesting feature on the broad portfolio of possible intermolecular interactions in ionic liquids, that is, hydrogen bond between like-charge ions.10 Cation-cation hydrogen bonding has been shown to promote cluster formation in ionic liquids containing an imidazolium cation with a hydroxyl functionalized side chain.11,12 A distinctive feature of hydrogen-bonded [HSO4]- anions is the developing of chains of anions in the bulk with the concomitant high viscosity of [C2C1im][HSO4],

η = 2390 mPa.s at T = 293 K.13

H3C

N

+

N

CH3

Figure 1. Molecular structures of 1-ethyl-3-methylimidazolium, [C2C1im]+, and hydrogen sulfate, [HSO4]-.

In a previous work,10 the occurrence of hydrogen-bonded [HSO4]- anions has been revealed by a detailed discussion of Raman spectra of ionic liquids within the context of the literature on vibrational spectroscopy of simple salts of alkaline metal cations, MHSO4, where M+ = Li+, Na+, K+, Cs+. The large body of literature related to infrared and Raman spectroscopies of ionic liquids, the assignment of vibrations based 3 ACS Paragon Plus Environment


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on quantum chemistry ab initio calculations, and the spectral changes following phase transitions has been recently revised.14 Vibrational spectroscopy of ionic liquids based on simple anions, for instance, [NO3]-, [HSO4]- etc., is indeed a touchpoint between the well-known literature on high temperature molten salts and the more recent field of ionic liquids. For easier reference, Figure 2 shows the spectral range covering the anion totally symmetric mode νs(S=O) of [C2C1im][HSO4] in liquid, supercooled, and glassy phases.10 Analogous to vibrational spectra of simple (high temperature) MHSO4 molten salts, the νs(S=O) mode impinges two characteristic bands in the Raman spectrum of [C2C1im][HSO4], ∼1045 and ∼1010 cm-1. The Raman band at 1045 cm-1 corresponds to

νs(S=O) of anions interacting with neighboring cations and anions, eventually forming dimers ([HSO4]-)2. The Raman band at 1010 cm-1 is enhanced and shifted to lower frequency as the temperature decreases because it is due to chains of hydrogen-bonded anions, ([HSO4]-)n.

T/K 400 293 270 210 120

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Figure 2. Raman spectra obtained along cooling of [C2C1im][HSO4]. From top to bottom, the temperatures are: 400, 293, 270, 210, and 120 K. The downward arrow at

∼1045 cm-1 indicates the band assigned to the totally symmetric stretching mode

νs(S=O) of anions interacting with neighboring cations and anions, and the upward arrow at ∼1010 cm-1 indicates the νs(S=O) mode of hydrogen-bonded anions in (HSO4-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation. The spectra have been vertically shifted for helping visualization.

There is a large body of literature using vibrational spectroscopy to characterize temperature15,16,17 and pressure18 dependent crystal polymorphs of alkaline metal hydrogen sulfates. For instance, the crystalline phase of CsHSO4 at room temperature has extended ([HSO4]-)n chains, phase transition at 318 K leads to crystals with ([HSO4]-)2 dimers, and another solid phase above 417 K with orientational disorder and relatively high conductivity (10-2 Ω -1cm-1) precedes the melting (Tm = 487 K).16 Even though ([HSO4]-)n chains are present in the room temperature crystalline phase of CsHSO4, the superprotonic conduction characteristic needs the anion-anion hydrogen bond for the proton conductivity and simultaneously freedom of fast reorientation allowed for by the high temperature crystalline phase. It was not an issue of the previous work10 a detailed analysis of phase transitions of [HSO4]- based ionic liquids. Thus, this work is a further investigation on the consequences of anion-anion hydrogen bonding aiming an understanding at molecular level of the thermal events found along cooling and heating of [C2C1im][HSO4]. The discussion will be based not only on the spectral range of the νs(S=O) mode, but also on 5 ACS Paragon Plus Environment


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the low-frequency range of the Raman spectrum, which is directly related to intermolecular vibrations and thus provides clear signature of glass transition, crystallization, solid-solid transition, and melting. Another motivation of this work is that an X-ray determination of the single-crystal structure of a related ionic liquid, 1-butyl-2,3-dimethylimidazolium hydrogen sulfate, [C4C1C1im][HSO4],19 indicated the occurrence of dimers ([HSO4]-)2, rather than infinite chains of hydrogen-bonded anions as in alkaline metal hydrogen sulfates. Here, we show that Raman spectroscopy gives strong evidences of two crystalline phases of [C2C1im][HSO4], crystal I and crystal II, the first having extended ([HSO4]-)n chains. The solid-solid transition of [C2C1im][HSO4] is barely noted in a calorimetric experiment at usual rate of temperature variation because it does not appear as two well-separated peaks in the DSC scan. On the other hand, we will show that the solid-solid transition is evident in the Raman scattering and X-ray diffraction measurements of [C2C1im][HSO4]. Furthermore, the finding that the structural motif of ([HSO4]-)n chains is already present in the normal and supercooled liquid points to the kinetic origin of the glass-forming ability, that is, crystal growth is hampered by the sluggish molecular dynamics of [C2C1im][HSO4]. Ultraslow dynamics of crystallization has been found in ionic liquids containing imidazolium derivatives with longer alkyl chains, e.g. [C4C1im]Br,20 and it has been linked to conformational changes of the butyl chain. A distinctive feature that we emphasize in the case of [C2C1im][HSO4] is the anion–anion connectivity as the ultimate reason for ultraslow dynamics of cold-crystallization, solid-solid, and melting. The continuous melting of [C2C1im][HSO4] along a wide temperature range leads to a glacial state at room temperature, i.e. a mixture of crystals and amorphous phase. In other words, starting with liquid [C2C1im][HSO4], one ends, after a cooling/heating


