Physical and electrochemical properties of ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

Article abstract of DOI:10.1016/j.molliq.2010.08.005

The ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide were prepared and several of their properties including viscosities, thermal stability, surface tension, refractive index, pH and density were investigated as a function of temperature. The coefficients of thermal expansion of the ionic liquids were also calculated from the experimental values of the density at various temperatures. The electrochemical stability of the ionic liquids, as electrolytes for voltammetric aspects, was studied at glassy carbon electrode. The results revealed a wide voltage range of the electrochemical window, which allowed studies on compounds such as furaldehydes.

Full text of DOI:10.1016/j.molliq.2010.08.005

Journal of Molecular Liquids 157 (2010) 43–50
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Physical and electrochemical properties of ionic liquids 1-ethyl-3-methylimidazolium
tetrafluoroborate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate and
1
-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide
a,
a,b
b
Mojtaba Shamsipur ⁎, Ali Akbar Miran Beigi , Mohammad Teymouri ,
a
a
Sayed Mahdi Pourmortazavi , Mohsen Irandoust
Department of Chemistry, Razi University, Kermanshah, Iran
a
b
Oil Refinery Research Division, Research Institute of Petroleum Industry, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 May 2010
Received in revised form 12 August 2010
Accepted 15 August 2010
Available online 24 August 2010
The ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium trifluoro-
methanesulfonate and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide were prepared and
several of their properties including viscosities, thermal stability, surface tension, refractive index, pH and
density were investigated as a function of temperature. The coefficients of thermal expansion of the ionic
liquids were also calculated from the experimental values of the density at various temperatures. The
electrochemical stability of the ionic liquids, as electrolytes for voltammetric aspects, was studied at glassy
carbon electrode. The results revealed a wide voltage range of the electrochemical window, which allowed
studies on compounds such as furaldehydes.
Keywords:
Ionic liquids
Electrolytes
Physical and electrochemical properties
Temperature dependence
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Room temperature ionic liquids (RTILs) are organic salts that are
In spite of the interesting feature and practical importance of ILs,
there are limited literature reports on the accurate measurements of
many of their fundamental physical and chemical properties at
various temperatures [21–24]. Thus, in this paper we wish to report
the results of our studies on the physical, electrochemical, thermo-
dynamic and transport properties of 1-ethyl-3-methylimidazolium
liquid at ambient temperature, possessing an appreciable liquid range [1].
The most commonly investigated ILs contain ammonium, pyridinium,
phosphonium, or imidazolium cations, with varying heteroatom func-
tionality [2–4]. Very interestingly, the physicochemical properties of ILs
can be finely tuned by slight structural changes of the corresponding
cations and anions [5,6]. Due to their unusual properties, the attention of
a growing number of engineers and scientists has been focused on
different applications of ILs, as shown by the increasing number of
published papers in recent years [7–10]. Most of the ILs are nonvolatile,
nonflammable and are dissolvable in many organic and inorganic
materials. They have high thermal stabilities, and can remain liquid in a
wide range of temperature (usually over 200 °C). More importantly, the
ILs show low toxicity and high conductivity together with significant
electrochemical stability, so that they could be used as “green”
alternatives to volatile organic solvents in electrochemical, synthetic
and separation processes [11]. Typical ILs consisted of organic cations
with delocalized charges and organic or inorganic fluoroanions [7–15].
Due to these unique features which are not found in usual materials, the
ILs have received an increasing attention for a wide variety applications
tetrafluoroborate, [EMIM][BF
fluoromethanesulfonate, [BMIM][Otf], and 1-butyl-1-methylpyrroli-
dinium bis(trifluoromethylsulfonyl)imide, [BMPyrr][NTf ]), which
4
], 1-butyl-3-methylimidazolium tri-
2
are historically among the most important and commonly used ILs
[1,2,6,7]. Molecular structures of the three ILs are shown in Fig. 1.
The properties of these three ILs, accurately measured at atmo-
spheric pressure and several temperatures include density, viscosity,
thermal stability, surface tension, refractive index and pH. The thermal
expansion coefficients of the ILs were then calculated from the
measured densities as a function of the temperature from 10 to 90 °C.
The electrochemical stability of the ionic liquids, as electrolytes for
voltammetric purposes, was also studied at glassy carbon electrode.
2. Experimental section
[16–20].
2.1. Preparation of ionic liquids
All reagents and solvents were A.R. grade products from Merck
(
Germany), unless otherwise indicated. High purity helium and
⁎
Corresponding author. Tel.: +98 21 66908032; fax: +98 21 66908030.
E-mail address: mshamsipur@yahoo.com (M. Shamsipur).
nitrogen gases were obtained from Roham Gas Co. (Tehran, Iran).
