Ionic parachor and its application II. Ionic liquid homologues of 1-alkyl-3-methylimidazolium Propionate {[Cnmim][Pro] (n = 2-6)}

Article abstract of DOI:10.1021/jp301985g

Five propionic acid ionic liquids (PrAILs) [C_nmim][Pro] (n = 2-6) (1-alkyl-3-methylimidazolium propionate) have been prepared by the neutralization method and characterized by ¹H NMR spectroscopy and differential scanning calorimetry

Full text of DOI:10.1021/jp301985g

Article
pubs.acs.org/JPCB
Ionic Parachor and Its Application II. Ionic Liquid Homologues of
1
‑Alkyl-3-methylimidazolium Propionate {[C mim][Pro] (n = 2−6)}
n
Jing Tong,* Xue Ma, Yu-Xia Kong, Yan Chen, Wei Guan, and Jia-Zhen Yang
Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China
*
S Supporting Information
ABSTRACT: Five propionic acid ionic liquids (PrAILs) [C mim][Pro] (n =
n
2
−6) (1-alkyl-3-methylimidazolium propionate) have been prepared by the
1
neutralization method and characterized by H NMR spectroscopy and
differential scanning calorimetry (DSC). Their density, ρ, surface tension, γ,
and refractive index, n , were measured at (298.15 ± 0.05) K, and the
D
experimental values of parachor for the PrAILs were calculated. Using the
+
parachor values of [C mim] obtained by Guan et al., the anionic parachor
n
values of [C mim][Pro] (n = 2−6), [C mim][RBF ] (R = N-C H
(n = 1−
n
2
3
n
2n+1
5
)), [C mim][Gly] (n = 2−6), and [C mim][PF (CF CF ) ] (n = 1−6) were
n
n
3
2
3 3
determined. Then, the parachor, surface tension, and refractive index of the
ILs investigated in this work were estimated. The estimated values correlate
quite well with the corresponding experimental values.
1
6
1
. INTRODUCTION
ionic parachor has been proposed as a new concept, that is,
ions composed of ILs can be seen as independent descriptors of
In the past few decades, interest in ionic liquids (ILs) has
1
,2
parachor. The ionic parachor, P , can be defined by the
increased dramatically, especially because carboxylic acid
ionic liquids (CAILs) were successfully synthesized from
natural carboxylic acids. CAILs have attracted considerable
attention from industry and the academic community as new-
generation “greener ionic liquids”.
enzyme-“friendly” cosolvent for resolution of amino acids,
ultrasonic irradiation toward synthesis of trisubstituted
imidazoles, assisted transdermal delivery of sparingly soluble
drugs, and some catalytic reactions. Among CAILs, propionic
acid ionic liquids (PrAILs) are the important ones.
i
following equation
Pi = γ1/4V
(1)
i
3
−6
CAILs are useful for an3
where V is the molar volume of an ion in an IL. According to
i
the additivity principle, the parachor value of an IL is equal to
the sum of ionic parachors of the corresponding cation and
anion
4
5
6
P = P + P−
(2)
+
In recent years, there has been a developing trend in the
literature toward the estimation of the physicochemical
where P and P are the ionic parachor of the cation and anion,
+ −
respectively. Now, the key question is how the experimental
value of the parachor for an IL can be divided into the
7
−9
properties for ILs by semiempirical methods.
Although
16
these estimated results cannot be regarded as accurate
physicochemical data, this approach is recommended because
it provides valuable insight into the behavior of materials.
Among all of the semiempirical methods, parachor is the
corresponding values for the cation and anion. Guan et al.
recommended two extrathermodynamic assumptions for the
evaluation of an individual ionic parachor. The first of them is
the extrapolation method. For example, for the acetic acid ionic
liquid homologues of 1-alkyl-3-methylimidazolium acetate
7
,10−13
simplest.
The parachor, P, is a relatively old concept that
is available as a link between the structure, density, and surface
tension of the liquids. However, the vast majority of parachor
(
[C mim][OAc] (n = 2−6)), according to eq 3
n
1
4,15
1/4
studies have focused on the uncharged compounds.
