The conductivity of imidazolium-based ionic liquids from (248 to 468) K. B. variation of the anion

Article abstract of DOI:10.1021/je900793r

The effect of the anion, namely dicyanamide, hexafluorophosphate, trifluoroacetate, or trifluoromethanesulfonate, on the conductivity (κ) of 1-N-butyl-3-N-methylimidazolium-based room-temperature ionic liquids (RTILs) was studied over the temperature range (248 to 468) K. The uncertainty in κ was estimated to be less than 0.5 %. The conductivity values obtained are well-described by the Vogel-Fulcher-Tammann equation. Additionally, densities (ρ) and the corresponding molar conductivities (Λ) are reported for the temperature range (278 to 338) K. The data for Λ and the associated viscosities (η) were found to fit fractional forms of the Walden relationship.

Full text of DOI:10.1021/je900793r

1
774
J. Chem. Eng. Data 2010, 55, 1774–1778
The Conductivity of Imidazolium-Based Ionic Liquids from (248 to 468) K. B.
†
Variation of the Anion
Oliver Zech, Alexander Stoppa, Richard Buchner, and Werner Kunz*
Institut f u¨ r Physikalische und Theoretische Chemie, Universit a¨ t Regensburg, D-93040 Regensburg, Germany
The effect of the anion, namely dicyanamide, hexafluorophosphate, trifluoroacetate, or trifluoromethane-
sulfonate, on the conductivity (κ) of 1-N-butyl-3-N-methylimidazolium-based room-temperature ionic liquids
(
RTILs) was studied over the temperature range (248 to 468) K. The uncertainty in κ was estimated to be
less than 0.5 %. The conductivity values obtained are well-described by the Vogel-Fulcher-Tammann
equation. Additionally, densities (F) and the corresponding molar conductivities (Λ) are reported for the
temperature range (278 to 338) K. The data for Λ and the associated viscosities (η) were found to fit fractional
forms of the Walden relationship.
1
. Introduction
Over the last several years, ionic liquids (ILs) have gained
received. 1-Butylimidazole (ABCR, g 99 %), methyl trifluo-
roacetate (ABCR, g 99 %), and methyl trifluoromethane-
sulfonate (ABCR, g 98 %) were distilled prior to use.
Synthesis and Sample Handling. [bmim][DCA] and
more and more attention because of their unique properties, such
as negligible vapor pressure and high electrical conductivity and
thermal stability, coupled with a wide liquid range. These
properties render ILs, especially room-temperature ILs (RTILs),
excellent candidates for potential applications. ILs can be
applied as solvents in organic reactions or catalysis and as
media in extraction processes. Other studies concern the use
of ILs as electrolytes in batteries, double-layer capacitors, or
solar cells.
[
6
bmim][PF ] were obtained by metathesis of 1-N-butyl-3-N-
1
,2
methylimidazolium chloride ([bmim][Cl]) and appropriate alkali
21
metal salts, following the procedure of Cammarata et al. with
3
,4
19
slight modifications.
5
,6
7
[bmim][TfO] and [bmim][TA] were prepared by slowly
adding slight molar excesses (approximately 1:1.1) of methyl
trifluoromethanesulfonate and methyl trifluoroactetate, respec-
tively, to solutions of 1-butylimidazole (mass ratio 2:3) in
acetonitrile (Merck, g 99.9 %). Excess reactants were removed
by vacuum distillation.
