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Contents lists available at ScienceDirect

Journal of Molecular Liquidsjournal homepage: http://ees.elsevier.com

High-pressure crystal polymorph of the protic ionic liquid: Ethylammonium nitrateHiroshi Abe a,⁎, Yoshihiro Koyama b, Hiroaki Kishimura a, Kiyoto Matsuishi b

a Department of Materials Science and Engineering, National Defense Academy, Hashirimizu 1-10-20, Yokosuka, 239-8686, Japanb Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8573, Japan

A R T I C L E I N F O

Article history:Received 25 June 2020Received in revised form 27 July 2020Accepted 30 July 2020Available online xxx

KeywordsEthylammonium nitrateProtic ionic liquidCrystal polymorphHydrogen bonding

A B S T R A C T

A high pressure (HP) crystal polymorph was obtained for the protic ionic liquid ethylammonium nitrate (EAN).Crystal structures of the HP phases were determined by X-ray diffraction. The formation of either a single crystalor polycrystal depended on the temperature and pressure pathway. In a previous work, a low temperature (LT)crystal polymorph was identified, and multiple crystallization pathways were observed by simultaneous X-raydiffraction and differential scanning calorimetry measurements. The non-reversible HP-crystal polymorph wasdifferent from the LT-one. Despite the simple molecular structure of EAN, HP-crystal polymorph is observed inaddition to LT-crystal polymorph and multiple LT-crystallization pathways.

© 2020

1. Introduction

The first ionic liquid (IL), ethylammonium nitrate (EAN), was syn-thesized by Paul Walden in 1914 [1,2]. ILs are composed of a cation-an-ion ion pair with low melting points. Electrochemically, ILs can be di-vided into aprotic and protic ILs (pILs). EAN belongs to the latter. Re-cently, pILs have been utilized as fuel cell electrolyte [3,4]. The pro-ton transfer-based physicochemical properties of pILs have been thor-oughly investigated [5–8]. Proton thermodynamics of pILs was analyzedby voltammetry, pKa studies, ionicity, and Gurney diagrams consideringa Grotthuss-type proton transfer mechanism [6,7]. The Grotthuss trans-port mechanism was originally described to explain fast proton transferin water [9,10]. Considering the indirect mass transfer of proton, theionicity of pILs has been investigated both experimentally and theoreti-cally [11–13]. The hydrogen bonding network in pILs is crucial for theinterpretation of the Grotthuss-type transport.

Detailed liquid structures of partially deuterated EAN [14] andpropylammonium nitrate (PAN) [15] were determined by neutron dif-fraction. Using empirical potential structure refinement, sponge-likenano-structures in pILs were observed in the simulation box. More-over, the prepeak at low Q for EAN, PAN, and butylammonium nitrate(BAN) was observed by high energy X-ray diffraction [16]. Combinedwith molecular dynamics (MD) simulations, the liquid structures of pILswere analyzed, and strong hydrogen bonding was estimated [16]. Ra

⁎ Corresponding author.E-mail address: ab@nda.ac.jp (H. Abe)

man spectra and density functional theory (DFT) calculations showedclustering in EAN, PAN and BAN [17]. The DFT calculations, whichagreed with the observed Raman bands, indicate the existence of anasymmetric hydrogen bonding network. The acid–base properties ofEAN representing the proton behavior were obtained by potentiometrictitration [18]. The pH value windows of EAN were determined.

Generally, the crystal structure indicates the orientational and posi-tional orders of the molecules in a unit cell. Information of the molec-ular orders in a crystal reflects the molecular interactions in the liquidstates. The crystal structure of methylammonium nitrate (MAN) was de-termined by X-ray diffraction [19]. Its bonding distances were preciselyestimated by crystal structure analysis. In addition, Raman spectroscopyand DFT calculations allowed to determine the hydrogen bonding net-work. For EAN, the modulation of the [NO3]− anions occurred at lowtemperature (LT) in the unit cell [20]. It was interpreted that LT-crystalpolymorph is derived from molecular orientational disorders. Interest-ingly, a crystal polymorph and multiple crystallization pathways wereobserved at LT by the simultaneous X-ray diffraction and differentialscanning calorimetry (DSC) measurements [21]. Proton behavior couldinfluence the crystallization pathway. Crystal polymorphs of dimethy-lammonium nitrate (DAN), MAN and PAN were also detected by simul-taneous X-ray diffraction and DSC measurements [22]. The solid statesof DAN and MAN at room temperature possess plastic crystal charac-teristics, while the liquid states of EAN and PAN at room temperatureare characterized by multiple crystallization pathways. A few studies onpILs under high pressure (HP) have been carried out [23,24]. The crys-tallization of PAN at up to 2–4 GPa was examined by HP-Raman spec-troscopy, and its crystal domains were found to be inhomogeneous.

https://doi.org/10.1016/j.molliq.2020.1139590167-7322/© 2020.

