Density and Viscosity of Binary Mixtures of 1-Ethyl-3-methylimidazolium Heptachlorodialuminate and Tetrachloroaluminate Ionic Liquids

Article abstract of DOI:10.1021/acs.jced.7b00702

The densities and viscosities of binary mixtures of 1-ethyl-3-methylimidazolium heptachlorodialuminate and tetrachloroaluminate ionic liquids with different molar compositions were measured by high-precision vibrating tube densimeter and automated microviscometer from 293.15 to 353.15 K. On this basis, the excess molar volumes and excess viscosities of binary mixtures were also calculated for the first time. All the experimental data were well fitted by the empirical equations. According to the results of computational calculations, enhanced molecular symmetry and molar concentration of heptachlorodialuminate anion usually lead to higher density. The hydrogen bonding among ions is confirmed as one of the most important structural parameters in determining the viscosity of ionic liquids. It shows that Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate can be treated as the classic binary mixtures of heptachlorodialuminate and tetrachloroaluminate ionic liquids. Looser packing and/or weaker interionic interaction probably occurs after the formation of binary mixtures. The results and conclusions of this work will help to promote future research and application of chloroaluminate ionic liquids.

Full text of DOI:10.1021/acs.jced.7b00702

Article
pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX
Density and Viscosity of Binary Mixtures of 1‑Ethyl-3-
methylimidazolium Heptachlorodialuminate and
Tetrachloroaluminate Ionic Liquids
Yong Zheng,*,† Yongjun Zheng,† Qian Wang,‡ Zhen Wang,† and Dayong Tian†
†College of Chemistry and Environmental Engineering, Anyang Institute of Technology, Anyang, 455000 Henan, P. R. China
‡State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, P. R. China
S
* Supporting Information
ABSTRACT: The densities and viscosities of binary mixtures of 1-ethyl-3-
methylimidazolium heptachlorodialuminate and tetrachloroaluminate ionic
liquids with different molar compositions were measured by high-precision
vibrating tube densimeter and automated microviscometer from 293.15 to
353.15 K. On this basis, the excess molar volumes and excess viscosities of
binary mixtures were also calculated for the first time. All the experimental data
were well fitted by the empirical equations. According to the results of
computational calculations, enhanced molecular symmetry and molar
concentration of heptachlorodialuminate anion usually lead to higher density.
The hydrogen bonding among ions is confirmed as one of the most important
structural parameters in determining the viscosity of ionic liquids. It shows that
Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate can be treated as
the classic binary mixtures of heptachlorodialuminate and tetrachloroaluminate
ionic liquids. Looser packing and/or weaker interionic interaction probably
occurs after the formation of binary mixtures. The results and conclusions of this work will help to promote future research and
application of chloroaluminate ionic liquids.
tunable acidity, chloroaluminate ILs usually serve as efficient
electrolytes and catalysts in many chemical reactions.11−13
Among these ILs, one of the most commonly used systems is
Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate
([Emim]Cl/AlCl3) (see Figure 1).14−17 Although significant
progress has been achieved in relevant research studies, the
density, viscosity, excess molar volume, excess viscosity, and
structure of Lewis acidic [Emim]Cl/AlCl3 are still less
understood, which is probably attributed to the chemical
instability of chloroaluminate ILs in moisture and air.18 From
the perspective of industrial application, there is much work to
be done in this field.
1. INTRODUCTION
Ionic liquids (ILs), the so-called room-temperature molten
salts, have attracted worldwide attention over the past few
Figure 1. Chemical structure of the [Emim]+ cation.
decades. Owing to their novel structure and excellent
physicochemical properties, ILs can be used as green reaction
media in numerous research fields, including electrochemistry,
extraction, catalysis, synthesis, biomass processing, gas capture,
and so on.1−4
The history of first-generation ILs began in 1951, when
Hurley et al.5 reported that a mixture of organic halide salt and
aluminum chloride became liquid at low temperature. Since
then, extensive studies have been carried out to explore the
chemical composition of these systems.6−8 It was found that the
resulting liquids were chloroaluminate-based molten salts. As
the mole fraction of aluminum chloride increases from 0.5 to
0.66, the predominant form of chloroaluminate anion changes
from neutral tetrachloroaluminate ([AlCl4]−) to Lewis acidic
heptachlorodialuminate ([Al2Cl7]−) (see eq 1).9,10 With their
Cl− + AlCl3 → [AlCl4]− + AlCl3 → [Al2Cl7]−
(1)
Under the above background, this work aimed to make
perform further and systematic research on the typical
properties of [Emim]Cl/AlCl3. Therefore, a series of Lewis
acidic [Emim]Cl/AlCl3 ILs with different molar compositions
were prepared by first mixing precise molar quantities of
[Emim][AlCl4] and [Emim][Al2Cl7]. Then, the densities and
viscosities of these ILs mixtures were measured from 293.15 to
Received: July 31, 2017
Accepted: October 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jced.7b00702
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Journal of Chemical & Engineering Data
Article
Table 1. Information of Chemical Samples Used in This Work
initial mass
fraction purity
final mass
fraction purity
analysis
method
chemical name
CAS no.
source
purification method
a
b
1-ethyl-3-methylimidazolium
chloride
65039-09-0 Tokyo Chemical Industry
>0.980
recrystallization;
vacuum drying
>0.995
NMR ; EA ;
c
KF
d
anhydrous aluminum chloride
7446-70-0
Sinopharm Chemical
Reagent Co., Ltd.