The
P = P + (M γ )/ρ
(3)
−
+
parachor data obtained from the neutral molecule are difficult
to be applied to an IL because there is no consideration of the
Coulomb force between ions. Although a number of early
studies attempted to determine the parachor values for ions,
these studies were hampered by experimental difficulties
encountered in determining the surface tensions and densities
of high melting salts, and no other related investigations
when the molar mass of the cation, M , approaches zero, the
+
ionic parachor of the anion, P , could be obtained. The second
−
extrathermodynamic assumption was proposed as eq 4, that is,
the volume ratio of the cation and anion the in crystal equals
their volume ratio in the IL or their parachor ratio
9
followed. However, because numerous ILs are fluid at room
Received: February 29, 2012
Revised: April 20, 2012
Published: April 25, 2012
temperature, they offer a solution to determine the
experimental parachor of ionic compounds. Therefore, the
©
2012 American Chemical Society
5971
dx.doi.org/10.1021/jp301985g | J. Phys. Chem. B 2012, 116, 5971−5976

The Journal of Physical Chemistry B
Article
of this synthetic route. First, [C mim]Br (n = 2−6) were
V (Crystal)/V (Crystal)
n
−
+
19,20
synthesized according to the literature.
Then, aqueous 1-
=
V (IL)/[V − V (IL)]
alkyl-3-methylimidazolium hydroxides ([C mim][OH] (n = 2−
m−
m
m−
n
6
)) were prepared from [C mim]Br (n = 2−6) by use of an
n
=
P (IL)/[P − P (IL)]
(4)
−
−
activated anion-exchange resin in a 100 cm column. However,
where V (Crystal) and V (Crystal) are the corresponding
[C mim][OH] (n = 2−6) are not particularly stable, and they
+
−
n
cationic and anionic volumes derived from crystal structures
and can be obtained from the literature; Vm−(IL) is the ionic
volume of the corresponding anion in an IL, Vm is the
should be used immediately after preparation. These hydroxide-
containing aqueous solutions were added dropwise to a slightly
excess of propionic acid in the aqueous solution. The mixture
was stirred with cooling for 48 h. Then water was evaporated
under reduced pressure at 40−50 °C. To this reaction mixture
was added, a mixed solvent of acetonitrile/methanol (volu-
metric ratio =9/1), and the mixture was stirred vigorously. The
mixture was then evaporated under reduced pressure to remove
slightly excess propionic acid and solvents. The products of
molecular volume of the IL, P (IL) is the ionic parachor of the
−
corresponding anion, and P is the experimental value of the
−
parachor for the IL. Guan et al. used [OAc] as the reference
ion, the reference value of P = 83.9, which is the average of
−
that obtained from two extrathermodynamic assumptions, and
then, the values of the ionic parachor for all corresponding
+
[C mim][Pro] (n = 2−6) were dried in vacuo for 2 days at 80
n
imidazolium cations, [C mim] , were obtained. However, we
n
°
C. The structures of the resulting PrAILs were confirmed by
believe that the second extrathermodynamic assumption is
more reliable than the extrapolation. Because the measured
range of the molar mass of the cation is about 56 (from about
1
H NMR spectroscopy (see the figures in section A in the
Supporting Information).
Differential scanning calorimetric (DSC) measurements
167.2 to 111.2) and the results are extrapolated to zero (further
showed that [C mim][Pro] (n = 2−6) have no obvious
n
from 111.2), this extrapolation may have some uncertainty.
In order to use the new experimental data to prove the
reliability and authenticity of the second assumption proposed
by Guan et al., in this paper, we report that (1) PrAILs
melting point and are given in the figures in section B in the
Supporting Information. The water contents (w is the water
2
−3
−3
mass fraction, w = (8.10 ± 0.01) × 10 , (8.30 ± 0.01) × 10 ,
2
−3
−3
(
×
=
8.10 ± 0.01) × 10 , (8.20 ± 0.01) × 10 , and (8.10 ± 0.01)
[
C mim][Pro] (n = 2−6) (1-alkyl-3-methylimidazolium
n
−3
10 mass fraction, respectively) in the ILs [C mim][Pro] (n
n
propionate) have been prepared by the neutralization method.