0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2010.08.005

44
M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
Fig. 1. Chemical structures of the ionic liquids.
Water used was freshly deionized and distilled before use. All chemicals
were used without further purification.
(supplied by Cannon Co.) of known density, dynamic and kinematic
viscosity at several temperatures.
The ILs were prepared from the corresponding chlorides according
to the procedures reported in literature [25–28]. Generally, 1-alkyl-3-
methylimidazolium tetrafluoroborates are synthesized by ion-ex-
change reaction of 1-alkyl-3-methylimidazolium halide and sodium
tetrafluoroborate. Another route to their synthesis is the direct
reaction of fluoroboric acid with 1-alkyl-3-methylimidazolium halide.
This reaction forms a gaseous HX by-product, instead of AgX, which
can be removed easily to obtain high purity ILs at high yields. This
reaction also reduces the cost because fluoroboric acid is cheaper than
tetrafluoroborate salts. Similarly, 1-butyl-3-methylimidazolium tri-
fluoromethanesulfonate was prepared from drop-wise addition of a
solution of trifluoromethanesulfonic acid to a cooled aqueous solution
of [BMIM][Cl]. After addition of acid, the mixture was stirred at
ambient temperature for 24 h. The IL was extracted from the aqueous
phase using dichloromethane (5×50 mL). The dichloromethane phase
was then washed with distilled water (10×100 mL) to remove any
chloride salt and acid. The aqueous layer used for washing the organic
phase was tested for residual acid using a pH indicator paper and
The surface tension of the ILs was measured with a KRUSS-K9
tensiometer (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
uncertainty better than ± 0.1 mN m . The equipment was calibrated
and certified by the supplier. However, the calibration was checked
periodically with pure liquids of known surface tension.
Electrochemical measurements were carried out with a minidouble-
wall three electrode cell (max. capacity of 1.0 mL) designed in Research
Institute of Petroleum Industry (RIPI, Iran). The detector consisted of an
EG&G Princeton Applied Research company (PARC) Model 303A
equipped with glassy carbon electrode (o.d. 2.2 mm), and controlled
by a PARC Model 384B Potentiostat. The surface of the electrode was
polished before each measurement. A platinum wire obtained from
PARC was used as a quasi-reference electrode. A platinum wire was also
used as the counter electrode. This system was used to determine the
electrochemical window of the ILs and the diffusion coefficient of
environmental pollutants such as 2-furaldehyde (furfural).
−
1
3
AgNO was used to test for residual chloride. The organic phase was
subsequently heated under vacuum at 120 °C overnight to remove
residual water and dichloromethane, to produce a yellowish IL.
A general procedure for preparation of the ionic liquid [BMPyrr]
A Santon, model TR-01 thermobalance with sensitivity of 0.1 mg was
used to study the thermal stability of the ILs. Approximately 4.0 mg of
sample and reference (Pt foil) were placed in alumina pans and heated
−
1
[
NTf
propanol, yielding 1,1-butylmethylpyrolidinium chloride ([BMPyrr]
Cl]), followed by ion exchange with lithium bis(trifluoromethylsul-
2
] was the reaction of 1-methylpyrrolidine and 1-chlorobutane in
by a rate of 10 °C min from 25 to 700 °C. In this study, the flow rate of
−
1
air as purge gas was 10 mL min
at 1 bar. TG mass and temperature
[
calibrations were performed prior to the experiments [29].
fonyl)imide in dichloromethane. The corresponding two steps
preparation is shown in Scheme 1. All procedures were prescribed
in detail in a previous work [28].
The DSC curves were obtained at a temperature range of 25 to 700 °C
using a Du Pont differential scanning calorimeter model DSC 910s
Before use, the prepared ILs were treated by heating in a vacuum
oven at ≈−0.1 MPa, via a three-step process, 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 another hour. Finally, the
samples were kept at 105 °C for 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 immediately put into the
apparatus for further measurements.
Cl-
N+
N
2
-propanol
(1)
+
Cl
2
.2. Apparatus and procedure
-
(
CF SO ) N
3
2 2
The density (ρ) and dynamic viscosity (η) of the ILs were measured
Cl-
CH Cl
+ LiCl
(2)
2
2
with an automated Anton Paar SVM-3000 digital double-tube visco-
densimeter. The precision in experimental measurements found to be
+
Li (CF3SO2)2N
N+
N+
−
4
−1
−4
better than ± 2×10 g mL for the density and ± 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
Scheme 1. Two steps preparation of ionic liquids.

M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
Table 2
45
equipped with an aluminum crucible, at a heating rate of 10 °C min−1,
−
1
Fitting parameters of equations (1) and (2) and standard deviations in equation (3)
used for correlation of the physical properties of ILs studied.
under nitrogen atmosphere with a flow rate of 50 mL min
.
