Phase behaviour and heat capacities of selected 1-ethyl-3-methylimidazolium-based ionic liquids

Journal Pre-proofs Phase behaviour and heat capacities of selected 1-ethyl-3-methylimidazoliumbased ionic liquids Vojtěch Štejfa, Jan Rohlí ček, Ctirad Červinka PII: DOI: Reference:

S0021-9614(19)30870-5 https://doi.org/10.1016/j.jct.2019.106020 YJCHT 106020

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J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

3 October 2019 26 November 2019 27 November 2019

Please cite this article as: V. Štejfa, J. Rohlí ček, C. Červinka, Phase behaviour and heat capacities of selected 1ethyl-3-methylimidazolium-based ionic liquids, J. Chem. Thermodynamics (2019), doi: https://doi.org/10.1016/ j.jct.2019.106020

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Phase behaviour and heat capacities of selected 1ethyl-3-methylimidazolium-based ionic liquids. Vojtěch Štejfa a,*, Jan Rohlíček b, Ctirad Červinka a a

Department of Physical Chemistry, University of Chemistry and Technology, Prague,

Technická 5, 166 28 Prague 6, Czech Republic b

Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech

Republic

*Corresponding author: [email protected]

1

Abstract Ionic liquids start to leave the position of novel and astonishing compounds and become commercially available in a relatively high purity. Consequently, their thermodynamic properties should be described in a more thorough way than in the times of first pioneering studies revealing the importance of the water content. Available thermodynamic data, however, contradict this assumption keeping large discrepancies even among the data published in the most recent papers. Eight common ionic liquids based on the 1-ethyl-3-methylimidazolium cation, [C1C2Im], which are coupled with simple organic or inorganic anions are selected in this work and their phase behaviour, heat capacities, and crystal structures are studied. Special attention is paid to drying of the samples and evaluation of the effect of water content on the melting temperature. Resulting melting temperatures, fusion enthalpies, and heat capacities are compared to the available literature data. For the compounds, which melt above the room temperature, crystal structures were determined using the X-ray powder diffraction experiments to identify the studied crystalline phases. The thermodynamic and crystallographic data were used simultaneously to interpret the polymorphism of the investigated ionic liquids, where it occurred. Moreover, a crystal structure for the dimethyl phosphate salt [C1C2Im][Me2PO4] is reported for the first time.

Keywords: ionic liquids; heat capacity; phase behaviour; fusion properties, crystallography.

2

1

Introduction

Ionic liquids (ILs) are salts whose melting points are lower than 373 K. However, the real melting points of many compounds denoted as ionic liquids are unknown for one of two reasons: the IL does not crystallize at all or measurement of the melting point temperature was not determined with a sufficiently pure (especially dry) sample. The effect of the water content on properties of ionic liquids, mostly their melting point temperature and viscosity, is widely mentioned [1; 2]. Despite that, even recent studies were performed with undried samples [3; 4; 5; 6] or do not report information about the drying procedure and manipulation with the sample adequately [7; 8] to rule out undesirable influence on the thermodynamic properties [9]. Knowledge of the melting point temperature is crucial for assessing the technological suitability of ILs for chemical processes that are feasible only in the liquid/solid phase under given conditions. Furthermore, the melting point temperature is often a valuable experimental input for theoretical works [10; 11], exploiting classical or quantum molecular simulations to model behaviour and properties of ILs-related systems at finite temperatures [12; 13; 14; 15; 16; 17; 18; 19; 20]. Although the melting point temperature is in principle accessible from molecular simulations, e.g. through the pseudo-supercritical path approach [21; 22], such computations of the phase transition temperatures are costly and significantly less reliable than the experimental determinations. Contemporarily, representatives of the 1-alkyl-3-methylimidazolium family are probably the most widely used cations for ILs. Eight ILs, which contain the 1-ethyl-3-methylimidazolium cation coupled with relatively simple organic or inorganic anions, were chosen in this work for a detailed investigation of their phase behaviour and thermodynamic properties. The samples were dried carefully dried by applying vacuum and heating simultaneously followed by storage and manipulation under dry atmosphere. Phase behaviour of the compounds was studied with 3

samples after one and two drying procedures lasting about three weeks and with the original moist samples in some cases, to evaluate effect of drying on the melting temperature of the samples and to confirm sufficient drying for the final results. Many DSC runs were performed in order to make the samples crystalize, to obtain reliable values for their fusion temperature and enthalpy and to check for any signs of polymorphic behaviour. For two of the ILs, yet unreported polymorphs were observed and thermodynamically characterized. Out of the eight studied ILs, six are known to crystallize, but crystal structures for only three compounds were found in the Cambridge Structural Database (CSD) [23], namely two structures for [C1C2Im][NO3] [24; 25], and one for both [C1C2Im][OTf] [26] and [C1C2Im][OTs] [24]. Xray powder diffraction experiments were performed to identify the studied crystalline phases with the known structures and obtain the yet unreported crystal structure of [C1C2Im][Me2PO4].

4

2

Experimental and theoretical sections

The molar masses of all compounds were calculated based on the recommendation by IUPAC [27]. The molar gas constant was R = 8.314462618 JK-1mol-1 [28], and all temperatures reported here are based on the international temperature scale ITS-90. 2.1

Materials

The description of the samples used in this work is given in Table 1. The compounds were dried by simultaneous heating and application of vacuum. First drying cycle was performed at 343 K and 150 Pa for 15 days, second cycle at 353 K and 50 Pa for 20 days. Having finished the drying, flasks were refilled with dry nitrogen and preparation of the samples was performed under nitrogen atmosphere in a glovebox. Purity of the samples was determined by Karl-Fischer analysis (only the molar water content xH2O) and from DSC measurements (giving the overall purity including water, method is explained in Section 2.2). Comparison of DSC thermograms for the samples after first and second drying cycle did not indicate any decomposition of the samples, e.g. DSC-estimated purity remained equal or slightly increased in few cases discussed in Section 3.1.2. The water content of [C1C2Im][Ac] is significantly higher than expected, exceeding the values for the other compounds. We assume that the moisture could be introduced to the sample during the preparation and progress of the Karl-Fischer analysis, since the glass-transition temperature and heat capacities observed for [C1C2Im][Ac] sample definitely do not indicate lower purity than in the previous studies (see Section 3.1.2). The impurities and the residual water content detected for other compounds were estimated not to affect strongly the thermodynamic properties except for the melting temperature, which was corrected for the impurities content.