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HP-crystal polymorph is significant in pharmaceutical field [25]. Or-ganic molecules such as pharmaceuticals easily change the propertiesby the pressure-induced crystal polymorphs. Under HP, the metastablephases were formed by crystal packings [26], conformations varieties[25], and hydrogen bonding [27] on the process for the minimization oftotal potential energy. Recently, a new ethane polymorph under HP wasdiscovered [28]. The HP-polymorph of ethane was compared with theLT-one. Orientational disorder under HP was clarified by single crystalstructure analysis [28]. The crystal free energy between the LT and HPphases was estimated.

In this study, a HP-crystal polymorph of EAN was observed by X-raydiffraction. The non-reversible HP-crystal polymorph was different fromthe LT-one. Polycrystal and single crystal formations were sensitive tothe thermal and pressure treatments.

2. Experimental

The hydrophilic pIL was acquired from Kanto Chemical Co. (98%,Fig. 1(a)). A few ruby balls and the sample were placed into a dia-mond anvil cell (DAC) inside a glove bag with a continuous flow of drynitrogen gas with a relative humidity below 10% (silica gel was usedas desiccant). Using a stainless-steel gasket, the sample and ruby balls

were sealed in the DAC (diameter: 0.25–0.40 mm; thickness:0.15–0.25 mm).

HP X-ray diffraction measurements were performed in the DAC usingsynchrotron radiation at the beamline BL-18C of the Photon Factory atthe High Energy Accelerator Research Organization, Japan. The wave-length and camera length were calibrated at 0.8349 Å with a CeO2 stan-dard. An imaging plate (IP) system (BAS2500, FujiFilm Co., Japan) wasused to obtain the two-dimensional (2D) Debye rings of the sample [29].The 2D data were reduced to 1D diffraction patterns to minimize thepreferred orientation on the Debye rings. The pressure was monitoredfrom the R1 fluorescence line of the ruby balls.

The crystal structure was analyzed by a combination of the FOXsoftware [30] and Conograph software [31]. First, possible lattice pa-rameters were calculated using Conograph; then, a global optimizationachieving using FOX allowed to identified the space group.

3. Results and discussion

3.1. Single crystal and polycrystal formations

Crystallizations of EAN are sensitive to the thermal and compress-ing processes (Fig. 2). In this study, crystallization pathways (i) and (ii)were performed. With increasing pressure described by pathway (i) at

Fig. 1. (a) Molecular structures of ethylammonium cation, and (b) crystal formations on the temperature and pressure scales. scL reveals the supercooled liquid of EAN. Immediate crys-tallization of HP-single crystal and self-mediated LT-polycrystal are described by strong ion-paring and hydrogen bonding, respectively.

Fig. 2. Single crystal under HP and polycrystal formations at LT by optical microscope observations. HP-polycrystal was obtained from the LT-crystal.

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room temperature (Fig. 2), EAN easily crystallized at room tempera-ture. The single crystal domain appeared by optical microscopy, and thesharp Bragg spots were detected on the IP (Fig. 2). The transition pres-sure (PC) was 0.08 GPa (Fig. 2). Since DACs have a large blind angle,only a few Bragg spots can be obtained from the single crystal. Due tothe lack of information, crystal structure analysis was not possible usinga DAC and single crystal.

Pathway (ii) consists of two processes: one is cooling, and the otheris pressing. The cooling in pathway (ii) was considered to obtain theLT-polycrystal. In a previous study, LT crystallization of EAN was re-ported at −14.8 °C (TC) [21]. Here, liquid EAN was cooled below theTC in the DAC. After cooling, several crystal domains were observed un-der an optical microscope. Then, the LT-crystal was gradually pressedat room temperature. According to observations under the optical mi-croscope, the phase change occurred at 2.2 GPa, which was when theDebye rings appeared on the IP. The analysis indicated that the crys-tal structure of the new phase was different from that in the LT-phases(Table 1), demonstrating that the HP-polycrystal was successfully ob-tained using the LT-polycrystal. The new phase was named HP-α.