2) The density, surface tension, and refractive index for the
2−6) were determined by use of a Karl Fischer moisture
(
titrator (ZSD-2 type).
PrAILs were measured at (298.15 ± 0.05) K. Because the
PrAIL molecules can form strong hydrogen bonds with water
molecules, which is thus a problematic impurity, the standard
2
.3. Determination of the Density and Surface
Tension of the PrAILs. Because the PrAILs can form strong
hydrogen bonds with water molecules, small amounts of water
in the PrAILs are difficult to remove by conventional methods,
and it becomes the most problematic impurity. In order to
eliminate the effect of the impurity water, SAM was applied to
these measurements. According to the SAM, a series of samples
1
7
addition method (SAM) was applied in these measurements.
(
+
3) Using the parachor values of [C mim] obtained by Guan
n
16
et al., the anionic parachor values of [C mim][Pro] (n = 2−
n
6
), [C mim][RBF ] (R = N-C H
(n = 1−5)), [C mim]-
2
3
n
2n+1
n
[
Gly] (n = 2−6), and [C mim][PF (CF CF ) ] (n = 1−6)
n
3
2
3 3
of [C mim][Pro] (n = 2−6) with different water contents were
n
were determined. (4) Finally, in terms of the ionic parachor
prepared.
data, the parachor, surface tension, and refractive index n of
D
The density of degassed water was measured on a Westphal
balance at (298.15 ± 0.05) K and was in good agreement with
the investigated ILs were estimated.
2
1
−3
the literature within an experimental error of ±0.0002 g·cm .
Then, the densities of the samples were measured at (298.15 ±
2. EXPERIMENTAL SECTION
2
.1. Chemicals. Deionized water was distilled in a quartz
0
.05) K. The sample was placed in a cell with a jacket and was
−4
−1
still, and its conductance was (0.8−1.2) × 10 S·m . Anion-
exchange resin (type 717) was purchased from Shanghai
Chemical Reagent Co. Ltd. and activated by the regular method
before use. Propionic acid was distilled and dried under reduced
pressure. N-Methylimidazole (AR-grade reagent) was vacuum-
distilled prior to use. 1-Bromoethane, 1-bromopropane, 1-
bromobutane, 1-bromopentane, and 1-bromohexane (AR-grade
reagent) were distilled before use. Ethyl acetate and acetonitrile
were distilled and then stored over molecular sieves in tightly
sealed glass bottles.
thermostatted with an accuracy of ±0.05 K.
By using the tensiometer of the forced bubble method
DPAW type produced by Sang Li Electronic Co.), the surface
(
tension of water was measured at (298.15 ± 0.05) K and was in
21
good agreement with the literature within an experimental
−2
error of ±0.1 mJ·m . Then, the values of surface tension of the
samples were measured by the same method at (298.15 ± 0.05)
K.
2
.4. Determination of the Refractive Index of the
PrAILs. The refractive index, n , of the PrAILs was measured
D
2
.2. Preparation of the PrAILs. The PrAILs [C mim]-
n
by an Abbe refractometer. First, the refractive index of the
[
Pro] (n = 2−6) have been prepared by a neutralization
method according to Fukumoto et al. Figure 1 is a schematic
degassed water was measured by the instrument at (298.15 ±
18
21
0
.05) K and was in good agreement with the literature within
an experimental error of ±0.0001. Then, the refractive index of
a series of samples, which were prepared by the SAM, was
measured at 298.15 K with an accuracy of ±0.05 K.
3
. RESULTS AND DISCUSSION
.1. Values of Density, Surface Tension, and
3
Figure 1. Preparation of PrAILs by the neutralization method. 1,
Refractive Index for the Samples. The measured values
of the density, surface tension, and refractive index for the
[
C mim][Br]; 2, [C mim][OH]; 3, [C mim][Pro].
