Thermodynamic properties of 1-alkyl-3-methylimidazolium bromide ionic liquids

Article abstract of DOI:10.1016/j.jct.2006.05.008

Heat capacities and enthalpies of phase transitions for a series of 1-alkyl-3-methylimidazolium bromide ionic liquids have been measured by adiabatic calorimetry. Thermodynamic properties of the compounds were calculated in the temperature range of (5 to 370) K. Water was found to have an additive contribution to the heat capacities of [C₄mim]Br in the liquid state above T_fus and in the solid state below 160 K at w(H₂O) ≤ 5 · 10^-3.

Full text of DOI:10.1016/j.jct.2006.05.008

J. Chem. Thermodynamics 39 (2007) 158–166
www.elsevier.com/locate/jct
Thermodynamic properties of 1-alkyl-3-methylimidazolium
bromide ionic liquids
a,
a
a
b
b
Y.U. Paulechka *, G.J. Kabo , A.V. Blokhin , A.S. Shaplov , E.I. Lozinskaya ,
b
Ya.S. Vygodskii
a
Belarusian State University, Chemistry Faculty, 14 Leningradskaya Str., 220030 Minsk, Belarus
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), GSP-1, 28 Vavilov Str., 119991 Moscow, Russia
b
Received 17 February 2006; received in revised form 29 April 2006; accepted 8 May 2006
Available online 26 May 2006
Abstract
Heat capacities and enthalpies of phase transitions for a series of 1-alkyl-3-methylimidazolium bromide ionic liquids have been mea-
sured by adiabatic calorimetry. Thermodynamic properties of the compounds were calculated in the temperature range of (5 to 370) K.
Water was found to have an additive contribution to the heat capacities of [C4mim]Br in the liquid state above Tfus and in the solid state
below 160 K at w(H2O) 6 5 Æ 10ꢀ3
.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquids; Thermodynamic properties; 1-Alkyl-3-methylimidazolium bromides; Synthesis of high-purity ionic liquids
1. Introduction
2. Experimental
In recent years, 1-alkyl-3-methylimidazolium ([Cnmim]+)
ionic liquids (ILs) have been intensively studied [1–9].
[Cnmim]X (X = Cl, Br) compounds can be easily used for
synthesis of IL with different anions including organic ones
[1,5–7,10,11]. That is why [Cnmim]Br together with
[Cnmim]Cl represent a potential source of numerous ILs,
and the study of their thermodynamic properties is of great
importance.
It is known that [Cnmim]Br are very hygroscopic and
have low thermal stability in comparison with similar tetra-
fluoroborate and bis(trifluoromethanesulfonyl)imide ILs
[7,10–18]. This makes problems in the preparation of the
high-purity samples.
2.1. Synthesis of ILs
All alkyl bromides and 1-methylimidazole were pur-
chased from Acros Organics. Dry methanol of high purity
was obtained from Merck and used as received. ILs
investigated in this work were obtained using specially
developed method concluding in N-methylimidazole quart-
ernization. Such a method based on several procedures
[7,10,19] differs from the others in very mild process condi-
tions and solvent addition and results in the formation of
high-purity colorless bromide ILs.
2.1.1. 1-Ethyl-3-methylimidazolium bromide [C2mim]Br
Ethyl bromide was preliminary distilled under inert gas
atmosphere over CaH2, while N-methylimidazole was
freshly distilled from potassium hydroxide under reduced
pressure. Into a round-bottom flask equipped with a mag-
netic stirrer, a condenser and a CaCl2 tube, 41.8 cm3
(0.524 mol) of 1-methylimidazole and 70 cm3 of dry meth-
anol were placed under argon atmosphere. The reaction
mass was thoroughly mixed and cooled to T = (273 to
In this work, we report the results of measurements of
heat capacities in the temperature range of (5 to 370) K
and enthalpies of phase transitions for [Cnmim]Br
(Cn = C2H5,C3H7,C4H9). Thermodynamic properties for
([C4mim]Br + H2O) systems are studied as well.
*
Corresponding author. Tel.: +375 17 2095197; fax: +375 17 2003916.
E-mail address: paulechka@bsu.by (Y.U. Paulechka).
0021-9614/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jct.2006.05.008

Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
159
278) K in an ice bath. After 30 min an excess of 1-bromoe-
thane (58.7 cm3, 0.786 mol) was very slowly added to the
intensively stirred reaction mixture. Afterwards the reac-
tion mass was kept at T = (273 to 278) K for 24 h under
stirring and inert atmosphere. Then the temperature was
stepwise risen: to T = 293 K for 24 h, after this to T =
313 K for 24 h and finally to T = 328 K for 24 h. For
obtaining colorless IL the maximum temperature should
not exceed T = 328 K. After the reaction completion meth-
anol and the haloalkane excess were removed by distilla-
tion under reduced pressure at T = 328 K, the resultant
viscous liquid was treated at the same temperature under
p ꢁ 1 Pa until it was slowly crystallized. The product is
solid at T = 298 K and represents colorless crystals. Yield:
97.6 g. Anal. Calc. for C6H11N2Br (191.07): C, 0.3772; H,
0.0580; N, 0.1466; Br, 0.4182. Found: C, 0.3776; H,
0.0574; N, 0.1454; Br, 0.4175. Onset loss temperature:
493 K (according to t.g.a. data); 1H n.m.r. (400 MHz,
CDCl3): d = 1.44 (t, 3H, CH2CH3, J(HH) = 7.32 Hz),
3.95 (s, 3H, NCH3), 4.25 (m, 2H, NCH2CH3, J(HH) =
7.9 Hz), 7.53 (s, 1H, H5 (Im)), 7.54 (s, 1H, H4 (Im)),
10.08 (s, 1H, H2 (Im)); i.r. (KBr pellet): 3433 (m), 3138
(m), 3069 (m), 2955 (vs), 2934 (vs), 2857 (s), 1573 (vs),
1464 (s), 1378 (m), 1341 (m), 1172 (s), 1032 (m), 868 (m),
1472 (s), 1390 (m), 1350 (m), 1180 (m), 1040 (m), 770
(m) cmꢀ1
.