Moreover, we found that completely different crystal formations areinduced on the temperature and pressure scales, Fig. 1(b). Hydrogenbonding in pILs plays different roles at LT or HP. At ambient pressure,EAN existed as a supercooled liquid (scL) at a wide temperature range[21]; the crystallization temperature (TC) was −14.8 °C and the meltingpoint (Tm) was 12 °C. The dynamical nano-heterogeneity of pILs con-tribute to the scL. By quasielastic neutron scattering, dynamics of sin-gle-particle and cooperative components of the pILs is decomposed [32].Combined with the MD analysis, a relationship between dynamic het-erogeneity and nano-structure of the pILs were interpreted by stronghydrogen bonds. DFT calculations showed a strong hydrogen bondingfor EAN [17,33]. Despite being strongly hydrogen-bonded, nondiffu-sive reorientational dynamics was observed by dielectric relaxation (DR)spectroscopy and femtosecond-infrared spectroscopy [34]. Also, DR andoptical Kerr-effect spectra suggest that large-angle jumps occur in liq-uid EAN [35]. Furthermore, such large-angle jumps were analyzed byMD simulations [36], and the jump angles with hydrogen bond switch-ing were estimated. Additionally, hydrogen bond dynamics of MAN wasstudied by ab initio MD [37]. The dynamics is coupled with the ion pairformation of the methylammonium cation and the [NO3]− anion. There-fore, we can assess that proton behavers disturb the crystal nucleationat ambient pressure, Fig. 1(b).

Under HP, hydrogen bonding in crystals promotes the various net-works by changing the bonding distances. In a molecular system con-taining hydrogen bonds, the HP-crystal polymorph occurs coupled withmolecular conformations [27]. By single crystal X-ray diffraction, theaccurate bonding lengths of the hydrogen bonding at each HP-phases

were determined, and the hydrogen bonds are found to be compressibleparts in the crystals. The length-tunable hydrogen bonding is character-ized by the HP-polymorph. In case of EAN, under HP, EAN immediatelycrystallized at 0.08 GPa from liquid (Fig. 2). Moreover, the formationof a single crystal implies that ion pairing and hydrogen bonding areenhanced under HP. The HP leads to not only small intermolecular dis-tances but also compressible hydrogen bonding [27].

3.2. EAN HP-crystal polymorph

Pathway (ii) in Fig. 2 indicates the HP-polycrystallization of EAN.The HP-polycrystal was obtained from the LT-crystal. X-ray diffractionand optical microscopy were used to analyze the HP-crystal polymorphof EAN. As mentioned in Section 3.1, the HP-α phase was formed at2.2 GPa. One of the possible space groups was C2/c (monoclinic), aslisted in Table 1. The calculated density of the HP-α phase is compara-ble to that of the LT-phases. Further pressing led to a crystal morphol-ogy change at 3.0 GPa (Fig. 2). Because of the fine crystal domains,the sample color became darker. Also, new Bragg reflections appearedin the X-ray diffraction pattern (Fig. 3(a)). The new and small peaksare represented by the open circles in Fig. 3(a). The observed Bragg in-tensities at 0.8 Å−1 and 2.0 Å−1 were different from the calculated onesof the HP-α phase (red curve in Fig. 3(a)). In fact, high reliability fac-tors (wR and R) of the HP-α phase were obtained (Table 1). Thus, wefound another possible lattice using Conograph. The possible lattice isnamed HP-α’ phase, since the lattice parameters were similar to thoseof the HP-α phase except for the β angle (Table 1). The calculated in-tensities of the HP-α’ phase are expressed by the green curve in Fig.3(a), and peaks at 0.8 Å−1 and 2.0 Å−1 were more comparable to the ob-served ones. The smaller wR and R of the HP-α’ phase indicate that thephase transition from HP-α to HP-α’ phases occurred at 3.0 GPa. Whenfurther increasing the pressure, at 4.0 GPa, the sample became partlybright (Fig. 2). Also, the crystal domain structure was remarkably mod-ified. The X-ray structure analysis showed an increase in the density ofthe HP-α’ phase.