n
n
n
5
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The Journal of Physical Chemistry B
Article
−
3
−2
Table 1. Values of the Density (ρ /g·cm ), Surface Tension (γ/mJ·m ), and Refractive Index for [C mim][Pro] (n = 2−6)
n
a
Containing Various Amounts of Water at 298.15 K
[
[
[
[
[
C mim][Pro]
2
1
03w2
8.60
12.1
1.1937
14.5
1.1945
16.7
18.6
1.1957
0
r2
s × 105
3.8
−
3
ρ/g·cm
1.1926
8.30
1.1951
15.6
1.1900
0
0.999
r2
3
10 w
10.6
41.1
11.2
13.2
41.4
14.2
18.1
42.1
19.8
s
2
−
2
γ/mJ·m
40.7
41.7
39.6
0.99
r2
0.042
3
5
10 w
8.70
17.3
0
s × 10
2
nD
1.4882
1.4876
1.4871
1.4866
1.4862
1.4897
0.99
7.42
C mim][Pro]
3
1
03w2
8.80
11.7
1.1582
14.3
1.1589
17.1
18.7
1.1599
0
r2
s × 105
3.7
−
3
ρ/g·cm
1.1575
8.40
1.1595
16.1
1.1554
0
0.999
r2
3
10 w
10.8
39.6
11.3
13.2
39.9
14.0
19.0
40.6
19.6
s
2
−
2
γ/mJ·m
39.4
40.2
38.4
0.99
r2
0.042
3
5
10 w
8.80
17.0
0
s × 10
2
nD
1.4871
1.4865
1.4860
1.4855
1.4851
1.4886
0.99
7.85
C mim][Pro]
4
1
03w2
8.10
11.4
1.1307
13.6
1.1313
16.8
19.9
1.1329
0
r2
s × 105
5.1
−
3
ρ/g·cm
1.1300
8.90
1.1321
17.4
1.1279
0
0.999
r2
3
10 w
11.6
38.3
11.3
14.7
38.6
14.0
19.6
39.2
19.6
s
2
−
2
γ/mJ·m
38.0
38.9
37.0
0.99
r2
0.030
3
5
10 w
8.80
17.0
0
s × 10
2
nD
1.4847
1.4842
1.4838
1.4833
1.4828
1.4862
0.99
3.88
C mim][Pro]
5
1
03w2
8.40
11.5
1.1066
13.4
1.1071
16.3
19.6
1.1087
0
r2
s × 105
4.4
−
3
ρ/g·cm
1.1059
8.20
1.1078
15.4
1.1038
0
0.999
r2
3
10 w
10.3
37.2
11.2
12.6
37.6
13.9
17.5
38.2
19.5
s
2
−
2
γ/mJ·m
36.9
38.0
35.7
0.99
r2
0.053
3
5
10 w
8.90
16.9
0
s × 10
2
nD
1.4828
1.4824
1.4820
1.4815
1.4812
1.4841
0.99
4.47
C mim][Pro]
6
1
03w2
8.30
11.1
1.0865
13.7
1.0872
16.9
19.7
1.0888
0
r2
s × 105
3.2
−
3
ρ/g·cm
1.0857
8.20
1.0880
15.5
1.0835
0
0.999
r2
3
10 w
10.7
36.2
11.2
12.8
36.5
13.4
17.9
37.1
17.5
s
2
−
2
γ/mJ·m
35.8
36.8
34.8
0.99
r2
0.043
3
5
10 w
8.50
15.7
0
s × 10
2
nD
1.4816
1.4813
1.4810
1.4807
1.4804
1.4828
0.99
3.66
a
2
w is the water content. r is the correlation coefficient squared, and s is the standard deviation.
2
samples of ILs [C mim][Pro] (n = 2−6) containing various
Vm = M/(N·ρ)
(5)
n
contents of water are listed in Table 1. Each value in the table is
the average of three measurements. According to the SAM, the
values of the density or surface tension or refractive index of the
samples at a given temperature were plotted against the water
content, w (w is the mass percentage), so that a series of good
where M is the molar mass and N is the Avogadro constant, so
that V (n = 2−6) = 0.2572, 0.2851, 0.3126, 0.3406, and 0.3685
m
3
nm for [C mim][Pro] (n = 2−6) at 298.15 K. The average
n
difference between molecular volumes of [C mim][Pro] and
n
2
2
3
[
C mim][Pro] is 0.0278 nm , which can be seen as the
straight lines were obtained. The intercepts of the straight lines
are the values of the density or surface tension or refractive
n‑1
contribution to the molecular volume of one methylene
−CH −) group and was in good agreement with a mean
(
2
index of [C mim][Pro] (n = 2−6) without water and can be
n
3
contribution of 0.0272 nm per methylene (−CH −) group
2
seen as the experimental values, which are included in the
22
obtained by Glasser from ILs [C mim][BF ]. The values of
n
4
column where w = 0 in Table 1. Figures C1−C3 in the
2
the molecular volume, V , are listed in Table 2.