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cycle, with a solid sample in a glacial state for an indefinite period of time at room temperature. The paper is organized as follows. Section II gives experimental details of the calorimetric, Raman, and X-ray diffraction measurements. Section III of results and discussion is divided into three sub-sections. Section III.A first provides the basic findings of thermal properties of [C2C1im][HSO4] indicating that this ionic liquid is a good glass-forming liquid. The main question addressed in section III.A by Raman and X-ray spectroscopies is that a solid-solid transition is hidden within the relatively broad cold-crystallization peak seen in the DSC heating curve. Raman spectroscopy provides evidences that [C2C1im][HSO4] has a crystalline phase containing extended ([HSO4]-)n chains, which was not the case in the crystallographic study of [C4C1C1im][HSO4],19 but in line with well-known structures of simple alkaline metal hydrogen sulfates. Section III.B explores the consequence of the anion-anion hydrogen-bonded structures: ultra-slow dynamics of the processes of crystallization and melting of [C2C1im][HSO4]. In fact, the glass forming ability of [C2C1im][HSO4] resulting from its very high viscosity has the ultimate reason on these hydrogen-bonded structures, so that we argue in section III.B that the fragility index of [C2C1im][HSO4] is relatively lower than typical ionic liquids. Section III.C addresses one remaining question: there is an apparent contradiction between the proposition of this work, i.e. the crystalline phase of [C2C1im][HSO4] containing ([HSO4]-)n chains, in comparison to the previous crystallographic study of [C4C1C1im][HSO4] indicating the presence of ([HSO4]-)2 dimers.19 The DSC and Raman data of section III.C indeed show that just by replacing the cation [C2C1im]+ by [C4C1im]+ is enough for [C4C1im][HSO4] avoiding a crystalline phase with extended ([HSO4]-)n structures. The main conclusions of the work are summarized in section IV. 7 ACS Paragon Plus Environment


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II. EXPERIMENTAL The ionic liquids [C2C1im][HSO4] and [C4C1im][HSO4] (> 98 % purity) were purchased from Iolitec and used without further purification. The samples were dried under high vacuum (∼ 10-5 mbar) for several days before analyses. Calorimetric measurements were performed in the differential scanning calorimeter model Q500 (TA Instruments) under a dynamic N2 atmosphere (50 ml min-1). Approximately 10 mg of sample was hermetically sealed in an aluminum pan. The sample was heated to 323 K in order to remove eventual crystal nuclei present in the liquid. DSC scans were obtained using cooling and heating rates of 1 Kmin-1. Duplicate measurements were performed to confirm the results. Temperature dependent X-ray diffraction (XRD) measurements were performed for samples loaded in a copper sample holder. Samples were cooled using a He closedcycle cryostat (Cold Edge Technologies) under high vacuum. Uncertainty in temperature is ±0.1 K. XRD measurements were taken within the 150–300 K range using a D8 Discover-Bruker AXS equipment with a MoKα (λ = 0.7093 Å) radiation, tube voltage of 50 kV and current of 600 µA, from 4◦ ≤ 2θ ≤ 20◦, with 0.02◦ step size, and 50 s per step. Another set of XRD data at room temperature were recorded in a STADI-P powder diffractometer (Stoe®, Darmstadt, Germany) in transmission geometry by using a MoKα1 (λ = 0.7093 Å) wavelength selected by a curved Ge (111) crystal, with tube voltage of 50 kV and current of 40 mA. The diffracted intensities were measured by a silicon microstrip detector, Mythen 1 K (Dectris®, Baden, Switzerland). XRD measurements were carried out keeping the detector at a fixed position covering a range of 18.84◦. This diffractometer was used for the solid sample of [C2C1im][HSO4] obtained after a cooling/heating cycle performed in the cryostat used for Raman 8 ACS Paragon Plus Environment

