Low-temperature heat capacity and standard thermodynamic functions of 1-hexyl-3-methyl imidazolium perrhenate ionic liquid

Article abstract of DOI:10.1007/s10973-019-08312-7

Heat capacity for 1-hexyl-3-methyl imidazolium perrhenate ionic liquid [C₆MIM][ReO₄] in the temperature range from 79 to 396?K has been measured by a fully automated adiabatic calorimeter. For [C₆MIM][ReO₄], glass transition temperature, the melting temperature, standard molar heat capacity, enthalpy and entropy of solid–liquid phase transition were determined to be (202.164 ± 0.405) K, (226.198 ± 0.265) K, (480.702 ± 0.013) J?K^?1?mol^?1, (15.665 ± 0.195) kJ?mol^?1 and (69.250 ± 0.780) J?K^?1?mol^?1, respectively. In addition, the thermodynamic characteristics and solid–liquid phase change behavior of [C₆MIM][ReO₄] were compared with the ones of [C₇MIM][ReO₄] reported in the literature. The thermodynamic functions (H_T? H_298.15), (S_T? S_298.15) and (G_T? G_298.15), for the compound in the experimental temperature range were calculated.

Full text of DOI:10.1007/s10973-019-08312-7

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-019-08312-7(0123456789(). -, vo Vl
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Low-temperature heat capacity and standard thermodynamic
functions of 1-hexyl-3-methyl imidazolium perrhenate ionic liquid
Da-Wei Fang1 Kun-Hao Liang1 Xiao-Hong Hu1 Xiu-Ting Fan1 Jie Wei1
•
•
•
•
Received: 14 October 2018 / Accepted: 27 April 2019
´
´
Ó Akademiai Kiado, Budapest, Hungary 2019
Abstract
Heat capacity for 1-hexyl-3-methyl imidazolium perrhenate ionic liquid [C6MIM][ReO4] in the temperature range from 79
to 396 K has been measured by a fully automated adiabatic calorimeter. For [C6MIM][ReO4], glass transition temperature,
the melting temperature, standard molar heat capacity, enthalpy and entropy of solid–liquid phase transition were deter-
mined to be (202.164 0.405) K, (226.198 0.265) K, (480.702 0.013) J K-1 mol-1, (15.665 0.195) kJ mol-1 and
(69.250 0.780) J K-1 mol-1, respectively. In addition, the thermodynamic characteristics and solid–liquid phase change
behavior of [C6MIM][ReO4] were compared with the ones of [C7MIM][ReO4] reported in the literature. The thermody-
namic functions (HT- H298.15), (ST- S298.15) and (GT- G298.15), for the compound in the experimental temperature range
were calculated.
Keywords Perrhenate Á Thermodynamic functions Á Heat capacity Á Adiabatic calorimeter Á Ionic liquids
Introduction
[8–11]. The parameters of fusion of ILs are crucial for their
applicability. Many results, such as melting temperature,
glass transition temperature, standard molar heat capacity,
enthalpy and entropy of solid–liquid phase transition, and
other important information about the structure and ener-
getics of the ILs, can be obtained from the experimental
heat capacity data. All in all, the low-temperature heat
capacity has very significant role in the theoretical research
and application development of ILs [12–14].
Usually, the melting temperatures of ionic compounds are
higher than room temperature due to their strong and long-
range interionic interactions. However, it is found that
melting temperatures of a series of organic ionic com-
pounds are lower than room temperature, and they are
called room-temperature ionic liquids (ILs) [1, 2]. ILs are
made up of a bulky organic cation and a counterion and
possess special physical and chemical properties, making
them attractive for both pure scientific and applied studies
[3–7]. In recent years, all kinds of applications, extraction
and separation processes, synthetic chemistry, catalysis,
materials science and so on have been proposed for ILs; for
example, rhenium ionic liquids can be used for epoxidation
of olefins due to their excellent activity and selectivity
A report about the preparation and thermodynamic
properties of ionic liquid [C7MIM][ReO4] has been pub-
lished in our laboratory [1]; as a continuation of our pre-
vious investigation, this paper reports the following: (1)
1-hexyl-3-methyl imidazolium perrhenate ionic liquid
1
[C6MIM][ReO4] were prepared and characterized by H
NMR spectroscopy and Raman spectrum; (2) low-tem-
perature heat capacities of [C6MIM][ReO4] were measured
by a high-precision automated adiabatic calorimeter over
the temperature range from 79 to 396 K; (3) the thermo-
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10973-019-08312-7) con-
tains supplementary material, which is available to autho-
rized users.
dynamic functions (HT- H298.15), (ST- S298.15
) and
(GT- G298.15) were also calculated based on the experi-
mental results.
& Jie Wei
18809848370@163.com
1
Institute of Rare and Scattered Elements, College of
Chemistry, Liaoning University, Shenyang 110036, People’s
Republic of China
123

