Physicochemical properties of ionic liquids [C3py][NTf 2] and [C6py][NTf2]

Article abstract of DOI:10.1021/je200534b

Air- and water-stable hydrophobic ionic liquids N-alkylpyridinium bis(trifluoromethylsulfonyl)imide ([C_npy][NTf₂], n = 3, 6) were synthesized. The density, surface tension, dynamic viscosity, and electrical conductivity of [C₆py][NTf₂] were measured in the range of T = (283.15 to 338.15) K. The density, dynamic viscosity, and electrical conductivity of [C₃py][NTf₂] were measured in the range of T = (308.15 to 338.15) K. The melting and glass transition temperatures of the two ILs were determined according to the differential scanning calorimetry (DSC). The physicochemical properties, including molecular volume, standard molar entropy, lattice energy, parachor, molar enthalpy of vaporization, interstice volume, interstice fraction, and thermal expansion coefficient, were estimated in terms of empirical and semiempirical equations, as well as the interstice model theory on the base of the experimental values. The dynamic viscosity and electrical conductivity values were fitted by Vogel-Fulcher-Tammann (VFT) and Arrhenius equations for [C₆py] [NTf₂] and the Arrhenius equation for [C₃py][NTf ₂].

Full text of DOI:10.1021/je200534b

ARTICLE
pubs.acs.org/jced
Physicochemical Properties of Ionic Liquids [C py][NTf ] and
3
2
[
C py][NTf ]
6 2
†
†
†
‡
†
,†,§
Qing-Shan Liu, Miao Yang, Pei-Pei Li, Si-Si Sun, Urs Welz-Biermann, Zhi-Cheng Tan,* and
Qing-Guo Zhang*
,
‡
†
China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
People's Republic of China
‡
Department of Chemistry, Bohai University, Jinzhou 121000, People's Republic of China
§
Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
People's Republic of China
S Supporting Information
b
ABSTRACT: Air- and water-stable hydrophobic ionic liquids N-alkylpyridinium bis(trifluoromethylsulfonyl)imide
[C py][NTf ], n = 3, 6) were synthesized. The density, surface tension, dynamic viscosity, and electrical conductivity of
(
n
2
[
C py][NTf ] were measured in the range of T = (283.15 to 338.15) K. The density, dynamic viscosity, and electrical conductivity of
6 2
[
C py][NTf ] were measured in the range of T = (308.15 to 338.15) K. The melting and glass transition temperatures of the two ILs
3
2
were determined according to the differential scanning calorimetry (DSC). The physicochemical properties, including molecular
volume, standard molar entropy, lattice energy, parachor, molar enthalpy of vaporization, interstice volume, interstice fraction, and
thermal expansion coefficient, were estimated in terms of empirical and semiempirical equations, as well as the interstice model
theory on the base of the experimental values. The dynamic viscosity and electrical conductivity values were fitted by
VogelꢀFulcherꢀTammann (VFT) and Arrhenius equations for [C py][NTf ] and the Arrhenius equation for [C py][NTf ].
6
2
3
2
’
INTRODUCTION
In view of the above facts, the density, surface tension, dynamic
viscosity, and electrical conductivity of the ILs [C py][NTf ]
n
2
Ionic liquids (ILs) are organic salts that exhibit outstanding
(
n = 3, 6) were measured at different temperatures, except for the
physicochemical properties like low melting temperature, good
solvation, negligible vapor pressure, high electrical conductivity,
and high thermal stability. ILs have received more and more
attention as potential green solvents from industrial and scientific
surface tension of [C py][NTf ]. The speed of sound was esti-
3
2
1
7
mated by the Auerbach relation at the different temperatures.
The standard molar entropy and lattice energy were estimated in
terms of Glasser's theory. Parachor was estimated by the
18
1
ꢀ4
communities.
Most ILs are hydrophilic, so the anion for
ꢀ
equation reported in the ref 19. The molar enthalpy of vaporiza-
bis(trifluoromethylsulfonyl)imide [NTf ] ILs as a hydrophobic
compound has attracted much more attention in some fields.
2
20
5
ꢀ8
tion was estimated in terms of Kabo's empirical equation. The
9
interstice volume, interstice fraction, and thermal expansion
Tokuda et al. have studied the properties of a series of
imidazolium-based ILs, for example, the melting temperature,
glass transition temperature, dynamic viscosity, and electrical
conductivity. The melting temperatures are lower than room
temperature, so these ILs are liquid at room temperature; the
decomposition temperatures are higher than 690 K, so these ILs
exhibit excellent thermal stability. The electrical conductivities
are also high enough to be applied in electrochemical devices.