5

Table 1

Sample descriptions Compound 1-Ethyl-3-methylimidazolium tricyanomethanide 1-Ethyl-3-methylimidazolium dimethylphosphate 1-Ethyl-3-methylimidazolium thiocyanate 1-Ethyl-3-methylimidazolium tosylate 1-Ethyl-3-methylimidazolium acetate 1-Ethyl-3-methylimidazolium methylsulfate 1-Ethyl-3-methylimidazolium triflate 1-Ethyl-3-methylimidazolium nitrate

Abbreviation

CAS RN

M / g∙ mol-1

[C1C2Im][TCM]

666823-18-3

201.23

[C1C2Im][Me2PO4]

945611-27-8

236.21

[C1C2Im][SCN]

331717-63-6

169.25

[C1C2Im][OTs]

328090-25-1

282.36

[C1C2Im][Ac]

143314-17-4

170.21

[C1C2Im][MeSO4]

516474-01-4

222.26

[C1C2Im][OTf]

145022-44-2

260.24

[C1C2Im][NO3]

143314-14-1

173.17

Purity by supplier a > 0.98, xH2O = 0.0090 > 0.98, xH2O = 0.0080 > 0.98, xH2O = 0.0180 > 0.99, xH2O = 0.0112 > 0.95, xH2O = 0.0063 > 0.99, xH2O = 0.0014 > 0.99, xH2O = 0.0028 > 0.98, xH2O = 0.0040

Purity of dried sample b 0.993, xH2O = 0.0016 0.990, xH2O = 0.0019 N/A, xH2O = 0.0032 0.984, xH2O = 0.0049 N/A, xH2O = 0.0514 N/A, xH2O = 0.0007 0.988, xH2O = 0.0009 0.990, xH2O = 0.0031

Overall molar purity determined by the supplier (IoLiTec in all cases) using NMR. Water content determined by Karl-Fischer analysis and converted to mole fraction considering a mixture of pure sample and water. a

Overall molar purity (including water) determined from van’t Hoff plot of the melting peak from the DSC thermograms. This approach of purity determination is not applicable for compounds that do not crystalize. See Sections 2.2 and 3.1 for more details on purity determination by DSC. Water content was also determined by KarlFischer analysis by Metrohm 831 and converted to mole fraction for comparison (values are averages of three to five determinations). b

2.2

Phase behaviour measurements

The phase behaviour of the studied ILs was investigated from 183 K with a heat-flux differential scanning calorimeter (TA Q1000, TA Instruments, USA) calibrated to onset temperatures of water, gallium, naphthalene, benzoic acid, indium, and tin using the continuous method with various heating rates (most of the presented results were obtained at a heating rate of 2 K min-1). The sample load of about 5 mg was determined by an analytical balance with a readability of 0.01 mg, which was periodically calibrated.

6

Overall mole purity of the samples was determined from the DSC thermograms using the van’t Hoff method. Onset slope used for correction for the non-zero heating rate was determined as an average from calibration measurements for water, benzoic acid, and indium, since the shapes of melting peaks of other calibration compounds were found not to be acceptable. Estimation of the uncertainty of the purity is not available, it can be only stated that it was repeatable within 0.002 in terms of the molar fractions for each of the samples.

2.3

Heat capacity measurements

Condensed-phase heat capacities were measured with a Tian-Calvet calorimeter (SETARAM DSC IIIa, France) in the temperature range from (261 to 353) K applying the continuous method with the estimated combined expanded uncertainty (0.95 level of confidence) Uc( C p0 ,m ) = 0.01 C p0 ,m . See Štejfa et al. [29] for more details on the procedure and calibration. To extend the temperature range of the liquid-phase heat capacity data of the ILs crystalizing above 261 K (three of the compounds), experiments in the cooling mode were performed in addition to the routine measurements.

2.4

X-ray powder diffraction

X-ray powder diffraction (XRPD) measurements of [C1C2Im][Me2PO4] and [C1C2Im][OTs] were done on a powder diffractometer Smartlab of Rigaku. The diffractometer is equipped with a rotating Cu anode, a Johansson primary beam monochromator, a capillary holder, and a D/tex 250 detector. The sample was measured with the variable counting time strategy from 2θ = 3° to 2θ = 90° and with a step size of 0.01° in a total time of 20 hours.

7

The variable-temperature XRPD measurements were done on Empyrean of PANalytical, which was equipped with a sealed Cu X-ray tube, a capillary holder, a PIXCel3D detector, and an Oxford Cryostream cooling head 700 plus. The measurements were performed from 2θ = 3° to 2θ = 50° with a 0.013° step with every measurement lasting 15 minutes. The temperature was changed by the rate of 6 K∙min-1 between all measurements. On both instruments, the samples were placed into borosilicate capillaries with diameters of 0.5 mm. Samples were ground and placed into the capillaries in the glovebox, sealed by rubber, and kept in closed flasks to avoid moisturizing of the samples leading possibly to their melting or inability to crystallize.

