Isobaric and Isochoric Heat Capacities as Well as Isentropic and

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Thermodynamics, Transport, and Fluid Mechanics

Isobaric and Isochoric Heat Capacities as well as Isentropic and Isothermal Compressibilities of Di- and Trisubstituted Imidazolium-Based Ionic Liquids as Function of Temperature Edward Zorebski, Ma#gorzata Musia#, Karolina Ba#uszy#ska, Micha# Zor#bski, and Marzena Dzida Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00506 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Industrial & Engineering Chemistry Research

Isobaric and Isochoric Heat Capacities as well as Isentropic and Isothermal Compressibilities of Di- and Trisubstituted Imidazolium-Based Ionic Liquids as Function of Temperature

*

Edward Zorębski , Małgorzata Musiał, Karolina Bałuszyńska, Michał Zorębski, Marzena Dzida

Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland

Corresponding author. E-mail address: [email protected] Tel.: +48 323591625; Fax: +48 322599978

Abstract The isobaric and isochoric heat capacities of six disubstituted imidazolium-based ionic liquids with

different

anions

and

two

trisubstituted

imidazolium-based

bis(trifluoromethylsulfonyl)imides were determined at atmospheric pressure in the temperature range from 293.15 K to 323.15 K. The isobaric heat capacities were determined using differential scanning calorimetry. The isochoric heat capacities were determined indirectly by means of the acoustic method from the speed of sound and density measurements. Also, other connected with the speed of sound quantities such as isentropic and isothermal compressibilities as well as internal pressures were determined. The highest compressibilities show both trisubstituted imidazolium-based bis(trifluoromethylsulfonyl) imides, whereas 1-ethyl-3-methylimidazolium thiocyanate shows the lowest compressibility. The critical comparison of the isobaric heat capacity data with the available literature data makes possibility recommendation of the most reliable heat capacity values. Analyzed predictive capability of two heat capacity models based on group contribution method is poor in the case of [C(CN)3]¯ anion. Additionally, based on the speeds of sound the thermal conductivities were calculated using a modified Bridgman relation and compared with values estimated by a group contribution method. Here, the worst results is obtained in the case of [SCN]¯ anion. 1 ACS Paragon Plus Environment

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Keywords: Isobaric heat capacity; DSC; isochoric heat capacity, speed of sound, ionic liquids.

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1. Introduction Although isobaric and isochoric heat capacities are among the most important thermophysical properties of matter,1 papers reported simultaneously both heat capacities are very limited.2-6 Both quantities are measurable state functions, but while the values of the isobaric heat capacities of liquids can be obtained with relative ease by direct calorimetric determination, the rather scarce values of isochoric heat capacities are mostly obtained indirectly by the use of an acoustic method. There is very attractive route taking into account difficulties that exist during calorimetric determination of isochoric heat capacities of liquids in standard conditions.7,8 Using this route, we have reported very recently isochoric heat capacities for eleven aprotic imidazolium- and pyrrolidinium-based ionic liquids (ILs) together with their thermophysical material constants and isobaric heat capacities. 6 As ILs can be alternative fluids for heat transfer or heat storage media,9-13 therefore, the knowledge of their isobaric heat capacity as a function of temperature is crucial in many applications where heat flux is or can be present. Unfortunately, a number of isobaric heat capacity data sets reported in the literature is not extensive. Moreover, often significant deviations (up to 20% 6 and even 41% 14) between published data sets are observed. In the case of ILs, deviations of such order are generally not unusual because they are also observed for many other properties.15 Generally, a number of various factors influence the uncertainty of the experimental heat capacity data for ILs, however, all factors can be divided in two main categories. These two categories include: (i) sample purity and its chemical characteristics, and (ii) instrumentation problems. Therefore, a study of the heat capacity of ILs should follow a considered procedure to control the purity of the samples as well as a careful choice of experimental method and procedures employed in like manner as for all thermophysical properties.16 However, the water content should be checked best both before and after the measurements. Although imidazolium-based ILs are one of the most investigated families of ILs, the literature search shows that even in the cases of relatively popular ILs of this type, very limited heat capacity data are available or even lack such data. Therefore, we continue here an investigation of the isobaric heat capacities at atmospheric pressure in the temperature range from 293.15 K to 323.15 K for eight aprotic imidazolium-based ILs. Among investigated ILs are:

1-ethyl-3-methylimidazolium

methylimidazolium diethylphosphate

ethylsulfate ([C2C1im][DEP]),

thiocyanate

([C2C1im][SCN]),

([C2C1im][EtSO4]),

1-ethyl-3-



tricyanomethanide 3

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([C2C1im][C(CN)3]),

1-butyl-3-methylimidazolium

diethylimidazolium

ethylsulfate

acetate

[C2C2im][EtSO4],

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([C4C1im][OAc]),

1,3-

1-ethyl-2,3-dimethylimidazolium

bis(trifluoromethylsulfonyl)imide ([C2C1C1im][NTf2]), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

([C4C1C1im][NTf2]).

The

two

latter

are

so-called

trisubstituted imidazolium-based ILs. Note that from above ILs only [C2C1im][EtSO4] was extensively investigated but at the same time a deviation reaching 41% was reported.14 The obtained data are critically analyzed together with the available in the open literature data to recommend the most reliable isobaric heat capacities values for the above-mentioned imidazolium-based ILs. The effect of cation and/or anion structure (including influence of the alkyl chain length of the cation) on the heat capacity is analyzed too. Obtained data of isobaric heat capacity are also compared with the values calculated from two models based on a group contribution method, first proposed by Ge et al.

17

and second proposed by Oster et al.18

(exceeded version of Ge et al.17 model). Using the experimental molar isobaric heat capacities and the indirect acoustic method, the isochoric molar heat capacities, adiabatic indices, and connected thermodynamic quantities are determined by means of the additionally measured density and speed of sound. Lastly, based on the speed of sound, the heat conductivities are calculated according to modified by Wu et al.19 Bridgman relation and compared with literature values and heat conductivities calculated from a model based on group contribution method proposed by Oster et al.18

2. Materials and Methods 2.1. Chemicals. The supplier of the eight ILs was Iolitec (Germany). The CAS number, molar mass, and purity along with the water and halide contents are listed in Table 1. In the case of all samples, prior to measurements, the water content was determined using the coulometric Karl Fisher method. If necessary, the samples were dried and degassed under low pressure at temperatures not exceeding 343 K.

Table 1. Sample Table ILs

CAS number

M

mass

/ g⋅mol-1

fraction purity

water

halides/

/ ppm

ppm

a

[C2C1im][SCN]

331717-63-6

169.25

> 0.98

< 2000a / 1847b

< 2%a,c

[C2C1im][C(CN)3]

666823-18-3

201.23

> 0.98

< 1000a / 724b

< 5000a


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[C2C1im][EtSO4]

342573-75-5

236.29

> 0.99

< 500a / 22b

< 100a

[C2C1im][DEP]

848641-69-0

264.26

> 0.98

< 2500a / 627b

< 1%a,c,d

[C4C1im][OAc]

284049-75-8

198.26

> 0.98

< 10000a / 1512b

–

[C2C2im][EtSO4]

516474-04-7

250.32

> 0.99

< 500a / 13b

< 100a

[C2C1C1im][NTf2]

174899-90-2

405.34

> 0.99

< 100a / 85b

< 100a

[C4C1C1im][NTf2]

350493-08-2

433.39

> 0.99

< 100a / 62b

< 100a

a

Declared by supplier. b Coulometric Karl Fisher titration, TitroLine 7500. percent. d 1-methylimidazole content.

c

Mass fraction

Although the isobaric heat capacity measurement procedure is the same as in the previous report,6 the possibility of sample contamination with water during measurement procedure was checked in this work as well. The water content was probing directly before and after measurement for two by chance selected ILs. In the case of [C2C1im][EtSO4] and [C2C2im][EtSO4], the water contents after measurements were 24 and 14 ppm, respectively. Thus, the measuring procedure leads generally insignificant changes in water contents only (in comparison to water contents before the measurements given in Table 1).