D.-W. Fang et al.
C6H13
Br
C6H13
N
Experimental
N
N
N
N
C6H13Br
NH4ReO4
ReO4
Reagents
N
CH3
CH3
CH3
The purities and sources of the reagents are listed in
Table 1.
Scheme 1 Synthesis of the novel ionic liquid [C6MIM][ReO4]
Preparation of IL [C6MIM][ReO4]
The molar heat capacities of [C6MIM][ReO4]
(1.74589 g) were repeatedly measured by the adiabatic
calorimeter, established by Dalian Institute of Chemical
Physics [18–20]. Over the temperature range between 79
and 396 K, the first experimental data are listed in Table 2
and plotted in Fig. 1, and the second experimental data are
listed in Table S1 and plotted in Figure S3 in Supporting
Information, respectively.
In this work, [C6MIM][ReO4] was synthesized and the
scheme 1 shows the synthetic route. The N-methylimida-
zole (1 mol) and the bromohexane (1.2 mol) were placed
in a round-bottomed flask and stirred under reflux at 80 °C
for 48 h to obtain the [C6MIM] Br [15, 16]. Then,
[C6MIM] Br and 1.2 equiv. of NH4ReO4 were reacted in
acetone under argon and stirred at room temperature for
48 h to obtain the target product. In addition, ethyl acetate
and acetonitrile were used as an extractant in this experi-
ment. The content of Br- was determined by dripping the
silver nitrate solution; the results reveal that any yellow
deposition did not appear. The [C6MIM][ReO4] was
Results and discussion
Heat capacity
1
characterized by H NMR spectroscopy and Raman spec-
From Fig. 1, an endothermic step corresponding to a glass
transition occurred at glass transition temperature Tg-
= 201.760 K. From 210 to 230 K, a sharply endothermic
peak was observed with the peak temperature 225.934 K; it
corresponds to a melting process. And smooth heat
capacity curves without endothermic and exothermic peaks
were observed in other experimental temperature regions.
The molar heat capacities are fitted to two following
polynomial in reduced temperature (x) by means of the
least square fitting.
trum (see Figure S1 and S2); its purity is more than 99.8%.
And the water content was determined by a Karl Fischer
moisture titrator (ZSD-2 type), and it is less than
0.3 mass%.
Measurement of molar heat capacities of IL
[C6MIM][ReO4] by adiabatic calorimeter
To verify the dependability of the adiabatic calorimeter, the
molar heat capacities for reference standard material, a-
Al2O3 was measured. The deviations of our experimental
results from the recommended values by NIST are within
For the first temperature range (80–196) K:
x ¼ ðT À 138Þ=58
ð1Þ
Cp;m ¼ 254:54162 þ 76:73522x À 25:34498x2 þ 5:63153x3
0.5%, while the uncertainty is within
0.37%, as
þ 25:69092x4 þ 1:67611x5 À 5:19123x6:
ð2Þ
compared with the values given by the former National
Bureau of Standards [17] in the temperature range of
(78–400) K.
Table 1 The purities and
Reagent
Purity/%
Source
sources of the reagents
Deionized water
N-Methylimidazole
Bromohexane
Ethyl acetate
C 99.8
99.5
ACROS
Shanghai Reagent Co. Ltd
99.5
Tianjin Fuyu chemical limited company
Tianjin Fuyu chemical limited company
Jiangxi Copper Industry Company
National Institute of Standards
Tianjin Fuyu chemical limited company
Acetonitrile
99.5
Ammonium perrhenate
a-Al2O3(s)
99.9
[ 99.95
99.5
Acetone
123

