Investigation on physical and electrochemical properties of three imidazolium based ionic liquids (1-hexyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide and 1-butyl-3-methylimidazolium methylsulfate)

Article abstract of DOI:10.1016/j.molliq.2012.10.025

Three types of imidazolium based ionic liquids, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF₄]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][NTf₂]) and 1-butyl-3-methylimidazolium methylsulfate ([BMIM][MeSO₄]) were prepared and a variety of their fundamental properties such as kinematic (ν) and dynamic (η) viscosities, thermal stability, surface tension (σ), refractive index (n_D), pH and density (ρ) were investigated as a function of temperature. The coefficients of thermal expansion (α_p) of the pure liquids were also calculated from the experimental values of the density at different temperatures. Electrochemical studies of these pure fluids as media were also studied at screen printed glassy carbon electrode (SP-GCE). The measurements were performed on a single drop of ionic liquids at surface of a screen-printed three electrode cell. The results showed an ideal wide range of potential windows for studies of electrochemical behavior of some species such as hydrogen sulfide and thiols in the lipophilic and hydrophilic ionic liquids. These properties were studied for special purposes such as development of electroanalytical methods for trace determination of organosulfur compounds in petroleum and its products.

Full text of DOI:10.1016/j.molliq.2012.10.025

Journal of Molecular Liquids 177 (2013) 361–368
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Investigation on physical and electrochemical properties of three imidazolium
based ionic liquids (1-hexyl-3-methylimidazolium tetrafluoroborate,
1
1
-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide and
-butyl-3-methylimidazolium methylsulfate)
a,
b,
a,b
c
a
Ali Akbar Miran Beigi ⁎, Majid Abdouss ⁎, Maryam Yousefi , Seied Mahdi Pourmortazavi , Amir Vahid
a
Research Institute of Petroleum Industry, West Blvd. of Azadi Sport Complex, Tehran, Iran
Department of Chemistry, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran
Faculty of Material and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, Iran
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 August 2012
Received in revised form 12 October 2012
Accepted 18 October 2012
Available online 2 November 2012
Three types of imidazolium based ionic liquids, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]
BF ]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][NTf ]) and 1-butyl-3-
methylimidazolium methylsulfate ([BMIM][MeSO ]) were prepared and a variety of their fundamental prop-
erties such as kinematic (ν) and dynamic (η) viscosities, thermal stability, surface tension (σ ), refractive
index (n ), pH and density (ρ) were investigated as a function of temperature. The coefficients of thermal ex-
pansion (α ) of the pure liquids were also calculated from the experimental values of the density at different
[
4
2
4
D
p
Keywords:
temperatures. Electrochemical studies of these pure fluids as media were also studied at screen printed glassy
carbon electrode (SP-GCE). The measurements were performed on a single drop of ionic liquids at surface of a
screen-printed three electrode cell. The results showed an ideal wide range of potential windows for studies
of electrochemical behavior of some species such as hydrogen sulfide and thiols in the lipophilic and hydro-
philic ionic liquids. These properties were studied for special purposes such as development of electroanalyt-
ical methods for trace determination of organosulfur compounds in petroleum and its products.
Pure fluids
Room temperature ionic liquids
Physical properties
Temperature dependence
Scavengers
Hydrogen sulfide
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
liquids and also disagreement in reports were of major reasons for
present work. The aim of the present work is to study the physical and
electrochemical properties of three imidazolium based ionic liquids.
Eco-friendly imidazolium-based ionic liquids studied in this research
have been used as a catalyst and as a reaction medium in irreversible re-
Ionic liquids (ILs) are molten salts [1] composed entirely of ions, and
many of them are liquids at room temperature [2–8]. Room temperature
ionic liquids (RTILs), often referred to as ‘designer solvents’, have been
the great focus of scientists in various fields since they can be tuned for
specific applications [9–13]. Nowadays, the most commonly studied ILs
normally contain, imidazolium, ammonium, phosphonium, pyridinium,
and pyrrolidinium cations, and tetrafluoroborate, hexafluorophosphate,
bistrifluorosulfonylimide and triflate anions [3,14]. The physicochemical
properties of ILs can be finely tuned by slight structural changes of the
corresponding cations and anions [15–17]. To better understand the na-
ture of ionic liquids and rationally expand their applications especially as
pollutant scavenger and electrolyte, knowledge of their thermophysical
and electrochemical properties is required. The design of industrial pro-
cesses and new products based on ILs can only be achieved when their
thermophysical properties, such as viscosity, density, surface tension,
refractive index, and thermal decomposition, are adequately character-
ized [18,19]. The lack of some properties reported for the studied ionic
2
action of H S to form a thermally stable, oil-soluble alkyl sulfide. They
can be applied at a wide range of temperatures, from ambient up to
350 F (177 °C) [15,20]. The investigated ionic liquids included water
and oil-soluble scavengers which were characterized by 1H NMR and
IR spectra. Oil-soluble IL scavengers were used when water tolerance
of the crude petroleum is an issue. The results show that the
oil-soluble ILs were ideally used for viscous heavy oils and residuals.