2.2. Heat capacity measurements. The isobaric heat capacities were measured using a differential temperature-scanning microcalorimeter and procedure which have been already previously described in details.6 Therefore, only brief description is given. The main part of the microcalorimeter used is a semiconductor differential heat flux detector of the Tian-Calvet type. The time constant of the DSC used is equal to 9.2 s. The vessels (ca.3 mL constant volume) were made from Hastelloy C22. As the reference vessel, the full metal cylinder was used. The measuring vessel (the thickness of the vessel walls 0.2 mm) was always sealed with a Viton O-ring and exactly identically orientated, to avoid or at least minimizing any effects of change in surface contact of the measuring cell with the calorimeter’s furnace. The samples with volume in each case 2.7 mL, i.e., ca. (3.3 to 4.1) g were prepared by mass (precision ±6⋅10-7 kg) using an OHAUS balance (DV215CD). In each case before a proper measurement, the sample was heated up in the measuring vessel for a final degassing; the samples were not treated in any other manner to avoid the possible effect of the “thermal history”.20 The same three step temperature program ((i) isothermal phase (5⋅103 s) at starting temperature, (ii) temperature scanning at a rate of 1 mK s-1 in heating direction, and (iii) isothermal phase (5⋅103 s) at final temperature) has been applied to the empty sample vessel, reference sample, and the measured sample. As a reference standard 1-butanol (Aldrich, 5 ACS Paragon Plus Environment


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anhydrous, mass fraction purity 0.995, SureSeal) and the values reported in reference 21 were used. The control measurements were performed with n-hexane and benzene (details in ref. 6). During all experiments, the temperature in the laboratory was maintained within ±1 K. Based on the calibration procedure, test runs, measurement procedure, and neglecting vapor correction, the expanded uncertainty (coverage factor k = 2, confidence level 0.95) of the isobaric heat capacity values obtained in this work for the studied ILs was estimated to be of ±1%. 2.3. Density measurements. A vibrating-tube densimeter DMA 5000M (Anton Paar, Austria) was used for density measurements. Apparatus was calibrated (extended calibration procedure was used) with dry air and re-distilled water. The water was always freshly degassed (by boiling) before using; its electrolytic conductivity was 1⋅10-4 S⋅m-1 at T = 298.15 K. The densities were measured in the temperature range (283.15-363.15) K. In each case, an automatic viscosity correction was made. Standard uncertainty was ±0.05 kg⋅m-3; a repeatability was ±0.005 kg⋅m-3. 2.4. Speed of sound measurements. The speeds of sound were measured using commercial apparatus DSA 5000M (Anton Paar, Austria). As known, in the case of DSA 5000M, the wide-band technique (terse comparison of wide-band and narrow-band technique can be found for example in ref. 22) is adopted and the measurements can be correctly executed in non-dispersive regions only. Since ILs are often dispersive media,23,24 care must be taken into account during measurements of ILs; detailed discussion can be found elsewhere.25 In this work, speeds of sound were measured mostly in the temperature range (293.15–343.15) K. In some cases, however, to avoid ultrasound velocity dispersion the temperature range was finally significantly narrower; this temperature range restriction results from the analysis discussed in Section 3.3 and SI. The measuring cell (with transducers operating at frequency 3 MHz) was calibrated with re-distilled water which was prepared as above in the case of density measurements; the standard speed of sound values reported by Del Grosso and Mader26 were applied. Afterwards, the apparatus was checked with 1,2-ethanediol (Fluka, anhydrous, mass fraction purity 0.995). At 298.15 and 313.15 K, the discrepancies between the obtained experimental values and reported previously values27 are 0.7 m⋅s-1 and 0.8 m⋅s-1, respectively. The expanded uncertainty (k=2, level of confidence of 0.95) for measured ILs was estimated to be less than 1 m·s-1.