Refractive indices of the ILs were determined by using a
refractometer supplied by SUR BERLIN. The apparatus was calibrated
with iso-propyl alcohol and water at different temperatures from 10
to 90 °C. A DL-37 coulometric Karl Fischer apparatus supplied by
Mettler (Switzerland) was used for the determination of trace
amounts of water in ILs. The coulometric cell consisted of a two-
compartment cell separated by a ceramic diaphragm. Pyridine-free
catholyte and anolyte reagents were synthesized as prescribed in our
previous work [30] and used in all measurements.
Physical property
A
0
A
1
A
2
S.D.
[
EMIM][BF
4
]
ρ (g mL 1)
−
1.3119
1.4105
−0.0008
−0.0002
−23.176
−17.665
0.0154
3.0×10
−7
0.0020
0.0018
0.1982
0.1478
0.1570
−
7
n
D
−5.0×10
η (mPa s)
107.97
82.614
48.058
2
−1
ν (mm
s
)
σ (mN m−1)
−5.0×10−4
[BMIM][OTf]
−
1
−8
−
3
ρ (g mL
)
1.3252
1.4397
269.35
204.45
23.777
−0.0008
−0.0003
−59.8
−45.27
−0.0393
−4.0×10
−2.0×10
0.0021
0.0018
0.6481
0.4917
0.1796
Aqueous solutions with mass concentration of 0.01 g cm
of the
−
8
n
D
ILs were prepared and used for pH measurements. The pH and
chloride content measurements were obtained using a DL-70 Mettler
memotitrator. The instrument was calibrated with different pH
buffers and sodium chloride as a primary standard.
η (mPa s)
2
−1
ν (mm
s
)
σ (mN m−1)
−2.0×10−4
[
2
BMPyrr][NTf ]
−
1
−7
ρ (g mL
)
1.4218
1.4271
275.19
194.84
34.458
−0.001
−0.0003
−61.59
−43.49
0.0085
8.0×10
0.0110
0.0027
0.4810
0.5956
0.8330
−
7
3
. Results and discussion
n
D
−2.0×10
η (mPa s)
2
−1
ν (mm
s
)
3
.1. Physicochemical properties of ILs
σ (mN m−1)
−3.0×10−4
Due to needs for experimental data for temperature dependence of
physicochemical properties of [EMIM][BF
4
], [BMIM][OTf] and [BMPyrr]
and
[
NTf ], their density, refractive index, dynamic and kinematic viscosities,
2
pH, and surface tension were measured over a wide temperature range
of 10 to 90 °C. The experimental values are listed in Table 1.
The density, ρ, refractive index, n , dynamic viscosity, η, and
D
surface tension σ, values were fitted by the method of least squares
À∘
Á
ln ηðm Pa sÞ¼A0T C ꢀA
ð2Þ
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
using the following equations [24]:
D
o
1
2
correlation parameters, together with their standard deviations (S.D.),
are listed in Table 2. The S.D. values were calculated by applying the
following expression:
À∘
Á
À
Á
2 ∘
2
z = A0 + A1T
C
+ A2T
C
;
ð1Þ
Table 1
ꢂ
ꢃ
Density ρ, dynamic viscosity η, refractive index
expansion α
n
D
,
surface tension σ, thermal
ꢀ
ꢁ
1=2
2
S:D:¼ ∑ zexpꢀzcalc =n
;
ð3Þ
p
, and pH of the ionic liquids at different temperatures.
t (°C)
ρ
η
n
D
σ
104
(K)
α
p
pH of 1%
Solution
(
g mL−1)
(mPa s)
(mN m−1)
where, z and n are the property values and the number of
experimental points, respectively.
[
4
EMIM][BF ]
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
1.3041
1.2970
1.2893
1.2819
1.2745
1.2670
1.2597
1.2526
1.2456
57.362
38.208
27.083
20.008
15.35
12.103
9.7709
8.0817
6.795
1.4085
1.4072
1.4051
1.4026
1.4003
1.3989
1.3966
1.3933
1.3915
48.28
48.17
48.07
47.86
47.75
47.30
46.80
46.65
46.41
6.089
6.080
6.071
6.061
6.050
6.040
6.028
6.017
6.005
4.58
4.59
4.60
4.60
4.60
4.61
4.62
4.62
4.62
D
The surface tension σ, density ρ, refractive index n , dynamic
viscosity η, and thermal expansion α of the ILs at 25 °C are given in
Table 3. The percent relative error,%E, calculated by the equation %E=
(|Zexp −Zcalc|/Zexp).100 where, Zexp and Zcalc refer to the experimental
and calculated values, respectively, are also included in Table 3. As
seen, good agreements, ranging from 0.002% to 2.64%, between the
results obtained from fitting equations and the experimental results
was observed. The best fittings was observed for the density and the
refractive index, especially for [BMIM][OTf].