>0.990
sublimation
>0.995
PT ; KF
e
1-ethyl-3-methylimidazolium
tetrachloroaluminate
80432-05-9 synthesized
electrolysis vacuum
drying
>0.995
NMR; EA;
KF
e
1-ethyl-3-methylimidazolium
heptachlorodialuminate
synthesized
electrolysis vacuum
drying
>0.995
NMR; EA;
KF
a
b
c
d
e
Nuclear magnetic resonance. Elemental analysis. Karl Fisher titration. Potentiometric titration. Water content < 50 mg/kg.
353.15 K with high-precision vibrating tube densimeter and
automated microviscometer. On this basis, the resulting excess
molar volumes and excess viscosities of binary mixtures have
been calculated by empirical equations. For illustrating the
relationship between physicochemical properties and molecular
structure of ILs, computational calculations were also
conducted by the Gaussian09 program. Finally, all of the
experimental results were evaluated and analyzed in detail
according to the empirical equations and theory of the liquid
binary mixtures.
0.98). Prior to use, all of the chemicals were purified. Detailed
information on the chemicals is listed in Table 1.
2.2. Synthesis of ILs. All the following synthesis and
subsequent purification processes were performed in an argon-
filled glovebox (Universal, MIKROUNA, China) in which the
mass contents of water and oxygen were both less than 1 mg/
kg. The mass measurements were conducted by an electronic
balance (MS204S, Mettler Toledo, Switzerland) with accuracy
of 0.0001 g.
[Emim][AlCl4]. Dry [Emim]Cl was first mixed with
anhydrous aluminum chloride at the precise molar ratio of
1:1 around room temperature. Then, the resulting liquid was
filtered through a quartz glass frit and purified by potentiostatic
electrolysis method to remove the organic and/or inorganic
impurities. The electrolysis process was conducted in an
electrolytic cell with two aluminum electrodes through an
electrochemical workstation (CHI660D, Chenhua, China). The
final product was obtained as a colorless liquid and dried at
−0.1 MPa and 323 K in a vacuum oven (DZF-6020, Jinghong,
China) before use.
[Emim][Al2Cl7]. Dry [Emim]Cl was first mixed with
anhydrous aluminum chloride at the precise molar ratio of
1:2 around ambient temperature. After that, the resulting liquid
was filtered through a quartz glass frit and purified by
potentiostatic electrolysis method to remove the organic and/
or inorganic impurities. The electrolysis process was conducted
in an electrolytic cell with two aluminum electrodes through
the electrochemical workstation. The final product was
obtained as a colorless liquid and dried at −0.1 MPa and 323
K in the vacuum oven prior to use.
Lewis Acidic [Emim]Cl/AlCl3 (Binary Mixtures of [Emim]-
[Al2Cl7] and [Emim][AlCl4]). The binary mixtures used were
prepared by mixing precise molar quantities of [Emim][Al2Cl7]
and [Emim][AlCl4] at room temperature. The corresponding
standard uncertainty u(x) of the mole fraction is 0.0001.
2.3. Characterization of ILs. The molecular structure and
chemical composition of [Emim]Cl, [Emim][Al2Cl7], and
[Emim][AlCl4] ILs were characterized by NMR spectra,
elemental analysis, and water content analysis. NMR spectra
were obtained on an NMR spectrometer (av-400 MHz, Bruker,
Switzerland). In the measurement of NMR for [Emim][Al2Cl7]
and [Emim][AlCl4], acetone-d6 was used as an external
standard to suppress the interference from other solvents.
The elemental analyzer (Vario El cube, Elementar, Germany)
was used to determine the chemical composition of ILs.
[Emim]Cl. 1H NMR (DMSO-d6, 400 MHz): δ × 106 = 9.492
(s, 1H), 7.858 (d, 1H), 7.757 (d, 1H), 4.180 (m, 2H), 3.836 (s,
3H), and 1.370 (t, 3H). EA: C (w = 0.4895), N (w = 0.1903),
H (w = 0.0747), and Cl (w = 0.2412).