Thus, in this paper we wish to report the results of our studies on the
physical, electrochemical, thermodynamic and transport properties of
1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM][BF
3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [EMIM][NTf
and 1-butyl-3-methylimidazolium methylsulfate, [BMIM][MeSO ] which
4
], 1-ethyl-
2
],
4
are historically among the most important and commonly used ILs
[21–23]. Molecular structures of the three ILs are shown in Fig. 1.
The properties of these three ILs, accurately measured at atmo-
spheric pressure and temperature from 10 to 90 °C, include density,
viscosity, thermal stability, surface tension, refractive index and pH.
The electrochemical stability of the ionic liquids, as electrolytes for
⁎
Corresponding authors.
E-mail addresses: miranbeigiaa@ripi.ir (A.A. Miran Beigi),
majidabdouss@yahoo.com (M. Abdouss).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.10.025

362
A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
1
-Hexyl-3-Methylimidazolium
Tetrafuoroborate
HMIM][BF4]
1-Buthyl-3-Methylimidazolium
Methylsulfate
1-Ethyl-3-Methylimidazolium
bis(trifluoromethylsulfonyl)imide
[EMIM][NTf2]
[
[BMIM][MeSO4]
C10H19BF N2
C H N O S
C H F N O S
2
4
9
18
2
4
8
11
6
3
4
M.W. = 254.08
M.W. = 250.32
M.W. = 391.32
IL1
IL3
IL2
Hydrophilic Property
Lipophilic Property
Fig. 1. Chemical structures of the ionic liquids.
voltammetric purposes, was also studied at screen printed glassy
carbon electrode (SP-GCE).
another hour. Finally the samples were kept at 105 °C for further 2 h.
After cooling, the samples were kept in glass vials, closed with screw
caps to ensure a secure seal and prevent humidity. The samples were
taken from the vial with a syringe through a silicone septum and imme-
diately were put into the apparatus, for further measurements. The
samples were also used for characterization of the three tested ILs as
shown in Fig. S1 (supporting information section).
2
. Experimental section
2
.1. Preparation of ionic liquids
All reagents and solvents were A.R. grade products of Merck
(Germany), unless otherwise indicated. High purity helium and nitrogen
2.2. Apparatus and procedure
gases were obtained from Roham Gas Co. (Tehran, Iran). Water used
was freshly deionized and distilled before use. The chemicals were
used without further purification. The ILs were prepared from the corre-
sponding chlorides according to the procedures reported in literature
The density (ρ) and dynamic viscosity (η) of the ILs were mea-
sured with an automated Anton Paar SVM-3000 digital double-tube
visco-densimeter. The precision in experimental measurements has
−
4
−3
[24–27]. Generally, 1-alkyl-3-methylimidazolium tetrafluoroborates are
been found to be better than ± 2×10
g cm
for the density and
−
4
synthesized by ion-exchange reaction of 1-alkyl-3-methylimidazolium
halide and sodium tetrafluoroborate. Another route to their syn-
thesis is the reaction of fluoroboric acid directly with 1-alkyl-3-
methylimidazolium halide. This reaction also forms a gaseous HX
by-product, instead of AgX, which is removed easily and high puri-
ty ILs will be obtained at high yields. This reaction also reduces the
cost because fluoroboric acid is cheaper than tetrafluoroborate salts.
Similarly, the synthesis of 1-butyl-3-methylimidazolium methylsulfate
occurred in two steps. First, the imidazole was deprotonized using
sodium ethoxide and then alkylated with the hydrocarbon chain from
chlorobutane to make 1-butylimidazole. Second, dimethlysulfate was
added to the 1-butylimidazole, which reacted exothermically to create
± 1×10
mPa s for the dynamic viscosity. The instrument was
calibrated and certified by the supplier company. The calibration
was checked periodically with pure liquids (supplied by Cannon
Co.) with known density, dynamic and kinematic viscosity at several
temperatures.
The surface tension of the ILs was measured with a KRUSS-K9 ten-
siometer (Germany) by the ring method. The measurement cell was
thermostated in a temperature controller with a temperature stability
of ± 0.02 °C, regulated in a RC6 LAUDA thermostat. The equipment
has a digital control unit for precise measurements with an uncertain-
−
1
ty better than ± 0.1 mN m . The equipment was calibrated and
certified by the supplier. However, the calibration was checked peri-
odically with pure liquids of known surface tension.