Oliveira et al. studied the properties of a series of pyridinium-
based ILs. The density and dynamic viscosity of the dried and
saturated ILs were studied. The water content has a strong effect
on the dynamic viscosity, but only a very small effect on
density was observed. Jacquemin et al. have also studied the
density and dynamic viscosity of several pure and water-
saturated ILs. The same results were obtained for the effect
of water content on the density and dynamic viscosity. The ILs
with [NTf ] anion were studied widely because this type of
2
1,22
coefficient were estimated by the interstice model theory.
The VogelꢀFulcherꢀTammann (VFT) and Arrhenius equa-
tions were used to fit the temperature dependence on dynamic
viscosity and electrical conductivity values.
’
EXPERIMENTAL SECTION
1
0
Preparation of ILs [C py][NTf ] (n = 3, 6). N-Alkylpyridinium
n
2
bromide ([C py][Br], n = 3, 6) was synthesized according to the
n
23
procedure described in earlier paper. [C py][NTf ], n = 3, 6,
n
2
were synthesized according to the ion exchange reaction. The
0 g of salt [C py][Br], n = 3, 6 was placed in a 500 mL round-
1
1
8
n
bottomed flask and dissolved in 30 mL of distilled water; the
equivalent amount of bis(trifluoromethanesulfonyl)imide (HN-
(
SO CF ) (Rhodia Co., mass purity 99.9 %) was added dropwise
2 3
ꢀ
2
ILs has a relatively low viscosity, a wide electrochemical
window, and a high electrical conductivity to be applied in
the electrochemical devices.
Received: June 1, 2011
Accepted: September 27, 2011
Published: October 17, 2011
1
2ꢀ16
r 2011 American Chemical Society
4094
dx.doi.org/10.1021/je200534b
|J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
ARTICLE
Table 1. Molecular Weight and Thermal Properties of ILs
Table 3. Experimental Values of Density, G, and Surface
Tension, γ, of ILs [C py][NTf ] and [C py][NTf ]
[
C py][NTf ] and [C py][NTf ]
3
2
6
2
3
2
6
2
MW
T
g
T
m
[C
3
py][NTf
2
]
6 2
[C py][NTf ]
ꢀ1
ILs
g mol
K
K
a
b
3
T
F
γ
F
F
F
γ
[
C
3
py][NTf
2
]
402.33
317.6
276.4
ꢀ3
ꢀ2
ꢀ3
ꢀ3
ꢀ3
ꢀ2
K
g cm
mJ m
g cm
g cm
g cm
mJ m
3
3
3
3
3
3
[
C
6
py][NTf
2
]
444.06
196.6
2
2
2
2
3
3
3
3
83.15
88.15
93.15
98.15
03.15
1.4008
1.4078
1.4032
1.3987
1.3942
1.3897
1.3852
1.3807
1.3763
1.3718
1.3673
1.3629
1.3585
32.5
1.3966
1.3923
1.3877
1.3831
1.3789
1.3744
1.3699
1.3655
1.3615
1.3569
1.3525
32.2
32.0
31.7
31.6
31.4
31.2
31.0
30.8
30.6
30.3
30.1
3
Table 2. Mass Fraction, 10 w, of Water Before and After the
Measurement of Density, Surface Tension, Dynamic Viscos-
ity, and Electrical Conductivity
1.3865
1.3819
1.3774
1.3728
1.3683
1.3638
1.3592
1.3547
1.3502
08.15 1.4845
13.15 1.4800
18.15 1.4757
[
C
3
py][NTf
2
]
6 2
[C py][NTf ]
property
density
before
0.18
after
0.31
before
after
0.23
0.23
0.23
0.23
0.45
0.26
0.25
323.15 1.4710
328.15 1.4667
333.15 1.4623
338.15 1.4574
surface tension
viscosity
0.18
0.18
0.19
conductivity
a
The column values are the dried sample experimental values from
literature. The column values are the saturated sample experimental
values from literature.
3
0 b
into the flask at room temperature and stirred for 3 h. The lower
liquid was washed several times with 30 mL of distilled water
until no Br was present as shown by the solution of AgNO3/
HNO . The final product was dried under the vacuum at 353 K
for 24 h. H and C NMR spectra were recorded (see Tables 1,
, and 3 of the Supporting Information). From the NMR, the
3
0
of the density is the average value of three measurements. The
values are listed in Table 3.
