Thermodynamic investigation of room temperature ionic liquid : the heat capacity and thermodynamic functions of BMIPF6

Article abstract of DOI:10.1007/s10973-009-0610-6

The molar heat capacities of the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluoroborate (BMIPF₆) were measured by an adiabatic calorimeter in temperature range from 80 to 390 K. The dependence of the molar heat capacity on temperature is given as a function of the reduced temperature (X) by polynomial equations, C _P,m (J K ^-1 mol^-1) = 204.75 + 81.421X - 23.828 X ² + 12.044X ³ + 2.5442X ⁴ [X = (T - 132.5)/52.5] for the solid phase (80-185 K), C _P,m (J K^-1 mol^-1) = 368.99 + 2.4199X + 1.0027X ² + 0.43395X ³ [X = (T - 230)/35] for the glass state (195 - 265 K), and C _P,m (J K^-1 mol ^-1) = 415.01 + 21.992X - 0.24656X ² + 0.57770X ³ [X = (T - 337.5)/52.5] for the liquid phase (285-390 K), respectively. According to the polynomial equations and thermodynamic relationship, the values of thermodynamic function of the BMIPF₆ relative to 298.15 K were calculated in temperature range from 80 to 390 K with an interval of 5 K. The glass transition of BMIPF₆ was measured to be 190.41 K, the enthalpy and entropy of the glass transition were determined to be ΔH _g = 2.853 kJ mol^-1 and ΔS _g = 14.98 J K^-1 mol^-1, respectively. The results showed that the milting point of the BMIPF₆ is 281.83 K, the enthalpy and entropy of phase transition were calculated to be ΔH _m = 20.67 kJ mol^-1 and ΔS _m = 73.34 J K^-1 mol ^-1.

Full text of DOI:10.1007/s10973-009-0610-6

J Therm Anal Calorim (2010) 101:1143–1148
DOI 10.1007/s10973-009-0610-6
Thermodynamic investigation of room temperature ionic liquid
The heat capacity and thermodynamic functions of BMIPF6
•
Z. H. Zhang T. Cui J. L. Zhang H. Xiong
•
•
•
•
•
•
•
•
G. P. Li L. X. Sun F. Xu Z. Cao F. Li
J. J. Zhao
Received: 4 September 2009 / Accepted: 17 November 2009 / Published online: 15 December 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract The molar heat capacities of the room tempera-
ture ionic liquid 1-butyl-3-methylimidazolium hexafluorob-
orate (BMIPF6) were measured by an adiabatic calorimeter in
temperature range from 80 to 390 K. The dependence of the
molar heat capacity on temperature is given as a function of
the reduced temperature (X) by polynomial equations, CP,m
(J K-1 mol-1) = 204.75 ? 81.421X -23.828 X2 ? 12.044X3
? 2.5442X4 [X = (T - 132.5)/52.5] for the solid phase
(80–185 K), CP,m (J K-1 mol-1) = 368.99 ? 2.4199X ?
1.0027X2 ? 0.43395X3 [X = (T -230)/35] for the glass
state (195 - 265 K), and CP,m (J K-1 mol-1) = 415.01 ?
21.992X - 0.24656X2 ? 0.57770X3 [X = (T - 337.5)/52.5]
for the liquid phase (285–390 K), respectively. According to
the polynomial equations and thermodynamic relationship,
the values of thermodynamic function of the BMIPF6 rela-
tive to 298.15 K were calculated in temperature range from
80 to 390 K with an interval of 5 K. The glass transition of
BMIPF6 was measured to be 190.41 K, the enthalpy and
entropy of the glass transition were determined to be DHg =
2.853 kJ mol-1 and DSg = 14.98 J K-1 mol-1, respec-
tively. The results showed that the milting point of the
BMIPF6 is 281.83 K, the enthalpy and entropy of phase
transition were calculated to be DHm = 20.67 kJ mol-1 and
Z. H. Zhang ꢀ J. L. Zhang ꢀ H. Xiong ꢀ G. P. Li
Applied Chemistry Institute of R&D Center of PetroChina
Pipeline Company, 51 Jinguang Road, 065000 Langfang, Hebei,
People’s Republic of China
DSm = 73.34 J K-1 mol-1
.