At 6.2 GPa, Bragg intensities of the X-ray diffraction patternchanged, and the R-factors of the HP-α’ phase notably improved. The βangle of the HP-α’ phase gradually increased as the pressure increasedfrom 3.0 to 6.2 GPa (Table 1). Bragg reflections were still sharp evenat 6.2 GPa. Moreover, peak splitting of the Bragg reflections at around2 Å−1 was observed. Further increasing the pressure led to moving crys-tal domain boundaries, and the color of the sample became even darker(Fig. 2). A clear crystal-crystal phase transition was observed by opti-cal microscopy at 8.1 GPa. In the X-ray diffraction pattern, additionalpeaks denoted by the closed circles (Fig. 3(a)) appeared, and peakbroadening simultaneously occurred. This new phase was named HP-β.

Table 1Crystallographic data of low-temperature [21] and high-pressure phases. a, b, c, α, β, and γ reveal lattice parameters in the unit cell. Z is number of cation-anion pairs in the unit cell. wRand R indicate weighted reliability factor and reliability factor, respectively.

Spacegroup

a(Å)

b(Å)

c(Å)

α(°)

β(°)

γ(°) Z

ρ(g/cm 3)

wR(%)

R(%)

T (°C)−90.2 I P 7.521 9.653 11.479 111.6 100.4 98.0 6 1.427 9.9 11.8−28.6 II P21/c 9.785 11.447 9.857 90 112.9 90 8 1.412 8.4 9.5P (GPa)2.2 α C2/c 11.031 15.595 7.195 90 98.2 90 10 1.465 3.6 3.63.0 α C2/c 10.912 15.698 7.252 90 97.3 90 10 1.457 37.1 30.43.0 α’ C2/c 9.057 15.530 7.307 90 106.4 90 8 1.456 31.3 18.24.0 α’ C2/c 9.046 15.350 7.301 90 106.8 90 8 1.480 37.1 25.36.2 α’ C2/c 9.150 15.182 7.212 90 107.5 90 8 1.503 14.5 10.78.1 β C2/c 18.087 6.841 11.678 90 126.7 90 10 1.549 11.9 8.91.0 γ P21 7.669 24.106 8.769 90 114.8 90 12 1.463 3.9 4.1

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Fig. 3. Pressure dependence of X-ray diffraction patterns upon (a) compression and (b)decompression. Red, green, blue, and purple curves reveal the calculated X-ray diffractionpatterns of α, α’, β, and γ phases, respectively. Thick grey lines express the observed X-raydiffraction patterns. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

The crystal structure was also monoclinic, but the lattice constants weredifferent than those of the α and α’ phases (Table 1). Moreover, the βangle discretely increased at 8.1 GPa. During the compression process,the HP-crystal polymorph is described by the α’-β phase transition at8.1 GPa.

During the decompression process, crystal domain structures of theHP-β phase did not show any changes down to 2.8 GPa (Fig. 2). Oncean HP-crystal is stabilized by the strong ion pairing and short intermole-cular distances, phase changes were inhibited even upon decompression.However, at 1.0 GPa, a drastic morphological change observed by opti-cal microscopy suggests a decompression-induced phase transition (Fig.2). The pattern for this phase corresponded to a domain structure dif-ferent from that of HP-α, HP-α’, and HP-β. The phase transition is moreclearly observed in the X-ray diffraction pattern (Fig. 3), which is com-pletely different from those of the other phases. This indicates that themolecular arrangement in the crystal was induced by a large volumechange. The space group of the crystal structure was P21 (Table 1). Thisdecompression-induced phase was named HP-γ. Large pressure differ-ence and large volume change from the HP-β to the HP-γ phases meanthat the HP-β phase is much stabilized under HP. Decompression-in-duced crystal polymorphs were observed in many systems [38,39]. Incase of EAN, HP-crystal polymorph was not reversible. One of possiblereasons is the compressible hydrogen bonding under HP. Distance-tun-able hydrogen bonding can compensate the large volume change fromthe HP-β to the HP-γ phases by changing the hydrogen bonding network.It is worth noting that the crystal did not melt even at ambient pressure.This means that, for EAN, the ion pair formation and hydrogen bondingare enhanced under HP. It was also confirmed that the compressible hy-drogen bonding contributes to the HP-inherent crystallization.

