Heat capacities and electrical conductivities of 1-ethyl-3-methylimidazolium-based ionic liquids

Heat capacities and electrical conductivities of 1-ethyl-3-methylimidazolium-based ionic liquids

J. Chem. Thermodynamics 41 (2009) 103–108 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locat...

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J. Chem. Thermodynamics 41 (2009) 103–108

Contents lists available at ScienceDirect

J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct

Heat capacities and electrical conductivities of 1-ethyl-3-methylimidazolium-based ionic liquids Ya-Hung Yu, Allan N. Soriano, Meng-Hui Li * R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, Taiwan

a r t i c l e

i n f o

Article history: Received 1 July 2008 Received in revised form 16 July 2008 Accepted 16 July 2008 Available online 25 July 2008 Keywords: Heat capacity Electrical conductivity [Emim]-Based ionic liquids

a b s t r a c t We present the heat capacities and electrical conductivities of five [Emim] 1-ethyl-3-methylimidazoliumbased ionic liquids: [Emim][BF4] (tetrafluoroborate), [Emim][CF3SO3] (trifluoromethanesulfonate), [Emim][C2N3] (dicyanamide), [Emim][C2H5SO4] (ethylsulfate), and [Emim][MDEGSO4] (2-(2-methoxyethoxy) ethylsulfate). The heat capacities were measured using a differential scanning calorimeter (DSC) over the temperature ranging from (303.2 to 358.2) K. The electrical conductivities were measured over the temperature ranging from (293.2 to 353.2) K using a commercial conductivity meter. The estimated uncertainties of heat capacity Cp and electrical conductivity r measurements were ±0.015 kJ  kg1  K1 and ±0.001 mS  cm1, respectively. The measured Cp and r are presented as a function of temperature. The temperature dependency of the CP value was correlated using an empirical equation. A modified version of VTF-type (Vogel–Tamman–Fulcher) equation was used to describe the temperature dependency of r values. The correlations give satisfactory results. Also, the results of this study are in good agreement with the available literature data. The heat capacities and electrical conductivities presented in this work are in good agreement with the available literature data. The results of this study can be applied to numerous chemical processes, since Cp and r data are essential information for rational design. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Aqueous alkanolamine solutions are industrially effective for CO2 absorption, but this method usually requires higher energy consumption and operational cost, and also comes with solvent pollution. Ionic liquids (ILs), coined as green solvents, have been recognized as a versatile alternative to the aqueous alkanolamine solutions. The IL is a common name given to many different chemical compounds, which are composed solely of ions and have melting points around or below room temperature. The maximum melting temperature to be accepted for a compound to be included in the IL category is about 373 K [1]. The ILs also show thermal stability at high temperatures and at high solubility for both polar and non-polar organic as well as inorganic substances. Consequently, ILs could be in a position to replace flammable and volatile organic solvents in chemical processes [2]. Because of these appealing properties, they could be very useful in the future. A significant number of research groups have already done systematic measurement and collection of the thermophysical and transport properties of ILs [2–15].

* Corresponding author. Tel.: +886 3 265 4109; fax: +886 3 265 4199. E-mail address: [email protected] (M.-H. Li). 0021-9614/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2008.07.013

The heat capacity Cp and electrical conductivity r are two of the basic pure component properties for any substance, and their knowledge is necessary for many engineering applications. For most ILs, these values are still lacking. As in the case of Cp measurement, so far the thoroughly studied ILs are ammonium-, pyridinium-, pyrrolidinium-, bis[(trifluoromethyl)sulfonyl]amide-, and imidazolium-based ILs [2,3,5,7,10,12,13,16–25]. In the Cp measurement of imidazolium-based ILs, most of the work clustered on hexyl- and butyl-3-methylimidazolium [10,12–14,17,18]. For the r measurement, only those of ammonium-, pyrrolidinium-, and bis[(trifluoromethyl)sulfonyl]amide-based have been widely studied, while not much has been studied on imidazolium-based ILs [4,9,11,16,26–30]. Therefore heat capacities and electrical conductivities for five [Emim]-based ILs have been measured. The heat capacities were measured over the temperature ranging from (303.2 to 358.2) K at atmospheric pressure condition and the electrical conductivity over the temperature ranging from (293.2 to 353.2) K at atmospheric pressure condition. The ILs that have been investigated are listed in table 1. The structures of cations and anions are shown in figure 1. In order to check the accuracy of the apparatus and the experimental procedures used, heat capacity and electrical conductivity of standard materials have also been measured. Water was used to check the heat capacity measurements and the standard KCl solution for electrical conductivity measurements.