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spectroscopy measurements. The solid sample was mounted in a rotatory disc and a sequence of XRD data were recorded by fast accumulation along ca. 5 minutes. The Raman spectrometer used was a Horiba-Jobin-Yvon T64000 triple monochromator equipped with CCD. Raman spectra were excited with the 659.5 nm line of a solid-state Cobolt 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. We used two different cryostats allowing for ±0.1 K of temperature control, both working with liquid nitrogen. Most of the results shown in this paper were obtained with an OptistatDN cryostat (Oxford Instruments) in which the sample is contained in a small glass tube. Raman spectra obtained using the OptistatDN cryostat were recorded in the macro-chamber of the spectrometer. We also used a micro-cryostat Janis ST-500 coupled to a temperature controller model 325 of LakeShore. A small drop of the ionic liquid lies on the copper sample mount plateau of the micro-cryostat. Raman spectra obtained with the micro-cryostat were recorded by using the Olympus BX41 microscope coupled to the Horiba-Jobin-Yvon spectrometer. Raman spectra were obtained following different cooling and heating protocols as discussed in the next section.

III. RESULTS AND DISCUSSION

A. Phase transitions

The DSC scan presented in Figure 3 shows that [C2C1im][HSO4] is a good glassformer since no crystallization is seen when cooling at a rate of -1 Kmin-1. Only a slight change of baseline is seen at the low temperature end of the cooling curve and the glass



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transition temperature is better determined on the heating curve at Tg = 199 K. Interesting features of the DSC scan are seen by further heating of the supercooled liquid. Cold-crystallization gives a broad peak with maximum at 248 K, but it is clear a shoulder at the lower-temperature side of the peak. Curve fit of this exothermic peak indicates that there are two thermal events taking place within a small temperature range, TI = 244 K and TII = 248 K, when heating the supercooled liquid at a rate of +1 K min-1. Furthermore, although the melting peak in the DSC scan of [C2C1im][HSO4] is Tm = 300 K, actually melting of [C2C1im][HSO4] is a process covering an extended temperature range of ∼30 K before Tm. In a recent investigation of phase transitions of the ionic liquid triethylsulfonium bis(trifluoromethanesufonyl)imide, [S222][NTf2], we found melting along a sequence of steps of changes on the long-range structure, followed by changes in non-polar domains and then in polar domains until the final melting.21 In contrast to the stepwise melting of [S222][NTf2], the mechanism of melting of [C2C1im][HSO4] should be better characterized as a continuous melting.

-1

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TI = 244 K -1

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Tg = 199 K Tm = 300 K

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Figure 3. DSC scans of [C2C1im][HSO4] obtained by cooling (–1 K min-1, upper curve) followed by heating (+1 K min-1, bottom curve).

Figure 4 shows the low-frequency range of Raman spectra obtained along cooling of [C2C1im][HSO4] to supercooled liquid and glassy phases. It is well-known that the low-frequency Raman spectrum of a liquid is dominated by the quasi-elastic scattering (QES) resulting from polarizability fluctuations due to anharmonicity and complex intermolecular (translational and reorientational) fast relaxation dynamics. The QES intensity strongly decreases as the liquid is supercooled so that intermolecular vibrations become apparent as broad bands in the Raman spectrum of the glass (Ref. [14] and references therein). The Raman spectrum of [C2C1im][HSO4] at 200 K shows the characteristic feature of the glassy phase, that is, the so-called boson peak at ∼23 cm-1. Although the exact origin of the boson peak is still a lively debated issue in the literature concerning the dynamics of the glass transition, there is a consensus that it has a mixed nature of localized vibrations and transverse acoustic modes of high wavevectors (0.1 < Q < 0.5 Å-1).22,23 The low-frequency Raman spectrum of glassy [C2C1im][HSO4] exhibits other features that have been also observed in Raman and optical Kerr effect (OKE)24 spectra of other ionic liquids. The Raman band at ∼100 cm-1 is due to rattling, i.e. hindered rotation, of the imidazolium ring. This assignment is relatively well established in the literature because this band is absent in Raman spectra of ionic liquids based on cations without aromatic ring.14,24,25 In contrast, the band at

∼60 cm-1 is always observed in Raman and OKE spectra of ionic liquids irrespective of the cation (imidazolium, pyrrolidinium, tetraalkylammonium etc.) or the anion. Thus, the 60 cm-1 band seems to be inherent of the condensed phase as it does not depend on



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the actual molecular structure of the ions. In a previous investigation of Raman spectra of ionic liquids under high pressure, we proposed that the 60 cm-1 band also has a partial nature of acoustic modes on the basis of the same pressure induced frequency shift as the boson peak.26 Thus, once the boson peak has contributions of localized and transverse vibrations, the Raman band at ∼60 cm-1 may have partial nature of longitudinal acoustic modes of high wavevectors. This assignment will be reinforced by looking at the changes of the Raman spectra following the solid-solid transitions of [C2C1im][HSO4] to be discussed in the following. The weak band seen in Figure 4 at 165 cm-1 is a spectral feature observed in Raman spectra of [C2C1im]+ based ionic liquids that is assigned to torsional motion of the cation according to quantum chemistry calculations.27,28,29,30 A distinctive feature of the low-frequency Raman spectrum of [C2C1im][HSO4], in comparison to other ionic liquids, is that the spectrum of [C2C1im][HSO4] in the glassy phase clearly exhibits small bumps besides the main features at 23, 60, and 100 cm-1 (see inset of Figure 4). It is worth remembering that the spectral range covered in Figure 4 is the same in which one expects vibrations related to the hydrogen bonds, which are more intense in infrared than Raman spectrum. It has been shown indeed that far-infrared spectroscopy is a powerful tool to access the dynamics of stretching and bending modes of hydrogen bonds in ionic liquids.14,31 It would be interesting for a future work a far-infrared study addressing the intermolecular vibrations directly related to anion-anion hydrogen bonds in [C2C1im][HSO4].