2. EXPERIMENTAL SECTION
2.1. Chemicals. 1-Ethyl-3-methylimidazolium chloride
([Emim]Cl) was purchased from Tokyo Chemical Industry
with a mass fraction purity higher than 0.99. Anhydrous
aluminum chloride was obtained from Sinopharm Chemical
Reagent Co., Ltd. at analytic grade (mass fraction purity >
Table 2. Comparison of the Experimental Densities ρ and
Viscosities η of [Emim][Al2Cl7] and [Emim][AlCl4] at
a
Temperature T with Literature Values
ρ/g cm−3
η/mPa s
this work
T/K
this work
lit.
lit.
[Emim][Al2Cl7]
1.393419
1.384219
1.375019
1.365919
1.356719
1.347519
1.338419
[Emim][AlCl4]
1.298119
1.297920
1.290119
1.290820
1.282119
1.283320
1.274019
1.275920
1.266019
1.268920
1.258019
1.249919
293.15
303.15
313.15
323.15
333.15
343.15
353.15
1.395287
1.385833
1.376465
1.367190
1.357991
1.348876
1.339819
16.11
12.41
9.790
7.943
6.571
5.526
4.721
16.0619
12.3419
9.75219
7.90219
6.53819
5.50819
4.71319
293.15
303.15
313.15
323.15
333.15
1.299317
1.291130
1.283046
1.275029
1.267095
20.62
15.69
12.31
9.952
8.214
20.5319
15.5919
12.2519
9.89619
8.18619
343.15
353.15
1.259221
1.251412
6.927
5.945
6.90819
5.92819
a
Standard uncertainties u are u(T) = 0.01, and 0.01 K for ρ and η,
respectively, the relative standard uncertainties for the densities Ur(ρ)
= 0.0005, and the relative expanded uncertainties for the viscosities
Ur(η) = 0.005 (0.95 confidence level ).
B
DOI: 10.1021/acs.jced.7b00702
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Table 3. Experimental Values of Density ρ for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15
a
K as a Function of Mole Fraction x1 at 0.1 MPa
ρ/g cm−3
x1
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
353.15 K
0.0000
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
1.0000
1.299317
1.311437
1.323232
1.334146
1.344261
1.354516
1.363068
1.371773
1.380103
1.387334
1.395287
1.291130
1.303063
1.314695
1.325469
1.335448
1.345572
1.354021
1.362614
1.370832
1.377972
1.385833
1.283046
1.294785
1.306242
1.316875
1.326738
1.336733
1.345066
1.353545
1.361663
1.368703
1.376465
1.275029
1.286574
1.297853
1.308347
1.318086
1.327975
1.336207
1.344572
1.352559
1.359515
1.367190
1.267095
1.278441
1.289546
1.299898
1.309503
1.319285
1.327419
1.335662
1.343536
1.350405
1.357991
1.259221
1.270362
1.281285
1.291503
1.300986
1.310657
1.318694
1.326817
1.334583
1.341369
1.348876
1.251412
1.262347
1.273072
1.283154
1.292539
1.302085
1.310018
1.318032
1.325698
1.332392
1.339819
a
Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative standard uncertainties for the densities Ur(ρ) =
0.0005 (0.95 confidence level).
1
[Emim][AlCl4]. H NMR (acetone-d6, 400 MHz): δ × 106 =
takes p- and d-type polarization, as well as diffuse function for
hydrogen, carbon, nitrogen, and other elements into consid-
eration. This basis set is very important for improving the
accuracy of computations when investigating the interaction
energy and hydrogen bonding in ionic liquids. The optimized
structure and energy obtained by this method shows good
precision and reproducibility in calculating these structural
parameters of ion pairs. The results of computations are well
consistent with the experimental data. Consequently, the
structure of [Emim][AlCl4] and [Emim][Al2Cl7] ion pairs
was studied and optimized at the Becke−three−parameter−
Lee−Yang−Parr (B3LYP) level with 6-31+G(d,p) basis set.
Under such conditions, each optimized structure has been
obtained as a true minimum by frequency calculations. The
effect of basis set superposition errors (BSSE) on interaction
energy between ions has been considered through counterpoise
method.
Table 2 shows the comparison of densities and viscosities of
[Emim][Al2Cl7] and [Emim][AlCl4] measured in this work
with literature values at the same temperatures. The relative
error of densities and viscosities of [Emim][Al2Cl7] and
[Emim][AlCl4] between this work and literature is less than
0.15 and 0.65%, respectively.19,20 It can be concluded that all
the experimental values are well consistent with those reported
in the literature.
8.325 (s, 1H), 7.437 (d, 1H), 7.384 (d, 1H), 4.267 (m, 2H),
3.955 (s, 3H), and 1.522 (t, 3H). 27Al NMR (acetone-d6, 400
MHz): δ × 106 = 102.154. EA: C (w = 0.2570), N (w =
0.1002), H (w = 0.0393), and Cl (w = 0.5068).