1
-butyl-3-methylimidazolium methylsulfate.
A general procedure for preparation of the ionic liquid [EMIM][NTf
2
]
Electrochemical measurements were carried out by screen printed
voltammetry 910 P stat mini, supplied by Metrohm Co. (Switzerland)
using three electrode cell consisted of a glassy carbon working elec-
trode and Ag plate as the reference electrode. A graphic plate was
also used as counter electrode.
was also a reaction between 1-methylimidazole and 1-bromoethane in
toluene at 110 °C to form 1-ethyl-3-methylidinium bromide
(
[EMIM][Br]). The reaction followed by ion exchange with lithium
bis(trifluoromethylsulfonyl) imide in water. The corresponding two
step preparation is shown in Scheme 1. All procedures were prescribed
in detail in a previous work [28]. Before use, the prepared ILs were treat-
ed by heating in a vacuum oven at ≈−0.1 MPa, via a three-step pro-
cess, as follows. Firstly, the samples were dried at 50 °C for 1 h, then
the temperature was raised to 70 °C, and the samples were kept for
A simultaneous TG/DTA thermal analyzer (Perkin Elmer PYRIS
Diamond) was used to study thermal stability of the ionic liquids.
Approximately 20 mg of the sample and reference (Pt foil) was
−
1
placed in alumina pans and heated by a rate of 10 K min
from 25
to 600 °C. In this study, the flow rate of nitrogen as purge gas was
Scheme 1. Two-step preparation of ionic liquids.

A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
363
0 mL min− at 1 bar. TG mass and temperature calibrations were
1
viscosity (η), and surface tension (σ) values were fitted by the meth-
5
performed prior to the experiments [29].
od of least squares using the following equations [30]:
Refractive indices were determined by using a refractometer
Model J357 supplied by Rudolph Company. The apparatus was cali-
brated with iso-propyl alcohol and water at different temperatures
from 10 to 90 °C. A 851 Titrando, coulometric Karl Fischer apparatus
supplied by Metrohm (Switzerland) was used for the determination
of trace amounts of water in ILs. The coulometric cell was consisted
of a two-compartment cell separated by a ceramic diaphragm.
Pyridine-free catholyte and anolyte reagents were synthesized as pre-
scribed in our previous work [28] and were used in all measurements.
○
○
2
z ¼ A þ A t= C þ A ðt= CÞ
ð1Þ
0
1
2
and
○
ln ηðmPa sÞ ¼ A =ðt= CÞ−A
ð2Þ
0
1
where z is ρ/(g cm−3), n
and A , A and A are the adjustable parameters. The corresponding
or σ/(mN m−1), T/°C is the temperature
D
0
1
2
−
3
Aqueous solutions with mass concentration of 0.01 g cm
of the
correlation parameters are listed in Table 2, together with their stan-
dard deviations (S.D.). The S.D. values were calculated by applying the
following expression:
ILs were prepared and used for pH measurements. The pH and chlo-
ride content measurements were obtained using a CH 9101 Herisau,
Ion Analysis supplied by Metrohm (Switzerland). The instrument
was calibrated with different pH buffers and sodium chloride as a pri-
mary standard.
ꢂ
ꢃ
ꢀ
ꢁ
1=2
2
S:D: ¼ ∑ zexp−zcalc =n
ð3Þ
3
. Results and discussion
where, z and n are the property values and the number of experimen-
tal points, respectively.
3
.1. Physicochemical properties of ILs
A comparison between the experimental and literature data for the
physical and electrochemical properties of the studied ILs, at two tem-
peratures (i.e., 20 and 25 °C), is also made in Table 3. Experimental
Due to needs for experimental data for temperature dependence
of physicochemical properties of [HMIM][BF
4
], [EMIM][NTf
2
] and
4 2 4
data for [HMIM][BF ], [EMIM][NTf ] and [BMIM][MeSO ] are in good
[
BMIM][MeSO ], their density, refractive index, dynamic and kine-
4
agreement with available literature values.
matic viscosities, pH, and surface tension were measured over a
wide temperature range of 10 to 90 °C. The experimental values are
3.2. Density and thermal expansion
D
listed in Table 1. The density (ρ), refractive index (n ), dynamic
Data on density for the ILs at temperatures ranging from 10 to
0 °C are shown in Fig. 2. As seen, the densities of [HMIM][BF ] and
[EMIM][NTf ], in entire temperature range studied, and those of
BMIM][MeSO ], at temperatures lower than 55 °C are larger than
corresponding water densities. It is interesting to note that the densi-
ty of ILs studied decrease nearly linear with increasing temperature,
but at a rate less than that for molecular organic solvents. Density
data for some of studied ILs are already available in the open litera-
ture, and the relative deviations between the experimental data
obtained in this work are also presented in Fig. 2 (A, B and C) and
Table S1 (supporting information section).