m
Supporting Information are the plots of the density, surface
22
According to Glasser’s theory, the standard molar entropy
tension, refractive index, respectively, against w at (298.15 ±
0
2
for an IL, S , is given by
2
0.05) K. The square of the correlation coefficients, r , of all
linear regressions was larger than 0.99, and all values of
standard deviation, s, were within the experimental error. These
data show that use of the SAM is appropriate in this work.
0
−1
−1
3
S (298)/(J·K ·mol ) = 1246.5(V /nm ) + 29.5
(6)
m
0
−1
−1
The values of S (J·K ·mol ) for [C mim][Pro] (n = 2−6)
are listed in Table 2. These data imply that the entropy
contribution per methylene group to the standard entropy for
n
3
.2. Volumetric Properties of ILs [C mim][Pro] (n = 2−
n
6
). If the liquid state was the reference, from the experimental
−1
−1
values of the density, the molecular volumes (the sum of the
[C mim][Pro] is 34.7 J·K ·mol , and this value is in good
n
1
−
−1
volumes of the positive and negative ions), V , of [C mim]-
agreement with the values of 33.9 J·K ·mol for [C mim]-
n
1
22
m
n
−
−1
[
Pro] (n = 2−6), were calculated from the following equation
[BF ] and 35.1 J·K ·mol for [C mim][NTf ].
4 n 2
5
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The Journal of Physical Chemistry B
Article
Table 2. Values of the Molecular Volume, Standard Molar
Entropy, and Lattice Energy of the ILs at 298.15 K
3.3. Ionic Parachor. According to the definition of the
parachor
0
M/
ρ/
g·cm
S /
Upot
/
1/4
g·mol−
1
−3
Vm/nm3 J·K−1·mol
−1
−1
P = (Mγ )/ρ
(8)
ionic liquid
kJ·mol
[
[
[
[
[
C mim]
184.24
198.27
212.29
226.32
240.35
1.1900
1.1554
1.1279
1.1038
1.0835
0.2572
0.2851
0.3126
0.3406
0.3685
350.1
384.8
419.2
454.0
488.8
473
2
where γ is the surface tension, M is the molar mass, and ρ is the
density of a substance, the experimental values, PExp, of the
parachor for the PrAILs are listed in Table 3. In terms of the
[Pro]
C mim]
460
450
440
431
3
[Pro]
+
16
values of [C mim] , P , obtained by Guan et al., the ionic
C mim]
n
+
4
[Pro]
parachor of the anion, P , can be determined by eq 2 and are
−
C mim]
listed in Table 3. From Table 3, the average of the ionic
5
[Pro]
−
parachor for the anion [Pro] is (109.1 ± 2.0). Similarly, using
C mim]
+
6
the parachor values of [C mim] , the values of P for
n
−
[Pro]
[
(
C mim][RBF ] (R = N-C H
(n = 1−5)), [C mim][Gly]
2
3
n
2n+1
n
n = 2−6), and [C mim][PF (CF CF ) ] (n = 1−6) were
n
3
2
3 3
calculated and are listed in Table 3. From Table 3, it can be
−1
The lattice energy, U , in kJ·mol , can be estimated from
pot
seen that the relative standard deviations of P for [C mim]-
−
n
2
2
the following equation
[
gly] and [C mim][PF (CF CF ) ] were less than 2%.
n
3
2
3 3
UPOT/kJ·mol−1 = 1981.2(ρ/M) + 103.8
1/3
For [C mim][BF ], the ionic volumes of the cation and
2 4
27
(7)
3
anion in the crystal are V (Crystal) = 0.156 nm and
+
3
The estimated values of Upot for [C mim][Pro] are listed in
V (Crystal) = 0.073 nm , respectively, so that according to the
n
−
23
Table 2. Recently, Krossing et al. pointed out that eq 7, which
was an old formulation, provides lattice enthalpies of 20−140
kJ·mol− that are lower than the ones assessed by the Born−
Fajans−Haber (BFH) cycle. Even taking into account this
second extrathermodynamic assumption, P = 285.0 and P =
+ −
133.3 were obtained. In comparison with P = 283.1 and P =
+
−
1
135.2 in Table 3, the relative deviation is also less than 2%.