[Emim][Al2Cl7]. 1H NMR (acetone-d6, 400 MHz): δ × 106 =
8.077 (s, 1H), 7.158 (d, 1H), 7.118 (d, 1H), 4.032 (m, 2H),
3.726 (s, 3H), and 1.351 (t, 3H). 27Al NMR (acetone-d6, 400
MHz): δ × 106 = 103.386. EA: C (w = 0.1739), N (w =
0.0673), H (w = 0.0264), and Cl (w = 0.5989).
The structure of ILs was confirmed according to the peaks
and chemical shifts obtained in NMR spectra. The results of
elemental analysis were consistent with the chemical
composition of ILs. The water content of chloroaluminate
ILs was determined by Karl Fisher titration (751 GPD Titrino,
Metrohm, Switzerland). On the basis of the above measure-
ments, the mass fraction purity of ILs was higher than 0.995.
2.4. Density Measurements. A high-precision vibrating
tube densimeter (DMA 5000, Anton Paar, Austria) was applied
in the density measurement of ILs. The relative standard
uncertainty Ur(ρ) was 0.0005 (0.95 confidence level). Before
the experiment, the densimeter was calibrated with dry air and
ultrapure water. The following measurement was conducted
from 293.15 to 353.15 K at 0.1 MPa in the argon-filled
glovebox. The precision of the experimental temperature was as
high as 0.001 K, which was controlled by integrated Pt 100
platinum thermometers. Experimental pressure was regulated
with a programmable logic controller in the glovebox, and the
standard uncertainty U(p) is 0.1 KPa.
2.5. Viscosity Measurements. The viscosities of ILs were
measured by an automated falling ball microviscometer
(AMVn, Anton Paar, Austria) at 0.1 MPa in an argon-filled
glovebox. The relative expanded uncertainty Ur(η) was 0.005
(0.95 confidence level). Calibration was conducted with
ultrapure water and viscosity standard oils (No. H117, Anton
Paar, Austria). The experimental temperature ranged from
293.15 to 353.15 K with precision of 0.01 K, which was
controlled by a built-in precise Peltier thermostat. Experimental
pressure was regulated by programmable logic controller in the
glovebox, and the standard uncertainty U(p) is 0.1 KPa.
2.6. Computational Method. All the computational
calculations for ILs were conducted by the Gaussian09 program
based on density functional theory (DFT). The 6-31+G(d,p)
basis set is known as a split-valence double-ζ basis set, which
3. RESULTS AND DISCUSSION
3.1. Density. The densities ρ of [Emim][Al2Cl7] and
[Emim][AlCl4] binary mixtures obtained from 293.15 to
353.15 K are listed in Table 3. As shown in Figure 2, the
values of density fall in the range from 1.251412 to 1.395287 g
cm−3 and are well fitted by the classic equation
ρ = A + BT + CT2
(2)
in which A (g cm−3), B (g cm−3 K−1), and C (g cm−3 K−2) are
empirical parameters.21,22 The values of best-fit parameters can
be found in Table 4.
All of the densities of IL mixtures obviously decrease as the
experimental temperature increases, which indicates that the
increasing temperature enhances the volume of ions, resulting
in smaller mass per unit volume. Meanwhile, the chemical
composition of chloroaluminate ILs has a significant effect on
the density values. At the same temperature, the density of ILs
C
DOI: 10.1021/acs.jced.7b00702
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Table 5. Interaction Energy ΔE, Dipole Moment μ, and
Hydrogen Bonds of [Emim][AlCl4], [Emim][Al2Cl7],
[Bmim][AlCl4], and [Bmim][Al2Cl7] Ion Pairs Calculated
by DFT Method
C2−H···
C6−H···
C7−H···
a
a
a
ΔE
kJ mol−1
μ
Cl
Cl
Cl
b
IL
D
10−10
m
10−10
m
10−10
m
[Emim][AlCl4] 293.83
15.14
15.29
2.619
2.663
2.728
2.765
2.812
2.862
[Emim]
[Al2Cl7]
273.60
[Bmim][AlCl4] 290.987
15.327
15.567
2.5887
2.6167
2.7117
2.7477
2.6977
2.7257
[Bmim]
[Al2Cl7]
270.617
a
b
The distance of hydrogen bond between H and Cl atoms. D =
3.33564 × 10−30 C m.
Figure 2. Density ρ of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2)
binary mixtures as a function of temperature T at different mole
probably due to the enhanced molar concentration of [Al2Cl7]−
anion with larger volume and heavier molecular weight.7,23 The
difference between the molecular structure of [Emim][Al2Cl7]
and [Emim][AlCl4] can be seen in Figure 3.