Surface Tension Measurements. Using the tensiometer
DP-AW type produced by Sang Li Electronic Co.) of the forced
bubble method, the surface tension of the double-distilled freshly
degassed water was measured in range of T = (283.15 to 338.15) (
.05 K, which is in good agreement with the literature, and the
experimental error is ( 0.1 mJ m . The temperature was
controlled by a thermostat with an uncertainty of ( 0.05 K. The
attaining thermal equilibrium time was 30 min. The surface tension
of the samples was measured with the same method. The surface
tension values were recorded every 5 K. Each data point of the
surface tension is the average value of five measurements. The values
of the surface tension of the samples are listed in Table 3.
3
1
13
2
(
impurity of the samples was not found. The reaction equations
are the following:
24
0
ꢀ2
3
Thermodynamic Property. Differential scanning calorimetry
Dynamic Viscosity Measurements. The dynamic viscosity of
the ILs were measured using an Ostwald viscometer within the
range of T = (298.15 to 353.15) K. The temperature was
controlled by a thermostatic bath with an uncertainty of ( 0.1 K.
The attaining thermal equilibrium time in the viscometer was 30 min.
The values were recorded every 5 K. Each data point of the viscosity
is the average value of three measurements, and the uncertainties
were estimated to be ( 1 %. The results are listed in Table 4.
Electrical Conductivity Measurements. The electrical con-
ductivity measurments of the ILs were carried out by a MP522
(
DSC) was carried out on Q2000 V24.4 Build 116 at a heating
ꢀ
1
rate for 10 K min under nitrogen atmosphere over a tem-
3
perature window of (153 to 452) K. The data points of the
thermal properties in the two samples are listed in Table 1 (aee
the Figures A, B of the Supporting Information of DSC).
Water Content. The ILs [C py][NTf ] (n = 3, 6) are
n
2
hydrophobic compounds. Therefore, the property measurement
was carried out at atmospheric pressure except for the electrical
conductivity. Since traces of water still exist in the ILs after drying
by common methods, the presence of water becomes the most
problematic impurity and needs to be confirmed before and after
the measurement. The value of the water was the average of the
three times measurement by a Cou-Lo Aquamax Karl Fischer
moisture meter (v.10.06). The values are listed in Table 2.
Density Measurements. The densities of degassed water
were measured by a Westphal balance and the values were in
ꢀ
1
conductivity instrument with the cell constants of 1 cm
the cell was calibrated with the aqueous KCl solution) under the
(
high pure argon gas in the range of T = (283.15 to 338.15) K. The
uncertainties were estimated to be (1 %. The sample was placed in a
cell with a jacket. The temperature was controlled by a thermo-
stat with an uncertainty of ( 0.05 K. The attaining thermal
equilibrium time was 30 min. The values were recorded at per
24
good agreement with the literature within experimental error
ꢀ
3
5
K intervals. The results are listed in Table 4.
(
0.0002 g cm . Then, the densities of the ILs were measured
3
in the temperature range of T = (283.15 to 338.15) ( 0.05 K.
The sample was placed in a cell with a jacket. The density values
were recorded every 5 K. The temperature was controlled by a
thermostat with an uncertainty of ( 0.05 K. The attaining
thermal equilibrium time in the cell was 30 min. Each data point
’
RESULT AND DISCUSSION
10
Water Content. Oliveira et al. studied the properties of a
series of pyridinium-based ILs. The density and dynamic viscosity of
4
095
dx.doi.org/10.1021/je200534b |J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
ARTICLE
Table 4. Experimental Values of Electrical Conductivity, η,
and Dynamic Viscosity, σ, of ILs [C py][NTf ] and
3
2
[
C py][NTf ]
6 2
[C
3
py][NTf
2
]
[C
6
2
py][NTf ]
a
b
T
η
σ
η
η
η
σ
ꢀ
1
ꢀ1
K
mPa s mS cm
mPa s mPa s mPa s mS cm
3
3
3
3
3
3
2
83.15
88.15
93.15
98.15
03.15
08.15
13.15
18.15
23.15
28.15
33.15
38.15
0.76
1.00
1.30
1.66
2.08
2.57
3.11
3.87
4.46
5.38
6.21
7.12
2
2
2
3
3
3
3
3
3
3
3
84.5
66.4
53.2
43.1
35.2
29.1
24.5
20.9
17.9
96.206 63.143
75.215 50.410
59.862 40.912
48.419 33.686
39.742 28.104
33.039 23.728
27.794 20.244
23.642 17.430
20.298 15.157
33.0
27.6
23.3
19.8
17.1
14.9
13.5
Figure 1. Plot of T
m
vs n of ILs: 9, [C
n
py][NTf
2
] (n = 2, 3, 4, 5, 6); b,
7.19
[
C mim][NTf ] (n = 2, 4, 5, 6).
n
2
8.30
9.55
10.83
12.23
13.65
Table 5. Estimated Values of Physicochemical Properties of
[
C py][NTf ] at 298.15 K
6
2
property
value
property
value
a
The column values are the dried sample experimental values from
ꢀ
1
ꢀ3
ꢀ1
30 b
u/ m s
1095.5
41.82
44.1
V/cm mol
320.2
759.9
142.9
31.77
38.25
11.95
6.40
literature. The column values are the saturated sample experimental
values from literature.