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TABLE 1 Ionic liquids investigated in this work Ionic liquid 1-Ethyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium

tetrafluoroborate trifluoromethanesulfonate dicyanamide ethylsulfate 2-(2-methoxyethoxy) ethylsulfate

Abbreviation

Purity, Mass fraction

[Emim][BF4] [Emim][CF3SO3] [Emim][C2N3] [Emim][C2H5SO4] [Emim][MDEGSO4]

P0.970 P0.980 P0.999 P0.992 P0.982

was in the range (15 to 20) mg. Five replicate runs were carried out for each measurement. The apparatus and the experimental procedures are the same as those described by Chiu et al. [31]. 2.3. Electrical conductivity measurements The electrical conductivity was measured using a SC-170 conductivity meter manufactured by Suntex. The uncertainty of the conductivity measurement is ±0.001 mS  cm1 for readings below 50 mS  cm1. The temperature was monitored using a digital thermometer (model 3002, CROPICO), with an uncertainty of ±0.01 K. A fixed volume of sample (3 cm3) was placed in a test tube and placed in a water bath where the temperature was controlled. The conductivity cell was first adjusted to zero in the air followed by the calibration of standard KCl solution before the sample was put in. The calibration of this equipment is the same as those described by Widegren et al. [4]. The conductivity cell was then washed with deionised water and ethanol to remove any adhering IL, and dried. To preserve the conductivity cell it was placed in water or in dry air before it was used for the next measurement. The measurements were done in five replicate runs. 3. Results and discussion FIGURE 1. Cation and anions of the ionic liquids investigated.

3.1. Heat capacity 2. Experimental 2.1. Chemicals All ILs used in this work were supplied by TCI Co. Table 1 shows the purity of the ILs that were used without further purification since the experiments required a small amount of sample. The liquid water used for the calibration of the calorimeter was deionised, with a resistivity of 18.3 MX  cm and with a total organic carbon mass fraction of less than 1.5  1010 produced by Barnstead Thermodyne, model Easy Pure 1052. The standard KCl solution used for the calibration of the cell of the conductivity meter was supplied by Merck, with c = 0.1 mol  L1 and electrolytic conductivity of 1.415 mS  cm1 at T = 298.15 K. 2.2. Heat capacity measurements

The equations used to represent the Cp are the same as those described by Chiu et al. [31]. The Cp of liquid water was measured and compared to the data of Osborne et al. [32] to verify the accuracy of the DSC. Osborne and co-workers [32] were able to measure the heat capacity of water with an uncertainty of (±0.0001 to ±0.0002) kJ  kg1  K1 at close intervals of temperature (1 K) by using a large adiabatic calorimeter. Table 2 shows the comparison of Cp results obtained by Osborne et al. [32] and those obtained in this study. The average values given in the last column in table 2 have an average absolute deviation (AAD) of 0.11 from the available experimental data [32]. The (AAD) is defined as

TABLE 2 Heat capacities Cp of water T/K

The heat capacity was measured using the differential scanning calorimeter consisting of a DSC-2010 and a thermal analysis controller from TA Instruments. The DSC operating range is from the room temperature to T = 998 K. Both the temperatures and the heat flow associated with the transitions in materials can be easily and rapidly measured with the system. The DSC operates with a temperature repeatability of ±0.1 K. Calorimetric sensitivity is 1 lW (rms) with a precision of ±0.01 kJ  kg1  K1 based on the measurements of metal samples. The purge gas was nitrogen with a flow rate of 40 cm3  min1. The heating rate was set to be 5 K  min1. By using the sample encapsulating press, the liquid sample was prepared in a hermetic sample pan. The internal volume of the hermetic pan was approximately 10 mm3. Sample mass