9
4
Table 1
Density (ρ), dynamic viscosity (η), refractive index (n
expansion (α ), and pH of the ionic liquids at different temperatures.
D
), surface tension (σ), thermal
2
p
[
4
ρ/(g mL−1)
σ/(mN m−1
104
T/°C
η/(mPa s)
n
D
)
α
p
/K pH of 1%
solution
4
[HMIM][BF ]
1
2
2
3
4
5
6
7
8
9
9
0
0
5
0
0
0
0
0
0
0
5
1.1562
1.1492
1.1461
1.1425
1.1355
1.1280
1.1214
1.1122
1.1004
1.0874
1.0838
608.1
311.7
220.0
167.3
103.3
63.77
52.22
28.48
21.39
15.87
13.92
1.4265 41.4
1.4241 40.6
1.4223 40.4
1.4211 40.0
1.4183 39.8
1.4158 39.0
1.4137 38.2
1.4106 37.2
1.4080 36.6
1.4051 36.3
1.4038 35.9
5.343
5.525
5.623
5.718
5.909
6.104
6.302
6.504
6.709
6.918
7.025
6.41
6.25
5.77
5.12
The density data for [HMIM][BF
available literature values, the relative deviation values are ranging
], the deviations
4
] are in good agreement with
from 0.06 to −0.33% [31,32], and for [EMIM][NTf
2
4.49
4.17
from our data and the literature are ranging from 0.13 to 0.17%
Table 2
2
[EMIM][NTf ]
Fitting parameters of Eqs. (1) and (2) and standard deviations in Eq. (3) used for cor-
relation of the physical properties of ILs studied.
1
2
2
3
4
5
6
7
8
9
9
0
0
5
0
0
0
0
0
0
0
5
1.5311
1.5220
1.5168
1.5117
1.5020
1.4907
1.4780
1.4651
1.4494
1.4292
1.4230
55.92
37.27
31.13
26.28
19.67
14.96
11.84
9.506
7.820
6.539
6.054
1.4254 40.2
1.4232 39.8
1.4220 39.4
1.4206 39.0
1.4179 38.8
1.4153 38.6
1.4127 38.0
1.4101 37.2
1.4072 36.6
1.4045 36.0
1.4033 35.5
4.706
5.518
5.929
6.344
7.186
8.045
8.925
9.828
10.76
11.72
12.21
6.48
6.5
R2
Physical property
[HMIM][BF
ρ/(g mL−1)
A
0
A
1
A
2
S.D.
6.54
6.60
4
]
−7
1.1627
1.4290
−0.0006
−0.0003
−9.0×10−
−6.0×10
0.9997
0.9993
0.9948
0.9943
0.9906
0.0010
0.0008
0.0238
0.0240
0.0285
8
n
D
η/(mPa s)
−4.2624
−4.2067
41.981
1976.3
1942.1
−0.0611
2
−1
6.66
6.69
ν/(mm
s
)
σ/(mN m−1)
−4.0×10−5
[
EMIM][NTf
2
]
−
−6
[
BMIM][MeSO
4
]
ρ /(g mL 1)
1.5366
1.4284
−2.4101
−2.4606
40.406
−0.0006
−0.0003
1165.7
1126.2
−0.0268
−6.0×10
0.9991
0.9999
0.9962
0.9951
0.9903
0.0010
0.0011
0.0064
0.0065
0.1485
−6.0×10−
8
1
2
2
3
4
5
6
7
8
9
9
0
0
5
0
0
0
0
0
0
0
5
1.2117
220.6
122.3
93.78
73.35
46.85
32.21
23.02
17.05
13.14
10.34
9.257
1.4831 46.0
1.4777 44.4
1.4771 43.7
1.4765 43.4
1.4745 42.6
1.4719 42.0
1.4694 41.2
1.4673 39.8
1.4650 39.0
1.4626 38.1
1.4615 37.6
4.457
4.810
4.988
5.167
5.530
5.899
6.274
6.656
7.0453
7.443
7.645
n
D
1.2053
1.2019
1.1983
1.1923
1.1853
1.1774
1.1668
1.1538
1.1400
1.1356
7.60
7.55
η/(mPa s)
2
−1
ν/(mm
s
)
σ/(mN m−1)
−0.0003
7.39
7.18
[BMIM][MeSO
4
]
ρ/(g mL−
1
)
1.2167
1.4813
−3.5880
−3.5536
45.579
−0.0005
−0.0002
1660.3
1625.9
−0.063
2.0×10−6
0.9992
0.9979
0.9938
0.9928
0.9946
0.0036
0.0011
0.0086
0.0075
0.179
−5.0×10−
7
n
D
6.96
6.85
η/(mPa s)
2
−1
ν/(mm
s
)
σ/(mN m−1)
−0.0002

364
A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
Table 3
D d
Experimental and literature values of densities (ρ), refractive indices (n ), dynamic viscosities (η), surface tensions (σ), electrochemical windows (ΔE), thermal decomposition (t )
and pH of the ionic liquids at 20.0 and 25.0 °C.