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100 150 -1 wavenumber / cm

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Figure 4. Low-frequency Raman spectra obtained along cooling of [C2C1im][HSO4]. From top to bottom, the temperatures are: 293, 270, 250, 220, and 200 K. The spectra are normalized by intensities of the high-frequency bands. The inset highlights the lowfrequency Raman spectrum of [C2C1im][HSO4] at 200 K.

Heating the glass leads to cold-crystallization of [C2C1im][HSO4] and impinges the significant effects seen in Figure 5 in the spectral range of the anionνs(S=O) mode and low-frequency Raman spectra. The Raman band assigned to νs(S=O) mode of anions engaged in ([HSO4]-)n chains at 1010 cm-1, which is already observed in the glassy phase spectrum, becomes an intense and sharp band in the spectrum of the crystal at 245 K. As the temperature increases, while in the solid state, the intensity of the 1010 cm-1 band decreases. Thus, there exist two crystalline phases of [C2C1im][HSO4]: the crystal I has extended chains of hydrogen bonded anions, unlike crystal II. The lowfrequency Raman spectra of crystals I and II are also distinct. The low-frequency 13 ACS Paragon Plus Environment


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Raman spectrum of crystal I exhibits a lattice mode band at 35 cm-1, whereas its counterpart in the crystal II spectrum is found at 25 cm-1. The softening of the lattice mode from 35 to 25 cm-1 as the system experiences the solid-solid transition nicely illustrates the stiffer crystal I structure having the ([HSO4]-)n chains in comparison to crystal II in which such network is absent. Furthermore, the intermediate Raman band at 70 cm-1 in the crystal I spectrum also softens to 60 cm-1 in the crystal II phase. This finding strongly suggests that the characteristic feature at ∼60 cm-1 seen in Raman and OKE spectra of ionic liquids in the normal liquid phase14,24,25,26 should be assigned to an external mode having partial contribution of lattice-like vibrations. In contrast, the Raman band at ∼115 cm-1 is barely shifted during the crystal I to II transition as one expects from its assignment as rattling of the imidazolium ring.


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30 40 -1 wavenumber / cm

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Figure 5. Raman spectra obtained along heating from the glassy phase of [C2C1im][HSO4] in the spectral ranges of the anion νs(S=O) mode (top panel) and the low-frequency (bottom panel). In both the panels, spectra from bottom to top correspond to temperatures: 200, 245, 270, 280, and 310 K. The spectra have been 15 ACS Paragon Plus Environment


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vertically shifted for helping visualization. The dashed line at 1010 cm-1 in the top panel helps visualization of temperature effect on the νs(S=O) mode of hydrogen-bonded anions in ([HSO4]-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation. Dashed lines at 25, 35, 60, and 70 cm-1 in the bottom panel help visualization of temperature effect on the low-frequency bands characteristic of crystals I and II, respectively, black and red lines. The inset highlights the 10 – 45 cm-1 spectral range for [C2C1im][HSO4] at 245 and 270 K.

The occurrence of solid-solid transition is confirmed in Figure 6 by the X-ray diffraction patterns obtained during heating of [C2C1im][HSO4] in glassy, crystal, and normal liquid phases. The more complex X-ray pattern at 245 K of the crystal I, which has long (HSO4-)n chains, changes to a much more simple pattern as the transition to crystal II is complete. The relatively simple X-ray pattern strongly suggests that absence of (HSO4-)n chains in crystal II leads to a more symmetric structure mainly dictated by anion–cation charge alternation. Summing up the results of Figure 5 and 6, the Raman and X-ray data confirm that the asymmetric peak of cold-crystallization seen in the DSC scan of [C2C1im][HSO4] (see Figure 3) is due to crystallization soon followed by solidsolid transition within a narrow temperature range when the system was heated at 1 K min-1.


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320 K 310 K 295 K

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280 K 265 K 245 K 190 K

0.5

1.0

1.5

2.0

2.5 o

3.0

3.5

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Figure 6. X-ray diffraction data obtained along heating of [C2C1im][HSO4] from the glassy phase up to the liquid phase at the indicated temperatures.