These facts show that the second assumption proposed by
Guan et al. is more credible.
deviation, the lattice energy of ILs, [C mim][Pro], is less than
n
−1
that of fused salts; for example, U = 613 kJ·mol for fused
3.4. Predicting Parachor and Surface Tension. Using
pot
2
1
+
CsI, which is the lowest lattice energy among alkali halides.
The lower lattice energy is an essential ingredient for ILs
parachor values of the above anions and [C mim] , estimated
n
parachor values of the corresponding ILs, P , can be obtained
Cal
[
C mim][Pro], and the true driver for being liquid at these
and are listed in Table 3. In Table 3, PExp is the experimental
value. Figure 2 is a comparative plot of the estimated parachor
values as a function of the corresponding experimental values
n
conditions is the difference between the entropy in the solid
state and that in the liquid state.
2
4,25
Table 3. Values of Ionic Parachor of the ILs According to the Second Extrathermodynamic Method
ρ/g·cm−
3
γ/mJ·m−2
PExp
P+
a
P−
105.3
b
PCal
d
d
d
d
[
[
[
[
[
C mim] [Pro]
1.1900
1.1554
39.6
388.4
427.2
464.2
501.1
283.1
319.0
355.0
390.9
426.1
392.2
428.1
464.1
500.0
535.2
2
d
C mim] [Pro]
3
38.4
108.2
109.2
110.2
112.5
d
C mim] [Pro]
4
1.1279
d
37.0
d
C mim] [Pro]
5
1.1038
35.7
d
d
C mim] [Pro]
6
1.0835
34.8
538.6
P of [Pro]−
c
109.1 ± 2.0
153.0
−
e
e
[
C mim][CH BF ]
1.1536
45.2
436.1
468.8
498.1
527.8
566.5
457.6
418.3
758.0
790.3
826.3
860.6
894.8
283.1
283.1
283.1
283.1
283.1
283.1
283.1
252.6
283.1
319.0
355.0
390.9
426.1
2
3
3
e
e
[
C mim][C H BF ]
1.1329
42.5
185.7
2
2
5
3
e
e
[
[
C mim][N-C H BF ]
1.1068
38.0
215.0
2
3
7
3
e
e
C mim][N-C H BF ]
1.0818
34.2
244.7
2
4
9
3
e
e
[
C mim][N-C H BF ]
1.0645
33.8
283.4
2
5
11
3
e
e
[
C mim][CH CHBF ]
1.1614
44.3
174.5
2
2
3
e
e
[
C mim][BF ]
1.2853
54.4
135.2
2
4
f
f
[
[
[
[
[
[
C mim][PF (CF CF ) ]
1.75552
36.3
1
3
2
3 3
f
f
C mim][PF (CF CF ) ]
1.70 926
34.8
507.2
788.5
824.4
860.4
896.3
931.5
2
3
2
3 3
f
f
C mim][PF (CF CF ) ]
1.66756
34.1
507.3
3
3
2
3 3
f
f
C mim][PF (CF CF ) ]
1.62962
33.2
505.6
4
3
2
3 3
f
f
C mim][PF (CF CF ) ]
1.59516
32.4
503.9
5
3
2
3 3
f
f
C mim][PF (CF CF ) ]
1.56356
31.7
929.2
503.1
6
3
2
3 3
c
P of [PF (CF CF ) ]
505.4 ± 1.9
137.8
−
3
2
3
3
g
g
h
h
i
[
[
[
[
[
C mim][Gly]
1.1589
48.1
420.9
283.1
319.0
355.0
390.9
426.1
421.2
457.1
493.1
529.0
564.2
2
h
C mim][Gly]
3
1.1358
45.6
43.5
41.9
40.6
455.9
493.1
528.3
136.9
h
C mim][Gly]
4
1.1109
138.1
i
C mim][Gly]
5
1.0947
137.4
i
i
C mim][Gly]
6
1.0755
566.4
140.3
P of [Gly]−
c
138.1 ± 0.9
−
a
b
c
d
e
f
g
h
i
Reference 14. P = P − P . P is the average of P . This work. Reference 27. Reference 28. Reference 29. Reference 30. Reference 12.