8

3

Results and discussion

3.1

Phase behaviour

3.1.1

Crystallization and glass transition

Table 2 lists approximate crystallization temperatures Tcryst of the studied compounds as detected for 5 mg to 10 mg samples in aluminium pans. Two samples, namely [C1C2Im][Ac] and [C1C2Im][MeSO4], did not crystallize in any of the runs, which is in agreement with the literature. Glass transition temperature Tglass is listed in Table 2 for these two ILs and for [C1C2Im][TCM] and [C1C2Im][OTs] that crystalize during the heating. [C1C2Im][Me2PO4] and [C1C2Im][NO3] crystalize readily around 273 K and their Tglass cannot be determined. Although [C1C2Im][OTf] and [C1C2Im][SCN] undercool heavily, their Tglass was not detected being too low for [C1C2Im][SCN] [30; 31] and probably also for [C1C2Im][OTf]. [C1C2Im][SCN] unfortunately crystallized only in a single experiment out of more than 40 runs performed with five pans filled with samples dried either by vacuum drying or by molecular sieves. Table 2

Temperatures of crystallization Tcryst and glass transition Tglass (in K) at p = (100 ± 10) kPa. Compound [C1C2Im][TCM] [C1C2Im][Me2PO4] [C1C2Im][Ac] [C1C2Im][MeSO4] [C1C2Im][OTs] [C1C2Im][NO3] [C1C2Im][OTf] [C1C2Im][SCN]

Tcryst a/ K 223 274 N/A N/A 278 266 210 ~225

Tglass b/ K 188 ± 1 N/A 207 ± 1 205 ± 2 234 ± 1 N/A N/A N/A

Tglass, lit/ K 181.5 ± 0.1 [32], 179.5 [33] N/A 202 ± 1 [34], 198.5 [35; 36], 203 [37] N/A 214 [38; 39], 222 [31] N/A N/A 184 [40], 175.6 ± 0.1 [30], 174.5 [31]

Temperatures of crystallization strongly depend on the conditions and are listed mostly to facilitate repetition of the experiments. [C1C2Im][Me2PO4] and [C1C2Im][NO3] crystalize at cooling; [C1C2Im][TCM], [C1C2Im][OTs], [C1C2Im][OTf], and [C1C2Im][SCN] crystallize on heating. [C1C2Im][OTf] was also observed to slowly crystalize at isotherm around 213 K. [C1C2Im][SCN] was found to crystallize seldom. a

9

Glass transition temperatures detected during heating for samples after the 2nd drying step with their standard uncertainties (k = 1). Uncertainties are higher than for the melting temperatures due to impossibility of correction of the values for the impurity contents and higher scatter of the results. b

3.1.2

Effect of the water content

Literature temperatures of the melting point exhibit a large scatter, which can be caused by two principal sources or errors: the method of calibration employed and data evaluation and the purity of the samples, especially the water content. It is complicated to trace possible systematic errors related to the calibration and data evaluation in the published literature. However, considering the fact that Tglass and melting temperatures Tfus obtained in this work are generally higher than the corresponding literature values, we primarily focused on examination of the effect of water content on the phase behaviour of the ILs. Especially, the DSC measurements were performed at different stages of the drying procedure, as summarized in Table 3. Based on the variation of the melting temperature during the drying process, we can state that i) decomposition of the samples is disproved, ii) the water content would not decrease significantly with further drying, and iii) measurements performed with as-received samples or samples exposed to laboratory atmosphere after the drying procedure are systematically offset. The effect of the water content on the DSC thermograms is shown in Figure 1 and Figure 2. Beside the decrease of the melting temperature, the water content also broadens the melting peak and induces occurrence of an eutectic peak (see e.g. a small peak in Figure 2a). This eutectic peak is often incorrectly attributed to a crystal-crystal phase transition, as is probably the case of [C1C2Im][TCM] [32; 33] and [C1C2Im][OTf] [26]. This conclusion can be supported by several strong facts, viz. i) the phase transition was not observed by any other group; ii) the reported melting temperatures are lower compared to other studies; iii) the melting peaks on thermograms presented in [26; 33] are very broad and the signal does not return to the baseline between the first and second peaks (see reprinted thermogram for [C1C2Im][TCM] [33] in Figure 1b); and iv)

10

the Tfus and fusion enthalpy ΔHfus vary significantly between studies [33] and [32], while v) the eutectic temperature does not. To support this hypothesis further, a solid-liquid equilibria (SLE) diagram for the system [C1C2Im][TCM] {water in Figure S1 in the Supplementary data (SD)} was constructed based on the data from [33], [41], and this work. The samples studied by DSC in [32; 33] exhibit behaviour that would correspond well to mixtures with composition xIL = 0.68 and xIL = 0.79, respectively. Since the authors [32; 33] report a significantly lower water content, we assume that the once-dried sample absorbed some moisture when being loaded to the DSC pan, which can occur almost instantly considering the small amount of the manipulated sample. An interesting study of effect of the water content on the glass transition of [C1C2Im][Ac] was previously presented by Troshenkova et al. [37] showing it can decrease by as much 30 K. Water content of xH2O = 0.051, as determined by Karl-Fischer analysis for the [C1C2Im][Ac] sample used in this work, would decrease the glass transition by several degrees kelvin, but the observed value in Table 2 is higher than all previous values [34; 35; 36; 37]. This fact supports the assumption that the sample used for DSC studies had significantly lower xH2O and became moist during the preparation for Karl-Fischer analysis.

Table 3

Temperatures of melting Tfus (in K), at p = (100 ± 10) kPa observed for samples at different stages of the drying procedure Compound [C1C2Im][TCM] [C1C2Im][Me2PO4] [C1C2Im][Ac] [C1C2Im][MeSO4] [C1C2Im][OTs] [C1C2Im][NO3], crI

Observed Tfus (or Tglass, where noted)/K Moist sample No crystallization 308.6 b 196.6 Not tested No crystallization 295.3

After 1st drying No crystallization 312.5 205.2 c 196.4 c 327.1 315.9

After 2nd drying 274.6 312.5 206.5 c 204.8 c 327.5 316.0

Corrected Tfus a/K After 2nd drying 274.9 ± 0.3 312.9 ± 0.3 N/A N/A 328.2 ± 0.3 316.4 ± 0.4

11

[C1C2Im][OTf], crI [C1C2Im][SCN]

255.2 No crystallization

261.9 No crystallization

262.0 266

262.6 ± 0.5 266 ± 2

Tfus corrected for impurities according to the ideal-solubility equation together with their standard uncertainties (k = 1). a

b

Sample was dried, but the DSC pan was filled under lab conditions instead of in dry atmosphere.

c

Glass transition.

HeatFlow (endo up) / mW

6

a

moist sample after 1st drying after 2nd drying

4

2

0 280

290

300

310

320

330

HeatFlow (endo up) / mW

T/K

b

Domańska et al. [33] after 2nd drying

15

Eutectic peak 10

5

0 220

230

240

250

260

270

280

290

300

T/K

12

Figure 1. DSC Thermograms for [C1C2Im][Me2PO4] (a) and [C1C2Im][TCM] (b) at different stages of the drying procedure. [C1C2Im][TCM] did not crystallize in any of the runs with the undried sample and with the sample after the 1st drying step.