2.2. Sample preparation
Organic impurities were not found in the synthesized
samples, with water being the only impurity present in mea-
surable amounts. It was determined that these substances
start to decompose after long (30 h) exposition in vacuum
of p ꢁ 0.1 Pa at T = 330 K. So, the synthesized samples
were exposed to vacuum of p ꢁ 3 Pa at T = 290 K for a
few days and then kept over P2O5 for >1 month. The spec-
imens were loaded into containers for calorimetric measure-
ments in a dry box. Molar purities of the samples prepared
in such a way were determined by the fractional-melting
technique in an adiabatic calorimeter (table 1) with the
use of the equation
ꢀ
ꢁ
ꢀ
ꢀ
ꢁ
ꢁ
m
Dfus
H
1
DCp
ln
þ 1
¼
DT 1 þ
ꢀ
Tfus 2DfusH
DT
;
ð1Þ
RT2fus
f
where DT = Tfus ꢀ T; T is an equilibrium temperature cor-
responding to a melt fraction f; Tfus is a triple-point tem-
perature; DfusH(Tfus) is an enthalpy of fusion for a pure
compound; DCp is a heat-capacity jump at fusion of a pure
compound; m is an amount of impurities in a sample, mole
per mole of the main substance. In the calculations only
those experimental points were used for which mol frac-
tions of the impurities in a liquid phase were <0.12. The tri-
ple-point temperatures for the substances were determined
from the data for the purest samples. For the other samples
characterized by this technique, only mol fractions of
impurities were found from equation (1). Mass fractions
of impurities were calculated assuming water to be the
main impurity.
791 (m), 651 (m), 624 (m) cmꢀ1
.
2.1.2. 1-Propyl-3-methylimidazolium bromide [C3mim]Br
1-Propyl-3-methylimidazolium bromide was synthe-
sized in a similar manner as [C2mim]Br. The obtained
product is colorless liquid at T = 298 K. Yield: 0.98. Anal.
Calc. for C7H13N2Br (204.03): C, 0.4099; H, 0.0639; N,
0.1366; Br, 0.3896. Found: C, 0.4391; H, 0.0691; N,
0.1287; Br, 0.3649. Onset loss temperature: 503 K (accord-
1
ing to t.g.a. data); H n.m.r. (400 MHz, CDCl3): d = 0.77
(t, 3H, CH2CH2CH3, J(HH) = 14.8 Hz), 1.74 (m, 2H,
CH2CH2CH3, J(HH) = 14.6 Hz), 3.92 (s, 3H, NCH3),
4.15 (t, 2H, NCH2, J(HH) = 14.6 Hz), 7.46 (s, 1H, H5
(Im)), 7.55 (s, 1H, H4 (Im)), 10.06 (s, 1H, H2 (Im)); i.r.
(KBr pellet): 3437 (m), 3139 (m), 3068 (vs), 2966 (vs),
2877 (m), 1572 (vs), 1461 (s), 1429 (m), 1387 (m), 1345
(m), 1172 (vs), 1090 (m), 864 (m), 801 (m), 755 (m), 651
To obtain the [C4mim]Br samples with higher water con-
tent a certain amount of water was added to the sample with
calorimetrically determined purity. An exact mass of the
added water was determined by weighing. To distribute
the impurity over the volume the sample was kept in an adi-
abatic calorimeter at temperatures above Tfus for (2 to 3) h.
(m), 6241 (s) cmꢀ1
.
TABLE 1
Purities of the studied samples
Sample
m/g
xH O ꢂ 102
wH O ꢂ 102
Procedure
2
2
2.1.3. 1-Butyl-3-methylimidazolium bromide [C4mim]Br
This IL was synthesized in accordance with the same
procedures as described above and represents colorless crys-
tals at T = 298 K. Yield: 0.99. Anal. Calc. for C8H15N2Br
(219.14): C, 0.4381; H, 0.0684; N, 0.1289; Br, 0.3646.