■
△
fractions x1 of [Emim][Al2Cl7] = (▶) 0.0000, ( ) 0.1016, ( )
□
▲
○
⧫
▼
0.2046, ( ) 0.3035, ( ) 0.4003, ( ) 0.5041, (◀) 0.5962, (▷)
0.6955, ( ) 0.7963, (◁) 0.8901, and ( ) 1.0000. The solid curves are
On the other hand, the density of [Emim]Cl/AlCl3 is higher
than that of another well-known chloroaluminate IL, 1-butyl-3-
methylimidazolium chloroaluminate ([Bmim]Cl/AlCl3), at the
same temperature and molar composition.7,19 Consequently, it
can be found that [Emim]Cl/AlCl3 has lower molar volume
under similar conditions. For example, the molar volumes of
[Emim]Cl/AlCl3 and [Bmim]Cl/AlCl3 calculated from the
density at 293.15 K and x1 = 0.4 are 248.0 and 280.7 cm3 mol−1,
respectively.7 It is apparent that the structure of imidazolium
cation, especially N-alkyl side chains, is vital to the density of
chloroaluminate ILs. On the basis of the results of computa-
tional calculations, the dipole moment μ of ion pairs follows the
order [Emim][AlCl4] < [Bmim][AlCl4] and [Emim][Al2Cl7] <
[Bmim][Al2Cl7] (see Table 5). Consequently, it is proper to
state that the decreased length of N-alkyl side chain enhances
the geometric symmetry of chloroaluminate ILs, resulting in
higher packing density of ions. It is probably the main reason
why [Emim]Cl/AlCl3 ILs have higher densities than [Bmim]-
Cl/AlCl3 ILs under the same conditions.
correlated with eq 2, and the symbols represent experimental values.
Table 4. Fitted Values of the Empirical Parameters A−C and
Standard Deviation σ for the Densities of Binary Mixtures of
[Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) as a Function of
Mole Fraction x1 Based on Eq 2
A
103B
107C
x1
g cm−3
g cm−3 K−1
g cm−3 K−2
105σ
0.0000
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
1.0000
1.57157
1.58751
1.60203
1.61762
1.63306
1.64816
1.66091
1.67264
1.68495
1.69570
1.70816
−1.03713
−1.04458
−1.04682
−1.06448
−1.08754
−1.10821
−1.12589
−1.13510
−1.15063
−1.16521
−1.18601
3.69762
3.50714
3.26667
3.32500
3.49167
3.63214
3.74643
3.70952
3.77619
3.86429
4.05001
0.9805
0.9679
1.1555
1.0418
0.7643
1.4161
1.5626
1.2804
0.6976
0.8452
0.8025
3.2. Viscosity. Table 6 lists the viscosities η of [Emim]-
[Al2Cl7] and [Emim][AlCl4] binary mixtures measured in the
temperature range from 293.15 to 353.15 K. The temperature
dependency of viscosity at different mole fractions is presented
increases with increasing mole fraction of [Emim][Al2Cl7].
According to the literature and our previous work, this trend is
Figure 3. Optimized structure of [Emim][AlCl4] and [Emim][Al2Cl7] ion pairs calculated at the B3LYP/6-31+G(d,p) level. Hydrogen bonds are
indicated by dashed lines.
D
DOI: 10.1021/acs.jced.7b00702
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Table 6. Experimental Viscosities η of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures Measured from 293.15 to
a
353.15 K as a Function of Mole Fraction x1 at 0.1 MPa
η/mPa s
x1
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
353.15 K
0.0000
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
1.0000
20.62
19.96
19.38
18.85
18.40
17.95
17.57
17.19
16.82
16.47
16.11
15.69
15.23
14.81
14.44
14.10
13.77
13.49
13.21
12.93
12.68
12.41
12.31
11.98
11.67
11.38
11.12
10.86
10.64
10.42
10.21
10.01
9.790
9.952
9.697
9.461
9.239
9.033
8.826
8.650
8.468
8.286
8.127
7.943
8.214
8.016
7.821
7.643
7.474
7.304
7.157
7.008
6.859
6.726
6.571
6.927
6.762
6.598
6.447
6.306
6.161
6.037
5.906
5.777
5.660
5.526
5.945
5.806
5.664
5.533
5.408
5.282
5.173
5.059
4.946
4.842
4.721
a
Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative expanded uncertainties for the viscosities Ur(η) =
0.005 (0.95 confidence level).
in Figure 4. All the experimental data have been fitted according
to the well-known Vogel−Fulcher−Tamman (VFT) equation
η = η exp(B/(T − T ))
(3)
0
0
where η0 (mPa s), B (K), and T0 (K) are adjustable
parameters.24,25 The best fitted parameters are given in Table 7.