HeatFlow (endo up) / mW

8

a

moist sample after 1st drying (crI) after 2nd drying (crII) after 2nd drying (crI)

6

4

2

Eutectic peak Exothemic transition

0 200

220

240

260

280

300

320

T/K

HeatFlow (endo up) / mW

8

6

4

after 1st drying after 2nd drying (fast scan) after 2nd drying (heated to 263 K)

b

Phase transition / concomitant melting

2

0 240

250

260

270

T/K Figure 2. DSC Thermograms for [C1C2Im][NO3] (a) and [C1C2Im][OTf] (b) at different stages of the drying procedure.

13

3.1.3

Polymorphic behaviour

For two of the ILs, [C1C2Im][NO3] and [C1C2Im][OTf], yet unreported polymorphic behaviour was detected. Two scenarios were observed for [C1C2Im][NO3] when being crystallized and subsequently heated, as can be seen in Figure 2a: i) occurrence of a shallow exothermic peak around 250 K and melting at 314.5 K and ii) straightforward melting at 316.0 K. The polymorphs seem to be enantiotropically related, since the melting enthalpy in the first scenario is higher (18.4 kJ∙mol-1 compared to 17.6 kJ∙mol-1). Therefore, we denote the higher melting form (metastable at low temperatures) crI and the lower melting form (melting as super cooled crystal) crII. The enthalpy of the spontaneous transition was found to be 0.8 kJ mol-1, which excellently agrees with the difference in ΔHfus. The endothermic transition from crII to crI, occurring at or slightly above the triple point temperature (TcrII-crI) for many enantiotropic polymorphs, was not observed for [C1C2Im][NO3]. When considering equality of the Gibbs energies of the polymorphs at TcrII-crI, the following equation can be derived, if the difference in the heat capacities is neglected: TcrII-crI 

H fus,crI  H fus,crII H fus,crI H fus,crII  Tfus,crI Tfus,crII

(2)

which, combined with data from Table 4, gives TcrII-crI = (286 ± 10) K. The approximation of equal heat capacities is justified for liquid and crystalline phase over as short temperature interval as 1.5 K as well as for two crystalline phases. As a result, crI polymorph should be stable at the room temperature. Uncertainty of TcrII-crI is high, since the ΔHfus have very close values making Gibbs-energy curves almost parallel. It is also interesting to note that the DSC sample after the first drying step tended to melt as crI predominantly, while the double-dried sample more often transformed to crII. Melting of crI with the double-dried sample was induced by a slow heating (2 K min-1) from 213 K, giving more time for a spontaneous transition. For the moist sample, we 14

were not able to decide which of the polymorphs was observed. During the heat capacity measurements with the Tian-Calvet calorimeter, slightly different tendencies for forming of the polymorphs were observed, probably due to a different pan material and the sample amount. Slow cooling (0.35 K∙min-1) to 261 K resulted in formation of crII, while faster cooling (2 K∙min1)

in crI, although the sample crystallized in both cases at higher temperatures compared to DSC

measurements, around 278 K. Finally, a sample remaining for a long period at room temperature was observed to form the stable crI spontaneously. Example thermograms for [C1C2Im][OTf] are displayed in Figure 2b. During a standard heatingmode run, the melting peak always showed a left shoulder of a varying magnitude. This peak only disappeared, when the crystallized sample was heated to 263 K and quenched again before melting completely. The following scan in the heating mode showed a distinct melting peak (blue line in Figure 2b) with the peak area slightly smaller than for the peak with the shoulder. Reproducible Tfus and ΔHfus values in Table 4 for crI were determined from runs with this thermal history. Higher amount of water in the sample caused significantly better separation of the peaks, but could also switch their order or completely change their nature and we therefore avoid interpreting the corresponding measurements. Two possible phase-behaviour scenarios are suggested that agree with the observations, both including concomitant polymorphism: i) the shoulder belongs to an enantiotropic transition occurring about 0.5 K below the Tfus, and ii) the shoulder belongs to melting of a different polymorph. In both cases, the magnitude of the shoulder is expected to vary due to different polymorphic composition of the sample and the transition to crI is irreversible although crI is a metastable polymorph (at least below 260 K). Unfortunately, the DSC measurements do not enable to resolve the polymorphism of [C1C2Im][OTf] completely or determine the phase-change properties for the second observed polymorph (crII). 15

3.1.4

Final results and comparison to literature

Final values of the fusion temperatures Tfus and fusion enthalpies ΔHfus of the studied compounds, averaged from several DSC measurements with double-dried samples, are listed in Table 4. Values of Tfus presented are already corrected for impurities according to the ideal-solubility equation. Literature values are also listed for comparison to the ones being in an acceptable agreement with our results printed in bold. If there are any literature Tfus values higher than our results, the difference is within uncertainty of the values, but majority of the previously reported values of Tfus are significantly lower compared to our results. As discussed in Section 3.1.2, these are probably consequences of the measurements with samples not adequately dried or wetted during the manipulation following to the drying procedure. Literature values on ΔHfus are scarce and if available, in a very poor agreement with our results, again probably due to the water content of the used samples. The observed ΔHfus of [C1C2Im][SCN] was considerably lower when compared to literature [40], which can be a result of an incomplete crystallization of the sample. The value could not be reproduced since the sample did not crystallize again as discussed in Section 3.1.1. About half of the available literature values are those by group of Domańska, where the uncertainty of the results is not presented and the same compounds are studied repeatedly without discussing why the results differ excessively. Reliability of these studies is further arguable due to the fact that the authors published a study on solid-liquid equilibria of [C1C2Im][TCM] with water [32], without a proper interpretation of their recorded DSC thermogram [33], most probably containing an eutectic peak of the [C1C2Im][TCM] – water mixture as discussed in Section 3.1.2. Previous thermodynamic studies on [C1C2Im][NO3] did not report its polymorphism and did not include determination of the crystalline structure, which prevents from assigning the reported Tfus 16

and ΔHfus to one of the polymorphs. No previous study was found for [C1C2Im][Me2PO4], only the producer quoted Tfus = 296 K in the material safety data sheet, which was probably determined with a moist sample as the given value is 17 K lower than our result.