Found: C, 0.4391; H, 0.0691; N, 0.1287; Br, 0.3649. Onset
loss temperature: 548 K (according to t.g.a. data); 1H
n.m.r. (400 MHz, CDCl3): d = 0.90 (t, 3H, CH2CH2CH2-
CH3, J(HH) = 14.7 Hz), 1.34 (m, 2H, CH2CH2CH2CH3,
J(HH) = 14.6 Hz), 1.93 (m, 2H, CH2CH2CH2CH3, J(HH) =
14.6 Hz), 4.23 (s, 3H, NCH3), 4.52 (t, 2H, NCH2,
J(HH) = 14.6 Hz), 8.28 (s, 1H, H5 (Im)), 8.38 (s, 1H,
H4 (Im)), 10.02 (s, 1H, H2 (Im)); i.r. (KBr pellet): 3170
(m), 3120 (m), 2980 (m), 2950 (m), 2890 (m), 1580 (s),
[C2mim]Br
1
2
0.9821
1.0791
5.55
2.17
0.550
0.209
Fractional melting
Fractional melting
[C3mim]Br
3
4
5
1.2494
1.1132
1.1822
6.87
7.50
0.644
0.707
Fractional melting
Fractional melting
[C4mim]Br
6
7
8
9
10
0.8756
0.9890
0.9764
0.9857
0.9146
1.72
5.89
6.24
17.3
38.8
0.141
0.482
0.510
1.69
Fractional melting
Fractional melting
Fractional melting
Gravimetric
4.95
Gravimetric

160
Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
The purities of the samples used in the calorimetric mea-
ranges. The heat-capacity jumps, DliqCs;m, were found as
gl
surements are shown in table 1.
the differences of heat capacities for the metastable liquids
and the glasses extrapolated to Tg. The heat-capacity jumps
2.3. Calorimetric measurements for [Cnmim]Br
at fusion, DliqCs;m, were calculated in a similar way.
cr
Heat capacities of [C2mim]Br and [C4mim]Br were
extrapolated to T = 0 K with the use of the equation
Cp = Cv = aT3. The a parameters were calculated by the
least-squares fitting of the data in the range of T = (5 to
6) K. The assumption Cp = Cv was used since (Cp ꢀ Cv)
has stronger temperature dependence than Cp or Cv have,
and normally near T = 5 K (Cp ꢀ Cv) becomes negligible.
The residual entropy, DgcrlSmꢃ ðT ! 0Þ, for glass
[C4mim]Br was determined as the difference of entropies
of the liquid at Tfus obtained, on one hand, from the Cp
vs T dependence for the crystal and the entropies of phase
transitions, and, on the other hand, from the Cp vs T
dependence for the glass and the supercooled liquid. The
residual enthalpy DgcrlHꢃmðT ! 0Þ was found in the same
manner.
Heat-capacity measurements were conducted in a Ter-
mis BKT-10 adiabatic calorimeter. The calorimeter and
the procedure of the measurements have been described
elsewhere [20]. The container filled with a substance was
evacuated to vacuum of p ꢁ 3 Pa, and then it was filled
with helium to p = 10 kPa at T = 290 K (p = 102 kPa for
samples 9 and 10). Masses of the samples in vacuum are
listed in table 1.
After assembling, the measuring system was cooled in a
liquid nitrogen bath. If the measurements were performed
below 80 K, a liquid helium bath was used. Initial cooling
rate of the container with the sample was ꢁ20 mK Æ sꢀ1. In
these cooling conditions liquid [C3mim]Br and [C4mim]Br
were supercooled and formed glasses. To obtain crystalline
[C4mim]Br, the glass was heated to the temperatures (20 to
30) K above the glass transition temperatures. At these
temperatures crystallization was initiated.
3. Results
It was found that [C3mim]Br forms two crystalline mod-
ifications, but cannot be easily crystallized. In an adiabatic
calorimeter, the crystals after melting could not be
obtained again from the glass and the supercooled liquid.
Samples 4 and 5 of [C3mim]Br in the crystal state were
loaded in a container, and the purification procedures
described above were not applied to them.
To get the completely crystallized samples for all the
studied compounds the partly crystallized samples were
annealed at temperatures (20 to 30) K below their melting
temperatures until spontaneous heat evolution due to crys-
tallization had stopped. The process took less than 15 h.
The experimental heat capacities were corrected with
respect to the amount of helium in the container. The dif-
ferences between Cs and Cp were found to be negligible
compared with uncertainty of the measurements.
The triple-point temperatures of the substances were
determined by the fractional-melting technique. The
enthalpies of fusion for [C2mim]Br and [C4mim]Br were
measured in series of special experiments in which an initial
temperature, Tinit, was (40 to 50) K below the fusion tem-
perature, and a final temperature, Tfin, was (4 to 16) K
above Tfus. In the DfusH calculations Tinit was adjusted to
T = 295.00 K with the use of the experimental heat capac-
ities in the corresponding temperature range. Above this
temperature heat capacities of the crystals were extrapo-
lated by linear polynomials obtained from the experimental
data in the temperature ranges of (270 to 295) K for
[C2mim]Br and (271 to 296) K for [C4mim]Br correspond-
ingly. Heat capacities of the liquids were extrapolated to
Tfus by linear functions obtained from the experimental
data in a range of temperatures of (Tfus to 370 K).
3.1. Effect of water on heat capacity for [C4mim]Br
The experimental cs values for the [C4mim]Br samples
with different water content are presented in Tables S.1
and S.2. Effect of water differs in various temperature
ranges.