Low-temperature heat capacity and standard thermodynamic functions…
Table 2 The first series
T/K
Cp,m/J K-1 mol-1
T/K
Cp,m/J K-1 mol-1
T/K
Cp,m/J K-1 mol-1
experimental molar heat
capacities of [C6MIM][ReO4] in
the temperature range (79–396)
K
79.847
165.521
167.603
175.269
176.989
181.461
187.163
191.605
194.881
201.376
205.946
207.490
212.165
218.918
224.976
229.398
235.602
241.066
245.356
248.584
255.313
259.022
261.489
264.746
266.929
268.474
274.004
277.803
280.802
286.743
291.140
294.393
295.654
299.087
304.741
309.570
186.631
189.630
192.690
195.736
198.767
201.760
204.708
207.677
210.635
213.653
216.729
219.816
222.929
225.934
228.831
231.875
234.973
237.944
240.900
244.625
248.269
251.241
254.305
257.368
260.429
263.491
266.550
269.611
272.690
275.675
278.647
281.726
284.809
287.887
290.960
315.379
320.004
327.105
333.939
346.877
370.199
409.994
413.112
418.281
422.586
426.548
450.574
543.826
702.589
438.340
436.097
442.837
445.658
447.247
448.292
451.607
453.704
455.820
457.209
459.765
461.608
462.297
464.743
465.772
467.945
469.106
471.806
473.688
476.739
477.476
294.048
297.149
300.253
303.363
306.478
309.596
312.715
315.829
318.940
322.051
325.159
328.268
331.376
334.490
337.609
340.740
343.865
346.977
350.085
353.194
356.307
359.424
362.545
365.674
368.809
371.950
375.000
378.050
381.101
384.153
387.199
390.243
393.280
396.310
479.084
480.145
481.827
482.071
484.151
486.627
487.201
489.721
492.504
493.162
495.556
497.927
503.083
505.595
507.151
509.545
512.444
516.090
522.530
524.788
528.841
531.638
536.565
540.313
541.419
544.886
549.221
552.305
557.736
559.263
563.462
567.344
568.877
573.538
82.171
85.489
88.308
91.167
94.026
96.896
99.765
102.650
105.555
108.476
111.417
114.348
117.269
120.213
124.263
128.265
131.159
134.130
137.126
140.098
143.045
146.023
149.028
152.015
154.981
157.933
160.916
163.931
166.933
169.922
172.897
175.908
178.952
182.814
The standard uncertainty (0.68 level of confidence): u(T) = 0.001 K. u(p) = 0.001 MPa. The expanded
uncertainties Uc (0.95 level of confidence): Uc(Cp,m) = 0.005
The correlation coefficient of the fitting R2 = 0.9992.
x ¼ ðT À 312:5Þ=83:5
ð5Þ
Cp;m ¼ 487:88856 þ 52:43944x þ 38:07827x2 þ 31:55308x3
For the second temperature range (205–217) K:
À 31:39746x4 À 16:69594x5 þ 10:47850x6:
ð6Þ
x ¼ ðT À 211Þ=6
ð3Þ
Cp;m ¼ 418:57120 þ 9:91126x À 0:11770x2 À 1:64125x3:
The correlation coefficient of the fitting R2 = 0.9991.
Where x is the reduced temperature, x = [T - (Tmax-
ð4Þ
The correlation coefficient of the fitting R2 = 0.9958.
? Tmin)/2]/[(Tmax - Tmin)/2],
T
is the experimental
For the third temperature range (229–396) K:
temperature.
123