It is worth noting in Figures 5 and 6 that the low-frequency Raman spectrum and the X-ray diffraction pattern of [C2C1im][HSO4] at 310 K still belongs to the crystal II. The low-frequency Raman spectrum of [C2C1im][HSO4] at 310 K exhibits the lattice mode band at 25 cm-1 on the background of the QES intensity, which grows as the temperature increases. Therefore, the resulting solid phase of [C2C1im][HSO4] at room temperature should be characterized as a glacial state, that is, microcrystals surrounded



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by an amorphous phase. We found that once prepared, the glacial phase of [C2C1im][HSO4] remains for several weeks if the sample in the glass flask is kept at room temperature (∼ 293 K). It is clear from the X-ray data of Figure 6 that complete melting is accomplished when temperature is increased to 320 K. In other words, there is a huge hysteresis in the melting of the [C2C1im][HSO4] crystal in such a way that, after the cooling/heating cycle, the sample remains solid at room temperature.

B. Ultraslow dynamics of phase transitions

The Raman spectra shown in the previous section were obtained using a cryostat with a relatively large sample (∼ 1 mL) of [C2C1im][HSO4] inside a glass tube. As an alternative, we used a micro-cryostat that allow the early stage of crystallization of [C2C1im][HSO4] being followed. In the micro-cryostat, just a drop of the liquid is put on the copper plateau of the sample holder. The sample was again cooled to the glassy phase at 180 K then slowly reheated to the supercooled liquid phase. Interestingly, we observed crystallization of [C2C1im][HSO4] starting at 220 K at the border of the drop (see the photograph in Figure 7). It should be noted that 220 K is well below the temperature of cold-crystallization as observed in the DSC scan of Figure 3. This small amount of crystal formed at 220 K did not grow during a period of hours. Thus, Raman microscopy could be used for analyzing the crystal/supercooled liquid border. Figure 7 shows that the Raman spectrum of the small crystal formed at 220 K (red line) exhibits the intense and sharp band at 1010 cm-1 due to anion νs(S=O) mode of (HSO4-)n chains and the low-frequency range has the lattice mode at 35 cm-1. These are the characteristic features of the crystal I discussed in the previous section. The Raman spectrum of the liquid part of the sample at 220 K (black line in Figure 7) is the same as shown in Figs. 18 ACS Paragon Plus Environment

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2 and 4 for [C2C1im][HSO4] in the supercooled liquid state. The sharp Raman band of the νs(S=O) mode at 1010 cm-1 in the crystal I formed at 220 K is more intense than the nearby band at 1021 cm-1 belonging to the imidazolium ring (compare Figure 7 with Figures 2 and 5). This finding strongly suggests that the (HSO4-)n structures are better defined in the crystal I slowly formed at 220 K. From the fact that the drop of [C2C1im][HSO4] sample at 220 K remained the same as in the photograph of Figure 7, it became evident that the very high melt viscosity was precluding crystal growing. By slowly heating the sample in steps of 5 K, we found that crystal growing achieved usual rate of few minutes when the temperature was above 240 K. In other words, the peak of cold-crystallization of [C2C1im][HSO4] in the DSC scan is found at ca. 245 K (see Figure 3) because that is the temperature range at which the rate of crystal growing is commensurate with the 1 Kmin-1 heating rate used in the calorimetric measurement.

* 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

*

* T = 220 K

20 40 60 80 100 120 140

980 1000 1020 1040 1060

wavenumber / cm

-1

Figure 7. Raman spectra of [C2C1im][HSO4] at T = 220 K in the spectral ranges of the anion νs(S=O) mode (right) and the low-frequency (left). The upper right photograph shows the drop (diameter ∼ 0.5 cm) of the ionic liquid on the plateau of the micro19 ACS Paragon Plus Environment


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cryostat. The white spot seen in the left of the drop is a small crystal of [C2C1im][HSO4] formed at 220 K. The border between crystal and supercooled liquid is highlighted in the photograph (∼ 100 µm2) taken from the microscope of the spectrometer. Raman spectra of the crystal (red lines) and supercooled liquid (black lines) were obtained by focusing the laser on each side of the border indicated by red and black stars in the photograph. Arrows at 1045 and 1010 cm-1 indicate Raman bands assigned to anionνs(S=O) mode, the latter related to (HSO4-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation.

The solid-solid transition of [C2C1im][HSO4] is also a very slow process that can be followed by recording Raman spectra at isothermal condition as a function of time. Figure 8 shows the evolution of the Raman spectrum along a period of time of three hours while keeping the [C2C1im][HSO4] sample at 270 K. The intensity of the 1010 cm-1 band of νs(S=O) in (HSO4-)n chains decreases, and the 70 and 35 cm-1 lattice bands shift to 60 and 25 cm-1, as a function of time. Thus, both the νs(S=O) mode and the lowfrequency range slowly change from the pattern of crystal I to crystal II. Overall, Raman and X-ray data indicate that a mixture of crystals I and II can be formed depending on the heating rate because cold-crystallization is closely followed by solid-solid transition within a small temperature range.