303.2 308.2 313.2 318.2 323.2 328.2 333.2 338.2 343.2 348.2 353.2 (AAD) a

Cp/(kJ  kg1  K1) Osborne et al. [32]

This study

4.1785 4.1782 4.1786 4.1795 4.1807 4.1824 4.1844 4.1868 4.1896 4.1928 4.1964

4.182 ± 0.013a 4.183 ± 0.013 4.182 ± 0.012 4.184 ± 0.013 4.186 ± 0.013 4.187 ± 0.012 4.189 ± 0.012 4.191 ± 0.012 4.194 ± 0.012 4.197 ± 0.012 4.202 ± 0.012 0.11

Mean value ± standard deviation.

105

Y.-H. Yu et al. / J. Chem. Thermodynamics 41 (2009) 103–108 n 1X jðelit  eexpt Þji =elit ; n i

ð1Þ 600

525

· 450

·

where elit is the literature value, eexpt is the experimental value, and n is the number of data points. Hence, the measured Cp values of liquid water for temperatures (303.2 to 353.2) K are in good agreement with those reported by Osborne et al. [32]. On the basis of comparison with the literature values for water, the uncertainty of the Cp measurements was estimated to be ±0.015 kJ  kg1  K1. Upon the calibration, the Cp values of five [Emim]-based ionic liquids were then measured at temperatures ranging from (303.2 to 358.2) K. To further validate that the measured Cp results in this work were correct, available literature data were also compared. Among the studied ILs, only [Emim][BF4] and [Emim][CF3SO3] have literature values available. To this end, [Emim][BF4] was used to demonstrate the accuracy of our Cp measurements through comparison with the available literature data of Waliszewski et al. [5] At temperatures (303.2, 313.2, 323.2, 333.2, 343.2, and 353.2) K, we report the values of Cp for [Emim][BF4] of (1.547, 1.569, 1.588, 1.598, 1.613, and 1.631) kJ  kg1  K1, respectively. These values were very close to those reported by Waliszewski et al. [5], which were (1.548, 1.565, 1.582, 1.600, 1.619, and 1.639) kJ  kg1  K1 at the same temperature. As presented in table 4, the present Cp measurements are consistent with the Cp data of Waliszewski et al. [5], as shown by the values of (AAD) of 0.67 and 0.49, respectively. The data of Waliszewski et al. [5] appear to have slightly higher Cp values compared to the present Cp measurements. This may be attributed to the difference in the water content of the IL used in these two studies. The [Emim][BF4] used by Waliszewski et al. [5] has a water mass percent of about 0.04, while the [Emim][BF4] used in this study has a water mass percent of 60.9. This observation proved once more why purity is the most serious one among the many reasons to account for differences in the published experimental data for thermophysical properties [33]. The measured Cp values of the investigated ILs are presented in table 3 (specific heat capacities) and are also shown in figure 2 (molar heat capacities). Results are presented between T = (303.2 and 358.2) K. The heat capacity increases linearly with an increase in the temperature. Differences in the heat capacity for the investigated ILs are due to the changes in the anions. Since the investigated ILs have a common cation, the molar heat capacity varies with the anion, and decreases following the order [MDEGSO4] > [C2H5SO4] > [CF3SO3] > [C2N3] > [BF4]. The anion types appear to contribute independently to the Cp values of the ILs. As also shown in figure 2, [Emim][C2H5SO4] is observed to have the strongest temperature dependence of Cp, while [Emim][MDEGSO4] has the weakest temperature dependence of Cp. For the purpose of application, the measured Cp values of the investigated ILs are expressed as a function of temperature as follows:

Cp / (J mol-1 K-1)

ðAADÞ ¼

375

300

300

320

340

360

T/K FIGURE 2. Plot of molar heat capacities against temperature for [Emim]-based ionic liquids obtained from this work: ., [Emim][MDEGSO4] (M = 338.43); j, [Emim][CF3SO3] (M = 260.24); , [Emim][C2H5SO4] (M = 236.29); N, [Emim][BF4] (MW = 197.97); d, [Emim][C2N3] (M = 177.21); and lines, calculated using equation (2).