D.-W. Fang et al.
the alkyl chains of the ILs, and the change trend is
agreement with Tan’s work, that is mainly reported that the
heat capacities and melting points of ionic liquids
1-ethylpyridinium bromide (EPBr) and 1-propylpyridinium
bromide (PPBr) [16].
700
600
500
400
300
200
100
Thermodynamic functions
The thermodynamic functions related to the reference
temperature 298.15 K were calculated in the temperature
range (80–400) K with an interval of 5 K, using the
polynomial equation for heat capacity and thermodynamic
relationships as follows:
50
100
150
200
250
300
350
400
Z
T/K
T
HTÀH298:15
¼
Z
Cp;dT
ð7Þ
ð8Þ
ð9Þ
Fig. 1 The first series experimental molar heat capacities of [C6-
MIM][ReO4] in the temperature range (79–396) K
298:15
T
À
Á
STÀS298:15
¼
Cp=T dT
298:15
Z
Z
700
600
500
400
300
200
225.934 K
226.463 K
T
T
Series 1
Series
À
Á
2
GTÀG298:15
¼
Cp; dTÀT
Cp=T dT:
298:15
298:15
The calculated values of thermodynamic functions
(HT- H298.15), (ST - S298.15) and (GT - G298.15) are listed
in Table 3.
From Table 3, standard molar heat capacity at
T = 298.15 K is 480.715 J K-1 mol-1, which is lower than
the value of [C7MIM][ReO4] [1], that is, it can be seen that
the values of the standard molar heat capacity increase
along with increasing the number of methylene group in
the alkyl chains of the ILs, and it is also consistent with
Tan’s [16].
160
180
200
220
240
260
T/K
The molar enthalpy DfusHm and entropy DfusSm of fusion
of the compound were calculated from the following
equations:
Fig. 2 Both series of experimental molar heat capacities of [C6-
MIM][ReO4] in the glass transition and fusion region
R
R
R
Tm
Tf
Tf
Both series of heat capacity measurements of the sample
in the fusion region are carried out and shown in Fig. 2;
from the figure, not only is verified the almost perfect
reversibility and repeatability of the fusion process, but
also the repeated heat capacity measurements of the both
series. And the melting temperature is obtained to be
(226.198 0.265) K, and it is listed in Table 4. Compared
with [C7MIM][ReO4] [1], the melting temperature of [C6-
MIM][ReO4] is higher, which is possibly due to the more
carbonyl group in the cation, the more the steric hindrance
strengthened, which leads to the melting temperature
decrease with increasing the number of methylene group in
Q À n
CpðSÞdT À n
CpðLÞdT À
C0dT
Ti
Tm
Ti
D
fusHm
¼
n
ð10Þ
ð11Þ
D
fusHm
DfusSm
¼
Tm
where Ti is the temperature at the initial melting temper-
ature, Tf is the temperature at the final melting temperature,
Q is the total energy introduced to the sample cell from Ti
to Tf, Cp(S) is the heat capacity of the sample in the solid
phase at Ti, Cp(L) is the heat capacity of the sample in liquid
123

Low-temperature heat capacity and standard thermodynamic functions…
Table 3 The calculated values of thermodynamic functions data for [C6MIM][ReO4] in the temperature range (80–400) K
T/K
Cp,m/J K-1 mol-1
HT- H298.15/kJ mol-1
ST- S298.15/J mol-1 K-1
- (GT- G298.15)/kJ mol-1
80
165.402
174.468
179.236
188.902
195.301
205.410
209.448
220.404
229.087
236.619
243.802
250.355
258.941
263.631
267.211
274.039
279.619
288.534
294.444
297.698
306.245
312.849
320.775
331.924
353.849
411.956
416.681
423.020
452.516
692.993
402.480
442.765
447.245
448.568
452.873
456.145
459.389
461.840
464.939
467.499
470.227
473.859
477.301
479.433
480.715
481.744
482.905
486.761
- 75.698
- 74.775
- 73.850
- 72.917
- 71.971
- 71.007
- 70.019
- 69.004
- 67.955
- 66.871
- 65.746
- 64.578
- 63.364
- 62.100
- 60.785
- 59.416
- 57.993
- 56.512
- 54.974
- 53.378
- 51.723
- 50.009
- 48.237
- 46.406
- 44.519
- 42.576
- 40.579
- 38.530
- 36.430
- 34.234
- 32.092
- 29.860
- 27.589
- 25.285
- 22.951
- 20.593
- 18.213
- 15.819
- 13.415
- 11.008
- 8.602
- 404.008
- 393.446
- 383.166
- 373.135
- 363.322
- 353.699
- 344.237
- 334.911
- 325.696
- 316.571
- 307.513
- 298.504
- 289.525
- 280.561
- 271.597
- 262.620
- 253.619
- 244.585
- 235.508
- 226.383
- 217.206
- 207.972
- 198.681
- 189.331
- 179.927
- 170.470
- 160.966
- 151.421
- 141.843
- 132.243
- 122.632
- 113.023
- 103.430
- 93.831
- 84.362
- 74.925
- 65.579
- 56.348
- 47.256
- 38.330
- 29.597
- 21.087
- 12.830
- 4.859
- 43.377
- 41.332
- 39.365
- 37.469
- 35.639
- 33.869
- 32.153
- 30.489
- 28.871
- 27.300
- 25.769
- 24.280
- 22.831
- 21.419
- 20.045
- 18.710
- 17.414
- 16.155
- 14.938
- 13.761
- 12.626
- 11.534
- 10.488
- 9.486
- 8.534
- 7.630
- 6.776
- 5.974
- 5.225
- 4.479
- 3.887
- 3.300
- 2.766
- 2.296
- 1.861
- 1.487
- 1.162
- 0.887
- 0.656
- 0.467
- 0.315
- 0.196
- 0.106
- 0.037
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
220
225
230
235
240
245
250
255
260
265
270
275
280
285
290
295
298.15
300
305
310
- 6.206
- 3.827
- 1.470
0.889
3.301
5.728
3.010
10.909
18.786
0.014
0.026
0.096
123