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T = 270 K 1h 2h 3h

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

20

40

60

80 100 120 140

*

980 1000 1020 1040 1060 -1

wavenumber / cm

Figure 8. Raman spectra of [C2C1im][HSO4] in the spectral ranges of the anion νs(S=O) mode (right) and the low-frequency (left) obtained in isothermal condition while keeping the sample at T = 270 K by 1 h (black lines), 2 h (red lines), and 3 h (green lines). The arrow at 1010 cm-1 indicates the Raman band assigned to νs(S=O) mode of (HSO4-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation.

Melting of [C2C1im][HSO4] is the thermal event with the most evident consequence of the high melt viscosity. After a cycle of cooling and heating back to the room temperature, the liquid sample of [C2C1im][HSO4] ended as a solid. The photographs of Figure 9 show the sample at room temperature before and after the cooling/heating cycle. Raman spectra of liquid and solid [C2C1im][HSO4] samples at room temperature are shown in Figure 9 by black and red lines, respectively. The 21 ACS Paragon Plus Environment


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Raman spectrum of the solid in the high-frequency range of the anion νs(S=O) mode is characteristic of the crystal II phase proper to the absence of the band due to (HSO4-)n chains at 1010 cm-1. The lattice mode characteristic of the crystal II is seen only as a bump at 25 cm-1 on the QES intensity. Thus, the very slow melting of [C2C1im][HSO4], that is already evident in the DSC scan of Figure 3 as a continuous process, results in a macroscopically solid sample of [C2C1im][HSO4] at room temperature. However, the Raman spectra and simple visual inspection indicate that the solid sample of [C2C1im][HSO4] obtained after the cooling/heating cycle is a mixture of crystals and amorphous phase, i.e. a glacial state.

T = 294 K

* 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

20 40 60 80 100 120 140

980 1000 1020 1040 1060 -1

wavenumber / cm

Figure 9. Raman spectra in the spectral ranges of the anion νs(S=O) mode (right) and the low-frequency (left) of [C2C1im][HSO4] at room temperature (294 K). The photographs show the sample in the liquid phase (left) and the glacial phase obtained after the cooling/heating cycle (right). Raman spectra in black and red lines correspond to liquid and glacial phases, respectively, both at room temperature. Arrows at 1045 and


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1010 cm-1 indicate Raman bands assigned to anionνs(S=O) mode, the latter related to (HSO4-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation.

The sample of [C2C1im][HSO4] in glacial state shown in the photograph of Figure 9 is a soft solid, which melts upon mechanical perturbation such as scratching. We performed an X-ray diffraction measurement at room temperature by carefully taking a small amount of this soft solid. The X-ray patterns shown in Figure 10 clearly indicate that mechanical treatment by spinning the sample holder is enough for prompt melting of the soft solid. In fact, it has been shown decades ago32 that crystal of a simple salt, namely, cesium hydrogen sulfate, CsHSO4, is susceptible to mechanical treatment. Infrared and Raman spectroscopies showed that long structures of hydrogen bonded anions are destroyed by grinding or pressing crystalline CsHSO4.32 In the case of [C2C1im][HSO4], we found that the glacial state remains for an indefinite period of time if the solid sample in the flask is kept still at room temperature.



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T = 294 K

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

0.5

1.0

1.5

2.0 o

2.5

3.0

-1

Q/A

Figure 10. X-ray diffraction data of [C2C1im][HSO4] in the glacial phase at room temperature. From top to bottom, the figure shows the data collected at different times along ca. 5 min while spinning the sample until recovering the broad XRD pattern of the liquid phase.

The ultimate reason for slow dynamics of cold crystallization, solid-solid transition, and melting during heating of [C2C1im][HSO4] is formation and disruption of anion-anion hydrogen bonded structures. It is worth noting that the (HSO4-)n chains of the crystal I structure are already present in the liquid and they grow as the liquid is


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supercooled (see Figure 2). However, for the very same reason, the ionic liquid [C2C1im][HSO4] is a good glass-former because nucleation and crystal growth is hampered by the sluggish liquid dynamics. Furthermore, one expects that the hydrogen bond network impinges its signature on the fragility of the glass-forming ionic liquid [C2C1im][HSO4]. Fragility is usually quantified by the steepness of the temperature variation of viscosity η close to Tg in the plot logη vs. Tg/T.33,34,35 The reciprocal of temperature is normalized by Tg so that different glass-formers can be compared in the same plot. The fragility index is defined by the derivative of this plot at T → Tg. Experimental data of transport coefficients of ionic liquids close to Tg are scarce, but the available data indicate that ionic liquids are in general fragile glass-formers (see references cited in [36]). This is expected proper to the dominance of non-directional Coulombic and van der Waals intermolecular interactions in ionic liquids. On the other hand, once anion-anion hydrogen bonded structures occur in the normal liquid phase of [C2C1im][HSO4], one expects that it should be a relatively less fragile glass-forming liquid. Experimental viscosity data of [C2C1im][HSO4] have been published within the 283–373 K range,13 but in the authors knowledge no viscosity data are available for [C2C1im][HSO4] in the deep supercooled liquid state. It has been suggested that whether a glass-former is strong or fragile could the anticipated from the liquid viscosity at relatively high temperature.37,38 This proposition comes from the fact that the isochronous conditions at both the glass transition and very high temperatures constrain by construction the full shape of the normalized logη vs. Tg/T plot. Figure 11 shows for a comparison purpose η(T) data of [C2C1im][HSO4]13 and another common ionic liquid, namely [C4C1im][BF4].39 In a previous publication,40 we have performed fit of available