A comparison between the experimental and literature data for
the physical and electrochemical properties of the studied ILs at 20
and 25 °C has also made in Table 4. To the best of our knowledge, no
literature data on surface tensions were previously available for three
studied ILs. As is obvious from Table 4, the experimental data for
[
BMIM][OTf]
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
1.3168
1.3097
1.3014
1.2935
1.2848
1.2773
1.2685
1.2618
1.2530
137.5
90.31
63.60
42.12
28.14
20.82
16.21
11.92
8.780
1.4370
1.4342
1.4319
1.4292
1.4262
1.4231
1.4209
1.4182
1.4154
23.40
22.91
22.40
22.00
21.34
21.00
20.24
19.50
19.15
2.356
2.355
2.354
2.353
2.352
2.352
2.351
2.350
2.349
3.21
3.21
3.21
3.22
3.23
3.25
3.25
3.26
3.28
[
EMIM][BF
with available literature values. Experimental density values are
lower (within 0.6%) than Gardas et al. [31] for [EMIM][BF ]. Lower
deviations (ranging from −0.2 to 0.5%) were obtained for [BMIM]
OTf] and [BMPyrr][NTf ], when compared with the literature. Positive
deviations (ranging from −0.20 to +1.20) for dynamic viscosity and
refractive index are observed for [BMIM][OTf] and [BMPyrr][NTf ],
4 2
], [BMIM][OTf] and [BMPyrr][NTf ] are in good agreement
[
2
BMPyrr][NTf ]
4
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
1.4109
1.4030
1.3930
1.3816
1.3731
1.3637
1.3550
1.3464
1.3370
145.8
86.01
59.13
37.39
26.66
19.80
15.17
12.02
9.73
1.4241
1.4216
1.4191
1.416
1.4134
1.4108
1.4085
1.4058
1.4035
34.52
34.40
34.31
34.19
34.10
34.02
33.71
33.30
33.04
59.01
62.81
67.11
72.05
77.76
84.44
92.37
101.9
113.7
[
2
2
when compared with Zhang et al. [32], Nishida et al. [33] and
Matsumoto et al. [34]. The deviations can be due essentially to the
salt's impurities including water and halide content and also to the
experimental technique adopted.

46
M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
Table 3
Estimated density ρ, dynamic viscosity η, refractive index n
D
p
, surface tension σ, thermal expansion α and% relative error E% of the ionic liquids at 25 °C.
Ionic liquids
EMIM BF
ρ (g mL−1)
%E
η (mPa s)
%E
n
D
%E
σ (mN m−1
)
%E
104
α
p
(K)
%E
4
1.2931
1.3056
1.3980
0.080
0.16
0.79
32.35
76.96
72.57
2.64
0.84
0.66
1.4052
1.4322
1.4195
0.007
0.006
0.007
48.13
22.67
34.06
0.002
0.04
1.23
6.076
2.354
64.96
0.65
0.96
0.12
BMIM OTf
BMPyrr NTf
2
Table 4
Experimental and literature values of densities ρ, refractive indices n
D
, dynamic viscosities η, surface tensions σ, electrochemical windows ΔE and thermal decomposition t
d
of the
ionic liquids at 20.0 and 25.0 °C.
Property
[EMIM][BF
4
]
[BMIM][OTf]
Present work
[BMPyrr][NTf
2
]
Present work
Literature
20.0 °C
Literature
20.0 °C
1.29c
Present work
Literature
20.0 °C
20.0 °C
25.0 °C
1.2931
25.0 °C
1.28b
20.0 °C
1.3097
25.0 °C
25.0 °C
20.0 °C
1.4030
25.0 °C
1.3980
25.0 °C
ρ (g mL−1)
1.2970
1.2709
a
1.3056
1.2976l
1.4091o
1.3940p
i
n
1.3038m
1
1
.3055
1.3061
j
1.2796k
l
.2844
1.3016
d
d
1.4368m
75.28q
c
1.3970e
n
D
1.4072
38.21
1.4052
32.35
1.410
–
1.4342
90.31
1.4322
76.96
1.434
1.4216
86.01
1.4195
72.57
1.41(± 5%)
85.00g
η (mPas)
–
32f
92.09q
85(± 5%)c
e
3
7
σ (mN m−1)
ΔE (V)
48.17
4.6
422
4.59
48.13
4.6
–
–
22.91
5.3
22.67
5.3
–
–
–
–
–
–
34.40
6.0
34.36
5.9
–
–
4.1a,b,4.3f
6.0
300
c
5.5
g
h
b
t
d
(°C)
450
382
3.21
346
–
pH1% aqueous soln.