As illustrated in Figure 4, the viscosities of IL mixtures range
from 4.721 to 20.62 mPa s and decrease remarkably when the
temperature increases. It can be inferred that the increase in
temperature weakens ILs’ resistance to internal motion and
interionic friction, which mainly results from the hydrogen
bonding and electrostatic force among ions.26 At the same
temperature, the viscosities of [Emim]Cl/AlCl3 ILs decrease as
the mole fraction of [Emim][Al2Cl7] increases. According to
Table 5 and Figure 3, [Emim][Al2Cl7] has higher ion-pair
interaction energy than that of [Emim][AlCl4], and the
distances of hydrogen bonds in [Emim][Al2Cl7] are longer
than those in [Emim][AlCl4]. These experimental and
computational results reflect the presence of stronger cation−
anion interaction and hydrogen bonding in [Emim][AlCl4].
Consequently, the increased molar concentration of [Emim]-
[Al2Cl7] reduces the interionic force and friction, which leads to
lower viscosity of [Emim][AlCl4] and [Emim][Al2Cl7] binary
mixtures.
Compared with [Bmim]Cl/AlCl3 reported in our previous
work,7 [Emim]Cl/AlCl3 ILs have lower viscosities at the same
temperature and molar composition. Considering the relation-
ship between physiochemical properties and molecular
structure of ILs, it can be found that the strength of hydrogen
bonding is one of the most important intrinsic factors in
determining the viscosity of chloroaluminate ILs.27,28 As shown
in Table 5, the distances of hydrogen bonds in these ion pairs
follow the order [Emim][AlCl4] > [Bmim][AlCl4] and
[Emim][Al2Cl7] > [Bmim][Al2Cl7]. It reflects the presence of
weaker hydrogen bonding in [Emim][AlCl4] and [Emim]-
[Al2Cl7]. Therefore, the lower viscosities of [Emim][AlCl4] and
[Emim][Al2Cl7] binary mixtures are mainly ascribed to the
weaker interionic hydrogen bonding.
Figure 4. Viscosity η of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2)
binary mixtures as a function of temperature T at different mole
fractions x1 of [Emim][Al2Cl7] = ( ) 0.0000, ( ) 0.1016, ( )
■
▲
○
⧫
◊
□
▼
0.2046, (▷) 0.3035, ( ) 0.4003, ( ) 0.5041, (★) 0.5962, ( ) 0.6955,
(▶) 0.7963, (☆) 0.8901, and ( ) 1.0000. The solid curves are
correlated with eq 3, and the symbols represent experimental values.
Table 7. Fitted Values of the Empirical Parameters η0, B, T0,
and Standard Deviation σ for the Viscosities of
[Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures as
a Function of Mole Fraction x1 Based on the VFT Equation
η0
B
T0
x1
mPa s
K
K
σ
0.0000
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
1.0000
0.289
0.268
0.252
0.245
0.238
0.227
0.215
0.200
0.182
0.162
0.155
621.4
643.1
657.6
662.1
664.9
674.7
687.6
706.5
731.6
768.2
776.1
147.5
144.0
141.7
140.7
140.2
138.8
137.0
134.6
131.5
126.9
126.1
0.0099
0.0082
0.0075
0.0094
0.0086
0.0101
0.0110
0.0109
0.0070
0.0104
0.0111
3.3. Excess Molar Volume and Excess Viscosity. As
described in the literature, Lewis acidic [Emim]Cl/AlCl3 ILs
can be regarded as the binary mixtures of [Emim][AlCl4] and
[Emim][Al2Cl7].9,29 Thus, it is necessary to investigate the
corresponding excess molar volume VE and excess viscosity Δη
of [Emim]Cl/AlCl3 ILs.
E
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Table 8. Excess Molar Volumes VE for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures Calculated from 293.15 to
a
353.15 K as a Function of Mole Fraction x1 at 0.1 MPa
VE/cm3 mol−1
x1
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
353.15 K
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
0.1352
0.2027
0.2022
0.1807
0.1402
0.1051
0.0659
0.0259
0.0091
0.1407
0.2103
0.2095
0.1888
0.1477
0.1106
0.0707
0.0307
0.0126
0.1472
0.2207
0.2196
0.1957
0.1531
0.1163
0.0756
0.0329
0.0149
0.1544
0.2330
0.2318
0.2067
0.1588
0.1196
0.0787
0.0402
0.0195
0.1624
0.2454
0.2446
0.2201
0.1667
0.1245
0.0849
0.0468
0.0238
0.1723
0.2618
0.2605
0.2347
0.1772
0.1322
0.0940
0.0557
0.0300
0.1824
0.2815
0.2799
0.2480
0.1889
0.1424
0.1032
0.0629
0.0362
a
Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the expanded uncertainties for excess molar volumes U(VE) =
0.003 cm3 mol−1 (0.95 confidence level).