3
3
30
3
ꢀ1
ꢀ2
10 S
a
/ mJ K
m
p
3
3
ꢀ
2
g
0
ꢀ1
E
a
/ mJ m
l m
Δ H / kJ mol
24
3
3
3
3
the dried and saturated ILs were studied. The water content has
strong effect on the dynamic viscosity, but only a very small effect
on density was observed. Jacquemin et al. have studied the
density and dynamic viscosity of several pure and water-saturated
ionic liquids. There is a large change in water content from
Vm/nm
0.5320
692.6
1.316
1429
10 v/cm
0
ꢀ1
ꢀ1
3
S / J K mol
∑v/cm
3
3
11
7
ꢀ1
2
1
0 k/J K
10 ∑v/V
3
4
ꢀ1
T
T
U
c
/K
/K
pot/ kJ mol
10 α/K , (exp.)
4
ꢀ1
ꢀ
3
ꢀ3
b
857
393
10 α/K , (cal.)
6.01
0
.05 10 to 19.8 10 mass fraction for the IL [Emim][NTf2].
3
3
ꢀ1
3
The values of dynamic viscosity are also changed greatly, for
example: the dynamic viscosities are 28.7 mPa s for dried and
3
of [C py][NTf ] is higher than the other ILs [C py][NTf ]
3
2
n
2
1
6.8 mPa s for saturated at 303 K, respectively. The mass
3
ꢀ
3
(n = 2, 4, 5, 6). So, the asymmetry and oddꢀeven effect lead to
fractions of water content are 0.18 10 before measurement
and 0.19 10 after measurement in the present work. The
effect of absorbed water in the ILs can be ignored during the
3
ꢀ3
the high melting point for IL [C py][NTf ].
3
2
3
17
Speed of Sound. According to the Auerbach relation, the
speed of sound has the following relation with density and surface
tension (eq 1). The speed of sound of IL [C py][NTf ] can be
measurements.
6
2
Thermal Analysis. The glass temperature (T ), crystallization
g
estimated by the following equation at 298.15 K.
temperature (T ), and melting temperature (T ) can be ob-
c
m
tained from the DSC thermograms during the programmed
heat/cool/heat step. From the DSC, there are two exothermic
peaks for [C py][NTf ] at (231.6 and 243.4) K; it indicated that
ꢀ1
2=3
u=m s ¼ ðγ=ð0:00063FÞÞ
ð1Þ
3
6
2
where γ is surface tension, F is density, and u is the speed of
it has two crystal forms. The inflection value of the glass tem-
sound. The estimated value of [C py][NTf ] is listed in Table 5.
6
2
perature is T = 196.6 K. The peak value of the melting temper-
g
Volumetric and Surface Properties. Recently, Oliveira
25
ature is T = 276.4 K; the value is nearly the same as 277 K. The
10
m
et al. reported the density and dynamic viscosity values of
dried and water-saturated IL [C py][NTf ]. The values of present
work are in the range of the values of the dried and water-saturated
IL of the literature. The percent deviations of [C py][NTf ]
density of this work with the literature data were determined to
be: 0.43 % to 0.48 % for the dried sample, 0.09 % to 0.17 % for the
saturated sample; for dynamic viscosity: 12 % to 14 % for the
dried sample; 15 % to 25 % for the saturated sample. As expected,
the temperature dependence on the density and surface tension
in the range of the measurement can be fitted by the following
equation:
melting point (T ) of the IL [C py][NTf ] is 317.56 K. The
m
3
2
26
6
2
value is much higher than that of 303.61 K for [C py][NTf ],
2
2
26
26
2
99.05 K for [C py][NTf ], 272.84 K for [C py][NTf ], and
4 2 5 2
6
2
2
76.4 K for [C py][NTf ]. The temperature dependence on the
6
2
alkyl chain length of [C py][NTf ] (n = 2, 3, 4, 5, 6) is shown in
n
2
Figure 1. The melting point of this type ILs [C py][NTf ] (n = 2,
n
2
4
, 5, 6) is higher than the ILs 1-alkyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide [C mim][NTf ] (n = 2, 4, 5, 6)
that have the same side-chain.