All of the RTILs were dried under high vacuum (p < 10-8
bar) at 40 °C for 1 week, yielding (in the case of [bmim][DCA],
8
9
,10
In recent years, progress in the determination of thermody-
namic and transport properties of RTILs has been made, with
imidazolium-based compounds being in the focus of many
studies. Various groups have studied the conductivity (κ) of
imidazolium-based RTILs.
perature ranges investigated have been rather limited, and the
measurements have often been performed at a single frequency
1
1-20
In general, however, the tem-
[
bmim][TA], and [bmim][TfO] slightly yellow) liquids with
-6
water mass fractions of < 80·10 (as determined by coulometric
Karl Fischer titration using a Mitsubishi Moisturemeter MCI
CA-02). Potentiometric titration of freshly prepared aqueous
(thus neglecting electrode polarization) and/or with commercial
samples of limited purity. In this contribution, we report precise
conductivity data for the RTILs 1-N-butyl-3-N-methylimidazo-
lium dicyanamide ([bmim][DCA]), 1-N-butyl-3-N-methylimi-
dazolium hexafluorophosphate ([bmim][PF ]), 1-N-butyl-3-N-
6
methylimidazolium trifluoroacetate ([bmim][TA]), and 1-N-
butyl-3-N-methylimidazolium trifluoromethanesulfonate ([bmim]-
RTIL solutions against standardized AgNO
mass fractions of < 50·10 for [bmim][PF
[
3
(aq) yielded halide
] and 0.0021 for
bmim][DCA]. No AgCl precipitation was observed when
-6
6
3
AgNO (aq) was added to [bmim][TA] or [bmim][TfO]. No
1
13
contamination was detected by analysis using H, C, and
19
(
where applicable) F NMR spectroscopy. The samples were
[TfO]) over the temperature range (248 to 468) K. This contribution
stored in a N -filled glovebox; conductivity cells were filled
2
on the anion dependence of RTIL conductivity complements a
companion paper (DOI: 10.1021/je900789j) on the cation depen-
dence of κ for a series of imidazolium tetrafluoroborates.
under a nitrogen atmosphere.
20
Thermal stability was studied with a thermogravimetric
analyzer (PerkinElmer model TGA 7). Samples were heated at
-
1
a rate of 10 K ·min , applying a continuous nitrogen flow.
Decomposition temperatures (Tdec) were taken as the onset of
mass loss, defined as the intersection of the baseline before
decomposition and the tangent to the mass loss afterward. The
obtained Tdec values were 566 K for [bmim][DCA]; 710 K for
2
. Experimental Section
Chemicals. 1-Methylimidazole (Merck, g 99 %) was distilled
from KOH, stored over 4 Å molecular sieves, and redistilled
prior to use. 1-Chlorobutane (Fluka, g 99.5 %) was distilled
prior to use. Sodium dicyanamide (Fluka, g 96 %) was
recrystallized from methanol (Merck, g 99 %). Potassium
hexafluorophosphate (Fluorochem, g 99 %) was used as
[
[
(
bmim][PF
6
]; 689 K for [bmim][TfO]; and 461 K for [bmim]-
TA], which are in quite good agreement with literature data
22
13
13
6
573 K for [bmim][DCA]; 706 K for [bmim][PF ]; 682 K
2
2
13
and 665 K for [bmim][TfO]; and 449 K for [bmim][TA] ).
ConductiWity. Conductivity measurements were performed
over the temperature range (248 to 468) K in steps of 10 K.
†
Part of the “Josef M. G. Barthel Festschrift”.
*
To whom correspondence should be addressed. Tel: +49 941 943 4044.
Fax: +49 941 943 4532. E-mail: werner.kunz@chemie.uni-regensburg.de.
1
0.1021/je900793r  2010 American Chemical Society
Published on Web 01/19/2010

Journal of Chemical & Engineering Data, Vol. 55, No. 5, 2010 1775
Measurements in the low-temperature region [(248 to 298) K]
were carried out with the equipment developed in the laboratory
Table 1. Conductivities (K) of the Investigated RTILs
κ/S·m-1
23,24
of J. Barthel.