Table 9. Excess Viscosities Δη for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15 K as a
a
Function of Mole Fraction x1 at 0.1 MPa
Δη/mPa s
x1
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
353.15 K
0.1016
0.2046
0.3035
0.4003
0.5041
0.5962
0.6955
0.7963
0.8901
−0.2018
−0.3173
−0.4012
−0.4147
−0.3965
−0.3611
−0.2933
−0.2087
−0.1357
−0.1268
−0.2089
−0.2545
−0.2770
−0.2666
−0.2445
−0.1988
−0.1481
−0.0905
−0.0740
−0.1244
−0.1652
−0.1812
−0.1797
−0.1676
−0.1373
−0.0933
−0.0570
−0.0509
−0.0800
−0.1033
−0.1148
−0.1133
−0.1042
−0.0867
−0.0662
−0.0368
−0.0311
−0.0568
−0.0724
−0.0823
−0.0818
−0.0774
−0.0633
−0.0467
−0.0256
−0.0227
−0.0424
−0.0548
−0.0602
−0.0598
−0.0547
−0.0466
−0.0344
−0.0200
−0.0146
−0.0306
−0.0405
−0.0470
−0.0460
−0.0423
−0.0347
−0.0243
−0.0135
a
Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.1 KPa, and u(x) = 0.0001, and the relative expanded uncertainties for the excess viscosities
Ur(Δη) = 0.02 (0.95 confidence level).
Figure 5. Excess molar volume VE vs mole fraction x1 of
[Emim][Al2Cl7] for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary
Figure 6. Excess viscosity Δη vs mole fraction x1 of [Emim][Al2Cl7]
for [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) binary mixtures at
■
●
▲
▼
)
different temperatures T = ( ) 293.15, ( ) 303.15, (◀) 313.15, (
■
●
▼
▲
)
mixtures at different temperatures T = ( ) 293.15, ( ) 303.15, (
⧫
⧫
323.15, (▶) 333.15, ( ) 343.15, and ( ) 353.15 K. The solid curves
are calculated with the Redlich−Kister equation, and the symbols
represent experimental values.
313.15, ( ) 323.15, (▶) 333.15, (◀) 343.15, and ( ) 353.15 K. The
solid curves are calculated with the Redlich−Kister equation, and the
symbols represent experimental values.
⎛
⎝
⎞
⎠
x1M1 + x2M2
x1M1
x2M2
ρ2
VE =
−
+
⎜
⎜
⎟
⎟
ρ
ρ
(4)
(5)
1
On the basis of the experimental densities and viscosities
described above, the values of VE and Δη have been calculated
by the expression
Δη = η − (x1η1 + x2η2)
F
DOI: 10.1021/acs.jced.7b00702
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Table 10. Coefficients of the Redlich−Kister Equation A0, A1, A2, A3, and Standard Deviation σ for Excess Molar Volumes VE
and Excess Viscosities Δη of [Emim][Al2Cl7] (1) + [Emim][AlCl4] (2) Binary Mixtures from 293.15 to 353.15 K
property
T/K
A0
A1
A2
A3
σ
VE/cm3 mol−1
293.15
303.15
313.15
323.15
333.15
343.15
353.15
293.15
303.15
313.15
323.15
333.15
343.15
353.15
0.5793
0.6060
−0.8581
−0.8754
−0.9122
−0.9735
−1.0245
−1.0794
−1.1569
0.6457
0.3304
0.3630
0.3990
0.4638
0.5167
0.5874
0.6538
−0.2893
−0.1318
0.0597
−0.0178
0.0462
0.0204
0.0587
−0.0446
−0.0406
−0.0320
0.0402
0.0030
0.0028
0.0034
0.0033
0.0034
0.0040
0.0050
0.0059
0.0023
0.0024
0.0017
0.0006
0.0003
0.0004
0.6319
0.6597
0.6957
0.0820
0.7419
0.1273
0.7938
0.2003
Δη/mPa s
−1.5837
−1.0645
−0.7189
−0.4502
−0.3310
−0.2412
−0.1871
−0.2065
−0.0918
−0.0380
0.0567
0.3488
0.1708
0.0737
0.0556
−0.0082
−0.0486
−0.0537
0.0584
0.0487
in which ρ and η are the density and viscosity of binary
mixtures, respectively, and x1 and x2, M1 and M2, ρ1 and ρ2, and
η1 and η2 are the mole fractions, molar masses, densities, and
viscosities of [Emim][Al2Cl7] and [Emim][AlCl4], respec-
tively.30,31
hydrogen bonding, resulting in fewer negative values of excess
viscosity.33
In brief, the excess molar volumes and excess viscosities of
[Emim][Al2Cl7] and [Emim][AlCl4] binary mixtures were well
fitted by the Redlich−Kister polynomial equation. The
experimental data have been analyzed according to the theory
of the liquid binary mixtures, which shows that the strength of
hydrogen bonding and interaction among ions becomes weaker
after the formation of the mixture.
All the experimental values of VE and Δη were fitted on the
basis of the Redlich−Kister polynomial equation
k
Y = x (1 − x )
A (2x − 1)i
∑
1
1
i
1
(6)
i=0
4. CONCLUSIONS
where Y represents VE or Δη, Ai is an adjustable parameter, and
x1 is the mole fraction of [Emim][Al2Cl7] in binary mixtures.