n
2
9
,27ꢀ29
The result can be attributed
to the symmetry of the cation that has been demonstrated by
3
0
Seddon. The melting temperature of [C mim][NTf ] could
3
2
2
8,29
2
not be found.
n = 2, 3, 4, 5, 6) decreases with the larger the asymmetry of the
side chain except for the IL [C py][NTf ]. The oddꢀeven
The melting point of the ILs [C py][NTf ]
n 2
Y ¼ A þ BT þ CT
ð2Þ
(
where Y is the density or surface tension; A, B, and C are
adjustable parameters. The fitting curve can be obtained (see
3
2
3
1
effect can be used to explain the result that the melting point
4
096
dx.doi.org/10.1021/je200534b |J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
ARTICLE
2
Figure 4. Plot of V
m
vs n at 298.15 K. The correlation coefficient is R =
3 2 6 2
Figure 2. Plot of density, F, vs T/K: 9, [C py][NTf ]; b, [C py][NTf ].
0.9999.
Table 6. Fitting Parameters and Correlation Coefficient for
Density and Surface Tension
Table 7. Estimated Volumetric Properties of the ILs
[
C py][NTf ] (n = 2, 4, 5, 6) at 298.15 K
n
2
4
7
2
ILs
property
A
10 B
10 C
R
F
V
m
V
+
r
+
ꢀ
3
[
[
C
3
py][NTf
2
]
]
F/g cm
1.6768 ꢀ3.7466 ꢀ8.0952 0.9998
3
ꢀ3
3
3
g cm
nm
M
+
nm
nm
ꢀ
2
3
γ/mJ m
3
ꢀ
3
a
C
6
py][NTf
2
F/g cm
1.6655 ꢀ9.7932
43.5 368.5
1.5884 0.9999
[C
2
4
5
6
py][NTf
py][NTf
py][NTf
py][NTf
2
2
2
2
]
1.5375
1.4547
1.4214
1.3877
0.4196 108.16
0.4754 136.22
0.5030 150.24
0.5320 164.27
0.2164
0.2722
0.2998
0.3288
0.372
0.402
0.415
0.428
3
ꢀ
2
a
a
b
γ/mJ m
ꢀ0.7992 0.9952
[C
]
]
]
3
[
[
C
C
a
b
Reference 25. This work.
Additionally, the surface energy, E = γ ꢀ T(∂γ/∂T) , can be
a
p
estimated by the experimental value of the surface tension; the
ꢀ
2
value of [C py][NTf ] is 44.1 mJ m . Comparing with fused
6
2
3
salts and organic liquids, this value is lower than fused salts, for
ꢀ
2
example, E = 146 mJ m for NaNO , and is closed to organic
a
3
3
ꢀ2
liquids, for example, E = 67 mJ m for benzene and E = 51.1
mJ m for octane. This fact shows that the alkyl group of the
ionic liquid at the liquid/vapor interface is directed toward
a
3
a
ꢀ
2
32
3
the vapor.
The molecular volume, V , can be calculated from experi-
m
mental density using the following equation:
Vmð298:15KÞ ¼ M=ðN FÞ
ð4Þ
3
6 2
Figure 3. Plot of surface tension, γ vs T/K: 9, [C py][NTf ].
where M is molar mass, F is density, and N is Avogadro's
3
Figure 2). The best fitting values and the correlation coefficient
are listed in Table 6.
By plotting the ln F against T/K, a straight line can be
obtained, and the empirical equation is:
constant. The values of the molecular volume are 0.5320 nm
3
3
for [C py][NTf ], 0.4196 nm for [C py][NTf ], 0.4754 nm
6
2
2
2
3
for [C py][NTf ], and 0.5030 nm for [C py][NTf ] at 298.15
4
2
5
2
26
K. The plot of V against the number of the carbons, n, in the
m
alkyl chain of the samples ([C py][NTf ]) can be obtained (see
Figure 4). The value, 0.0280 nm , of the slope of the fitting line is
n
2
ꢀ
3
3
ln F=g cm ¼ b ꢀ α T=K
ð3Þ
3
3
the mean contribution of the methylene to the molecular volume
3
where b is an empirical constant and the negative value of the
at 298.15 K. It is close to the values of 0.0272 nm for ILs
1
8
3
18
slope, α = ꢀ(∂ ln F/∂T) , is the thermal expansion coefficient of
[C mim][BF ], 0.0282 nm for [C mim][NTf ], and
p
n
4
n
2
ꢀ
3
3
33
the sample. The fitted equation is ln F/g cm = 0.5183 ꢀ
0.0278 nm for [C mim][Ala]. The molar mass of cations of
3
n
ꢀ
4
2
6
.40 10 T/K for [C py][NTf ], R = 0.9999. So, the thermal
the ILs [C py][NTf ] (n = 2, 4, 5, 6) was listed in Table 7. The
3
6
2
n
2
ꢀ
4
ꢀ1
expansion coefficient is 6.40 10
For [C py][NTf ], by plotting the values of γ obtained from
K
for [C py][NTf ].