Property
[HMIM][BF
4
]
[EMIM][NTf
2
]
[BMIM][MeSO
Present work
4
]
Present work
Literature
20.0 °C
Present work
Literature
20.0 °C
Literature
20.0 °C
20.0 °C
25.0 °C
1.1461
25.0 °C
20.0 °C
1.5220
25.0 °C
25.0 °C
20.0 °C
1.2053
25.0 °C
1.2019
25.0 °C
ρ/(g mL−1)
1.1492
1.1531a
1.1453c
1.5168
1.5240f
1.5233g
1.5190f
1.5132g
1.21295
1.2018
h
1.20956h
.1485b
1.1493
d
i
1
e
1
.1481
1.4292j
1.4237
k
1.4232
1.4220
1.4235l
1.4225l
1.4777
1.4771
1.48076m
1.47942m
n
D
1.4241
1.4223
n
o
1.4793
1.4792
n
1
.4782
q
1.591r
36.94
s
Log η/(mPa s)
2.494
40.6
2.342
40.4
2.497p
38.64u
2.447
36.8
1.571
39.8
1.493
39.4
1.5051
2.14240
44.4
2.03574
43.7
2.461t
2.329
t
σ/(mN m−
1
)
v
x
41.62
y
z(1)
z(1)
44.1
–
43.3
w
35.71z
3
9.2
ΔE/V
4.56
310
–
5.38
390
4.5Z(2)
5.66
317
z(3)
4
.78
z(4)
z(6)
z(7)
t
d
/°C
283
2
455
440
–
–
39z(5)
z(8)
pH 1% aqueous soln.
6.41
6.25
6.61
6.48
6.5
7.60
7.55
–
a
Ref. [31], bRef. [32], Ref. [1], Ref. [31], Ref. [52], Ref. [33], Ref. [35], Ref. [36], Ref. [37], Ref. [53], Ref. [54], Ref. [42], Ref. [17], Ref. [44], oRef. [43], pRef. [31], Ref. [55], Ref.
c
d
e
f
g
h
i
j
k
l
m
n
q
r
s
t
u
v
w
x
y
z
z(1)
z(2)
z(3)
z(4)
Ref. [56], z(5)Ref. [1],
z(6)
z(7)
z(8)
[
[
38], Ref. [56], Ref. [17], Ref. [53], Ref. [1], Ref. [57], Ref. [58], Ref. [59], Ref. [60],
65].
Ref. [17],
Ref. [61],
Ref. [62],
Ref. [63],
Ref. [64],
Ref.
[
33–35], and for [BMIM][MeSO
to −0.6% [36,37].
The coefficients of thermal expansion for [HMIM][BF
4
NTf ] and [BMIM][MeSO ] are defined by the following equation:
4
], the deviations are ranging from 0.2
A
1.2
.18
.16
.14
.12
1
1
1
1
4
], [EMIM]
[
2
ꢄ
ꢀ
ꢁ
ꢀ
ꢁ
À
Á
−
3
−3
ꢀ
α ¼ −1=ρ g:cm
∂ρ g:cm
=∂ t= CÞ p
ð5Þ
p
ꢀ
ꢀ
ꢀ
ꢁ
À
Á
ꢀ
ꢀ
ꢀ
2
¼
− A þ 2A t= CÞ =A þ A t= CÞ þ A t= CÞ
;
1
2
0
1
2
1.1
where, α
p
is the coefficient of thermal expansion, ρ is the IL density, and
are the adjustable parameters calculated in Eq. (1). The
thermal expansion coefficients for each of ILs at several temperatures
1.08
0
20
40
60
80
100
0 1 2
A , A and A
o
T/ C
are summarized in Table 1, and temperature dependence of the α
values is illustrated in Fig. S2 (supporting information section). The α
p
B
1.58
p
−
4
−1
values at 25 °C are 5.623, 5.929, and 4.988×10
°C
for [HMIM]
1.54
[
BF ], [EMIM][NTf ] and [BMIM][MeSO ], respectively. The results
4
2
4
show that the variation of the volume expansivity quantified by the
thermal expansion coefficient could be considered as independent of
the temperature for these ILs.
1
.5
1
1
1
.46
.42
.38
3.3. Viscosity
Data on viscosity for the ILs at temperatures ranging from 10 to
0
20
40
60
80
100
90 °C are also shown in Fig. 3. Unfortunately, the high viscosity of
ILs will negatively affect most of their operational processes [34].