3.3. Complicated phase behaviors and crystal structures

In the HP-crystals, the unit cells were relatively large despite thesmall size of molecules (Table 1). The molecular positional and orien

tational orders of the [NO3]− anions caused the lattice modulations bothat LT and HP. Crystal structures and molecular arrays of the [NO3]−

anions are shown in Fig. 4(a)-(c). For simplicity, hydrogen is omitted;also, it is quite difficult to probe hydrogen in X-ray diffraction. For com-parison, one of LT-phases (phase I) obtained in a previous study [21]is also shown in Fig. 4(a). The space group of phase I was P (Table1). A zigzag 1D chain-like order was developed for the [NO3]− anion inthe unit cell. O O distance between each [NO3]− anions along 1D was2.55–2.87 Å. The 1D chain develops along the [01 ] direction. AnotherLT-crystal structures of EAN was previously determined by single crystalX-ray diffraction [20]. The space group was P21/c (phase II) in Table 1,and the crystal structural feature was described by the sinusoidal mod-ulation of [NO3]− anions. In addition, a 2D network of [NO3]− anionsdescribes the HP-α’ phase at 6.2 GPa (Fig. 4(b)). The 2D [NO3]− sheetson the (h0ℓ) planes stack along the b-direction. The O O distancebetween each [NO3]− anions on the 2D plane was 2.59–3.37 Å, whilethe O O distance along the b-direction was 8.58 Å. In contrast, a 3Dnetwork of [NO3]− appeared in the HP-γ phase at 1.0 GPa (Fig. 4(c)).The 3D network is described by the O O distance of 3.42–4.54 Å. Thelargest unit cell in the HP-γ phase was characterized by the longest bconstant.

LT-crystal polymorph and multiple crystallization pathways areschematically illustrated in Fig. 5, in order to compare the HP-crys-tal polymorph in this study. Simultaneous X-diffraction and DSC mea-surements can distinguish the complicated phase behaviors of EAN atLT [21]. The bifurcation temperature TB determined the crystallizationpathways at LT. In the phase I′ and phase II', HNO3 partially coexisteddue to the local proton transfer. As expected, the non-reversible HP-crys-tal polymorph of EAN was different from the LT-one. At LT, the disor-dered rotator crystalline phase caused the LT-crystal polymorph [20].On the other hand, HP-crystal polymorph could be induced by the strongion pairing and compressible hydrogen bonding to prefer higher molec-ular packing under HP. In fact, densities of the HP-crystal phases wererelatively large as listed in Table 1. According to the LT- and HP-crys-tal polymorphs, and the multiple LT crystallization pathways, EAN is acomplicated liquid in spite of its small molecular size and little molec-ular conformations. The variety of phases reflects the various molecularinteractions in liquid EAN.

4. Conclusions

Crystallization of EAN as a pIL occurred immediately after applyinga low pressure. The superpressurized liquid existed at a narrow pressureregion. Under HP, a single crystal was formed due to the strong ion pair-ing and hydrogen bonding. The non-reversible HP-crystal polymorphwas detected in addition to the LT-crystal polymorph. The HP-crystalpolymorph is derived from the strong ion pairing and the compressiblehydrogen bonding due from higher molecular packing. The large unitcells of the HP-crystals were caused by the specific molecular orders ofthe [NO3]− anions. Several kinds of solid phases correspond to the com-plicated molecular interactions of EAN.

CRediT authorship contribution statement

Hiroshi Abe: Conceptualization, Writing - original draft, Writing -review & editing, Validation. Yoshihiro Koyama: Data curation, For-mal analysis, Validation. Hiroaki Kishimura: Data curation, Formalanalysis, Validation. Kiyoto Matsuishi: Supervision, Validation.

Declaration of competing interest

There are no conflicts to declare.

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Fig. 4. Unit cells of (a) phase I at LT (P ), HP-α’ phase (6.2 GPa), and (c) HP-γ phase (1.0 GPa).

Acknowledgements

We appreciate the helpful discussion with Dr. Takekiyo and Prof.Y. Yoshimura of National Defense Academy. We also thank Prof. N.Hamaya of Ochanomizu University for experimental supports. We ac-knowledge the supports of Photon Factory (Proposal Nos. 2010G546,2013G017, 2015G083, 2017G021, and 2019G006).

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Fig. 5. Schematic illustration of LT- and HP-crystal polymorphs, and LT-multiple crystal-lization pathways in EAN. TB reveals the bifurcation temperature.

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