plot of Vm against the molar mass of cations of the samples
3
6
2
([C py][NTf ]) can be obtained. The intercept of the linear
6
2
n
2
the experiment against T/K, a linear equation was obtained (see
regression can be approximately regarded by the volume of the
ꢀ
3
Figure 3). From Figure 3, the surface entropy, S = ꢀ(∂γ/∂T) ,
anion, NTf2 ; the value is 0.2032 nm . The volume value of
a
p
ꢀ
3
ꢀ1
ꢀ2
ꢀ
3
ꢀ34
4
can be obtained, and the value is 41.82 10 mJ K
m
.
NTf2 is much higher than 0.1390 nm for AlCl
and
3
3
3
4
097
dx.doi.org/10.1021/je200534b |J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
ARTICLE
2/3
2
Figure 5. Plot of γV
m
vs T (K). The correlation coefficient is R =
0
.9918. b, [C py][NTf ].
6
2
Figure 6. Plot of dynamic viscosity, η, vs T/K: 9, [C py][NTf ]; b,
3
2
3
ꢀ 35
0
.1548 nm GaCl
.
The volume values of the cations and the
[C
6 2
py][NTf ].
4
radii are listed in Table 7.
18
According to Glasser's theory, the standard molar entropy
can be estimated from the equation:
Parachors and Molar Enthalpy of Vaporization. The parachor,
P, was estimated from the following equation:
19
0
ꢀ1
ꢀ1
3
S ð298:15KÞ=J K mol ¼ 1246:5ðVm=nm Þ þ 29:5
3
3
1=4
Pð298:15KÞ ¼ ðM 3 γ Þ=F
ð8Þ
ð5Þ
where M is molar mass, F is density, and γ is surface tension.
where V is the molecular volume. The value of [C py][NTf ] is
m
6
2
The value of molar enthalpy of vaporization was estimated in
20
listed in Table 5.
terms of Kabo's empirical equation:
The surface tension, γ, has a relationship with the temperature
32
g
0
ꢀ1
2=3
N1=3Þ þ 2:4
ð9Þ
in terms of the E €o tv €o s equation:
Δl Hm ð298:15KÞ=kJ mol ¼ 0:01121ðγV
3
3
γVm2=3 ¼ kðTc ꢀ TÞ
ð6Þ
where V is molar volume, γ is surface tension, and N is
Avogadro's constant.
The data of parachor and molar enthalpy of vaporization are
listed in Table 5.
where V is the molar volume of the liquid, T is the critical
c
temperature, and k is an empirical constant.
ꢀ
3
The water mass fractions of [C py][NTf ] are 0.23 10
6
2
3
ꢀ
3
before measurement and 0.26 10 after measurement of the
3
Interstice Model Theory. According to the interstice
surface tension in the present work. The water content mass
fraction increases by 13 % during the measurement. The effect of
absorbed water in the ILs on surface tension during the measure-
21,22
model,
the interstice volume, v, can be estimated by classical
statistical mechanics:
ments can be evaluated from the fluctuation around the linear fit
3=2
2
v ¼ 0:6791ðkb T=γÞ
ð10Þ
3
(with R = 0.9918) seen in Figure 5. Customarily, a fit with a
correlation coefficient larger than 0.995 is considered a good fit.
where k is Boltzmann constant, T is the thermodynamic
temperature, and γ is the surface tension of ILs.
The molar volume of ionic liquids, V, consists of the inherent
volume, V , and the volume of the interstices; the molar volume
b
2
/3
By plotting the γVm of [C py][NTf ] against T, a straight line
6
2
was obtained (see Figure 5). From the plot, the value of empirical
constant (k) and critical temperature (T ) can be obtained
c
i
ꢀ
7
according to the fitting equation. The values of k = 1.316 10
J K and T = 1429 K were obtained. Rebole et al. have
reported that the normal boiling point, T , is approximately 0.6T
3
of the interstice, ∑v = 2N v:
ꢀ
1
36
3
3
c
V ¼ Vi þ 2N v
ð11Þ
b
c
3
for ILs. Herein, the normal boiling point, T , can be calculated,
b
If the expansion volume of IL only results from the expansion
of the interstices when the temperature increases, the thermal
expansion coefficient, α, can be predicted from the interstice
model:
and the value is T = 857 K. For the majority of organic liquids,
b
ꢀ
7
ꢀ1
ꢀ7
ꢀ1
k ≈ 2.1 10 J K , but for fused salts, k = 0.4 10 J K for
3
3
2
3
3
3
fused NaCl. It indicated that [C py][NTf ] has a medium
6
2
polarity between organic liquids and fused salts in terms of the
value of k.