For the high-temperature region [(308 to 468)
T/K
[bmim][DCA] [bmim][PF
6
]
[bmim][TfO] [bmim][TA]
K], a precision thermostat combined with a symmetrical
Wheatstone bridge with Wagner earth, sine generator, and
resistance decade was set up. Both thermostats were stable to
2
2
2
48.15
58.15
68.15
0.0483
0.1180
0.00858
0.0244
0.242
0.433
0.702
1.052
1.483
1.991
2.57
3.22
3.93
4.69
5.41
0.0164
0.0385
0.0792
0.1465
0.248
0.390
0.578
0.815
1.102
1.440
1.825
2.26
0.0539
0.1038
0.1808
0.290
0.436
0.621
0.848
1.117
1.426
1.775
2.16
0.0569
0.1144
0.205
0.333
0.504
0.723
0.988
1.301
1.658
2.06
<
0.003 K over the investigated temperature range. As bath
278.15
2
2
3
3
88.15
98.15
08.15
18.15
fluids, silicone oil M5 (Roth) and silicone oil M50 (Roth) were
used for the low- and high-temperature regions, respectively.
A set of four three-electrode capillary cells with cell constants
-1
(a) between (25 and 470) m was used for the low-temperature
328.15
338.15
348.15
3
3
3
region (e 298 K), whereas a set of four two-electrode capillary
-
1
cells with cell constants ranging from (1260 to 4610) m was
used at high temperatures. The cells were calibrated with
aqueous KCl solutions having molalities ranging from (0.01 to
58.15
68.15
78.15
2.50
6.34
2.60
-
1
0
.05) mol ·kg according to the procedure described by Barthel
388.15
7.26
2.74
3.04
2
5
3
4
4
4
98.15
08.15
18.15
28.15
8.19
9.15
3.25
3.81
4.39
5.01
5.65
6.31
7.00
7.69
3.52
4.03
4.56
5.12
5.69
6.28
6.88
7.50
et al. The accuracy of the cell constants was better than
0
.5 %.
10.12
11.12
12.10
13.09
13.75
14.54
To eliminate electrode polarization effects, resistances (R)
were measured at frequencies (ν) between 480 Hz and 10 kHz
and extrapolated to R ) limνf∞R(ν) to yield κ ) a/R , as
438.15
448.15
458.15
∞
∞
2
0
described in the companion paper. Repeated measurements
on selected samples agreed within δR/R ) 0.3 %. The relative
uncertainty of the electrical conductivities was estimated to be
4
68.15
2
0
<
0.5 %.
Since the experiments covered an extremely wide temperature
The present data for [bmim][DCA] (Figure 1) are in reason-
able agreement with the κ value given by Yoshida et al. for
range, possible changes in the cell constants a had to be
considered. The temperature dependence of a(T) is essentially
determined by the thermal expansion coefficients of the materials
used for cell construction (platinum and Pyrex glass) and by
16
2
98 K. However, they are systematically lower than those
28
reported by Hunger et al., with relative deviations [δLit
)
1
00·(κ - κLit)/κ] given by |δLit| e 11.3 (Figure 1). A possible
reason is the amount of halide impurities, for which ref 28
reported a mass fraction of 0.005, whereas that of the present
sample is only 0.0021 (corresponding to a mole fraction of 0.02).
Since the halide content of the present sample was still rather
high, it is suspected that (currently unavailable) very pure
2
3,26
the size and shape of the cell.
a(T) ) a0 1 + ꢀ(T - 298.15 K)
is the cell constant at 298.15 K and ꢀ is the temperature
coefficient
It can be expressed as
[
]
(1)
where a
0
[
bmim][DCA] would have a notably lower conductivity.
1
da
For [bmim][PF ] (Figure 2), conductivity data were reported
6
1
ꢀ
)
(2)
4
(
)
a dT
by Widegren et al. between (288.15 and 323.15) K, Suarez et
0
1
2
29
al. between (273.50 and 349.50) K, and Yu et al. between
For capillary cells like those used in the present work, ꢀ has
30
been found to be -3.5·10- K . Thus, the temperature
6
-1 23
(303.20 and 353.20) K. Furthermore, Kanakubo et al. reported
κ values obtained with two different cell types, namely, low-
pressure cells used between (298.15 and 353.12) K and high-
pressure cells covering (298.15 to 343.16) K. Whereas the data
variation of a was negligible in the present measurements.