Above Tfus specific heat capacity of the samples is a sum
of specific heat capacities of IL and liquid water [21]. Max-
imum deviations between the results for the different sam-
ples is 61.2 Æ 10ꢀ2 Æ cp and do not systematically depend on
the concentration of water (figure 1). This observation is in
agreement with the results for ([C4mim][BF4] + H2O)
reported by Rebelo et al. [22], and allows one to assume
that such an additivity will hold all over the temperature
range of existence of the liquid. To apply this below Tfus
one must know cp of liquid water down to 220 K. For
determination of the heat capacities of water and IL in
([C4mim]Br + H2O) and subsequent calculation of cp for
the pure IL, the following equation was used
2
cp=J ꢂ Kꢀ1 ꢂ gꢀ1 ¼ wILðb0 þ b1ðT=KÞ þ b2ðT=KÞ Þþ
2
wH Oðb00 þ b01ðT=KÞ þ b02=ðT=KÞ Þ;
ð2Þ
2
where wIL and wH O are mass fractions of IL and water, bi
2
are parameters of the equation for the heat capacity of IL,
b0i are parameters of the equation for water. These param-
eters (table 2) were calculated by the least-squares method
using all the experimental points for the liquid samples
([C4mim]Br + H2O). Maximum deviation of the experi-
mental heat capacities from function (2) was 6.7 Æ 10ꢀ3 Æ cp.
The two approaches give cp of the pure liquid [C4mim]Br in
agreement within 1 Æ 10ꢀ3 Æ cp above Tfus for wH O
<
2
1:7 ꢂ 10ꢀ2
.
The glass-transition temperatures, Tg, were determined
as the temperatures corresponding to the average values
of the heat capacity of the substances in the glass-transition
Heat capacities for liquid [C4mim]Br were measured by
d.s.c. in a range of temperatures of (298 to 323) K [23].

Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
161
FIGURE 2. Effect of water on heat capacity of crystal [C4mim]Br below
160 K. Specific heat capacities of (a) samples with water; (b) pure samples
calculated assuming cp = (1 ꢀ w(IL)) Æ cp(ice) + w(IL) Æ cp(IL). Heat capac-
FIGURE 1. Effect of water on heat capacity of liquid [C4mim]Br. Specific
heat capacities of (a) samples with water; (b) pure samples calculated
assuming cp = (1 ꢀ w(IL)) Æ cp(water) + w(IL) Æ cp(IL). Heat capacities are
designated as d for the sample with water content of 0.14 Æ 10ꢀ2; n,
ities are designated as d for the sample with water content of 0.14 Æ 10ꢀ2
;
n, 0.48 Æ 10ꢀ2; s, 0.51 Æ 10ꢀ2; h, 1.7 Æ 10ꢀ2; +, 5.0 Æ 10ꢀ2
.
0.48 Æ 10ꢀ2; s, 0.51 Æ 10ꢀ2; h, 1.7 Æ 10ꢀ2; +, 5.0 Æ 10ꢀ2
.
TABLE 2
Parameters of heat-capacity equations for liquid ([C4mim]Br + H2O)
i
b
b0
0
1
2
1.186
ꢀ2.962
ꢀ8.806 Æ 10ꢀ5
1.870 Æ 10ꢀ2
2.935 Æ 10ꢀ6
6.159 Æ 104
These results differ from those obtained in this work by
ꢀ5 Æ 10ꢀ2 Æ cp at T = 298 K and by 0.12 Æ cp at T = 323 K.
The glass-transition range shifts down with increasing
water content. Heat capacities of the supercooled liquid
could be measured at T > 225 K for the purest sample, at
T > 222 K at water content of 5 Æ 10ꢀ3, and at T > 210 K
at water content of 5 Æ 10ꢀ2 (figure 1).
The effect of water on heat capacity of the crystal is
more pronounced than that for the liquid. However, at
wH O 6 5 ꢂ 10ꢀ3 the additivity takes place (figure 2). Water
2
FIGURE 3. Effect of water on heat capacity of crystal [C4mim]Br above
150 K. Heat capacities are designated as d for the sample with water
content of 0.14 Æ 10ꢀ2; n, 0.48 Æ 10ꢀ2; s, 0.51 Æ 10ꢀ2; h, 1.7 Æ 10ꢀ2; +,
has the strongest effect in the pre-melting range
T = (166 K to Tfus) (figure 3) where the crystal and the
solution ([C4mim]Br + H2O) are in equilibrium. That is
5.0 Æ 10ꢀ2
.

162
Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
TABLE 3
Eutectic temperatures for [Cnmim]Br
Substance
Te/K
[C2mim]Br
[C3mim]Br
crI
crII
[C4mim]Br
170
173
165
166
why it was impossible to check additivity of the heat capac-
ity of the studied system in this range.
As the water content increases the peak with the onset
temperature of 166 K and the anomaly in the temperature
range of (217 to 274) K become more pronounced. The first
peak corresponds to ([C4mim]Br + H2O) eutectic. The
compositions of the eutectic and the phases forming it
are unknown. To determine the compositions the samples
with higher water content are to be studied. However, even
at the water content of 5 Æ 10ꢀ2 mass fraction crystallization
is problematic. So, we did not study the samples with
wH O > 5 ꢂ 10ꢀ2
.