Table 4

Temperatures of fusion Tfus, (in K) and fusion enthalpies ΔHfus (in kJ mol-1) at p = 100 ± 10 kPa Compound a

Tfus b,c/K

[C1C2Im][TCM]

274.9 ± 0.3

[C1C2Im][Me2PO4]

312.9 ± 0.3

[C1C2Im][OTs]

328.2 ± 0.3

[C1C2Im][NO3]e

316.4 ± 0.4 (crI) 314.9 ± 0.4 (crII)

[C1C2Im][OTf]e

262.6 ± 0.5 (crI)

[C1C2Im][SCN]

266 ± 2

Tfus, lit c/K 257.2 [33], 263.9 ± 0.1 [32] N/A 322.3 [31], 322.9 [38; 39], 326.6 [31], 327.4 ± 0.3 [42], 328 ± 1 [43] 303.9 ± 0.7 [44], 311.8 ± 0.3 [45] 247.5 [26], 259.1 ± 1 [3], 262.2 ± 0.3 [46], 263.1 [47], 264 [48] 267 [40]

ΔHfusc/ kJ∙mol-1 21.5 ± 0.4

ΔHfus, lit d/ kJ∙mol-1 6.5 [33], 9.0 ± 0.5 [32] N/A

20.1 ± 0.4

20.49 [31], 27.83 [38; 39]

12.6 ± 0.3

17.6 ± 0.4 (crI) 18.4 ± 0.4 (crII)

19.5 ± 0.2 [45]

11.7 ± 0.5 (crI)

N/A

16.6 ± 2

27.0 ± 1.3 [40]

a

[C1C2Im][Ac] and [C1C2Im][MeSO4] do not crystallize.

b

Presented Tfus are already corrected for impurities considering the ideal-solubility equation.

Presented standard uncertainties (k = 1) are reflecting uncertainty of calibration, scatter of the results, and experimental difficulties, where relevant. c

Values listed in ascending order with standard uncertainties (if presented). References in acceptable agreement with our results printed in bold. d

e

Two polymorphs were observed in this work. It is not clear which polymorphs were studied in the previous works.

17

3.2

Condensed-phase heat capacities

Experimental liquid-phase heat capacities C p ,m of the studied ILs are listed in Table 5. For the three ILs melting above the room temperature, it was also possible to determine C p ,m of he crystalline phase listed in Table 6. For [C1C2Im][NO3], C p ,m of both polymorphs is reported and although their Tfus are almost equal, C p ,m of crII seems to be more noticeably affected by premelting. Available literature determinations of C p ,m of the studied ILs are listed in Table 7 and compared to our results in Figure 3 through Figure 6. Consistent data highlighted in bold were selected and correlated together using a polynomial equation: 2

C p ,m

3

T  T  T  R  a  b   c   d   ,      

(3)

where a, b, c, and d are correlation parameters, and θ = 298.15 K. The cubic parameter was only included in the case of crystalline phases, since they exhibit complex temperature dependence. Developed parameters of equation (3) are presented in Table 8. The literature data on C p ,m are numerous, but their scatter is significant and does not always agree with the stated uncertainties. Generally, only the data by Freire et al. [49], Su et al. [50], Diedrichs and Gmehling [51] and García-Miaja et al. [52] were found to be of comparable reliability as our measurements and were included in the correlations. Data presented by Navarro et al. [5; 6] and Aparicio et al. [2] were found to deviate significantly with increasing temperature, which could be a result of evaporation of residual water in the sample.

18

Table 5

Experimental liquid-phase heat capacities C p ,m at p = (100 ± 10) kPa a [C1C2Im][TCM] m = 0.42834 g T/K

Cp,m b/

Cp,m / J∙K ∙mol -1

270.0 275.0 280.0 285.0 290.0 295.0 300.0 305.0 310.0 315.0 320.0 325.0 330.0 335.0 340.0 345.0 350.0

T/K 270.0 275.0 280.0 285.0 290.0 295.0 300.0 305.0 310.0 315.0 320.0 325.0 330.0 335.0 340.0 345.0 350.0

[C1C2Im][SCN] m = 0.39957 g

-1

J∙K ∙mol -1

-1

350.6 c

0.0 351.8 0.1 353.1 0.0 354.3 0.0 355.6 0.0 356.9 -0.1 358.2 -0.1 360.0 0.3 361.5 0.2 362.6 -0.1 364.2 0.0 365.9 0.2 367.2 0.0 368.7 -0.2 370.3 -0.2 372.0 -0.2 374.2 0.3 [C1C2Im][MeSO4] m = 0.35110 g

Cp,m b/

Cp,m / J∙K ∙mol -1

[C1C2Im][OTf] m = 0.40908 g

-1

J∙K ∙mol -1

-1

278.4 0.3 279.6 0.3 280.7 0.2 281.8 0.1 283.0 0.1 284.3 0.1 285.7 0.2 287.5 0.7 288.8 0.6 290.3 0.7 291.6 0.6 292.8 0.3 294.1 0.1 295.5 0.0 296.8 -0.2 298.4 -0.2 300.3 0.0 [C1C2Im][Me2PO4] m = 0.41728 g

Cp,m b/

Cp,m / J∙K ∙mol -1

[C1C2Im][OTs] m = 0.40532 g

-1

J∙K ∙mol -1

-1

356.1 2.7 357.9 2.2 359.8 1.8 361.8 1.5 363.9 1.3 365.9 1.0 368.0 0.8 370.5 1.1 372.9 1.1 374.6 0.5 376.7 0.3 378.8 0.2 380.6 -0.3 382.6 -0.6 384.4 -1.0 386.6 -1.2 389.3 -0.8 [C1C2Im][Ac] m = 0.40095 g