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In this study, always the temperature scale ITS-90 is used and all measurements are made at atmospheric pressure p0 = 0.101 MPa. The molar masses are reported in terms of the 2001 IUPAC relative atomic masses. 2.5. Predictive models. Predictive ability of selected models proposed by Ge et al.17, Oster et al.18 and Wu et al.19 to prediction of the isobaric heat capacity and thermal conductivity are tested. In case of heat capacity, the experimental values are compared with values calculated from two models based on group contribution method, proposed by Ge et al.17 and improved version proposed by Oster et al.18 In turn, in case of thermal conductivity, the models chosen for calculations are Wu et al.19 (modified Brindgman relation using ρ and c data) and Oster et al.18 (group contribution method).

3. Results and Discussion 3.1. Isobaric heat capacities. Since the results were recorded with the step 0.01 K over the temperature range studied, the raw experimental data points is equal ca. 3000 in each case. All the molar isobaric heat capacities Cp in the investigated temperature range were approximated by the polynomials in a form: 3

C p = ∑ Ai ⋅ ((T − 293.15) / 100)

i

(1)

i =0

where T is the temperature. The determined by the use of unweighted least-square method statistically significant coefficients Ai and the mean standard deviations from the regression lines δ are reported in Table S1 of the Supporting Information (SI). In turn, in Table 2 the selected Cp values in the temperature range from 293.15 K to 323.15 K are listed. In the following sections, we discuss separately the results for each group of ILs together with the available, in the investigated in this work temperature range, literature data sets; all uncertainties of the literature data given in this work are the original values reported by authors. In most cases, the agreement between our and literature data is demonstrated also in deviation plots. Note that a compilation of the oldest Cp data (up to 2010) for ILs can be found in papers of Zábranský et al.28 and Paulechka.29 At present helpful can be also database IL Thermo.30


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Table 2. Molar Isobaric Heat Capacitiesa Cp for ILs Studied within Temperature range (293.15 to 323.15) K at Atmospheric Pressure Cp/ J⋅K-1⋅mol-1

T/K

[C2C1im][SCN] [C2C1im][C(CN)3] [C2C1im][EtSO4] [C2C1im][DEP] [C4C1im][OAc] [C2C2im][EtSO4] [C2C1C1im][NTf2] [C4C1C1im][NTf2] 293.15

276.1

337.7

380.9

463.2

368.2

410.8

524.1

588.0

298.15

277.4

339.3

383.5

465.9

370.6

413.4

527.1

591.9

300.15

277.9

340.0

384.5

467.0

371.7

414.5

528.3

593.3

303.15

278.7

341.0

385.9

468.6

373.2

416.1

530.1

595.2

308.15

280.1

342.7

388.1

471.3

375.9

418.9

533.3

598.1

310.15

280.7

343.4

388.9

472.4

377.0

420.1

534.7

599.2

313.15

281.6

344.4

390.2

474.1

378.7

421.8

536.7

600.9

318.15

283.2

346.2

392.5

476.9

381.4

424.9

540.1

604.0

320.15

283.9

347.0

393.5

478.0

382.5

426.1

541.5

605.3

285.0 348.1 395.1 479.7 384.1 -1 -1 Expanded uncertainty (k = 2, confidence level 0.95) Uc(Cp) is ±0.01⋅Cp J⋅K ⋅mol

428.0

543.7

607.5

323.15 a

8

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3.1.1. Disubstituted imidazolium-based ILs. For the six such ILs studied in this work, the number of published data on Cp varies strongly. For example, in the case of [C2C1im][C(CN)3] only one data set exists

31

and no data were found for [C2C2im][EtSO4].