Since the viscosities of ILs are governed essentially by van der
Waals interactions and H-bonding and, thus, an increased alkyl
chain length and presence of bulkier side chains, which can reduce
the rotational freedom of molecules, or use of organofluoroanions
makes the salt more viscous, as can be seen from Table 1. On the
other hand, it is obvious that the ionic liquids with larger cations
o
T/ C
C
1.3
.25
1
1
1
1
.2
.15
.1
+
+
+
(
[HMIM] >[BMIM] >[EMIM] ) show higher dynamic and kine-
1
matic viscosities.
As is obvious from Fig. 3, the viscosity of ILs will decrease more or
less exponentially with increasing temperature. The empirical Eq. (2)
is thus applicable to describe the temperature dependence of the vis-
cosity of ionic liquid systems. On the other hand, the presence of a
more planar structure is known to result in less viscosity because
the planarity allows relatively facile slip between molecules.
Muhammad et al. [1] reported temperature dependence of viscos-
.05
1
0
20
40
60
80
100
o
T/ C
Fig. 2. Comparison of density data for [HMIM][BF
31]; ( ) Taguchi et al. [32], for [EMIM][NTf ] (B): ( ) this work; ( ) Tokuda et al. [33];
] (C): ( ) this work; ( ) Fernandez et al. [36];
4
] (A): ( ) this work; ( ) Seddon et al.
[
(
(
2
4
ity and density for dried [HMIM][BF ] with its chloride and water
) Yao et al. [35], for [BMIM][MeSO
) Sanchez et al. [37].
4
impurities. Some large deviations were due essentially to the salt

A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
365
A
et al. [33]. The deviations can be due essentially to the salt's impuri-
ties including water and halide content and also to the experimental
technique adopted.
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
3.4. Surface tension
Available data on the surface tension of the studied ILs are very
limited. In general, the liquid/air surface tension values for ILs are
somewhat higher than conventional solvents such as hexane
0
20
40
60
80
100
−
1
−1
o
(18 mN m ), but not as high as water (73 mN m ). The tempera-
Temperature ( C)
ture dependence of surface tension (σ) for the ionic liquids studied is
B
C
shown in Fig. 4. As seen, for imidazolium derivatives [HMIM][BF
4
],
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
[
EMIM][NTf ] and [BMIM][MeSO ], the surface tension decreases
2
4
with increasing lipophilic property of the ILs. It is necessary to say
that only two reports were found in the survey on the literature for
pure [HMIM][BF
.5. Refractive index and pH
The refractive index of a substance describes its ability to refract
4
].
3
0
20
40
60
80
100
o
Temperature ( C)
light as it moves from one medium to another and, therefore, the
higher the refractive index of a compound, the more the light
refracted. For most practical purposes, a high refractive index is con-
sidered to be a value greater than 1.4 (i.e., above the range of most
common organic materials).
As an application, in order to non-destructively locate inclusions in
unpolished diamonds and other minerals by optical means, it is desir-
able to immerse these minerals in a fluid with the same refractive
index [40]. Unfortunately, however, many available high refractive
2
2
1
1
50
00
50
00
5
0
0
3 4
index immersion compounds, such as AsI (RI=2.2) or SnI (RI=2.1),
0
20
40
60
80
100
are solid at room temperature, poisonous, unstable, and extremely un-
pleasant. Diiodomethane saturated with sulfur is another commercially
available high refractive index (RI=1.78) room temperature liquid, but
this is also a harmful material [41]. Since the physical properties of ionic
liquids (as green alternative compounds) can be tuned by varying their
cationic and/or anionic parts, it is possible to develop immersion fluids
for mineralogical studies which are relatively benign.
Fig. 5 shows temperature dependence of refractive index for the
studied ILs as a set of fluids which have been designed to have refrac-
tive indices>1.4, and also possess low environmental impact and
good shelf life. As can be seen from Fig. 5, for all three ILs, the
refracted index linearly decreases with increasing temperature. It is
noteworthy that, in all ranges of temperatures, the ILs used possess
refractive indices>1.4. In survey on the literature, only one literature
o
Temperature ( C)
Fig. 3. Temperature dependence of kinematic viscosity (ν, ) and dynamic viscosity
η, ) for [HMIM][BF ] (A), [EMIM][NTf ] (B) and [BMIM][MeSO ] (C). Comparison of
viscosities as a function of temperature (K) for [HMIM][BF ] (A) ( ) this work;
] (B): ( ) this work; ( ) Tokuda et al. [38];
] (C): ( ) this work; ( ) Pereiro et al. [43];
(
4
2
4
4
(
(
(
) Seddon et al. [31], for [EMIM][NTf
2
) Camper et al. [39], for [BMIM][MeSO
) Fernandez, A. et al. [36].