2
Water affects the heat capacities of [C2mim]Br and
[C3mim]Br in the same way. The eutectic temperatures
are presented in table 3.
3.2. Thermodynamic properties of [Cnmim]Br
[C2mim]Br forms only crystalline and liquid phases (fig-
ure 4, Table S.3). Sample 1 was used for the measurements
in the liquid helium range, and sample 2 for those in the
liquid nitrogen range (table 1). The experimental enthalpies
of fusion are given in table 4, and the fusion temperature,
as well as other thermodynamic characteristics for the pure
compound including the a parameters are presented in
table 5. The enthalpy of fusion for the substance from this
work essentially exceeds the values of 15.7 kJ Æ molꢀ1 [24]
and 15.5 kJ Æ molꢀ1 [25] determined by d.s.c.
FIGURE 4. Experimental heat capacities for [C2mim]Br. d are experi-
mental points for crystal; h, for liquid. Extrapolated heat capacities are
shown with a dashed line. A solid line is a baseline for the low-temperature
transition.
TABLE 4
An anomaly with Tmax = 13 K occurs in the heat-capac-
ity curve for this compound. The enthalpy and the entropy
Results of determination of enthalpy of fusion for the studied compounds
ꢂ
ꢃ
Tfin
fus
Tinit/K Tfin/K DTT h=
DTT hðcrÞ þ DT hðliqÞ =
D
fush/
fin
fus
of the phase transition DtrsH = 3.5 0.9 J Æ molꢀ1
DtrsS = 0.29
,
init
init
ðJ ꢂ gꢀ1
Þ
ðJ ꢂ gꢀ1
[C2mim]Br (sample 2)
Þ
(J Æ gꢀ1
)
0.08 J Æ Kꢀ1 Æ molꢀ1were determined by
numerical integration of the excess Cp and Cp/T in the
range of T = (7 to 15) K. The uncertainties were estimated
from the uncertainties of heat-capacity measurements in
this temperature range.
295.00
295.00
295.00
355.68 168.81
366.09 184.02
358.73 173.27
72.67
87.39
76.95
96.14
96.63
96.32
ÆDfushæ/(J Æ gꢀ1
)
96.36 0.62
67.88
[C3mim]Br forms two crystalline phases. The heat capac-
ities of the phase with higher Tfus = 314.7 K exceed those
for the phase with Tfus = 310.4 K (figure 5, Table S.4).
The purest sample of this compound contains ꢁ6 Æ 10ꢀ3
mass fraction of water, that causes appearance of anomalies
connected with the formation of eutectic ([C3mim]Br +
H2O) in cp vs T curve near 170 K. In the determination of
the enthalpies of fusion for both crystals the amount of
energy required to heat the crystals from Tinit to Tfin was
calculated as the total energy spent for heating the samples
from T ꢁ 169 K to the temperature above Tfus (table 4) in
all the consecutive experiments. The heat capacity of the
Crystal I [C3mim]Br (sample 4)
130.36
168.85
169.59
313.73 198.24
Crystal II [C3mim]Br (sample 5)
129.12
313.48 235.36
106.24
[C4mim]Br (sample 6)
295.00
295.00
295.00
355.22 179.35
355.63 179.61
355.49 179.53
74.46
74.73
74.49
104.89
104.89
105.04
ÆDfushæ/(J Æ gꢀ1
)
104.94 0.21

Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
163
TABLE 5
Thermodynamic characteristics of [Cnmim]Br
[C2mim]Br
Crystal
[C3mim]Br
Crystal I
[C4mim]Br
Crystal II
Crystal
a Æ 103/(J Æ Kꢀ4 Æ molꢀ1
Tfus/K
)
2.61
349.91
18.26 0.12
52.62 0.34
4.85
351.35
22.88 0.05
65.12 0.13
314.7
13.5
42.8
310.4
21.4
68.8
DfusHꢃ ðTfusÞ=ðkJ ꢂ molꢀ1
Þ
DfusSꢃmðTfusÞ=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
m
DlcirqCꢃp;m ðTfusÞ=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
21.7 1.8
207.5 0.8
64
67
51.8 2.2
241.6 1.0
Cꢃp;m ð298:15 KÞ=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
Sꢃm ð298:15 KÞ=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
248.7 1.1
300.3 1.3
D0298Hꢃm=ðkJ ꢂ molꢀ1
Þ
35.04 0.16
41.39 0.19
Glass
a Æ 103/(J Æ Kꢀ4 Æ molꢀ1
Tg/K
)
8.74
218.9
209
DlgilqCꢃp;m ðTgÞ=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
81
2
84.2 1.4
16.6 2.5
11.3 0.5
DgcrlSꢃm ðT ! 0Þ=ð J ꢂ Kꢀ1 ꢂ molꢀ1
Þ
DgcrlHꢃm ðT ! 0Þ=ðkJ ꢂ molꢀ1
Þ
Liquid at T = 360 K
299.7 1.2
Cpꢃ;m=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
268.6 1.1
344.2 1.5
67.56 0.30
336.4 1.3
416.4 1.9
81.07 0.36
Sꢃm=ðJ ꢂ Kꢀ1 ꢂ molꢀ1
Þ
D0360Hꢃm=ðkJ ꢂ molꢀ1
Þ
FIGURE 5. Experimental heat capacities for [C3mim]Br, where j are
heat capacities for crystal I; s, for crystal II; d, for glass and supercooled
liquid; ·, for liquid.
crystals was extrapolated to Tfus with linear equations
obtained from the experimental data in a range of temper-
atures of (120 to 170) K for crystal I and (130 to 170) K
for crystal II. It is impossible to compare the present results
with the obtained earlier value [24] DfusH(32.8 ꢁC) =
16.3 kJ Æ molꢀ1 since in the cited paper the crystal modifica-
tion of the compound was not specified.