−
Exp
+
−
−
5
974
dx.doi.org/10.1021/jp301985g | J. Phys. Chem. B 2012, 116, 5971−5976

The Journal of Physical Chemistry B
Article
Figure 3. Plot of the estimated surface tension for the ILs versus their
experimental values: ■ [C mim][Pro] (n = 2−6); ● [C mim]-
Figure 2. Plot of the estimated parachor for the ILs versus their
experimental values: ■ [C mim][Pro] (n = 2−6); ● [C mim]-
n
n
n
n
[
PF (CF CF ) ] (n = 2−6); ▲ [C mim][Gly] (n = 2−6). γ = −0.96
3
2
3
3
n
Cal
[
PF (CF CF ) ] (n = 2−6); ▲ [C mim][Gly] (n = 2−6). P = 0.524
2
3
2
3
3
n
Cal
+
1.03γ ; s = 0.57; r = 0.99.
Exp
2
+
0.999P ; s = 1.94; r = 0.999.
Exp
2
4
Table 5. Experimental Values of R , n , and 10 α and
m
D
p
Predicted Values of nD Cal for [C mim][Pro] at 298.15 K
n
for the ILs [C mim][Pro], [C mim][PF (CF CF ) ], and
n
n
3
2
3 3
[
C mim][Gly], and it shows that the estimated parachor and
ΔnD = nD Exp −
n
2
4
ionic liquid
Rm
nD Exp
10 α
nD Cal
nD Cal
the experimental values are highly correlated (correlation
coefficient squared, r = 0.999; standard deviation, s = 1.94) and
very similar (gradient = 0.999; intercept = 0.524).
p
2
[C mim]
44.74
1.4897
17.75
19.64
21.45
23.28
25.13
20.84
22.73
24.50
25.94
21.12
22.84
24.65
26.47
28.44
1.4896
0.0001
2
[
Pro]
[
[
[
C mim]
49.50
54.05
58.68
63.33
52.53
57.28
61.75
65.37
53.22
57.57
62.14
66.72
71.67
1.4886
1.4862
1.4841
1.4828
1.5069
1.5085
1.5066
1.4944
1.5106
1.5044
1.5019
1.4984
1.4970
1.4886
1.4861
1.4841
1.4827
1.5091
1.5081
1.5044
1.4193
1.5099
1.5052
1.5026
1.4998
1.4991
0
In terms of the estimated parachor values of the ILs, P , in
3
Cal
[
Pro]
Table 3, the predicted values of surface tension using eq 8 can
be obtained and are listed in Table 4. In Table 4, γExp is the
corresponding experimental value, and Δγ is the difference
between the experimental value and the corresponding
estimated one of the surface tension of the ILs, that is, Δγ =
C mim]
0.0001
0
4
[Pro]
C mim]
5
[Pro]
[C
mim]
Pro]
0.0001
−0.0022
0.0004
0.0022
0.0751
0.0007
−0.0008
−0.0007
−0.0014
−0.0021
6
[
γ
− γ . Figure 3 is a plot of the predicted surface tensions
Exp
Cal
[
C mim]
3
versus their corresponding experimental values, and it shows
that the values are highly correlated (r = 0.99) and extremely
[
Gly]
2
[
C mim]
4
similar (gradient = 1.03; intercept = −0.96), which not only
imparts confidence in the application of the ionic parachor but
also provides a method to closely predict the former and latter
physical properties for ILs using ionic parachor data.