η(T) data of ionic liquids by the Vogel-Fulcher-Tamman (VFT) equation with 25 ACS Paragon Plus Environment


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constraints η = 10-3 Poise at T → ∞ and η = 1011 Poise at Tg. The VFT equation,

η = AeB/(T-To), has A = 10-3 Poise, and B and To parameters limited to a narrower range when the isochronous conditions are imposed on the fit procedure of data around and above the room temperature. The fragility index is given by the VFT parameters as m = B/2.303×[Tg/(Tg-To)2]. Taking for instance [C4C1im][BF4], we have found B = 935 K, To = 161 K, thus m = 92.40 This fragility index was first predicted and then indeed obtained from a detailed rheology analysis carried out for the supercooled liquid and the glassy phases of [C4C1im][BF4].41 In the case of [C2C1im][HSO4], the constrained VFT fit shown in Figure 11 gives B = 1390 K, To = 156 K, therefore m = 65. In a recent dielectric spectroscopy study of a related system, 1-butylimidazolium hydrogen sulfate, [C4im][HSO4], a similar value (m = 63) has been found from the temperature dependence of ionic conductivity.42 The absence of the methyl group in the [C4im]+ cation allows for another site for hydrogen bonding in the ionic liquid [C4im][HSO4]. However, the closeness of m strongly suggests that the most important factor determining the liquid fragility of [C2C1im][HSO4] and [C4im][HSO4] is the connectivity given by the anion-anion hydrogen bonds. Nevertheless, the predict fragility index m = 65 for [C2C1im][HSO4] entitles this system as a moderately strong glass-forming ionic liquid.


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12 10 log (η / Poise)

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

8

[C2C1im][HSO4] [C4C1im][BF4]

6 4 2 0 -2 -4 0.0

0.2

0.4

0.6

0.8

1.0

Tg / T Figure 11. Experimental viscosity data of [C2C1im][HSO4] (circles, Ref. [13]) and [C4C1im][BF4] (triangles, Ref. [39]). The full lines are VFT fit to experimental data with constraints at high temperature and Tg.

C. Comparing [C2C1im][HSO4] and [C4C1im][HSO4]

A crystallographic study of single crystal of 1-butyl-2,3-dimethylimidazolium hydrogen sulfate, [C4C1C1im][HSO4], revealed the presence of ([HSO4]-)2 dimers rather than ([HSO4]-)n chains.19 It is not clear whether there is another crystalline structure [C4C1C1im][HSO4] besides the one investigated in Ref. [19]. Nevertheless, the molecular structure of the [C4C1C1im]+ cation has two significant differences in comparison to the [C2C1im]+ cation considered in this work. The methylation in the carbon atom C(2) of the imidazolium ring in [C4C1C1im]+ blocks the most important site for cation–anion hydrogen bonding. Furthermore, the butyl chain allows for more van 27 ACS Paragon Plus Environment


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der Waals interactions and eventual development of non-polar domains. It would be interesting to check whether one of these factors alone might preclude formation of crystals with ([HSO4]-)n chains. Thus, in order to single out the alkyl chain length effect, we provide here a brief comparison between the [C2C1im][HSO4] data discussed above with the counterpart 1-butyl-3-methylimidazolium hydrogen sulfate, [C4C1im][HSO4]. The effect of longer alkyl chain is enough for one significant difference in the DSC scan of [C4C1im][HSO4] in comparison to [C2C1im][HSO4]. Figure 12 shows the DSC scan of [C4C1im][HSO4]. The cooling curve shows that [C4C1im][HSO4] is also a good glass-former as expected, and the heating curve shows that melting is a continuous process. Alike [C2C1im][HSO4], we found that a cycle of cooling and heating of [C4C1im][HSO4] also ends at a glacial state at room temperature. However, the peak of cold crystallization of [C4C1im][HSO4] at 241 K does not exhibit any shoulder that could suggest a solid-solid transition.

-1

-1Kmin

Heat flow- exo up

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

Tc = 241 K

+1Kmin

-1

Tg = 204 K

200

220

Tm = 293 K

240

260 T/K

280

300

Figure 12. DSC scans of [C4C1im][HSO4] obtained by cooling (–1 K min-1, upper curve) followed by heating (+1 K min-1, bottom curve).