Density. To calculate molar conductivities (Λ), densities (F)
of the RTILs were determined over the temperature range (278
to 338) K using a vibrating-tube densimeter (Anton Paar DMA
6
0). The instrument was calibrated at each measurement
temperature with purified dry nitrogen and degassed Millipore
2
7
water using precise density values from the literature. The
-3
uncertainty in F was estimated to be < 0.05 kg m . The relative
uncertainty of Λ was thus estimated to be < 0.5 %.
3
. Results and Discussion
The conductivity data obtained for the four RTILs are
summarized in Table 1. [bmim][DCA] was investigated over
the full temperature range (248 to 468) K, whereas [bmim][PF
6
]
and [bmim][TfO] could only be studied between (268 and 468)
K because samples crystallized in the conductivity cells upon
cooling to 258 K. For [bmim][TA], data between (248 and 368)
K are reported, since the formation of bubbles was observed
above 375 K. This effect may be related to a partial degradation
of [bmim][TA] at these temperatures, although TGA experi-
ments yielded a decomposition temperature of Tdec ) 461 K.
As even small amounts of impurities, including degradation
products, can have significant effects on the conductivity of
Figure 1. Specific conductivities (κ) of [bmim][DCA] from this study (b)
along with literature κ values and their relative deviations from the present
data [δLit ) 100·(κ - κLit)/κ]: 4, Hunger et al.; 0, Yoshida et al. The
solid line is a fit of κ using eq 3 (see Table 2); dashed lines indicate an
(arbitrary) δLit ) ( 3 margin.
28
16
[
bmim][TA], we have abstained from reporting κ values for T
368 K.
>

1
776 Journal of Chemical & Engineering Data, Vol. 55, No. 5, 2010
Figure 2. Specific conductivities of [bmim][PF
6
] from this study (b) along
Figure 3. Specific conductivities of [bmim][TfO] from this study (b) along
with literature κ values and their relative deviations from the present data
with literature κ values and their relative deviations from the present data
1
4
15
15
29
[δ
Lit ) 100·(κ - κLit)/κ]: 4, Widegren et al.; 0, Tokuda et al.; ], Suarez
[δLit ) 100·(κ - κLit)/κ]: 4, Tokuda et al.; +, Yu et al.; 0, Bonh oˆ te et
1
2
29
11
et al.; +, Yu et al.; O, high pressure cell, and ×, low pressure cell,
al. The solid line is a fit of κ using eq 3 (see Table 2); dashed lines indicate
3
0
from Kanakubo et al. The solid line is a fit of κ using eq 3 (see Table 2);
dashed lines indicate an (arbitrary) δLit ) ( 3 margin.
an (arbitrary) δLit ) ( 3 margin.
Table 2. Parameters of the VFT Fits (Equation 1) with the
y
Corresponding Standard Uncertainties of the Fit [σ , where y )
-
1
ln(K/S·m )]
[
[
bmim]
DCA]
[bmim]
[PF
[bmim]
[TfO]
[bmim]
[TA]
[bmim]
a
6
]
4
[BF ]
Α/S· m-
B/K
1
104.3
131.1
-818.1
177.4
1.6
95.1
-767.2
165.6
0.3
103.5
-737.3
169.8
0.7
166.0
-806.6
167.0
3
-572.0
173.7
1.0
T
0
/K
1
00· σ
y
a
Data from ref 20.
of Widegren et al. are in excellent agreement with our results
-0.6 e δLit e 0.6), the values of Suarez et al. deviate
(
considerably (-10.4 e δLit e 7.5). The conductivities reported
by Yu et al. are systematically lower, with δLit e 40.4, whereas
the κ values determined by Kanakubo et al. with their low-
pressure cells are lower (δLit e 10.3) and those obtained with
the high-pressure cells are higher (δLit e -4.5); the value
reported for 343.16 K with δLit ) -390.7 seems to be an outlier.