D.-W. Fang et al.
Table 3 (continued)
T/K
C
p,m/J K-1 mol-1
HT- H298.15/kJ mol-1
ST- S298.15/J mol-1 K-1
- (GT- G298.15)/kJ mol-1
315
320
325
330
335
340
345
350
355
360
365
370
375
380
385
390
395
400
488.862
492.783
495.439
500.780
505.834
508.901
513.463
522.406
527.078
532.383
539.799
542.380
549.221
556.000
560.098
567.143
570.682
593.754
8.170
10.629
13.106
15.603
18.121
20.660
23.224
25.812
28.426
31.068
33.738
36.439
39.171
41.936
44.735
47.570
50.441
53.350
26.596
34.343
42.030
49.661
57.241
64.772
72.258
79.704
87.113
94.489
101.836
109.157
116.457
123.740
131.008
138.266
145.518
152.767
0.208
0.361
0.554
0.785
1.055
1.362
1.705
2.084
2.499
2.948
3.432
3.949
4.500
5.085
5.703
6.354
7.039
7.757
Table 4 Results of phase
transition of the
[C6MIM][ReO4] obtained from
both series of heat capacity
measurements
Thermodynamic functions
Tm/K
Series 1, x1
Series 2, x2
x Æ ra
225.93
15.47
68.47
226.46
15.86
70.03
226.198 0.265
15.665 0.195
69.250 0.780
D
D
fusHm/kJ mol-1
fusSm/J K-1 mol-1
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
n
2
ðxiÀxꢀÞ
i¼1
ra ¼
measurements; x, the mean value of a set of measurement results
, in which n is the experimental number; xi, a single value in a set of calorimetric
nðnÀ1Þ
J K-1 mol-1, respectively. It may due to the fact that the
longer carbon chain in imidazolium cation, the more steric
hindrance and lattice energy, which results in a decrease in
melting temperature but an increase in standard molar heat
capacity. And the variation trends of the melting temper-
ature and standard molar heat capacity at 298.15 K are
consistent with Tan’s.
phase at Tf and C0 is the average heat capacity of the empty
sample cell at temperature (Ti ? Tf)/2.
The results of the melting point, molar enthalpy and
entropy of two phase transitions obtained from every series
of repeated experiments have been listed in Table 4.
Conclusions
In addition, the corresponding thermodynamic functions
(HT - H298.15), (ST - S298.15) and (GT - G298.15) were
calculated in the temperature range from 80 to 400 K.
In this paper, the heat capacities of [C6MIM][ReO4] were
measured in the temperature range (79–396) K by adiabatic
calorimeter. And according to heat capacity values, glass
transition temperature, the melting temperature, standard
molar heat capacity, enthalpy and entropy of solid–liquid
phase transition were determined to be (202.164 0.405)
Acknowledgements This project was financially supported by
National Nature Science Foundation of China NSFC (Nos. 21673107
and 21703090) and Liaoning BaiQianWan Talents Program (2017).
K, (226.198 0.265) K, (480.702 0.013) J K-1 mol-1
(15.665 0.195) kJ mol-1 and (69.250 0.780)
,
123

Low-temperature heat capacity and standard thermodynamic functions…
and applications for chiral recognition and asymmetric synthesis.
Tetrahedron Asymmetry. 2005;16:3921–45.
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