At cooling the liquid forms glass with the characteristics
presented in table 5. Measurements of heat capacity for the
supercooled liquid were possible all over the temperature
range of (Tg to Tfus) K.
[C4mim]Br forms one crystalline phase, glass, and liquid
(figure 6, Table S.1). The experimental results on the DfusH
determination are shown in table 4. The fusion temperature
and the other thermodynamic parameters for the pure
FIGURE 6. Experimental heat capacities for [C4mim]Br, where j are
heat capacities of crystal; n, for glass and supercooled liquid; ·, for liquid.
Extrapolated heat capacities are shown with a dashed line. A solid line
designates interpolated heat capacities for supercooled liquid.

164
Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
TABLE 7
compound are presented in table 4. The results on DfusH
from [24] are underestimated by 0.24 Æ DfusH compared
with the present data.
Thermodynamic properties of [C4mim]Br
T/K
Cꢃp;m=R
Sꢃm=R
DT0 Hꢃm=RT
ꢀDT0 Gꢃm=RT
The characteristics of glass [C4mim]Br are presented in
table 5. The heat capacities of the glass in the temperature
range of (25 to 60) K lie below those for the crystal (figure
6). This is unusual for organic compounds since normally
heat capacities for glass exceeds those for crystal all over
the temperature range of their existence. The residual
entropy for the glass [C4mim]Br is rather low (table 4),
which is evidence of high degree of ions’ ordering in
Crystal
0
5
10
15
20
25
30
35
40
45
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
298.15
300
310
320
330
340
350
351.35
0
0
0
0
0.0714
0.601
1.581
2.793
4.054
5.267
6.371
7.348
8.219
8.998
10.33
11.45
12.42
13.29
14.08
14.81
15.49
16.15
16.80
17.45
18.10
18.76
19.44
20.13
20.86
21.64
22.44
23.22
23.99
24.80
25.64
26.56
27.47
28.36
29.08
29.24
30.12
31.01
31.89
32.77
33.65
33.77
0.0243
0.205
0.623
1.241
2.000
2.847
3.744
4.660
5.576
6.483
8.246
9.925
11.52
13.03
14.47
15.85
17.17
18.43
19.66
20.84
21.98
23.10
24.19
25.26
26.31
27.35
28.37
29.39
30.39
31.39
32.38
33.36
34.35
35.33
36.12
36.30
37.28
38.25
39.21
40.18
41.14
41.27
0.0182
0.154
0.457
0.8872
1.395
1.940
2.496
3.043
3.570
4.075
5.010
5.852
6.614
7.309
7.947
8.538
9.089
9.607
10.10
10.57
11.02
11.45
11.88
12.29
12.70
13.11
13.52
13.92
14.32
14.73
15.13
15.54
15.95
16.36
16.70
16.77
17.19
17.61
18.03
18.45
18.87
18.93
0.0061
0.0506
0.165
0.353
0.605
0.907
1.248
1.617
2.006
2.409
3.236
4.072
4.904
5.724
6.528
7.313
8.080
8.828
9.558
10.27
10.97
11.65
12.32
12.97
13.61
14.24
14.86
15.47
16.07
16.66
17.25
17.83
18.40
18.97
19.42
19.53
20.08
20.64
21.18
21.73
22.27
22.34
TABLE 6
Thermodynamic properties for [C2mim]Br
T/K
Cꢃp;m=R
Sꢃm=R
DT0 Hꢃm=RT
ꢀDT0 Gꢃm=RT
Crystal
0
5
10
15
20
25
30
35
40
45
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
298.15
300
310
320
330
340
349.92
0
0
0
0
0.0387
0.4012
1.103
2.022
3.007
3.979
4.893
5.717
6.467
7.155
8.367
9.385
10.28
11.08
11.80
12.48
13.12
13.73
14.33
14.92
15.50
16.08
16.67
17.27
17.88
18.51
19.15
19.81
20.49
21.19
21.91
22.65
23.46
24.29
24.96
25.11
25.94
26.76
27.59
28.42
29.24
0.0131
0.1202
0.433
0.873
1.430
2.065
2.748
3.456
4.173
4.891
6.306
7.674
8.987
10.25
11.45
12.61
13.72
14.80
15.84
16.84
17.83
18.78
19.72
20.64
21.54
22.42
23.30
24.17
25.02
25.87
26.72
27.56
28.40
29.23
29.92
30.07
30.91
31.75
32.58
33.42
34.25
0.0098
0.0920
0.327
0.633
1.009
1.424
1.855
2.287
2.710
3.121
3.897
4.611
5.265
5.868
6.425
6.945
7.433
7.894
8.333
8.753
9.156
9.547
9.926
10.30
10.66
11.02
11.37
11.73
12.08
12.43
12.78
13.13
13.48
13.84
14.14
14.20
14.57
14.94
15.31
15.68
16.05
0.0033
0.0282
0.106
0.240
0.421
0.641