4.60
–
3.21
–
aRef. [55]. b Ref. [56]. c Ref. [32]. d Ref. [57]. e Ref. [33]. f Ref. [34]. g Ref. [58]. h Ref. [59]. Ref. [31]. Ref. [36]. Ref. [37]. Ref. [41]. mRef. [42]. nRef. [43]. oRef. [39]. pRef. [40]. qRef. [44].
i
j
k
l
3
.2. Density and viscosity
been shown that the presence of even low concentrations of chloride
in the [EMIM][BF ] substantially increases the viscosity. For example
[EMIM][BF ] with a chloride content of 0.01 mol Kg
4
−
1
Data on viscosity and density for the ILs at temperatures ranging
4
−
(equal to
from 10 to 90 °C are shown in Figs. 2 and 3. As it is seen, the densities
of [BMIM][OTf] and [EMIM][BF ], in the entire temperature range
studied, and those of [BMPyrr][NTf ], at temperatures lower than
5 °C are larger than the corresponding water viscosities. It is
355 ppm Cl ) possessed a viscosity of 66.5 mPa s at 20 °C. However,
−
4
our synthetic ionic liquids had only b10 ppm Cl so that the observed
2
large deviations could be essentially due to the presence of salt
impurities in the samples. We have reported some of impurities
available in our studied RTILs in Table 5, and their characterization in
the supporting information section. However, a good agreement was
observed with data reported by Matsumoto et al. (within 1.0%) [34].
Fig. 4 shows a comparison of the logarithm of experimental and
reported viscosities as function of reciprocal temperature for [BMIM]
5
interesting to note that the density of ILs studied decrease nearly
linearly with increasing temperature, but at a rate less than that for
molecular organic solvents. However, the observed trends for
viscosity of ILs are reversed, in comparison with that of the
conventional organic solvents [35]. Unfortunately, the high viscosity
of ILs will negatively affect most of their operational processes.
Although the density data for some of the studied ILs are already
[OTf] and [BMPyrr][NTf
were in agreement with those reported by Ge et al. [38] and Tokuda et
al. [48]. In the case of [BMPyrr][NTf ], although the reported viscosity
2
]. As seen, the [BMIM][OTf] viscosity values
available in the open literature, such data for [BMPyrr][NTf
2
] is scarce.
2
The relative deviations between the experimental density data
obtained in this work are also presented in Fig. 3. As seen, the density
data at different temperatures were scarce, the results shown in
Fig. 4B revealed that the corresponding data obtained are slightly
lower (within 0.3%) than those reported by MacFarlane et al. [16] and
Tokuda et al. [45].
data for [EMIM][BF
values, the relative deviation values are ranging from (−0.8 to 0.6)%
31,36–38], and for [BMPyrr][NTf ], the deviations of our data and the
literature are in the range of −0.2 to 0.5% [39,40]. Finally, for [BMIM]
OTf], the agreement between our values with all available literature
data at atmospheric pressure [41–46] is quite satisfactory (within
.5%).
Since the viscosities of ILs are essentially governed by the van der
4
] are in good agreement with available literature
[
2
3.3. Surface tension
[
Available data on the surface tension of ILs are very limited. In
general, theliquid/air surface tension values for ILsare somewhat higher
0
−
1
than those of conventional solvents such as hexane (18 mN m ), but
−
1
Waals interactions and H-bonding, 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 salts
more viscous (see Table 1). On the other hand, it is obvious that for
not as high as water (73 mN m ). The temperature dependence of
surface tension (σ) for the ionic liquids studied is shown in Fig. 5. As
seen, in the case of imidazolium derivatives [BMIM][OTf] and [EMIM]
[BF
4
], the surface tension decreases with increasing carbon number, and
+
+
−
certain cations, such as [EMIM] and [BMIM] , the resulting ionic
liquids with larger anions show higher dynamic and kinematic
viscosities.
a lower surface tension is found for the OTf salt. It should be noted that,
the experimental surface tension values are reported for the first time in
this work, and no experimental surface tension data was found for three
studied ILs.
As is obvious from Fig. 2, the viscosity of ILs decreases more or less
exponentially with increasing temperature. The empirical equa-
tion (2) is thus applicable to describing the temperature dependence
of the viscosity 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.