Cp,m b/

Cp,m / J∙K ∙mol -1

-1

J∙K-1∙mol-1

434.5 c

0.2 437.1 c -0.1 439.9 c -0.2 c 442.8 -0.2 445.7 c -0.3 448.7 c -0.3 451.8 c -0.1 454.9 c 0.1 457.7 c -0.1 461.1 c 0.4 464.1 c 0.4 466.9 c 0.3 469.7 0.1 472.6 0.0 475.4 -0.1 478.3 -0.2 481.3 -0.2 [C1C2Im][NO3] m = 0.42811 g

Cp,m /

Cp,m b/

Cp,m /

Cp,m b/

Cp,m /

Cp,m b/

Cp,m /

Cp,m b/

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

336.7 338.3 340.1 342.1 344.1 346.1 348.3 350.7 352.5 354.6 356.6 358.7 361.0 363.4 365.5 368.0 370.8

0.2 -0.1 -0.2 -0.1 -0.1 0.0 0.1 0.5 0.2 0.1 0.0 -0.2 -0.1 0.0 -0.2 -0.1 0.3

crystallized 403.2 c 405.0 c 407.0 c 408.5 c 410.1 c 411.8 c 413.5 c 415.5 c 417.8 419.9 421.8 424.0 426.2 428.2 430.5 433.2

-0.2 0.0 0.3 0.0 -0.1 -0.1 -0.4 -0.3 0.0 0.1 0.0 0.1 0.0 -0.2 -0.2 0.2

315.1 316.4 317.8 319.4 321.0 322.6 324.3 326.6 328.2 330.1 331.9 333.5 335.2 337.0 338.8 341.0 343.4

0.4 0.2 0.0 0.0 0.0 -0.1 -0.1 0.4 0.2 0.4 0.3 0.0 -0.2 -0.3 -0.4 -0.2 0.2

crystallized crystallized crystallized crystallized 291.3 c 292.6 c 293.9 c 295.1 c 296.7 c 298.3 c 299.9 301.3 302.9 304.5 306.0 307.7 309.3

0.0 0.0 -0.1 -0.2 -0.1 0.1 0.1 0.0 0.1 0.1 0.0 -0.1 -0.2

19

a

The standard uncertainty of the temperature is u(T) = 0.05 K, and the combined expanded uncertainty of the heat

capacity is Uc( Cp,m ) = 0.01 Cp,m (k = 2, 0.95 level of confidence). Mean values of three determinations performed in this work. b

Cp ,m = Cp ,m  Cp ,m is the absolute deviation from the correlation represented by equation (3) with parameters from calc

Table 8. c

Super cooled liquid.

Table 6

Experimental crystalline-phase heat capacities Cp,m at p = 100 ± 10 kPa a [C1C2Im][Me2PO4] m = 0.41728 g T/K 270.0 275.0 280.0 285.0 290.0 295.0 300.0 305.0 310.0 a

Cp,m /

Cp,m

b/

[C1C2Im][OTs] m = 0.40532 g

[C1C2Im][NO3] (crII) m = 0.42811 g

[C1C2Im][NO3] (crI) m = 0.42811 g

Cp,m /

Cp,m b/

Cp,m /

Cp,m b/

Cp,m /

Cp,m b/

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

J∙K-1∙mol-1

340.4 345.9 350.0 353.5 357.4 melting

0.5 0.3 -0.3 -0.6 0.4

336.5 341.2 345.9 350.9 356.2 362.1 368.9 377.4 388.7

-0.1 -0.2 -0.1 0.1 0.3 0.2 -0.1 -0.2 0.7

250.6 258.9 267.1 277.5 290.8 308.7 336.4 melting

0.1 -0.4 -0.2 0.7 0.7 -0.4 0.2

229.3 233.4 237.9 243.0 248.6 255.2 263.8 Melting

0.1 0.4 0.3 0.1 -0.3 -0.5 0.6

The standard uncertainty of the temperature is u(T) = 0.05 K, and the combined expanded uncertainty of the heat

capacity is Uc( Cp,m ) = 0.01 Cp,m (k = 2, 0.95 level of confidence). Mean values of three determinations performed in this work. b

Cp ,m = Cp ,m  Cp ,m is the absolute deviation from the correlation represented by equation (3) with parameters from calc

Table 8.

Table 7

Condensed phase heat capacities C p ,m reported in the literature. Reference a

Nb

T range / K

U  C p ,m  , k  2

c

Method d

[C1C2Im][TCM] Navarro et al. 2013 [5]

20

296 – 372

4 J K-1 mol-1

HF-DSC

Navarro et al. 2014 [6]

20

296 – 372

10 J K-1 mol-1

HF-DSC

Zorębski et al. 2018 [7]

10

293 – 323

0.01 C p ,m

TC-DSC

20

[C1C2Im][Me2PO4] T/K Freire et al. 2011 [49]

1

298

1.7 J∙K-1∙mol-1

Ren et al. 2011 [8]

4

298 – 323

Not specified

Drop calorimeter Simple isoperibolic calorimeter

[C1C2Im][SCN] Ficke et al. 2010 [53]

7

283 – 343

0.08 C p ,m

HF-DSC

Freire et al. 2011 [49]

1

298

1.3 J∙K-1∙mol-1

Drop calorimeter

Navarro et al. 2013 [5]

20

296 – 372

4 J∙K-1∙mol-1

HF-DSC

Zorębski et al. 2018 [7]

10

293 – 323

0.01 C p ,m

TC-DSC

Not specified

TC-DSC

[C1C2Im][OTs] Aparicio et al. 2009 [2]

6

318 – 368 [C1C2Im][Ac]

Freire et al. 2011 [49]

1

298

0.6 J∙K-1∙mol-1

Drop calorimeter

Su et al. 2016 [50]

10

303 – 393

0.0098 C p ,m

Flow calorimeter

[C1C2Im][MeSO4] Aparicio et al. 2009 [2]

6

318 – 368

Not specified

TC-DSC

Ficke et al. 2010 [53]

7

283 – 343

0.08 C p ,m

HF-DSC

Requejo et al. 2014 [1]

3

288 – 308

2 J∙K-1∙mol-1

HF-DSC

Alkhaldi et al. 2017[4]

4

298 – 313

0.10 C p ,m

HF-DSC

[C1C2Im][OTf] Diedrichs and Gmehling 2006 [51]