0.893
1.169
1.463
1.770
2.408
3.063
3.722
4.378
5.025
5.662
6.288
6.901
7.502
8.092
8.670
9.236
9.793
10.34
10.88
11.41
11.93
12.44
12.95
13.45
13.94
14.43
14.91
15.39
15.78
15.87
16.34
16.81
17.27
17.74
18.19
Liquid
351.35
360
370
40.00
40.45
41.00
49.10
50.08
51.20
26.76
27.08
27.45
22.34
23.00
23.74
Glass
0
5
0
1.999
2.043
2.314
2.827
3.498
4.277
5.121
6.000
6.894
7.790
8.680
10.43
12.11
0.136
0.820
1.798
2.934
4.086
5.193
6.229
7.174
8.042
8.849
10.31
11.61
272.5
136.5
91.4
ꢀ270.4
ꢀ134.1
ꢀ88.6
10
15
20
25
30
35
40
45
50
60
70
69.1
ꢀ65.6
56.01
47.45
41.49
37.14
33.86
31.32
27.70
25.31
ꢀ51.74
ꢀ42.33
ꢀ35.49
ꢀ30.25
ꢀ26.07
ꢀ22.64
ꢀ17.27
ꢀ13.20
Liquid
349.92
350
360
31.85
31.85
32.31
32.76
40.52
40.53
41.43
42.33
22.33
22.33
22.60
22.87
18.19
18.20
18.83
19.45
370

Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
165
TABLE 7 (continued)
[C4mim]Br crystal. One may speculate that the structures
of the two crystals are also similar. However, the enthalpy
of fusion for crystal I [C3mim]Br is essentially lower than
that for crystal II. This can serve as the evidence of lower
stability of crystal I.
T/K
Cꢃp;m=R
Sꢃm=R
DT0 Hꢃm=RT
ꢀDT0 Gꢃm=RT
80
90
12.79
13.83
14.78
15.67
16.50
17.29
18.06
18.82
19.59
20.36
21.14
21.96
22.83
23.98
30.52
34.82
35.16
35.52
35.89
36.28
36.68
37.10
37.45
37.53
37.98
38.44
38.92
39.42
39.93
40.00
13.74
15.31
16.82
18.27
19.67
21.02
22.33
23.60
24.84
26.05
27.24
28.40
29.56
30.70
31.89
33.41
34.90
36.34
37.74
39.10
40.43
41.72
42.76
42.99
44.23
45.44
46.63
47.80
48.95
49.10
23.67
22.52
21.70
21.11
20.70
20.40
20.21
20.09
20.04
20.03
20.07
20.15
20.27
20.42
20.65
21.24
21.81
22.36
22.87
23.36
23.83
24.28
24.63
24.71
25.13
25.54
25.94
26.33
26.71
26.76
ꢀ9.93
ꢀ7.21
ꢀ4.88
ꢀ2.84
ꢀ1.03
0.62
2.12
3.51
4.81
6.02
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
298.15
300
310
320
330
340
350
351.35
4. Conclusions
In this work heat capacities and enthalpies of phase
transitions were measured for three 1-alkyl-3-methylimi-
dazolium bromides. Thermodynamic properties of these
compounds were calculated from these data. It was found
that the ability to vitrify and to crystallize among the stud-
ied ILs strongly depends on the length of the alkyl chain in
the imidazolium ring. Along with this, it was concluded
from the measurements in the ([C4mim]Br + H2O) system
that at concentrations of water <5 Æ 10ꢀ3 mas. the equation
cp = cp(water) + cp(IL) describes cp within 1 Æ 10ꢀ3 Æ cp for
the liquid above Tfus and the crystal below 160 K.
7.17
8.25
9.29
10.28
11.24
12.17
13.08
13.99
14.87
15.74
16.60
17.45
18.12
18.28
19.09
19.90
20.69
21.47
22.24
22.34
Acknowledgments
The authors are grateful to INTAS Foundation for
financial support of this work (Grant No. 03-50-5526).
This work was also supported by Russian Foundation
for Basic Research (RFBR Grant No. 06-03-33061_a and
05-03-08073-ofi_a) as well as by Grant of President of Rus-
sian Federation for Young Scientists (MK-3689.2005.3).
Special acknowledgement is to Prof. L. Komarova for i.r.
spectra and Dr. M. Buzin for t.g.a. measurements.
the glass. The glass transition temperature is 0.69 Tfus
,
which is in agreement with the empirical correlation
Tg/Tfus ꢄ 2/3.
Appendix A. Supplementary data
A very small anomaly with the maximum temperature
of Tmax = 11 K and DH = 0.9 0.7 J Æ molꢀ1 was observed
in the glass. It is not clear if the anomaly is related with the
nature of the compound or with the uncertainty in the mea-
surements. That is why it was not taken into account in cal-
culation of the thermodynamic functions.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jct.2006.
05.008.