Seddon et al. [47] have reported the influence of chloride, water
3.4. Refractive index and pH
The refractive index of a substance describes its ability to refract
light as it moves from one medium to another; the compounds of
higher refractive index will refract more light. In general, higher
density materials have greater concentrations of loosely bound
4
and organic solvents on the physical properties of [EMIM][BF ]. It has

M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
47
A 1.37
A 160
20
1
1
.33
.29
1
8
4
0
0
0
1.25
1
1
.21
.17
5
25
45
65
85
105
125
o
0
0
0
20
20
20
40
60
80
80
80
100
100
100
Temperature ( C)
o
Temperature ( C)
B 1.34
B
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1
.32
.3
1
1
1
1
1
.28
.26
.24
.22
5
25
45
65
85
105
125
o
Temperature ( C)
40
60
o
C 1.42
Temperature ( C)
C 160
1.40
1
.38
1
20
1.36
1
1
1
.34
.32
.30
8
4
0
0
0
5
25
45
65
85
105
125
o
Temperature ( C)
40
60
Fig. 3. Comparison of density data for [EMIM][BF
[31]; (▲) Taguchi et al. [36]; (+) Navia et al. [37]; (S) Wong et al. [38], for [BMIM][OTf]
B): (w) this work; (□) Tokuda et al. [45]; (×) Gardas et al. [43]; (o) Ge et al. [44]; (Δ)
Fredlake et al. [46]; (+) Soriano et al. [42]; (S) Garcia et al. [41], for [BMPyrr][NTf ] (C):
♦) this work; (S) Gardas et al.[39]; (+) MacFarlane et al. [16]; (▲)Tokuda et al. [45];
o) Kato et al. [40].
4
] (A): (♦) this work; (■) Gardas et al.
o
Temperature ( C)
(
Fig. 2. Temperature dependence of kinematic viscosity (ν, ■) and dynamic viscosity (η,
) for [BMIM][OTf] (A), [EMIM][BF ] (B) and [BMPyrr][NTf ] (C).
2
(
(
♦
4
2
electrons and, thus, larger refractive indices. For most practical
purposes, a high refractive index is considered 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
desirable to immerse these minerals in a fluid with the same refractive
index [48]. Unfortunately, most of available high refractive index
literature was found for comparing the refractive index data [42].
Fig. 6 shows also comparison of experimental refractive indices with
the literature. For [BMIM][OTf], the refractive index data were
obtained slightly lower (within 0.27%) than Soriano et al. [42]. To
the best our knowledge, no literature data on refractive indices were
4
previously available for [EMIM][BF ] and [BMPyrr][NTf
2
].
3 4
immersion compounds, such as AsI (RI=2.2) or SnI (RI=2.1), are
Table 5
solid at room temperature, poisonous, unstable, and extremely
unpleasant. Diiodomethane saturated with sulfur is another commer-
cially available high refractive index (RI=1.78) room temperature
liquid, but this is also a harmful material [49]. 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. 6 shows temperature dependence of refractive index for the
studied ILs as a set of fluids which have been designed to have
refractive indices N1.4, and also possess low environmental impact
and good shelf life. As it can be seen from Fig. 6, for all three ILs, the
refractive index decreases linearly with increasing temperature. It is
noteworthy that, at temperatures lower than 50 °C, the ILs used
possess refractive indices N1.4. In a survey on the literature, only one
Impurity contents of the studied ILs as mass fraction, w.
Species
Test method
Results
EMIM]
[
[BMIM]
[OTf]
[BMPyrr]
[NTf2]
[BF4]
O) a
100 w (H
2
Karl Fischer
0.06
0.02
b0.25
b10
b10
b5.0
0.003
b0.25
b10
b10
b5.0
1
00 w (Cl)
Flask Combustion
Potentiometry
Potentiometry
Turbidimetry
b0.25
b10
b10
b5.0
6
−
10 w (Cl
)
)
6
−
1
1
1
0
0
0
w (Br
w (SO
6
6
2−
4
)
w (Heavy metals Turbidimetry
b0.2
b0.2
b0.2
(as Pb))
100 w (Ash content) Electric furnace and
gravimetry
b0.001
b0.001
b0.001
a
The water content results are obtained after drying treatment described in the
experimental section.

48
M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
1
1
1
.44
.42
.40
A 2.5
2
1
.5
1
η
0
.5
0
2
2.5
3
3.5
4
-
1
1
000/T (K )
1.38
0
20
40
60
80
100
o
B 2.5
Temperature ( C)
Fig. 6. Plots of temperature dependence of experimental values of refractive indices
) of [BMIM][OTf] (■) this work; (S) Soriano et al. [42], [BMPyrr][NTf ] (▲) and
EMIM][BF ] (♦).
2
1
1
0
0
.0
.5
.0
.5
.0
(
[
n
D
2
4
η
of the lipophilic ILs, or to do measurements by using a universal non-
aqueous glass electrode.
2
.5
2.7
2.9
3.1
3.3
3.5
3.7
3.5. Thermal expansion
-
1
1
000/T (K )
The coefficients of thermal expansion for [EMIM][BF
4
], [BMIM]
Fig. 4. Comparison of logarithm of viscosities as a function of reciprocal temperature (K)
for [BMIM][OTf] (A) ((w) this work; (□) Ge et al. [44]; (▲) Tokuda et al. [45]) and
[OTf] and [BMPyrr][NTf ] are defined by the following equation:
2
[
BMPyrr][NTf
2
] (B) ((□) this work; (w) MacFarlane et al. [16]; (Δ) Tokuda et al. [45]).