23

313 – 423

0.01 C p ,m [54]

TC-DSC

Diedrichs and Gmehling 2006 [51]

23

315 – 425

0.10 C p ,m

HF-DSC

Diedrichs and Gmehling 2006 [51]

18

328 – 413

0.10 C p ,m

M-HF-DSC

Ficke et al. 2008 [55]

7

283 – 343

0.08 C p ,m

HF-DSC

García-Miaja et al. 2008 [52]

6

293 – 318

0.006 C p ,m

TC-DSC

Yu et al. 2009 [56]

12

303 – 358

7.8 J∙K-1∙mol-1

HF-DSC

Freire et al. 2011 [49]

1

298

0.78 J ∙-1∙mol-1

Drop calorimeter

a

References in bold were accepted for the correlation.

b

Number of presented heat capacity data points.

c

Standard uncertainties presented by the authors were recalculated to expanded uncertainties, U  C p ,m  , k  2 .

d

Used shortcuts are: TC: Tian-Calvet, HF: heat flux, M: modulated run.

21

Table 8

Parameters of equation (3) for the liquid phase of the studied ILs at p = 100 kPa Compound

Included literature none [C1C2Im][TCM], l [49] [C1C2Im][Me2PO4], l none [C1C2Im][Me2PO4], cr [49] [C1C2Im][SCN], l [C1C2Im][OTs], l none [C1C2Im][OTs], cr none [C1C2Im][Ac], l [49; 50] [C1C2Im][MeSO4], l none [C1C2Im][OTf], l [49; 51; 52] none [C1C2Im][NO3], l none [C1C2Im][NO3], crI none [C1C2Im][NO3], crII

a 39.505 47.435 -171.52 32.078 33.425 -1166.5 33.325 35.200 27.462 32.615 133.37 -7521.5

b -2.853 -8.996 424.48 -4.939 20.437 3816.6 -1.069 -1.419 16.773 -4.459 -258.54 24302.3

a

Applicability temperature range of the correlation equation.

b

2  n       C p0 ,m  C p0,calc ( n  m)  ,m  i  i1 

c 6.390 11.030 -209.74 7.138 0.359 -4053.9 6.686 8.004 -0.176 7.138 156.48 -26125.4

d 0 0 0 0 0 1447.8 0 0 0 0 0 9383.7

(Tmin – Tmax)a / σ b/ K J∙K-1∙mol-1 266 – 353 0.17 275 – 353 0.25 264 – 292 0.70 266 – 353 1.00 265 – 353 0.24 265 – 311 0.40 265 – 393 0.55 267 – 353 0.20 266 – 423 1.96 287 – 353 0.12 265 – 302 0.45 264 – 302 0.73

1/2

is the standard deviation of the fit; n is the number of fitted data points and

m is the number of adjustable parameters.

340

420

Tfus = 266 K

Cp0m / J mol-1 K-1

Cp0m / J mol-1 K-1

Tfus = 274.9 K 400 380 360

300

280

[C1C2Im][TCM]

340 260

320

280

300

320

T/K

340

360

380

[C1C2Im][SCN] 260 260

280

300

320

340

360

380

T/K

Figure 3. Experimental heat capacities Cp,m of [C1C2Im][TCM] and [C1C2Im][SCN].

22

, This work, liquid, , Navarro et al. [5; 6], , Freire et al. [49], , Ficke et al. [53], , Zorębski et al. [7], , final correlation by equation (3) based on sources highlighted in bold. Error bars correspond to expanded uncertainties (k = 2).

600

Tfus = 328.2 K

Cp0m / J mol-1 K-1

Cp0m / J mol-1 K-1

400

350

300

[C1C2Im][MeSO4] 280

260

300

320

340

360

550 500 450 400

[C1C2Im][OTs]

350 380

260

280

300

340

320

360

380

T/K

T/K

Figure 4. Experimental heat capacities Cp,m of [C1C2Im][MeSO4] and [C1C2Im][OTs]. , This work, liquid, , this work, crystal, , Aparicio et al. [2], , Ficke et al. [53], , Requejo et al. [1], , Alkhaldi et al. [4], , final correlation by equation (3) based on sources highlighted in bold.

340

440

320

420

300

Tfus,crII = 314.9 K Tfus,crI = 316.4 K

280 260 240 220

[C1C2Im][NO3] 260

280

300

T/K

320

340

360

Cp0m / J mol-1 K-1

Cp0m / J mol-1 K-1

Error bars correspond to expanded uncertainties (k = 2).

Tfus = 262.6 K

400 380 360 340 320

[C1C2Im][OTf] 280

320

360

400

440

T/K

Figure 5. Experimental heat capacities Cp,m of [C1C2Im][NO3] and [C1C2Im][OTf]. , This work, liquid, , this work, crystal I, , this work, crystal II, , Ficke et al. [55], , Freire et al. 2011 [49], , García-Miaja et al. [52], , Yu et al. [56], , , Diedrichs and Gmehling [51] (TC-DSC, 4 lowest points excluded due to inconsistency with other data), , Diedrichs and Gmehling [51] (M-HF-DSC), , Diedrichs and Gmehling [51] (HF-DSC), , final correlation by equation (3) based on sources highlighted in bold. Error bars correspond to expanded uncertainties (k = 2).

23

450

Cp0m / J mol-1 K-1

Cp0m / J mol-1 K-1

360 400

Tfus = 312.9 K

350

340

320

[C1C2Im][Ac]

[C1C2Im][Me2PO4] 300 260

280

320

300

T/K

340

360

300 260

280

300

320

340

360

380

400

T/K

Figure 6. Experimental heat capacities Cp,m of [C1C2Im][Me2PO4] and [C1C2Im][Ac]. , This work, liquid, , this work, crystal, , Ren et al. [8], , Freire et al. [49], , Su et al. [50], , final correlation by equation (3) based on sources highlighted in bold. Error bars correspond to expanded uncertainties (k = 2).