α ¼ ð1=VÞð∂V=∂TÞ ¼ 3N v=ðV TÞ
ð12Þ
p
3
3
The lattice energy, UPOT, was estimated according to the
18
following equation:
All of the data obtained from the empirical and semiempirical
equations are listed in Table 5
ꢀ
1
1=3
UPOTð298:15KÞ=kJ mol ¼ 1981:2ðF=MÞ þ 103:8
From Table 5, the value of estimation of the thermal expansion
coefficient is in good agreement with experimental value. It also
can be seen that the estimated value of interstice fraction, ∑v/V, is
11.95 % for [C py][NTf ]; the value is in good agreement with
3
ð7Þ
where M is molar mass and F is the density.
All of the data obtained from the empirical and semiempirical
equations are listed in Table 5.
6
2
the values of volume expansion resulted in the process from solid
to liquid state for majority materials which exhibit (10 to 15) %.
4
098
dx.doi.org/10.1021/je200534b |J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
Table 8. Fitted Values of Electrical Conductivity, Dynamic Viscosity, and Molar Conductivity of σ , E , σ , η , E , η , Λ , B, T ,
ARTICLE
∞
σ
0
∞
η
0
0
0
and R According to the VFT and Arrhenius Equations
Arrhenius equation
VFT equation
4
1
0 η
∞
E
η
η
0
B
T
0
ꢀ
1
2
2
property
IL
mPa s
kJ mol
R
mPa s
K
K
R
3
3
3
η/ mPa s
[C
3
py][NTf
2
2
]
]
11.77
1.65
26.19
32.51
0.9957
0.9984
3
[
C
6
py][NTf
0.0645
1036.0
153.8
/K
0.9999
ꢀ1
ꢀ1
2
ꢀ1
2
1
0σ
∞
/S cm
E
σ
/kJ mol
R
σ
0
/S cm
B/K
T
0
R
3
3
3
ꢀ
1
σ/mS cm
[C
3
py][NTf
2
2
]
]
4.31
22.63
0.9991
0.9970
3
[
C
6
py][NTf
77.95
32.44
0.81
803.1
168.4
/K
166.2
0.9996
ꢀ2
2
ꢀ1
2
10
Λ
0
/ S cm mol
B/K
T
0
R
3
3
2
ꢀ1
Λ/ S cm mol
[C
6
py][NTf
2
]
3.14
841.7
0.9996
3
3
39
the Arrhenius equation:
ln σ ¼ ln σ∞ ꢀ Eσ=ðRTÞ
ð15Þ
where σ is conductivity, σ is an empirical constant, and E is the
∞
σ
activation energy for electrical conduction (which indicates the
energy for an ion jump to a free hole). The values of σ , E , and
∞
σ
2
correlation coefficient, R , are listed in Table 8.
The values of electrical conductivity of ILs [C py][NTf ] were
6
2
fitted by using the Fulcher equation, which is commonly used
9,38,39
for ILs:
σ ¼ σ0 expð ꢀ B=ðT ꢀ T0ÞÞ
where σ , B, and T are empirical constants. The best fit values of
ð16Þ
0
0
2
σ , B, and T and the correlation coefficient, R , are listed in
0
0
Figure 7. Plot of electrical conductivity, σ, vs T/K: 9, [C
b, [C py][NTf ].
3 2
py][NTf ];
Table 8.
6
2
From Table 8, the values of the correlation coefficient, R, of the
IL [C py][NTf ] based on the VFT equation are better than that
6
2
This means that the interstice model is reasonable and the
interstice model theory can be applied to calculate the thermal
expansion coefficient of pyrinidium-based ILs.
from the Arrhenius equation. The VFT equation can be con-
sidered to be more suitable for application to ILs than the
Arrhenius equation. The dynamic viscosity and conductivity of
The temperature dependence on dynamic viscosity was plotted
for ILs [C py][NTf ] (n = 3, 6) (see Figure 6). By fitting ln η
the IL [C py][NTf ] were not fitted by the VFT equation within
3 2
the narrow temperature range from (308.15 to 338.15) K.