ꢀ
ꢁꢀ
ꢀ
ꢁ
ꢁ
À
Á
ꢀ
3
ꢀ3
∘C
α ¼ ꢀ1 = ρ gcm
∂ρ g cm
=∂T
Á
ð4Þ
p
p
ꢀ
À
Á
À
À∘CÁ
∘
∘
2
2
Owing to the fact that the presence of impurities can significantly
¼
ꢀ A þ 2A T C = A þA T C þA T
1
2
0
1
2
affect some properties of the ILs such as refractive index, pH and
electrochemical window, we measured several common impurities in
the prepared ILs and the results are summarized in Table 5. 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 reactions where the pH is important to the proper
metabolisms and functioning of animals and plants.
where, α
p
is the coefficient of thermal expansion, ρ is the IL density,
and A are the adjustable parameters calculated by
o
and A , A
1
2
equation (1). The thermal expansion coefficients for each of ILs at
several temperatures are summarized in Table 1, and the temperature
dependence of the α
p
values is illustrated in Fig. S3 (supporting
values at 25 °C are (6.075, 2.355, and
65.31)×10 °C for [EMIM][BF ], [BMIM][OTf] and [BMPyrr][NTf ],
information section). The α
p
−
4
−1
4
2
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
evaluate the temperature dependence of pH values of the ILs used. The
temperature effect on pH appears to be almost negligible as shown in
Fig. S2 (supporting information section). Although the pH values
provided by the literature (Table 4) 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% aqueous
solution for hydrophilic and intermediate ILs, and saturated solutions
respectively. The results show that the variation of the volume
expansively quantified by the thermal expansion coefficient could be
considered as independent of the temperature for these ILs.
3.6. Thermal analysis
As expected, the thermal decomposition of ILs is strongly
dependent on their structures. A previous study [50] shows that the
thermal decomposition of ILs may be catalyzed by pan composition. In
this work, we focused on the data obtained using aluminum pans.
Different from organic solvents, many kinds of ILs can be kept in the
liquid state above 300 °C (with T
give them good dynamic properties and excellent catalytic activity. In
this study, the thermal stability of ILs ([EMIM][BF ]) and ([BMIM]
[OTf]), ([BMPyrr][NTf ]) was investigated by thermogravimetry (TG)
and differential scanning calorimetry (DSC), and the results are shown
in Fig. S4 (supporting information section).
d
ranging from 300 to 600 °C), which
6
5
4
3
2
1
0
0
0
0
0
0
0
4
2
4
The DSC and TG curves of [EMIM][BF ] (Fig. S4A) showed a broad
endothermic behavior commencing near 422 °C, which is the first step
for its decomposition. As is obvious from the corresponding DSC
curve, here is an interval of temperature of about 50 °C between the
end of first decomposition step at 422 °C and beginning of a second
step for exothermic decomposition at 515 °C. Thus, the TG curve of
4
[EMIM][BF ] shows a significant mass loss over a broad temperature
0
20
40
60
80
100
o
range of 410 to 520 °C, with a Δm of 85%. Upon further increase in
temperature (i.e., N520 °C), the TG curve indicates a slow and
continuous mass loss caused by elementary carbon produced from
the previous steps.
Temperature ( C)
Fig. 5. Temperature dependence of surface tension (σ) for the ILs [EMIM][BF
♦), [BMPyrr][NTf ] (▲) and [BMIM][OTf] (■).
4
]
(
2

M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
49
The DSC and TG curves of [BMIM][OTf] (Fig. S4B) show that the IL
is thermally stable up to about 370 °C. However, at higher tempera-
tures, [BMIM][OTf] presents a significant mass loss step between 370
and 450 °C, with Δm=75% and a DSC peak temperature of 382 °C.
Over 450 °C, the TG curve indicates a slow and continuous mass loss
caused by elementary carbon formed from the previous step.
2
The DSC and TG curves for [BMPyrr][NTf ] are shown in Fig. S4C. As
is obvious from the DSC curve, no thermal event was observed prior to
the 346 °C, where the IL undergoes an exothermic decomposition.
This result agrees with the corresponding TG curve for this sample. A
mass loss (Δm=89%) occurs in the range 340–410 °C. Such behavior
2
suggests that pure [BMPyrr][NTf ] is kinetically stable until 340 °C.
Based on the thermoanalytical data obtained for the investigated
ILs in this study, using non-isothermal thermogravimetery and
differential scanning calorimetry experiments, the following stability
4 2
order was stablished: [EMIM][BF ] N [BMIM][OTf] N [BMPyrr][NTf ].