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The indication in the DSC scan that no solid-solid transition takes place in [C4C1im][HSO4] is corroborated by the Raman spectra as a function of temperature. Figure 13 shows Raman spectra obtained during heating of [C4C1im][HSO4] from the glassy phase. The Raman spectrum of the supercooled liquid at 230 K (black line) exhibits the 1010 cm-1 band seen as a shoulder in the lower frequency side of the imidazolium ring band at 1025 cm-1. The low-frequency Raman spectrum of [C4C1im][HSO4] at 230 K has the typical pattern of the deep supercooled liquid. The 1010 cm-1 Raman band is the signature of νs(S=O) mode of anions in ([HSO4]-)n chains in supercooled [C4C1im][HSO4]. The Raman spectrum of [C4C1im][HSO4] at 245 K (red line) belongs to the system after cold crystallization. It is clear that the 1010 cm-1 Raman band is absent in the spectrum of the crystalline phase of [C4C1im][HSO4]. The Raman spectrum of [C4C1im][HSO4] crystal, including the low frequency band at ∼ 25 cm-1, is analogous to the spectrum discussed in the previous sections for [C2C1im][HSO4] in the crystal II phase, which does not exhibit ([HSO4]-)n chains. The spectrum of [C4C1im][HSO4] keeps essentially the same pattern when heating from 245 K to 294 K (green line in Fig. 13) proper to the solid phase remaining at room temperature after the cooling/heating cycle. Alike the glacial state of [C2C1im][HSO4], it is also needed warming up to ∼320 K for complete melting of the glacial [C4C1im][HSO4].



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*

T/K 230 245 294

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

50

100

150

960

990

1020

1050

1080

-1

wavenumber / cm

Figure 13. Raman spectra obtained along heating from the glassy phase of [C4C1im][HSO4] in the spectral ranges of the anion νs(S=O) mode (right) and the lowfrequency (left). Raman spectra in black, red, and green lines correspond, respectively, to 230, 245, and 294K. The arrow at 1010 cm-1 indicates the Raman band assigned to

νs(S=O) mode of (HSO4-)n chains. The Raman band at 1021 cm-1 marked with asterisk is assigned to an imidazolium ring vibration of the [C2C1im]+ cation.

Summing up the comparison between the DSC and Raman data for [C2C1im][HSO4] and [C4C1im][HSO4], the latter does not pass through a crystalline phase with ([HSO4]-)n chains. In line with the crystallographic study of [C4C1C1im][HSO4],19 we proposed that [C4C1im][HSO4] forms crystal with ([HSO4]-)2 dimers, but not ([HSO4]-)n chains as [C2C1im][HSO4]. It is worth stressing that the previous Raman spectroscopic study of liquid, supercooled liquid, and glassy phases had already shown that both [C2C1im][HSO4] and [C4C1im][HSO4] have extended anion-anion hydrogen bonded structures.10 The far reaching conclusion drawn from the comparison of the cold crystallization processes between [C2C1im][HSO4] and 30 ACS Paragon Plus Environment

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[C4C1im][HSO4] is that just the presence of a longer alkyl side chain in the latter disfavors the ([HSO4]-)n chains of being maintained in the crystalline structure. Larger number of sites for van der Waals interactions between butyl chains of [C4C1im][HSO4] in comparison to [C2C1im][HSO4] seems to dominate over formation of extended chains of hydrogen bonded anions during crystal packing. This finding nicely illustrates the subtle competition between intermolecular interactions of different nature existing simultaneously in ionic liquids.

IV. CONCLUSIONS We showed by Raman spectroscopy that one crystalline phase of [C2C1im][HSO4] has extended ([HSO4]-)n chains of anions, whereas such chains are disrupted in another crystalline phase. Anion-anion hydrogen bonds in [C2C1im][HSO4] imply a series of interrelated consequences on the liquid dynamics and phase transitions: very high viscosity of the melt, good glass forming ability, relatively low fragility index, sluggish dynamics of crystallization and solid-solid transition, continuous melting, and glacial state at room temperature. The interesting difference we found between [C2C1im][HSO4] and [C4C1im][HSO4] is that there is no evidence of a crystalline phase with ([HSO4]-)n chains in the latter. Therefore, larger contribution of van der Waals interactions between butyl chains of [C4C1im]+ cations is competing with formation of long ([HSO4]-)n chains during ionic packing into the crystalline arrangement. Irrespective of considering the [C2C1im]+ or the [C4C1im]+ cation, the presence of structures of hydrogen-bonded anions, and its consequence on the fragility of the glass-forming liquid, deserves addressing the elastic and viscous properties of [HSO4]- based ionic liquids. Rheology measurements of these interesting ionic liquids will be the subject of future studies. 31 ACS Paragon Plus Environment


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ACKNOWLEDGMENT

The authors are indebted to FAPESP (Grants 2015/05803-0, 2014/15049-8, and 2012/13119-3) and CNPq for financial support. They also thank the CEM – UFABC for DSC and temperature dependent XRD measurements.

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