The conductivities published by Tokuda et al. for the tem-
perature range (273.15 to 373.15) K also agree well with the
present results (-3.4 e δLit e 5.2), except for the point at 263.15
K (δLit ) -144.9).
Figure 4. Specific conductivities of [bmim][TA] from this study (b) along
with literature κ values and their relative deviations from the present data
[δLit ) 100·(κ - κLit)/κ]: 4, Tokuda et al.; 0, Bonh oˆ te et al. The solid
line is a fit of κ using eq 3 (see Table 2); dashed lines indicate an (arbitrary)
1
5
11
1
5
δ
Lit ) ( 3 margin.
2
0
T
0
, which are similar to those for [bmim][BF
4
], are sum-
Neglecting their lowest temperature, where our data were
extrapolated using eq 3, the conductivities reported by Tokuda
marized in Table 2 together with the corresponding uncertainties
-1
of the fit [σ , where y ) ln(κ/S·m )]. The fitted curves are
y
1
5
et al. for [bmim][TfO] (Figure 3) over the temperature range
263.15 to 373.15) K are in very good agreement with the
present data (|δLit| e 3.8). On the other hand, the κ values of
shown as solid lines in Figures 1 through 4; deviations of the
experimental data from the fits are too small to be shown in
the lower panels of Figures 1 through 4.
(
2
9
27
Yu et al. deviate considerably (δLit e 36), as does the result
Consistent with the literature, the VFT temperatures (Table
) are (20 to 30) K below the glass transition temperatures (T
1
1
of Bonh oˆ te et al. for 293 K (δLit e -60).
2
g
)
16
For [bmim][TA], comparison is only possible with the
determined using differential scanning calorimetry: 179 K and
1
5
2
2
13
22
32
conductivity data of Tokuda et al. over the temperature range
263.15 and 373.15) K and the value at 293 K from Bonh oˆ te et
1
[
[
83 K for [bmim][DCA]; 196 K, 197 K, and 196 K for
1
5
13
(
bmim][PF
bmim][TA]. Tokuda and co-workers reported T
], 162 K for [bmim][TfO], and 172 K for
bmim][TA]. In view of the different temperature ranges of
the two studies, the agreement of these T values with the VFT
6
]; 191 K for [bmim][TfO], and 195 K for
1
1
al. (Figure 4). While the former are all systematically smaller
0
values of 174
1
5
11
(δLit e 10.4), the latter is larger by 22 %.
K for [bmim][PF
6
3
1
1
3
Similar to supercooled systems, the conductivity of the
[
present RTILs can be well-described by the empirical Vogel-
Fulcher-Tammann (VFT) equation:
0
temperatures of Table 2 is very good and reflects the generally
excellent agreement of the experimental conductivities from the
-
1
-1
B
ln(κ/S · m ) ) ln(A/S · m ) +
(3)
two investigations. In contrast to the behavior of the
T - T0
20
[
C
n
mim][BF
where a linear dependence of the strength parameter B/T
the number of carbon atoms in the N-alkyl chain, was observed,
4
] compounds studied in the companion paper,
with fit parameters Α, Β, and T
called VFT temperature. The values obtained for Α, Β, and
0
, where the latter is the so-
0
on n,
2
6

Journal of Chemical & Engineering Data, Vol. 55, No. 5, 2010 1777
Table 3. Densities (G) and Molar Conductivities (Λ) of the Investigated RTILs
[
bmim][DCA]
[bmim][PF
6
]
[bmim][TfO]
[bmim][TA]
4
4
4
4
F
10 ·Λ
F
10 ·Λ
F
10 ·Λ
F
10 ·Λ
kg ·m-
3
S·m ·mol
2
-1
kg·m-3
S·m ·mol
2
-1
kg·m-3
S·m ·mol
2
-1
kg·m-3
S·m ·mol
2
-1
T/K
2
2
2
3
3
3
3
78.15
88.15
98.15
08.15
18.15
28.15
38.15
1072.22
1066.15
1059.62
1053.35
1047.26
1040.87
1034.72
0.830
1.352
2.038
2.89
3.90
5.07
1383.38
1376.92
1368.35
1359.96
1351.63
1343.16
1334.73
0.079
0.163
0.304
0.518
0.820
1.222
1.735
1313.19
1305.51
1297.43
1289.31
1281.61
1273.62
1265.83
0.228
0.399
0.644
0.974
1.398
1.920
2.54
1230.37
1223.40
1215.48
1207.71
1200.23
1192.56
1185.09
0.235
0.422
0.692
1.053
1.519
2.09
6.39
2.77
3
0
no general trends in Α, Β, T
anion of the [bmim] RTILs could be found.