On the other hand, in the case of [C2C1im][EtSO4], over a dozen data sets exist.14,32-44 Below we critically analyzed each one of the above imidazolium-based ILs. [C2C1im][SCN]. For this IL three data sets are available.31,45,46 The maximal difference between the Cp values is about 4% (Figure 1 and Table S2). Thus, the consistency is relatively good.

An

average

absolute

relative

deviation

AARD

(AARD

=

N

100 ⋅ N −1 ⋅ ∑ (C p ,lit − C p ,exp ) / C p ,exp , where N is the number of data points, Cp,exp denotes i =1

values obtained in this work, and Cp,lit denotes literature values) is here close to 2.6%. Generally, the best accordance with our data show however the single Cp value (298.15 K) reported by Freire et al. obtained from drop calorimetry (DC);45 here the difference is close to 1.5%. Fairly good accordance show also data from DSC calorimeters by use of very small samples (≤ 0.1 mL) enclosed in sample pans.31,46 The differences between the reported values can be connected with the contamination of the sample by varying amounts of water; note that drying in the case of this IL is difficult.

Figure 1. Deviations between determined in this work (eq.1) Cp,exp values and available literature values Cp,lit shown as (Cp,lit - Cp,exp)/Cp,exp in the temperature range from (293.15 to 323.15) K for: △, [C2C1im][SCN], ref. 31; ○, [C2C1im][SCN], ref. 46; □, [C2C1im][SCN], ref. 45; and (red) ■, [C2C1im][C(CN)3], ref. 31.

[C2C1im][C(CN)3]. For this IL only one data set reported by Navarro et al.31 is available (Table S3). The deviations (Figure 1) between their values (DSC, sample < 0.1 mL) and those obtained by us are positive and increase slightly with the increasing temperature; an AARD is



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close to 6.1%. The differences result from instrumentation (Navarro et al.31 used DSC with speed scan rate 20 K⋅min-1 and small sample < 0.1 mL) and water content. We recommend our values.

[C2C1im][EtSO4]. numerous.

14,32-44

The literature data (Table S4 and Figure 2) are here relatively

Unfortunately, as mentioned in the introduction, the experimental data sets

for [C2C1im][NTf2] have a very high degree of scatter, reaches even 41%.14 However, excluding the extreme values of Fernandez et al.,35 where poor agreement (deviations up to 21% at 323.15 K) results most probably from the low purity of sample (≥0.95, water content 2000 ppm), as well as the values of Yu et al.36 (deviations up to 12% at 323.15 K), the rest of data show a satisfactory agreement. At 298.15 K the maximal difference between these values is equal ca. 5.0%.34 The values obtained in this work agree within experimental uncertainty with those reported by Paulechka et al.14,44 The AARD was calculated to be close 0.21%. Curious and very unfortunate is that one and the same data set is reported by some authors two or three times, i.e., in the independent papers (refs 14 and 44 as well as refs 38, 39, and 40). As reported data are the same, in this work, above data are treated by us only once.

[C2C1im][DEP]. For this IL three polythermal literature data sets are available.46-48 All three of data sets are obtained using DSC (speed scan rates) and very low samples (≤ 0.1 mL).46-48 The results, however, are very different. The relative deviations between our Cp values (Figure 3 and Table S5) and literature values are from 14%46 to -14%47 at 298.15 K. Generally, AARD close to 6.7% was found. Thus, the agreement is poor, however, two data sets46,47 are responsible for such situation. When these non-consistent data sets are omitted, the remaining set of data48 agree with data obtained in this work very good (AARD close to 0.7%). Thus, the consistency can be here considered as excellent.

Figure 2. Deviations between determined in this work (eq.1) Cp,exp values and available literature values Cp,lit shown as (Cp,lit - Cp,exp)/ Cp,exp in the temperature range from (293.15 to 10 ACS Paragon Plus Environment

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323.15) K for [C2C1im][EtSO4]: □, ref. 34; ◇, refs 38–40; ○, ref. 37; ■, ref. 43; △, ref. 32; ◆,

refs 14,44; ●, ref. 41; ✱, ref. 42; ▲ ref. 33.