Keywords Room temperature ionic liquid ꢀ BMIPF6 ꢀ
Heat capacity ꢀ Adiabatic calorimetry ꢀ Glass translation ꢀ
Thermodynamic function
T. Cui
PetroChina Pipeline Company, 51 Jinguang Road, 065000
Langfang, Hebei, People’s Republic of China
L. X. Sun (&)
Thermochemistry Laboratory, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, 116023 Dalian,
People’s Republic of China
Introduction
Room temperature ionic liquids (RTILs) have emerged as a
new kind of media alternative to the conventionally used
organic solvents, which are generally volatile, flammable
and hazardous chemicals [1–3]. Even though ionic liquids
such as [EtNH3][NO3], which has a melting point of 285 K,
have been known since 1914 [4]; it is only very recently that
the usefulness of these systems as a new kind of media for
synthetic electrochemical and catalytic applications are
being explored [5–11]. Some of the properties that make the
room temperature ionic liquids attractive media for various
applications are the wide liquid range, non-volatility (neg-
ligible vapor pressure), non-flammable nature, less reactiv-
ity, and the ability to dissolve a large variety of organic and
e-mail: lxsun@dicp.ac.cn
F. Xu (&)
Faculty of Chemistry and Chemical Engineering, Liaoning
Normal University, 116029 Dalian, People’s Republic of China
e-mail: xufen@lnnu.edu.cn
Z. Cao
School of Chemistry and Environmental Engineering, Changsha
University of Science and Technology, 410076 Changsha,
People’s Republic of China
F. Li ꢀ J. J. Zhao
School of Physics and Optoelectronic Technology and College
of Advanced Science and Technology, Dalian University of
Technology, 116024 Dalian, People’s Republic of China
123

1144
Z. H. Zhang et al.
inorganic substances including even the polymer materials at
high concentration. Many of these properties have made the
room temperature ionic liquids a nature-friendly ‘‘Green
Solvent’’. The room temperature ionic liquids that are cur-
rently the focus of extensive investigation are generally a
substituted imidazolium or a pyridinium salt. Even though
the solvent properties of different halogenoaluminate(III)
ionic liquids (ILs) were studied as early as in 1986 [12], very
little is known about the various properties of the contem-
porary ionic liquids despite their extensive usage in synthetic
applications. We refer, for instance, to that data of heat
capacity, standard enthalpy of formation, thermodynamic
functions and so on, which are paramount for the design of
any technological processes, are even scarcer [13].
(120 mL) and 1-chlorobutane (153 mL) were added into the
reaction vessel with continuous magnetic stirring. The
reaction mixture was heated under nitrogen N2 at 353.2 K for
96 h with stirring until two phases were formed. The top
phase, containing unreacted starting material, was decanted,
and ethylacetate (150 mL) was added with thorough mixing.
The ethylacetate was decanted followed by the addition of
fresh ethylacetate and this step was repeated twice. After the
third decanting of ethylacetate, any remaining ethylacetate
was removed by heating to 343 K and stirring on a vacuum
line. The product is slightly yellow and may be crystalline at
room temperature, depending on the amount of water present
in that phase. The product was recrystallized twice from dry
acetonitrile and dried under vacuum at 343 K for 12 h to
yield the pure crystalline 1-butyl-3-methylimidazolium
chlorate (BMIC). HPF6 was dropwise added into a solution
of BMIC in water and stirred for 36 h. The two-phase system
was separated, and the lower phase (BMIPF6) was extracted
thrice with 100 mL of deionized water to remove residual
HCl. The final ionic liquid product was dried in vacuo at
80 °C for several hours to yield the resulting BMIPF6. The
chemical shifts for 1H-NMR spectrum (DMSO, TMS) of the
product appear as follows:
As a continuation of our previous investigations of
RTILs [14–17], in this study, the molar heat capacities of
RTIL, BMIPF6 were measured by an adiabatic calorimeter
in temperature range from 80 to 390 K. Based on the
measured heat capacity data and thermodynamic relation-
ship, the values of thermodynamic function of the BMIPF6
relative to 298.15 K were calculated. The glass translation
of BMIPF6 was observed at 190.41 K, the enthalpy and
entropy of the transition were calculated. The melting point
of the BMIPF6 is 281.83 K; the enthalpy and entropy of
phase transition were calculated.
d = 9.113(s,H2), d = 7.744(d,H4), d = 7.693 (d,H5),
d = 4.167 (t,NCH2), d = 3.836 (s,NCH3), d = 1.741
(m,NCH2CH2), d = 1.278 (m,NCH2CH2CH2), d = 0.933
1
(t,CH3). The H-NMR spectra of the product are the same
Experimental
as those in literature [25].