0
, or B/T
0
with variation of the
0.91 reported by Kanakubo et al. No significant effect of the
anion on the exponent could be observed.
+
-1
To calculate the molar conductivity, defined as Λ ) κ·M·F ,
where M is the molar mass of the RTIL, F values were measured
between (278.15 and 338.15) K with a vibrating-tube densim-
4
. Conclusion
In this study, where we benefited from the excellent thermal
stability and wide liquid range of RTILs, the effect of different
anions on the conductivity of highly pure [bmim] RTILs has
been investigated over the exceptionally wide temperature range
248 to 468) K, supplementing a companion paper on the cation
dependence of κ for imidazolium-based RTILs. At a given
3
3
eter. A viscosity correction was not applied to the raw data
2
0
for the reasons detailed in the companion paper. The values
+
obtained for F and Λ are summarized in Table 3.
34
35
According to Voronel et al. and Veliyulin et al., fractional
Walden relations of the form
(
temperature, the conductivity decreases in the order [bmim]-
[DCA] > [bmim][TA] > [bmim][TfO] > [bmim][PF ]. The
6
temperature dependence is well-described by the VFT equation.
While our data compare favorably with some literature results,
significant deviations from others were noted. For the limited
temperature range (278 to 338) K, where fluidities are available
T t
η
κ · T ∝ D ∝
(4)
(
)
where D is the diffusion coefficient and η is the viscosity, are
typical for ionic melts, with exponent values of t ≈ 0.8 claimed
3
0
to be universal. For imidazolium-based ILs, Kanakubo et al.
3
6
and Harris et al. recently applied fractional Walden relations
of the form
6
for [bmim][PF ], [bmim][TfO], and [bmim][TA], the molar
conductivity data were found to fit fractional forms of the
Walden relationship. No systematic effect of the anion on the
exponent t could be observed.
T t
)
Λ · T ∝
(5)
(
η
Acknowledgment
Both groups found good fits over the temperature ranges studied
and obtained exponent values of t ≈ 0.9 independent of pressure
This work is dedicated to Professor J. Barthel, who set standards
for high-precision conductivity measurements that are still valid.
The authors thank R. M u¨ ller for help with the TGA measurements
and H. Hilbinger for the development of the high-temperature
precision thermostat. We are further grateful to J. Hunger for help
with the syntheses and potentiometric titrations of the RTILs.
6
and temperature. For the RTILs [bmim][PF ], [bmim][TfO], and
-
1
13
[
bmim][TA], for which fluidities (η ) are available, fractional
Walden plots according to eq 5 are shown in Figure 5. Good
fits were obtained, with t values of 0.86 for [bmim][PF ], 0.88
for [bmim][TfO], and 0.86 for [bmim][TA]. The present
exponent for [bmim][PF ] compares well with the value t )
6
6
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Received for review October 1, 2009. Accepted December 18, 2009.
JE900793R