The additivity found for ([C4mim]Br + H2O) at water
content of 6 5 Æ 10ꢀ3 allowed us to calculate the thermody-
namic properties for pure [C4mim]Br and [C2mim]Br (tables
6 and 7). The difference between the entropies of liquid
[C4mim]Br and [C2mim]Br is 72 J Æ Kꢀ1 Æ molꢀ1 at T = 360
K. This value is lower than the average increment per two
CH2-groups in the series of normal alkanes C14–C16–C18
References
[1] T. Welton, Chem. Rev. 99 (1999) 2071–2083.
[2] K.R. Seddon, J. Chem. Technol. Biotechnol. 68 (1997) 351–356.
[3] R. Sheldon, Chem. Commun. 23 (2001) 2399–2407.
[4] Ya.S. Vygodskii, E.I. Lozinskaya, A.S. Shaplov, Vysokomol. Soed.
(Polymer Science), Part C 43 (2001) 236–251, CA 2002, 136, 386412.
[5] P. Wasserscheid, W. Keim, Angew. Chem. 112 (2000) 3926–3945.
[6] R.D. Rogers, K.R. Seddon, Ionic Liquids. Industrial Applications to
Green Chemistry, American Chemical Society, 2002.
[7] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH,
2003.
[8] A. Heintz, J Chem. Termodyn. 37 (2005) 525–535.
[9] J.A. Widegren, E.M. Saurer, K.N. Marsh, J.W. Magee, J. Chem.
Thermodyn. 37 (2005) 569–575.
[10] P. Bonhote, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M.
Gratzel, Inorg. Chem. 35 (1996) 1168–1178.
2DCH S = 76.7 J Æ Kꢀ1 Æ molꢀ1 calculated from the thermo-
2
dynamic tables [26]. The difference in the heat capacities of
the mentioned ILs is 67 J Æ Kꢀ1 Æ molꢀ1 and is comparable
with the similar value 2DCH Cp = 68 J Æ Kꢀ1 Æ molꢀ1 for
2
n-alkanes.
[C4mim]Cl forms two crystals. The alkyl chain in the
cation is in a gauche conformation in one crystal, and in
a trans conformation in the other [27]. Only the [C4mim]Br
crystal with gauche conformation of the alkyl chain is sta-
ble [27]. The temperature dependence of the heat capacity
for crystal I of [C3mim]Br is similar to that for the
[11] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A.
Broker, R.D. Rogers, Green Chem. 3 (2001) 156–164.
[12] B.K.M. Chan, N.-H. Chang, M.R. Grimmett, Aust. J. Chem. 30
(1977) 2005–2013.

166
Y.U. Paulechka et al. / J. Chem. Thermodynamics 39 (2007) 158–166
[13] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwan, Thermochim.
Acta 357–358 (2000) 97–102.
[21] M.W. Chase Jr., NIST-JANAF Thermochemical Tables. J. Phys.
Chem. Ref. Data Monograph 9, 1998, 1-1951.
[14] J.D. Holbrey, K.R. Seddon, J.Chem. Soc., Dalton Trans. (1999)
2133–2139.
[22] L.P.N. Rebelo, V. Najdanovich-Visak, Z.P. Visak, M. Nunes da
´
Ponte, J. Szydlowski, C.A. Cerdeirina, J. Troncoso, L. Romanı,
˜
[15] S. Takahashi, N. Koura, S. Kohara, M.-L. Saboungi, L.A. Curtiss,
Plasmas Ions 2 (1999) 91–105.
J.M.S.S. Esperanc¸a, H.J.R. Guedes, H.C. de Sousa, Green Chem. 6
(2004) 369–381.
[16] E.I. Lozinskaya, A.S. Shaplov, M.V. Kotseruba, L.I. Komarova,
K.A. Lyssenko, M.Yu. Antipin, D.G. Golovanov, Ya.S. Vygodskii,
J. Polym. Sci. A 44 (2006) 380–394.
[17] K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M. Pringle, J.L.
Scott, Aust. J. Chem. 57 (2004) 145–147.
[18] D.M. Fox, J.W. Gilman, H.C. De Long, P.C. Trulove, J. Chem.
Thermodyn. 37 (2005) 900–905.
[19] P. Nockemann, K. Binnemans, K. Driesen, Chem. Phys. Lett. 415
(2005) 131–136.
[23] K.-S. Kim, B.-K. Shin, H. Lee, F. Ziegler, Fluid Phase Equilibria 218
(2004) 215–220.
[24] E.A. Turner, C.C. Pye, R.D. Singer, J. Phys. Chem. A 107 (2003)
2277–2288.
[25] H.A. Every, A.G. Bishop, D.R. McFarlane, G. Ora¨dd, M. Forsyth,
J. Mater. Chem. 11 (2001) 3031–3036.
[26] TRC/NIST Thermodynamic Tables, Hydrocarbons, 2005.
[27] J.D. Holbrey, W.M. Reichert, M. Nieuwenhuyzen, S. Johnston,
K.R. Seddon, R.D. Rogers, Chem. Commun. (2003) 1636–1637.
[20] A.V. Blokhin, G.J. Kabo, Y.U. Paulechka. J. Chem. Eng. Data 51
(2006) (in press).
JCT 06-39