Heat-capacity measurement
Chemicals
The ethylacetate (AR), acetone (AR), acetonitrile (AR) and
sodium hexafluoroborate (AR) were of analytical grade and
provided by Tianjing Chemical Agent Factory. 1-methyl-
imidazole was vacuum distilled prior to use. 1-chlorobu-
tane (AR) was purified by standard method.
Heat-capacity measurements were carried out in a high-
precision automatic adiabatic calorimeter described in
detail elsewhere [26, 27]. The principle of the calorimeter
is based on the Nernst stepwise heating method. The cal-
orimeter mainly consists of a sample cell, an adiabatic (or
inner) shield, a guard (outer) shield, a platinum resistance
thermometer, an electric heater, two sets of chromel–cop-
per thermocouples and a high vacuum can. The sample cell
was made of gold-plated copper and had an inner volume
of 48 cm3. Eight gold-plated copper vanes of 0.2 mm
thickness were put into the cell to promote heat distribution
between the sample and the cell. The platinum resistance
thermometer was inserted into the copper sheath, which
was soldered in the middle of the sample cell. The heater
wire was wound on the surface of the thermometer. The
evacuated can was kept within ca. 1 9 10-3 Pa during the
heat-capacity measurements so as to eliminate the heat loss
due to gas convection. Liquid nitrogen was used as the
cooling medium. One set of chromel–copper thermocou-
ples was used to detect the temperature difference between
the sample cell and the inner shield. Likewise, the other set
of thermocouples was installed between the inner and outer
Synthesis of BMIPF6
BMIPF6 was synthesized through alkylation of 1-butyl-
3-methylimidazolium chlorate (BMIC) with HPF6 in water as
an inert solvent according to the following reaction scheme
BMIPF6 was prepared according to literature procedures
[18–25]. We fitted a 500-ml three-neckedround-bottom flask
with a water condenser, provided it with a Teflon-coated
magnetic bar, and equipped with a gas inlet, through which
N2 can be purged in case of necessary. 1-methylimidazole
123

Thermodynamic investigation of room temperature ionic liquid
1145
shields. The temperature difference between them was kept
within 0.5 mK during the whole experimental process. The
sample cell was heated by the standard discrete heating
method. The temperature of the cell was measured by a
platinum-resistance thermometer. The thermometer was
made by the Instrument Manufactory of Yunnan, China,
and calibrated at the National Institute of Metrology in
terms of the IPTS-90. The temperature increment in a
heating period was 2–4 K, and temperature drift was
maintained at about 10-4 K min-1 in equilibrium period.
All the data were automatically acquired through a data
acquisition/switch unit (Model: 34970A, Aglient, USA)
and processed online by a computer.
Table 1 The experimental molar heat capacities of ionic liquid
BMIPF6
T/K
Cp/J K-1
T/K
Cp/J K-1
T/K
Cp/J K-1
mol-1
mol-1
mol-1
79.64
82.90
88.97
97.98
190.61 314.78
193.41 356.86
196.28 365.65
199.25 367.56
202.27 366.90
205.27 367.94
208.25 367.50
211.21 368.15
213.96 368.36
217.01 368.83
220.00 367.73
223.10 368.23
226.48 368.65
229.34 368.81
232.43 369.28
235.66 369.42
237.41 369.22
240.75 370.65
244.26 370.12
247.45 370.46
250.13 370.55
252.68 371.06
255.56 371.85
258.43 371.59
260.93 371.77
263.09 372.92
266.68 374.92
269.79 403.37
272.58 496.48
275.01 760.63
276.88 1047.91
278.18 1289.97
279.33 1707.73
280.20 2300.34
280.85 2898.28
281.36 3617.05
281.72 4860.56
281.83 7174.48
281.98 5740.88
283.07 778.96
285.98 392.26
289.29 394.10
292.34 395.77
295.45 397.38
298.56 398.63
301.67 399.74
304.83 400.81
307.98 402.32
311.14 403.74
314.30 404.97
317.41 406.50
320.54 407.94
323.74 409.38
326.84 410.64
330.56 411.97
333.87 413.38
337.63 414.76
340.56 416.18
343.74 417.86
346.97 419.05
349.95 420.42
353.28 421.78
356.52 422.98
359.74 424.27
362.95 425.70
366.03 426.97
369.66 428.33
372.92 429.89
376.30 431.27
379.53 432.87
382.72 434.15
385.89 435.58
389.15 436.90
85.77 106.42
88.70 114.54
91.68 122.61
94.71 130.40
97.67 137.55
100.57 144.14
103.51 150.79
106.51 157.30
109.44 163.40
112.42 169.31
115.46 175.26
118.44 180.80
121.37 186.09
124.35 191.49
127.39 196.44
130.38 201.13
133.33 205.78
136.24 210.38
139.20 214.87
142.22 219.26
145.20 223.62
148.15 227.53
151.14 231.53
154.20 235.37
157.22 239.36
160.19 243.48
163.21 247.20
166.24 250.53
169.23 254.52
172.29 258.43
175.33 262.40
178.34 266.68
181.41 271.93
184.54 276.81
187.62 288.26
The sample mass used for the heat capacity measure-
ment was 48.7349 g. To verify the reliability of the adia-
batic calorimeter, molar heat capacities for the reference
standard material, a-Al2O3, were measured. The deviations
of our experimental results from the values reported by
NIST [28, 29] were within 0.2% in the temperature
range of 80–400 K.