[C4C1im][OAc]. The discrepancies in the reported values (Figure 3 and Table S6) are moderate and the AARDs close to 2.9%49 and 3.4%50 for each data sets were found. Both values reported by Strechan et al.50 and Safarov et al.49 are higher than obtained in this study. Interestingly, Strechan et al. used AC, whereas Safarov et al. used DSC with a small sample. At the same time declared by authors mass fraction purity (> 0.995 and ≥ 0.95, respectively) of the samples and water content in ppm (3000 and 300, respectively) are very different. Surely, the purity on the level of 0.95 mass fraction as in the case of the sample used by Safarov et al.49 is too low. In turn, the water content in the case of the sample used by Strechan et al.50 is twice higher than in present study. All things considered, give the not clear picture, however, the analysis indicates that the obtained in this work values can be treated as recommended values.

Figure 3. Deviations between determined in this work (eq.1) Cp,exp values and available literature values Cp,lit shown as (Cp,lit - Cp,exp)/Cp,exp in the temperature range from (293.15 to 323.15) K for: □, [C2C1im][DEP], ref. 48; △, [C2C1im][DEP], ref. 46; ○, [C2C1im][DEP], ref. 47; ■, [C4C1im][OAc], ref. 49; ◆, [C4C1im][OAc], ref. 50; and +, [C2C1C1im][NTf2], ref. 51.

[C2C2im][EtSO4]. This is the so-called symmetric disubstituted imidazolium-based IL. According to our best knowledge, in the available literature lack the Cp data for this IL. For this reason, the values reported in this work are recommended. Comparison of the heat capacities for [C2C1im][EtSO4] and [C2C2im][EtSO4] shows that substitution of a methyl 11 ACS Paragon Plus Environment


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group by ethyl group in position C3 of the imidazolium ring leads to increase of heat capacity. It is in accordance with the general trend that a molecule containing more atoms would have a higher heat capacity because such molecule would have more energy storage modes.

3.1.2. Trisubstituted imidazolium-based bis(trifluoromethylsulfonyl)imides. Although there are interesting ILs based on trisubstituted imidazolium cation, the Cp data are very scarce.10,51,52

[C2C1C1im][NTf2]. In this case, apart from one relatively old polythermal data set of Fredlake et al.51 obtained from DSC (Table S7), no other literature Cp data are available. An agreement between above literature values and the values determined in this work is rather poor (Figure 3). As in other cases analyzed recently,6 the values reported by Fredlake at al.51 (DSC, small sample < 0.1 mL) show the systematic deviations; here, the relative deviations are from -7.7% (309.15 K) to -8.3% (323.15 K) and AARD close to 8% was found. Thus, our data are recommended. Comparison of the heat capacity for [C2C1C1im][NTf2] and [C2C1im][NTf2]6 shows that adding a methyl group on the C2 carbon increases the heat capacity. This result indicates that stated by Fredlake et al.51 conclusion that

[C2C1C1im][NTf2] has a lower heat capacity than [C2C1im][NTf2] was connected with errors in determined heat capacities for these ILs (too low for [C2C1C1im][NTf2] and too high for [C2C1im][NTf2]).