3
.7. Electrochemical window
The electrochemical window is defined as the potential interval
−
1
Fig. 8. Typical differential pulse (1) and cyclic (2) voltammograms of 100 μg g
of 2-
]. Conditions: scan rate, 10 mV s− ; temperature, 25 °C;
1
furaldehyde in [BMPyrr][NTf
purge time, 120 s.
2
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 elec-
trochemical power applications. Most ILs possess a wide potential
window of N4.0 V. It has been found that, for some ionic liquids, the
potential of anode limit proportionally decrease with increasing
highest occupied molecular orbit energy calculated for the anion [51].
It has also been shown that the quantum chemical calculations are an
excellent tool to predict the electrochemical window of ionic liquids,
which can be correlated to the calculated difference in energy level of
LUMO of the cation (reductive stability) and HOMO of the anion
studied are ranging from about 4 V to 6 V, and decrease in the order
[BMPyrr][NTf ] N [BMIM][OTf] N [EMIM][BF ].
2
4
Moreover, we also carried out the electrokinetic experiments for
−
7
2-furaldehyde in the studied ILs. Diffusion coefficients of 1.09×10
,
3.60×10 , 2.30 × 10 mm2
−
9
−8
s
−1
were obtained in [EMIM][BF4],
[BMIM][OTf], [BMPyrr][NTf ], respectively. The results are in line with
2
the viscosity data but not with the conductivity values. Fig. 8 shows
cyclic voltammograms and differential pulse voltammograms of 2-
(
oxidative stability) [52,53]. Furthermore, these calculations can be
used to predict the possible electrochemical breakdown products of
ionic liquids. On this basis, [BMPyrr][NTf ] decomposes into 1-
furaldehyde in pure [BMPyrr][NTf ]. Although the analyte was
2
electroactive in all three ILs, a better defined 2-electron irreversible
2
peak was observed at −1.93 V in [BMPyrr][NTf ], which is related to
2
methylpyrrolidine, octanes, 2-butanol, dibutylmethylamine and 1-
butylpyrrolidine. While, in the electrochemical breakdown of [EMIM]
the reduction of 2-furaldehyde to furfuryl alcohol. Besides, [BMPyrr]
[NTf ] is a hydrophobic and electrochemically stable solvent for
2
4
[BF ], 1-ethyl-3-methylimidazolium radicals are formed, which can
organic and even inorganic matters such as aluminum chloride [54].
react with each other via a radical–radical coupling reaction as well as
a disproportionation reaction.
For example, [BMPyrr][NTf ] dissolves up to 1.5 M AlCl3 as a clear
solution. Since this ionic liquid has a very low viscosity and is
2
In this work, the electrochemical windows of the investigated ILs
were experimentally obtained using cyclic voltammetric studies at
glassy carbon electrode (GCE). The corresponding voltammograms,
obtained in a double-wall three electrode cell at 25 °C at a scan rate
thermally stable, it can be easily dried under high vacuum at 100 °C.
Water contents below 50 ppm can be reached very easily, which make
[BMPyrr][NTf ] an ideal ionic liquid for electrochemistry.
2
−
1
1
00 mV s
by using a Pt wire as a quasi-reference electrode, are
shown in Fig. 7. As seen, the electrochemical windows for the ILs
4. Conclusions
The data of physical properties on ionic liquids are essential for
both theoretical research and industrial application. The establish-
ment of the databases in this respect will definitely promote the
research and development of ionic liquids. Thus, in this work, we have
carefully measured several important physical properties of ionic
liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF
-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]
OTf], and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfo-
nyl)imide [BMPyrr][NTf ] over a wide range of temperature. Obvi-
4
],
1
[
2
ously, much more attention should be paid on the measurement of
physicochemical properties of ionic liquids. Also because of inherent
advantages of the ionic liquids, new electroanalytical methods should
be developed in these media, especially for the organic and inorganic
compounds that are inactive at different electrode surfaces and/or in
aqueous solutions. The electrochemical results obtained for the three
ILs studied revealed an ideal wide range of the windows for the study
of redox behavior of furaldehydes.
Fig. 7. Cyclic voltammograms of the fluoroanions of the pure ionic liquids [EMIM][BF
4
] (1),
[
BMIM][OTf] (2) and [BMPyrr][NTf ] (3). Conditions: quasi-reference electrode, Pt wire;
2
−
1
Supplementary materials related to this article can be found online
at doi:10.1016/j.molliq.2010.08.005.
working electrode, GCE (O.D. 2.2 mm); temperature, 25 °C; scan rate, 100 mV s ; cell
capacity, 1 mL.

50
M. Shamsipur et al. / Journal of Molecular Liquids 157 (2010) 43–50
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