Results and discussion
Molar heat capacity of BMIPF6
As a continuation of our previous investigations on ther-
modynamics for new materials significant in science and
technology [30–37], the molar heat capacities of the
BMIPF6 were determined by using the adiabatic calorim-
eter in temperature range from 80 to 390 K. The results of
the molar heat capacities are listed in Table 1 and shown in
Fig. 1. No thermal anomaly was observed or no phase
transition took place for solid phase from 80 to 185 K, for
glass phase from 195 to 265 K, for liquid phase from 285
to 390 K, respectively.
In order to fit well the heat capacity data to a polynomial
equation, the temperature T was replaced by the reduced
temperature X which was defined
X ¼ ½T ꢁ 0:5ðTmax þ TminÞꢂ=½0:5ðTmax ꢁ TminÞꢂ
ð1Þ
where T is thermodynamic temperature, Tmax and Tmin are
the maximum and the minimum of the temperature in the
experimental temperature range. Then -1 B XB1.
The values of molar heat capacities of the BMIPF6 were
fitted to the following polynomial expressions by the least
square method. For the solid phase (80–185 K):
uncertainty of 0.3%. The correlation coefficient of the
fitted curve, R2 = 0.9999. For the glass phase (195–265 K):
ꢀ
ꢁ
CP;m J Kꢁ1molꢁ1 ¼ 368:99 þ 2:4199X þ 1:0027X2
þ 0:43395X3
ð3Þ
ꢀ
ꢁ
CP;m J Kꢁ1molꢁ1 ¼ 204:75 þ 81:421X ꢁ 23:828X2
where reduced temperature X = (T - 230)/35. The above
equation is valid from 195 to 265 K with an uncertainty of
0.4%. The correlation coefficient of the fitted curve,
R2 = 0.9485. For the liquid phase (285–390 K):
þ 12:044X3 þ 2:5442X4
ð2Þ
where reduced temperature, X = (T - 132.5)/52.5. The
above equation is valid from 80 to 185 K, with an
123

1146
Z. H. Zhang et al.
8000
7000
6000
5000
4000
3000
2000
1000
0
Thermodynamic functions of ionic liquid BMIPF6
400
350
300
250
200
10000
8000
6000
4000
2000
0
Phase Transition
Thermodynamic functions of BMIPF6 were calculated
based on the empirical Eqs. 2–4, and the relationships of
the thermodynamic functions:
Glass Transition
T
Z
160 170 180 190 200 210 220
250
260
270
280
290
300
T/K
HT ꢁ H298:15
¼
Cp;mdT
ð7Þ
ð8Þ
T/K
298:15
BMIPF6
T
Z
Cp;m
T
ST ꢁ S298:15
¼
dT
50 100 150 200 250 300 350 400 450 500
298:15
T/K
The thermodynamic functions relative to the reference
temperature (298.15 K) were calculated by Eqs. 7 and 8 in
the temperature range from 80 to 390 K with an interval of
5 K and are listed in Table 2.
Fig. 1 The experimental molar heat capacities of ionic liquid
BMIPF6
ꢀ
ꢁ
CP;m J Kꢁ1molꢁ1 ¼ 415:01 þ 21:992X ꢁ 0:24656X2
þ 0:57770X3
Table 2 The thermodynamic functions of BMIPF6
ð4Þ
T/K
Cp/J K-1
HT - H298.15
kJ mol-1
/
ST- S298.15/J K-1
where reduced temperature X = (T - 337.5)/52.5. The
above equation is valid from 285 to 390 K with an
uncertainty of 0.2%. The correlation coefficient of the
fitted curve, R2 = 0.9999.