C p;m ¼ a þ bðTÞ þ cðTÞ2 ;

ð2Þ

where Cp,m is the molar heat capacity in (J  mol1  K1) and T is the absolute temperature in K. The parameters a, b, and c were determined by fitting all the data from this work and from the available literature data that were consistent with the present measurements. Table 4 shows the results of the calculation of Cp,m using equation (2) based on the selected Cp data. As presented in table 4, the agreement of Cp measurements among different investigators is very satisfactory. The determined empirical parameters a, b, and c for each ionic liquid represent well the present experimental results and the selected literature data as shown by the overall (AAD) of about 0.43 for a total of 140 data points, as presented in table 4. The determined parameters a, b, and c of equation (2) are also presented in table 4 along with the correlation coefficient (R2). The correlation as in equation (2) represents the present experimental data satisfactorily as well as the selected literature data for the investigated systems as shown by the value of R2 equal to unity. 3.2. Electrical conductivity The r values of the standard KCl solution were measured to calibrate the cell of the conductivity meter. The measurements were done before the measurements on each sample to ensure that the conductivity cell was functioning properly. Table 5 contains a

TABLE 3 Heat capacities Cp of the investigated ionic liquids T/K

303.2 308.2 313.2 318.2 323.2 328.2 333.2 338.2 343.2 348.2 353.2 358.2 a

Cp/(kJ  kg1  K1) [Emim][BF4]

[Emim][CF3SO3]

[Emim][C2N3]

[Emim][C2H5SO4]

[Emim][MDEGSO4]

1.547 ± 0.018a 1.558 ± 0.018 1.569 ± 0.014 1.580 ± 0.017 1.588 ± 0.018 1.593 ± 0.017 1.598 ± 0.018 1.608 ± 0.022 1.613 ± 0.022 1.623 ± 0.022 1.631 ± 0.021 1.634 ± 0.021

1.458 ± 0.011 1.466 ± 0.011 1.473 ± 0.011 1.481 ± 0.010 1.489 ± 0.011 1.497 ± 0.012 1.505 ± 0.011 1.513 ± 0.009 1.521 ± 0.011 1.529 ± 0.011 1.536 ± 0.013 1.544 ± 0.014

1.851 ± 0.018 1.862 ± 0.018 1.879 ± 0.018 1.891 ± 0.018 1.911 ± 0.018 1.923 ± 0.018 1.934 ± 0.018 1.944 ± 0.018 1.955 ± 0.018 1.969 ± 0.018 1.984 ± 0.018 1.999 ± 0.019

1.819 ± 0.009 1.827 ± 0.010 1.841 ± 0.011 1.850 ± 0.012 1.868 ± 0.011 1.881 ± 0.012 1.893 ± 0.012 1.905 ± 0.009 1.917 ± 0.010 1.932 ± 0.008 1.951 ± 0.010 1.962 ± 0.008

1.696 ± 0.014 1.701 ± 0.014 1.706 ± 0.014 1.711 ± 0.015 1.716 ± 0.015 1.720 ± 0.015 1.726 ± 0.014 1.731 ± 0.014 1.736 ± 0.013 1.742 ± 0.013 1.747 ± 0.012 1.753 ± 0.012

Mean value ± standard deviation.

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TABLE 4 Molar heat capacity Cp,m of the investigated ionic liquids using equation (2) System

T/K

No. of data points

Reference

(AAD)A

aA/ (J  mol1  K1)

b A/ (J  mol1  K2)

105 cA/(J  mol1  K3)

R2

[Emim][BF4]

283.15 to 358.15 303.2 to 358.2 313.13 to 425.15 303.2 to 358.2 303.2 to 358.2 303.2 to 358.2 303.2 to 358.2

16 12 64 12 12 12 12 140

Waliszewski et al. [5] This study Diedrichs and Gmehling [2] This study This study This study This study

0.67 0.49 0.60 0.20 0.11 0.09 0.02 0.43

279.24

0.10312

66.350

1.0

271.10

0.35907

0.307

1.0

104.71 401.52 531.92

0.96220 0.35939 0.03759

74.546 148.871 58.252

1.0 1.0 1.0

[Emim][CF3SO3] [Emim][C2N3] [Emim][C2H5SO4] [Emim][MDEGSO4] Overall A

Calculated from equation (2).