4
impurities especially halides. We have reported some of impurities
available in our studied RTILs in Table 4, and their characterization
in the supporting information section. However, excellent agree-
ments were observed for [HMIM][BF
38,39] when comparing experimental viscosities with available liter-
ature values. Fig. 3 (A, B and C) shows comparison of viscosity as
function of temperature. [HMIM][BF ] viscosity values were lower
within 0.1%) than those of Seddon et al. [31]. Positive deviations
ranging from 0.12 to 1.2%) for dynamic viscosity were also observed
4 2
] [32] and [EMIM][NTf ]
[
was found for comparing refractive index data for [EMIM][NTf
in a wide range of temperature. Fig. 5 also shows comparison of ex-
perimental refractive indices with the literature for [EMIM][NTf
and [BMIM][MeSO ]. The refractive index data obtained were slightly
lower (within 0.02–0.03%) than Tariq et al. [42] for [EMIM][NTf ] and
2
] [42]
4
2
]
(
(
4
2
for [EMIM][NTf
2
] when compared with Seddon et al. [31] and Tokuda
lower (within 0.07–0.2%) than Pereiro et al. [43] and Singh et al. [44].
Table 4
48
46
Impurity contents of the studied ILs.
4
4
4
2
Species Test method
Results
[
[
HMIM]
BF
[EMIM]
[NTf
[BMIM]
4
[MeSO ]
40
4
]
2
]
38
a
36
Water content , mass%
Total chlorine, mass%
Chloride, ppm
Bromide, ppm
Sulfate, ppm
Karl Fischer
0.017
b0.25
b10
b10
b5.0
0.026
b0.25
b10
b10
b5.0
0.194
b0.25
b10
b10
b5.0
34
Flask combustion
Potentiometry
Potentiometry
Turbidimetry
32
30
0
20
40
60
80
100
120
140
Heavy metals (as Pb), ppm Turbidimetry
b0.2
b0.001
b0.2
b0.001
b0.2
b0.001
o
T/ C
Ash content, mass%
Electric furnace
and gravimetry
Fig. 4. Temperature dependence of surface tension (σ) for [HMIM][BF
( ) Ghatee, M.H. et al. [55], for [EMIM][NTf
) this work; ( ) Pereiro et al. [43].
4
]: ( ) this work;
a
The water content results are obtained after drying treatment described in the
Experimental section.
2 4
]: ( ) this work, for [BMIM][MeSO ]:
(

366
A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
1
1
1
1
1
1
1
1
1
.49
.48
.47
.46
.45
.44
.43
.42
.41
Table 5
Summary of TG/DTG/DTA results for investigated ionic liquids.
Ionic liquid
Maximum DTA
peak T/°C
Decomposition
T-Range/°C
Mass
loss/Δm%
[
[
[
HMIM][BF
EMIM][NTf
BMIM][MeSO ]
4
4
]
452
418
383
310–500
390–510
317–438
98
94
92
2
]
Therefore, TG/DTG curves confirm DTA data that shows main thermal
decomposition of the ionic liquid sample happened in this tempera-
ture range.
The DTA and TG/DTG curves of [EMIM][NTf
this ionic liquid is thermally stable up to about 380 °C. However, at
higher temperatures, TG/DTG curves for [EMIM][NTf ] present a sig-
nificant mass loss step between 390 and 510 °C, with Δm=94%.
DTA curve exhibits an exothermal event in this temperature range
and minimum DTA peak temperature for this ionic liquid was ob-
served at 418 °C. Over 510 °C, the TG/DTG/DTA curves indicate no
considerable thermal event for this sample.
1.4
0
20
40
60
80
100
2
] (Fig. S4b) show that
o
T/ C
2
Fig. 5. Plots of temperature dependence of experimental values of refractive indices
) of [HMIM][BF ] ( ) this work (synthesized sample); (○) this work (Aldrich
sample); [EMIM][NTf ] ( ) this work; ( ) Tariq et al. [42], [BMIM][MeSO ] ( ) this
work; ( ) Pereiro et al. [43]; ( ) Singh et al. [44].
(n
D
4
2
4
Owing to the fact that the presence of impurities can significantly
affect some properties of the ILs such as refractive index, pH and elec-
trochemical window, we measured several common impurities in the
prepared ILs and the results are summarized in Table 4. In the case of
pH, It is well documented that many chemical reactions are sensitive
to pH values, that is, the yield of products and even the nature of the
products may be altered appreciably if the pH changes significantly
during the course of reaction. This is especially true in biochemical re-
actions where the pH is important to the proper metabolism and
functioning of animals and plants.