mol-1
mol-1
80.00
90.00
-135.03
-133.36
-131.62
-129.81
-127.96
-126.06
-124.12
-122.16
-120.17
-118.16
-116.12
-114.07
-112.00
-109.90
-107.77
-105.60
-103.38
-101.10
-98.74
-396.46
-382.10
-368.54
-355.76
-343.73
-332.44
-321.85
-311.92
-302.61
-293.87
-285.65
-277.89
-270.53
-263.51
-256.74
-250.16
-243.68
-237.21
-230.66
-223.94
-216.93
-209.53
85.00 104.36
90.00 117.93
95.00 130.71
100.00 142.73
105.00 154.02
110.00 164.62
115.00 174.55
120.00 183.86
125.00 192.60
130.00 200.82
135.00 208.57
140.00 215.93
145.00 222.96
150.00 229.72
155.00 236.30
160.00 242.78
165.00 249.25
170.00 255.80
175.00 262.53
180.00 269.54
185.00 276.93
190.41 Glass transition
195.00 367.14
200.00 367.38
205.00 367.61
210.00 367.85
215.00 368.10
220.00 368.37
225.00 368.66
Thermodynamic parameters of glass transition
It can be seen from Fig. 1 that the heat capacity jump,
corresponding to the glass transition of the BMIPF6, took
place in the range from 185 to 195 K. The temperature of
the glass transition, Tg, was determined to be 190.41 K.
The molar enthalpy, DHg, and entropy, DSg, of the glass
transition were derived by the following equation [35]:
R
R
R
Tf
Ti
Tg
T1
T2
Tg
Q ꢁ
H
celldT ꢁ n
CpðTiÞdT ꢁ n
n
CpðTfÞdT
DHg ¼
ð5Þ
ð6Þ
DSg ¼ DHg=Tg
where Ti is a little lower than Tg and Tf a little higher than Tg,
Hcell is the heat capacity of the empty calorimeter, Q is the
total energyintroduced into the calorimeter during the course
of glass transition. The results of the calculation were
-96.29
DHg = 2.853 kJ mol-1 and DSg = 14.98 J K-1 mol-1
.
-93.71
-91.00
The molar heat capacity reached maxima in temperature
from 265 to 285 K as shown in Fig. 1, the phase transition
took place in the temperature range. The phase transition
temperature was determined to be Tm = 281.83 K which
was corresponding to the solid–liquid phase transition of
the ionic liquid. Using similar Eqs. 5 and 6, the values of
molar enthalpy of fusion, DHm, and of molar entropy of
fusion, DSm, can be calculated. The results were
-67.78
-65.90
-64.02
-62.13
-60.23
-58.34
-56.43
-104.35
-104.11
-103.87
-103.63
-103.38
-103.11
-102.82
DHm = 20.67 kJ mol-1 and DSm = 73.34 J K-1 mol-1
.
123

Thermodynamic investigation of room temperature ionic liquid
1147
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T/K
Cp/J K-1
HT - H298.15
kJ mol-1
/
ST- S298.15/J K-1
mol-1
mol-1
230.00 368.99
235.00 369.36
240.00 369.77
245.00 370.25
250.00 370.78
255.00 371.39
260.00 372.07
265.00 372.85
281.83 Phase transition
285.00 392.19
290.00 394.48
295.00 396.74
298.15 398.14
300.00 398.97
305.00 401.16
310.00 403.34
315.00 405.49
320.00 407.63
325.00 409.75
330.00 411.86
335.00 413.96
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355.00 422.33
360.00 424.44
365.00 426.55
370.00 428.67
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390.00 437.33
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Acknowledgements The authors gratefully acknowledge the finan-
cial support for this work from the National Natural Science Foundation
of China (No. 2083309, 20873148, 20903095,50671098 and
U0734005), 863 projects (2007AA05Z115 and 2007AA05Z102), the
National Basic Research Program (973 program) of China (2010
CB631303) and IUPAC (Project No. 2008-006-3-100).
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