TABLE 5 A sample measurement of electrical conductivity r of the standard KCl solution

r/(mS  cm1)

T/K

298.2 299.2 301.2 303.2 308.2 313.2 318.2 323.2 a

Merck Co.a

This study

1.408 1.434 1.491 1.547 1.685 1.836 1.981 2.137

1.411 1.430 1.494 1.550 1.680 1.830 1.983 2.140

the accuracy of the present r data. The discrepancy in the measurements may be attributed to the differences in purity of the ILs used in these two studies. Vila et al. [6] used [Emim][BF4] having a water mass percent of 61.0, while this study used [Emim][BF4] with a water mass percent of 6 0.9. Again, another proof that purity was one of the most serious reasons for the differences in the experimental data of thermophysical properties [33]. Our values of the r for the investigated ILs are presented in table 6 and are also shown in figure 3. Among the investigated

Merck Calibration Laboratory for pH value and electrical conductivity.

7.5

6.0

4.5

·

σ / (S m-1)

sample of the measured r values of standard KCl solution along with the r values from the standard analysis done by the chemical supplier (Merck Calibration Laboratory for pH value and electrical conductivity). As presented in this table, the present r measurements were very close to those by the Merck Calibration Laboratory, thus validating the present experimental procedure for the measurements of r data. Upon the calibration of the conductivity meter cell, the r values of the ionic liquids investigated were then measured for temperatures ranging from (293.2 to 353.2) K. To justify further that the r values obtained in this work are correct, available literature data were also compared. Of the five ILs investigated, only two of them have literature values for comparison, viz. [Emim][BF4] and [Emim][C2H5SO4]. To this end, [Emim][BF4] was used to show the accuracy of the present r values by comparing the r data from this work to the available literature data of Vila et al. [6]. For temperatures of (303.2, 313.2, 323.2, 333.2, 343.2, and 353.2) K, this study found that the values of r for [Emim][BF4] were (1.789, 2.320, 2.910, 3.590, 4.350, and 5.180) S  m1, respectively. These values are very close to those values reported by Vila et al. [6], which were (1.788, 2.280, 2.850, 3.560, 4.350, and 5.220) S  m1 at the same temperature, thus validating

3.0

1.5

0.0

288

306

324

342

360

T/K FIGURE 3. Plot of electrical conductivities against temperature for [Emim]-based ionic liquids obtained from this work: ., [Emim][MDEGSO4] (M = 338.43); j, [Emim][CF3SO3] (M = 260.24); , [Emim][C2H5SO4] (M = 236.29); N, [Emim][BF4] (MW = 197.97); d, [Emim][C2N3] (M = 177.21); and lines, calculated using equation (3).

TABLE 6 Electrical conductivities r of the investigated ionic liquids T/K

293.2 298.2 303.2 308.2 313.2 318.2 323.2 328.2 333.2 343.2 353.2 a

r/(S  m1) [Emim][BF4]

[Emim][CF3SO3]

[Emim][C2N3]

[Emim][C2H5SO4]

[Emim][MDEGSO4]

1.377 ± 0.009a 1.569 ± 0.010 1.789 ± 0.003 2.040 ± 0.010 2.320 ± 0.013 2.590 ± 0.013 2.910 ± 0.012 3.250 ± 0.012 3.590 ± 0.011 4.350 ± 0.013 5.180 ± 0.011

0.834 ± 0.010 0.979 ± 0.010 1.155 ± 0.012 1.340 ± 0.013 1.523 ± 0.011 1.737 ± 0.011 1.936 ± 0.012 2.170 ± 0.014 2.420 ± 0.013 2.940 ± 0.010 3.510 ± 0.013

1.890 ± 0.013 2.200 ± 0.015 2.550 ± 0.013 2.900 ± 0.010 3.290 ± 0.013 3.680 ± 0.013 4.100 ± 0.014 4.540 ± 0.011 5.020 ± 0.011 5.930 ± 0.012 6.860 ± 0.010

0.310 ± 0.008 0.398 ± 0.009 0.493 ± 0.003 0.603 ± 0.010 0.722 ± 0.010 0.852 ± 0.010 1.002 ± 0.009 1.167 ± 0.009 1.341 ± 0.009 1.735 ± 0.010 2.150 ± 0.010

0.110 ± 0.011 0.147 ± 0.009 0.194 ± 0.009 0.244 ± 0.010 0.307 ± 0.013 0.378 ± 0.011 0.459 ± 0.012 0.545 ± 0.012 0.641 ± 0.011 0.859 ± 0.013 1.124 ± 0.013

Mean value ± standard deviation.