From Table 8, it can be seen that the activation energies of
dynamic viscosity and conductivity are increased with the exten-
sion of the alkyl side chain of the cation; the discrepancies
between the activation energies of dynamic viscosity and elec-
n
2
ꢀ
1
against T , a fitting equation can be obtained according to the
Arrhenius equation, and the Arrhenius equation is:
12,37
ln η ¼ ln η∞ þ E =ðR TÞ
ð13Þ
η
3
4
0
trical conductivity are relatively small. Hagiwara et al. obtained
the same result; it suggests that the high conductivity can be
explained by the low viscosity without introducing some special
conduction mechanism such as ion hopping.
where η is dynamic viscosity, η is an empirical constant, and Eη
∞
is the activation energy for viscous flow. The values of η , E ,
∞
η
2
and correlation coefficient, R , are listed in Table 8.
The values of dynamic viscosity of ILs [C py][NTf ] were
The molar conductivity was calculated according to the follow-
ing equation:
6
2
fitted by using the VogelꢀFulcherꢀTamman (VFT) equation,
9,38,39
which is commonly used for ILs:
ꢀ
1
Λ ¼ σ M F
ð17Þ
3
3
η ¼ η expðB=ðT ꢀ T ÞÞ
ð14Þ
0
0
where Λ is the molar conductivity, σ is the conductivity, M is the
molar mass, and F is the density. The values of the molar
conductivity are listed in Table 9. The temperature dependence
where η , B, and T are empirical constants. The best fit values of
0
0
2
η , B, and T and the correlation coefficient, R , are listed in
0
0
on molar conductivity of [C py][NTf ] was also fitted according
Table 8.
6
2
to the VFT equation:
The temperature dependence on electrical conductivity was
studied for ILs [C py][NTf ] (n = 3, 6) (see Figure 7). By fitting
ln σ against T , a fitting equation can be obtained according to
n
2
ꢀ1
Λ ¼ Λ expð ꢀ B=ðT ꢀ T ÞÞ
ð18Þ
0
0
4
099
dx.doi.org/10.1021/je200534b |J. Chem. Eng. Data 2011, 56, 4094–4101

Journal of Chemical & Engineering Data
ARTICLE
Table 9. Molar Conductivity, Λ, of ILs [C py][NTf ] (n = 3, 6)
electrical conductivity values of [C py][NTf ] (n = 3, 6) were
n
2
n
2
experimentally determined except for the surface tension of
C py][NTf ]. The properties of the IL [C py][NTf ] were
2
ꢀ1
Λ/ S cm mol
3
3
[
3
2
6
2
T/K
283.15 288.15 293.15 298.15 303.15 308.15
calculated by the empirical and semiempirical equations. The
VFT and Arrhenius equations were used to fit the dynamic
viscosity and electrical conductivity values on temperature. The
activation energies of visicous flow and electrical conduction
were obtained according to the Arrhenius equation. The values
increase with the extension of the alkyl side chain of the cation.
The relationship of the molar conductivity and dynamic viscosity
was described by the Walden rule.
[
3
C py][NTf
2
]
]
[
6
C py][NTf
2
0.24
0.32
0.41
0.53
0.67
0.83
T/K
313.15 318.15 323.15 328.15 333.15 338.15
[
[
C
3
py][NTf
2
]
]
1.95
1.00
2.26
1.25
2.61
1.45
2.97
1.75
3.36
2.03
3.77
2.34
C
6
py][NTf
2
’
ASSOCIATED CONTENT
Supporting Information. H NMR spectral data pf
1
S
b
C pyrBr (Table 1) and [C py][NTf ] amd [C py][NTf ]
6
3
2
6
2
1
3
(
[
Table 2) in DMSO and C NMR spectral data of
C py][NTf ] amd [C py][NTf ] in DMSO (Table 3); DSC
3
2
6
2
curves of [C py][NTf ] amd [C py][NTf ] (Figures A and B,
3
2
6
2
respectively). This material is available free of charge via the
Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
Corresponding Author
*
E-mail: tzc@dicp.ac.cn (Z.-C. Tan).
Figure 8. Walden plots for ILs [C
from (298.15 to 338.15) K: 9, [C
solid straight line is the ideal line for a 0.01 M aqueous KCl solution.
n
py][NTf
2
] (n = 3, 6) at temperatures
]; b, [C py][NTf ]; the
Funding Sources
This work was financially supported by the National Nature
Science Foundation of China under Grant NSFC Nos. 21073189,
3
py][NTf
2
6
2
2
0901076, and 21003081 and the Knowledge Innovation Pro-
where Λ, B, and T are constants. The best-fitted parameters are
listed in Table 8.
0
gram of the Chinese Academy of Sciences under Grant DICP
K2009D03.
The relationship of the molar conductivity and dynamic
41ꢀ44
viscosity has been described by the Walden rule:
’
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