Since ILs, as a new class of buffers, can maintain the apparent pH
for a non-aqueous system at an optimum value, it was useful to pro-
vide the pH values and temperature dependence of ILs used in this
experiment. The temperature effect on pH appears to be almost neg-
ligible as shown in Fig. S3 (supporting information). Although the pH
values provided by the literature (Table 3) are almost the same for all
the ionic liquids investigated, these values are misleading, as most pH
meters are only suitable for determining the proton concentrations of
the aqueous liquids, not those of novel liquids like ionic liquids. We
recommend to measure pH of defined solutions of ILs (i.e., 1% aque-
ous solution for hydrophilic and intermediate ILs), and saturated so-
lutions of the lipophilic ILs, or measurement by using a universal
non-aqueous glass electrode.
4
The DTA and TG/DTG curves for [BMIM][MeSO ] are shown in Fig.
S4c. As is obvious from the DTA curve, a main thermal decomposition
occurred above 310 °C, where the ionic liquid sample undergoes an
endothermic decomposition. This result agrees with the correspond-
ing TG/DTG curves for this sample. The mass loss (Δm=92%) occurs
in the temperature range of 310–440 °C. Such thermal pattern sug-
4
gests that [BMIM][MeSO ] is thermally stable until 300 °C. However,
a small mass loss was observed in the temperature about 250 °C;
which is due to the trace amounts of water remained in the ionic
liquid sample. The thermoanalytical data given in Table 5 for the
studied ionic liquids in this investigation shows the following thermal
stability order: [EMIM][NTf
2
]>[BMIM][MeSO
4 4
]≥[HMIM][BF ].
3.7. Electrochemical window
The electrochemical window is defined as the potential interval
observed between the reduction potential of the organic cationic
part and the oxidation potential of the anionic part of pure ILs. A
wide electrochemical window makes ILs promising electrolytes for
electrochemical power applications. Most ILs possess a wide potential
window of >4.0 V. It has been found that, for some ionic liquids, the
potential of anode limit proportionally decreases with increasing
highest occupied molecular orbit energy calculated for the anion
[48]. It has also been shown that the quantum chemical calculations
3
.6. Thermal analysis studies
are an excellent tool to predict the electrochemical window of ionic
Data on the thermal behavior of ionic liquids such as ([HMIM][BF
4
]),
(
[EMIM][NTf ]), and ([BMIM][MeSO ]) is required in order to obtain
2
4
thermal stability information for their use in chemical process, develop-
ment of contacting equipments and storage conditions [45,46]. Thermal
analysis has an important role in the characterization and determina-
tion of thermal stability of novel materials such as ionic liquids [47].
Therefore, thermal properties of the investigated ionic liquids have
been characterized by TG/DTG/DTA techniques. Thermograms of the
studied ILs, ([HMIM][BF
shown in Fig. S4 (supporting information section).
The TG/DTG and DTA curves of [HMIM][BF ] (Fig. S4a) showed no
4 2 4
]), ([EMIM][NTf ]), and ([BMIM][MeSO ]), are
4
thermal event before 300 °C, which confirms that this ionic liquid is
thermally stable in the temperature range of 25–300 °C. Above this
temperature DTA curve shows that the sample undergoes an
endothermal phenomenon during temperature range of about
3
10–500 °C; which corresponds to the thermal decomposition of
HMIM][BF ]. The maximum DTA peak temperature for thermal de-
composition of this ionic liquid was observed at 452 °C. Meanwhile,
the TG/DTG curves of [HMIM][BF ] show a significant mass loss
over this broad temperature range (310 to 500 °C), with Δm=98%.
[
4
Fig. 6. Cyclic voltammograms of the purified tested ionic liquids. Conditions: quasi-
4
reference electrode, Ag wire; working electrode, SP-GCE (O.D. 2.2 mm); temperature,
25 °C; scan rate, 100 mV s−
1
.

A.A. Miran Beigi et al. / Journal of Molecular Liquids 177 (2013) 361–368
367
database will definitely promote research and development of ionic liq-
uids. Thus, here, we carefully measured several important properties of
ionic liquids 1-hexyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-
3
3
-methylimidazolium bis(trifluoromethylsulfonyl) imide and 1-butyl-
-methylimidazolium methylsulfate in a wide range of temperature.
So, more attention should be paid on the measurement of physico-
chemical properties of ionic liquids. Also because of inherent advan-
tages of the ionic liquids, such as a scavenger for some environmental
pollutants (i.e. hydrogen sulfide), we considered to study some impor-
tant properties like thermal decomposition, thermal expansion, electro-
chemical window, solubility, pH and water content more than the
others. These properties will be used for objectives in our further
researches.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.molliq.2012.10.025.
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