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Y.-H. Yu et al. / J. Chem. Thermodynamics 41 (2009) 103–108 TABLE 7 Electrical conductivity of the investigated ionic liquids using equation (3) System

T/K

[Emim][BF4]

258.1 293.2 258.1 293.2 293.2 293.2 293.2

[Emim][CF3SO3] [Emim][C2N3] [Emim][C2H5SO4] [Emim][MDEGSO4] Overall a

to to to to to to to

433.1 353.2 433.1 353.2 353.2 353.2 353.2

No. of data points

Reference

(AAD)a

r1a/(S  m1)

Eaa/meV

Tga/K

12 11 11 11 11 11 11 78

Vila et al. [6] This study Vila et al. [6] This study This study This study This study

0.93 0.74 0.50 0.50 0.35 0.29 0.44 0.54

361.20

91.60

102.66

98.28

57.34

153.52

81.77 77.86 64.93

37.24 52.95 57.80

178.62 181.85 187.89

Calculated from equation (3).

ILs, [Emim][C2N3] has the largest value of r and [Emim][MDEGSO4] has the smallest value of r. The IL [Emim][C2N3] has the strongest temperature dependence on r values, while [Emim][MDEGSO4] has the weakest temperature dependence on r values. From the present experimental results, the influence of the anion sizes in the temperature dependence of the electrical conductivity was also interpreted. Generally, lower size molecules usually have higher ionic mobility and so higher electrical conductivity. This was generally the major observation in the ILs studied as shown in figure 3, with some inconsistent behaviour in the case of [Emim][C2H5SO4] and [Emim][CF3SO3] with molar masses as 236.29 and 260.24, respectively. This same behaviour, the increase of r value with the anion size, had been observed and explained previously by Vila et al. [34] when studying the electrical conductivity of a highly concentrated aqueous solution of aluminium halide salts and imidazolium-based ionic liquids. In that work, Vila et al. [34] concluded that the anion size has two effects in electrical conductivity, i.e., the decrease of the surface electrical charge density and the effect of size for dynamical movement (hopping to adjacent holes); thus, a decrease or an increase of r value with the anion size could be observed. For the purpose of comparison and application, the r values of the studied [Emim]-based ILs are estimated using the method employed by Vila et al. [6], in which they employed a modified version of VTF-type (Vogel–Tamman–Fulcher) equation, which consequently reads



r ¼ r1 exp 

 Ea ; kB ðT  T g Þ

ð3Þ

where r1 is the maximum electrical conductivity (that it would have at infinite temperature) in S  m1, Ea is the activation energy for electrical conduction (which indicates the energy needed for an ion to hop to a free hole) in meV, kB is the Boltzmann’s constant, and Tg is the glass transition temperature in K. Using equation (3), the parameters r1, Ea, and Tg were determined by fitting the present r measurements and selected literature data, in which same criteria were used in the selection as discussed previously. Table 7 contains the results of the calculation of r using equation (3) from the different investigators. As presented in table 7, the agreement of r measurements among different investigators is satisfactory. The determined parameters r1, Ea, and Tg for the investigated ionic liquids are also presented in table 7. The determined parameters r1, Ea, and Tg for each ionic liquid correlated well with the present r measurements and the available literature data as shown by the overall (AAD) of about 0.54% for a total of 78 data points. It was also observed in table 7 that the maximum conductivity value r1, the maximum Ea, and the minimum Tg correspond to [Emim][BF4]. 4. Conclusions The heat capacities and electrical conductivities of five [Emim]based ILs were measured over the temperature ranging from

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JCT 08-235