[C4C1C1im][NTf2]. For this IL, apart from results reported by Paul et al.,52 we have not found any other literature data in the open literature (Table S8). Since data reported by Paul et al. (mostly graphically) seems small reliable (at 298.15 K deviation reaches -26% and temperature dependence is somewhat curious), the values reported in this work can be treated as recommended values. Also here, adding a methyl group on the C2 carbon increases the heat capacity in relation to values reported previously6 for [C4C1im][NTf2]. Comparison of the Cp values for [C2C1C1im][NTf2] and [C4C1C1im][NTf2] shows that elongation of the side alkyl chain in position C1 is connected with increasing the Cp values; it is according with the general rule. The increase is typical, i.e., similar as in the case of [CnC1im][NTf2] homologues and other homolog series.6 Taking into account the data reported for [C3C1C1im][NTf2] by Fredlake et al.,51 the effect of increasing the alkyl chain length of the cation on the Cp values for trisubstituted imidazolium-based bis(trifluoromethylsulfonyl)imides is equal to 32.4 ± 2.6 J·K−1·mol−1 per CH2 group at 298.15 K. Relatively great uncertainty is connected with rather small reliability of the Fredlake et al. data.51 Unfortunately, when also relatively old data reported for [C6C1C1im][NTf2]10 are taken into account, result is still worse (the mutual


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consistency is poor). This indicates rather poor quality of results reported for [C6C1C1im][NTf2].10 Summing up, for the investigated ILs agreement of literature values with the data presented in this work is fairly different. The best agreements are observed in the case of [C2C1im][SCN] and [C4C1im][OAc]. In contrast, the deviations up to 21%35 are observed for [C2C1im][EtSO4].

Generally,

however,

rejecting

the

largest

outliers

(applies

to

[C2C1im][EtSO4], [C2C1im][DEP], [C2C1im][C(CN)3], and [C2C1C1im][NTf2]), the deviations fall to below ±3%. In most cases, our data are located in the middle or are somewhat lower (always within ±3%) than the literature data. The conclusions are similar as in the case of previously studied 11 ILs,6 i.e., the most consistent and reliable results are obtained mainly from AC and TC DSC with samples and scan rates similar as in this work. In other words, TC DSC calorimetry with a slow (and very slow) scan rates (smaller than 1-2 K⋅min-1) and relatively large samples (1 to 10 mL) give generally very reliable results that are consistent with the results obtained by AC and DC. At the same time it is clearly noticeable that the greatest deviations are most often associated with DSC calorimetry using speed scanning rates (e.g., 10 K⋅min-1 and more) and small samples (≤ 0.1 mL).31,51,35,36,46 Such conclusions have been also previously reported,6,53,54 and it can be treated as a general rule. Thus, although standard DSC is not time-consuming in comparison to TC DSC, the accuracy is lower. Therefore, it is always a good idea to consider the more time-consuming TC DSC measurement technique. Such conclusion was also reported by Diedrichs and Gmehling.55 Obviously, other, significant source of errors can be impurities and sample treatment. However, unfortunately, even an in-depth inspection of original works does not give clear and uniform picture in this matter because information on impurities are un-unified and incompletely, and in some papers lack even any information about purity. In general, in older papers the purity of the samples used was however rather generally worse. According to the study of Paulechka et al.14, the level of impurities (such as residual solvents and reagents carried over from the synthesis) typical for the best available samples had rather a little impact on the specific isobaric heat capacity cp in all cases with exception of water. Thus, only water is an impurity which shifted the cp of the sample significantly. Hence, great attention related to the possible absorption of water during an experiment is especially important in the case of small samples. Moreover, because the measured heat capacity can depend also on the previous “thermal history” of the sample,20 the samples used for Cp measurements should not



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have such “history” (e.g., the sample should not be previously cooled down below the melting point).

3.1.3. Effect of anion and cation structure on heat capacity. The specific isobaric heat capacities for 8 ILs studied are different; at 298.15 K the lowest and highest values are (1.300 and 1.868) J⋅g-1⋅K-1 for [C2C1C1im][NTf2] and [C4C1im][OAc], respectively. At the same time an average cp (for 8 ILs) at 298.15 K is equal 1.612 J⋅g-1⋅K-1. The cp values for ILs studied can therefore be classified as moderate. The Cp values reported in this work and previously6 confirm that the anion structure has great influence on heat capacity. For example, in the case of [C2C1im]+ cation, the Cp values change in the row [SCN]-

Isobaric and Isochoric Heat Capacities as Well as Isentropic and

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