All the data of VE and Δη are listed in Tables 8 and 9. The plots
of VE and Δη vs mole fraction of [Emim][Al2Cl7] are shown in
Figures 5 and 6, respectively.32
In this study, Lewis acidic [Emim]Cl/AlCl3 ILs with different
mole fractions were prepared by mixing precise molar
quantities of [Emim][Al2Cl7] with [Emim][AlCl4]. The
densities and viscosities of binary mixtures were measured
from 293.15 to 353.15 K. On this basis, the excess molar
volumes and excess viscosities of binary mixtures were
calculated. All the experimental data were well fitted by the
empirical equations. For the relationship between molecular
structure and properties of ILs to be investigated, the optimized
geometry and parameters of [Emim][Al2Cl7] and [Emim]-
[AlCl4] ion pairs were obtained according to the computational
method, which shows that the enhanced molar concentration of
[Al2Cl7]− and higher molecular symmetry could increase the
density of binary mixtures. Meanwhile, the interionic
interaction, especially hydrogen bonding, has a significant
influence on the viscosity of ILs. Looser packing and/or weaker
interionic interaction probably occurs after [Emim][Al2Cl7] is
mixed with [Emim][AlCl4]. It is hoped that this work may
provide important experimental data and conclusions for future
research and application of Lewis acidic [Emim]Cl/AlCl3 ILs.
The standard deviation σ has been determined according to
the equation33,34
1/2
∑ (Y 2− Y )
⎡
⎢
⎢
⎤
⎥
⎥
cal
exp
σ(Y) =
n − p
⎢
⎣
⎥
⎦
(7)
in which n and p are the number of experimental data and
coefficients of eq 6, respectively. The values of parameters Ai
and standard deviations σ are listed in Table 10.
As shown in Figure 5, all the excess molar volumes for the
binary mixtures of [Emim][Al2Cl7] and [Emim][AlCl4] are
positive at the experimental temperatures and molar
compositions. The values of excess molar volume increase
with temperature increasing from 293.15 to 353.15 K, which
reaches the maximum value around mole fraction x1 = 0.2. It
reflects that the mixing process of [Emim][Al2Cl7] and
[Emim][AlCl4], as well as increase in temperature, could
both lead to less close-packing and/or weaker hydrogen
bonding and interionic interaction in [Emim]Cl/AlCl3 ILs.32,33
By contrast, the excess viscosities of binary mixtures are
negative under the studied experimental conditions, which also
increase as temperature increases. According to Figure 6, the
minimum value is obtained around mole fraction x1 = 0.4 for all
of the ILs. It can be concluded that the strength of hydrogen
bonding among ions becomes weaker after the formation of
binary mixtures.32 The increase in temperature weakens the
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.7b00702.
1
1H NMR spectrum of [Emim]Cl, H and 27Al NMR
spectra of [Emim][AlCl4] and [Emim][Al2Cl7], molar
volume, fractional deviations of experimental density,
viscosity, and excess molar volume and excess viscosity
from the values reported in the literature and calculated
by empirical equations (PDF)
G
DOI: 10.1021/acs.jced.7b00702
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Journal of Chemical & Engineering Data
Article
(15) Bollin, P. M.; Viamajala, S. Reactive extraction of triglycerides as
fatty acid methyl esters using Lewis acidic chloroaluminate ionic
liquids. Energy Fuels 2012, 26, 6411−6418.
(16) Bakkar, A.; Neubert, V. Electrodeposition and corrosion
characterisation of micro- and nano-crystalline aluminium from
AlCl3/1-ethyl-3-methylimidazolium chloride ionic liquid. Electrochim.
Acta 2013, 103, 211−218.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yzheng@ayit.edu.cn. Tel.: +86 372 2909705. Fax: +86
372 2592068.
■
ORCID
Yong Zheng: 0000-0002-6224-4073
Notes
The authors declare no competing financial interest.
(17) Yeung, K. S.; Qiu, Z.; Farkas, M. E.; Xue, Q.; Regueiro-Ren, A.;
Yang, Z.; Bender, J. A.; Good, A. C.; Kadow, J. F. An efficient one-pot
synthesis of 3-glyoxylic acids of electron-deficient substituted
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ACKNOWLEDGMENTS
■
(18) Veder, J. P. M.; Horne, M. D.; Ruther, T.; Bond, A. M.;
̈
This work was supported financially by the National Natural
Science Foundation of China (21406002, 51404230) and the
Science & Technology Development Program of Anyang. The
authors gratefully acknowledge Prof. Suojiang Zhang and Dr.
Kun Dong from Institute of Process Engineering, Chinese
Academy of Sciences for their guidance and assistance in the
computational calculations.
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