.5. Predicting Molar Refraction and the Refractive
Index. The Lorentz−Lorenz relationship between the
refractive index and the mean molecular polarizability, α ,
[Gly]
[C mim]
5
[
Gly]
[
[
C mim]
6
[
Gly]
3
C mim]
2
[
Ala]
p
[C mim]
3
31
leads to the definition of molar refraction Rm
[Ala]
[
[
C mim]
2
2
4
Rm = [(nD − 1)/(nD + 2)]·(M/ρ) = (4πN/3)·αp
[Ala]
C mim]
(9)
5
[
Ala]
According to eq 9, values of R and α were calculated from n
values of ILs without water and are listed in Table 5.
m
p
D
[C mim]
6
[Ala]
Because R has the property of additivity, its values for the
m
homologues [C mim][Pro] (n = 2−6) can be predicted by a
n
semiempirical method. The contribution per methylene
of R of [C mim][Pro] calculated from eq 9 are listed in Table
m n
(
−CH −) group is 4.65. It is very consistent with 4.60,
5. In addition, refractive index data of [C mim][Ala] and
2
n
32
32
which was obtained using AAILs [C mim][Ala]. The values
[C mim][Gly] from the literature are also listed in Table 5.
n
n
Table 4. Parachor and Surface Tension of the Investigated ILs
[
C mim]X
[C mim][Pro]
n
[C mim][PF (CF CF ) ]
[C mim][Gly]
n
n
n
3
2
3
3
γExp
γCal
Δγ
γExp
γCal
Δγ
0.3
γExp
γCal
Δγ
[
[
[
[
[
C mim]X
39.6
38.4
37.0
35.7
34.8
41.2
38.7
37.0
35.4
33.9
−1.6
−0.3
0.0
34.8
34.1
33.2
32.4
31.7
34.5
33.8
33.2
32.6
32.0
48.1
45.6
43.5
41.9
40.6
48.2
46.1
43.5
42.1
40.0
−0.1
−0.5
0.0
2
C mim]X
3
0.3
0.0
C mim]X
4
C mim]X
5
0.3
−0.2
−0.3
−0.2
0.6
C mim]X
6
0.9
5
975
dx.doi.org/10.1021/jp301985g | J. Phys. Chem. B 2012, 116, 5971−5976

The Journal of Physical Chemistry B
Article
3
3
Tripathi combined eq 9 with the parachor equation, eq 8,
to yield an expression
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/4
2
2
γ
= (P/R )[(nD − 1)/(nD + 2)]
Using eq 10, the refractive index of an IL can be estimated from
(10)
Thermodyn. 2010, 42, 904−908.
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(
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1133−11140.
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nD Cal, for [C mim][Pro], [C mim][Ala], and [C mim][Gly]
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n
n
were calculated from eq 10 and are listed in Table 5. Figure 4 is
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(
6
(
1
(
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5, 655−659.
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Figure 4. Plot of the estimated refractive index for ILs [C mim][Pro]
n
versus their experimental values: ■ [C mim][Pro] (n = 2−6); ●
n
[
+
C mim][Gly] (n = 2−5); ▲ [C mim][Ala] (n = 2−6). n
= 0.073
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n
n
D Cal
−3
2
0.95n
; s = 1.6 × 10 ; r = 0.98.
D Exp
(24) Preiss, U. P.; Beichel, W.; Erle, A. M. T.; Paulechka, Y. U.;
a plot of the predicted values of the refractive index for the ILs
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the predicted values of the refractive index and the
corresponding experimental values are correlated (correlation
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(
6
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ASSOCIATED CONTENT
(28) Liu, Q.-S.; Tong, J.; Tan, Z.-C.; Urs, W.-B.; Yang, J.-Z. J. Chem.
■
Eng. Data 2010, 55 (7), 2586−2589.
*
S
Supporting Information
1
(29) Yang, J.-Z.; Zhang, Q.-G.; Wang, B.; Tong, J. J. Phys. Chem. B
H NMR, DSC traces, and density, surface tension, and
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
(32) Fang, D.-W. Ph.D. Dissertation. Xining: Institute of Salt Lakes,
Chinese Academy of Sciences, 2008.
(33) Tripathi, R. C. J. Indian Chem. Soc. 1941, 18, 411−427.
ACKNOWLEDGMENTS
■
This project was supported by NSFC (20903053, 21173107),
the Education Bureau of Liaoning Province (LS2010069,
LS2010068), and the Foundation of 211 Project for Innovative
Talents Training, Liaoning University.
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