24

3.3

XRPD study

Three of the studied compounds have Tfus above the room temperature enabling preparation of samples suitable for XRPD measurements. X-ray study of the low-melting ILs, [C1C2Im][TCM], [C1C2Im][OTf], and [C1C2Im][SCN], could not be performed. A simple phase analysis was performed for powdered sample of [C1C2Im][OTs], which was identified with the phase with CSD’s ref. code UYUJUV03 [24] (see Figure S2 in the SD). Crystal structure of [C1C2Im][Me2PO4] was not known, thus we decided to solve it from XRPD. The sample in the capillary was slightly wet during the manipulation at T = (298 to 303) K, most probably due to the residual water content and a slight leakage of the closed capillary. The liquid content did not cause significant problems for the data collection; however, its influence to the refinement of the crystal structure is questionable. The resulting unit-cell parameters are described in Table 9 and the whole unit cell, containing two ionic pairs, is depicted in Figure 7. Furthermore, the asymmetric unit-cell containing a single ionic pair is displayed in Figure S3 in the SD, showing also magnitudes of the anisotropic displacement parameters (ADP). According to the high values of the ADP parameters, the ethyl group in the 1-ethyl-3-methylimidazolium seems to be disordered. However, attempted splitting of the atomic positions did not lead to a stable refinement and the influence on the agreement factor was lower than expected. For that reason, at the final stage of the refinement, one shared ADP parameter was introduced for the description of the disordered atoms. See Figure S4 in the SD and its legend for more details on the refinement. Two different structures were previously reported for [C1C2Im][NO3], but they were not explicitly designated as polymorphs [24]. To identify the polymorphs observed during the calorimetric study of [C1C2Im][NO3] with one or both of the structures and to confirm the

25

observation that the cooling rate influences the phase composition of the sample, we decided to perform the variable-temperature XRPD measurements. The sample was melted in the capillary at 333 K, quenched to 213 K with a heating rate of 6 K∙min-1, and measured stepwise at every 5 K from 213 K to 298 K. In all resulting XRPD patterns (Figure S5 in the SD), only the crI phase (ref. code KUCPED [25]) was observed. Note that the same structure was also determined for a sample crystallized, grinded, and filled to the capillary at room temperature (confirmed as crI phase by DSC). In a second experiment, the sample was melted again at 333 K and cooled down to 213 K with a heating rate of 0.1 K∙min-1. After that, the sample was again measured stepwise every 5 K from 213 K to 298 K as in the previous case. In all XRPD patterns (Figure S6 in the SD), only the crII phase (ref. code KUCPED02 [24]) was observed. The phase content in the sample in both experiments was confirmed by a simple Rietveld fitting, where the atomic model was fixed and only the unit cell parameters were refined (see Figures S7 and S8 in the SD). In both cases, strong preferred orientation was observed in the collected data, most probably due to the crystallization from melt in the capillary. The observed XRPD patterns had to be modelled by March-Dollase approach (crI: MD100 = 2.4; crII: MD001 = 1.6) to fit the theoretical patterns.

26

Figure 7. Illustration of the unit-cell of [C1C2Im][Me2PO4] crystal determined experimentally in this work.

Table 9

Crystallographic information of the [C1C2Im][Me2PO4] phase a Formula

C6H11N2·C2H6O4P

Crystal system Space group a /(Å) b /(Å) c /(Å) α /(°) β /(°) γ /(°) V /(Å3) calculated density Z radiation Rp/Rwp/Rexp χ2 parameters restraints/constraints temperature pressure CCDC deposition no.

Triclinic

P1 7.63167 ± 0.00010 8.63994 ± 0.00014 10.02787 ±0.00011 91.6247 ±0.0013 100.4313 ±0.0012 111.9354 ±0.0012 599.86 ± 0.02 1.308 g∙cm-3 2 Cu Kα1 0.044/0.046/0.029 2.43 87 27/79 295 ± 3 K 100 ± 10 kPa 1956337

Specified uncertainties of the unit-cell parameters are the standard error estimations of the Rietveld fitting. Uncertainties of temperature and pressure are standard uncertainties (k = 1). Uncertainty in density, when calculated through the error propagation law, would yield unrealistically small, about 0.00004 g∙cm-1. a

27

4

Conclusions

Samples of eight 1-ethyl-3-methylimidazolium-based ILs were thoroughly vacuum-dried and subjected to phase-behaviour studies and heat capacity measurements. Comparison of the obtained results to literature values indicated a superior purity of the samples used in this work. This hypothesis was further supported by water-content measurements, and by comparison of the shapes of the DSC thermograms, and of the temperature dependence of the heat capacities. [C1C2Im][SCN] was found to crystallize rarely, which significantly reduced the achievable accuracy of the fusion temperature and enthalpy. Special attention was payed to the polymorphic behaviour and assignment of the calorimetrically studied crystalline phases to the crystallographic structures. Polymorphic behaviour of [C1C2Im][NO3] was detected confirming the existence of both structures reported previously in the crystallographic literature, which were assigned melting point temperatures and fusion enthalpies. The phases with close melting points are monotropically related, however the endothermic transition between them does not occur. Crystallographic structure of [C1C2Im][Me2PO4] is reported for the first time. Polymorphic behavior detected for [C1C2Im][OTf] could not be fully resolved with the used techniques.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, …

Acknowledgements Authors V.Š. and C.Č. acknowledge financial support from the Czech Science Foundation (GACR No. GJ19-04150Y). Author J.R. acknowledges financial support for the X-ray experimental part from the project of the Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760). 28

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Phase behaviour and heat capacities of eight [C1C2Im] ionic liquids are studied calorimetrically. Special attention is payed to drying of the samples and evaluation of the effect of water content. Calorimetric results are critically compared with the literature data proving superior purity and lower uncertainty. Crystal structures are determined for [C1C2Im][Me2PO4], [C1C2Im][OTs], and two polymorphs of [C1C2Im][NO3].

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Vojtěch Štejfa: Conceptualization; Methodology; Software; Validation; Formal analysis; Investigation; Resources; Data Curation; Writing - Original Draft; Visualization. Jan Rohlíček: Software; Validation; Formal analysis; Investigation; Data Curation; Writing - Original Draft; Writing - Review & Editing; Visualization. Ctirad Červinka: Conceptualization; Writing - Review & Editing; Visualization; Supervision; Project administration; Funding acquisition.

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