Ionic parachor and its application in acetic acid ionic liquid homologue 1-alkyl-3-methylimidazolium acetate {[Cnmim][OAc](n = 2,3,4,5,6)}

Article abstract of DOI:10.1021/jp207882t

Five acetic acid ionic liquids (AcAILs) [C_nmim][OAc](n = 2,3,4,5,6) (1-alkyl-3-methylimidazolium acetate) were prepared by the neutralization method and characterized by ¹HNMR spectroscopy and differential scanning calorimetry (DSC). The values of their density and surface tension were measured at 298.15 ± 0.05 K. Since the AcAILs can strongly form hydrogen bonds with water, the small amounts of water are difficult to remove from the AcAILs by common methods. In order to eliminate the effect of the trace water, the standard addition method (SAM) was applied to these measurements. As a new concept, ionic parachor was put forward. [OAc] ^- was seen as a reference ion, and its individual value of ionic parachor was determined in terms of two extrathermodynamic assumptions. Then, the values of ionic parachors of a number of anions, [NTf₂] ^-, [Ala]^-, [AlCl₄]^-, and [GaCl ₄]^-, were obtained by using the value of the ionic parachor of the reference ion; the parachor and surface tension of the investigated ionic liquids in literature were estimated. In comparison, the estimated values correlate quite well with their matching experimental values.

Full text of DOI:10.1021/jp207882t

ARTICLE
pubs.acs.org/JPCB
Ionic Parachor and Its Application in Acetic Acid Ionic Liquid
Homologue 1-Alkyl-3-methylimidazolium Acetate
{[Cnmim][OAc](n = 2,3,4,5,6)}
Wei Guan, Xiao-Xue Ma, Long Li, Jing Tong, Da-Wei Fang, and Jia-Zhen Yang*
Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China
S Supporting Information
b
ABSTRACT: Five acetic acid ionic liquids (AcAILs) [Cnmim][OAc](n =
2,3,4,5,6) (1-alkyl-3-methylimidazolium acetate) were prepared by the neutra-
1
lization method and characterized by HNMR spectroscopy and differential
scanning calorimetry (DSC). The values of their density and surface tension were
measured at 298.15 ( 0.05 K. Since the AcAILs can strongly form hydrogen
bonds with water, the small amounts of water are difficult to remove from the
AcAILs by common methods. In order to eliminate the effect of the trace water,
the standard addition method (SAM) was applied to these measurements. As a
new concept, ionic parachor was put forward. [OAc]ꢀ was seen as a reference
ion, and its individual value of ionic parachor was determined in terms of two
extrathermodynamic assumptions. Then, the values of ionic parachors of a
number of anions, [NTf2]ꢀ, [Ala]ꢀ, [AlCl4]ꢀ, and [GaCl4]ꢀ, were obtained by
using the value of the ionic parachor of the reference ion; the parachor and
surface tension of the investigated ionic liquids in literature were estimated. In comparison, the estimated values correlate quite well
with their matching experimental values.
strongly form hydrogen bonds with water, which is problematic
as an impurity, the standard addition method (SAM) was applied
in these measurements;14 (3) as a new concept, the ionic parachor
was put forward and was applied to the AcAILs. [OAc]ꢀ was seen
as a reference ion, and its particular value of ionic parachor was
determined in terms of two extrathermodynamic assumptions,
then values of ionic parachor for the corresponding cations,
[Cnmim]+, in the AcAILs were calculated from the reference
value of [OAc]ꢀ ; (4) finally, in terms of the estimation of the
parachor and surface tension of the investigated ionic liquids in
literature using the ionic parachors for [OAc]ꢀ and [Cnmim]+,
the credibility of the new concept, ionic parachor, was tested and
verified.
1. INTRODUCTION
Ionic liquids (ILs) have several unique properties including
minuscule vapor pressure, nonflammability, and dual natural
polarity, making them greatly applicable in many physical and
chemical fields.1 Acetic acid ionic liquids (AcAILs) have attracted
considerable attention from industry and the academic commu-
nity as a new-generation greener ionic liquid. The AcAILs have
strong solubility and good catalytic properties, which are useful
for an enzyme-friendly cosolvent for the resolution of amino
acids,2 ultrasonic irradiation toward synthesis of trisubstituted
imidazoles,3 assisted transdermal delivery of sparingly soluble
drugs,4 and some catalytic reactions.5 Thus, AcAILs have been
expected to be applied in various fields, such as industrial
chemistry and pharmaceutical chemistry.
In recent years, there has been a developing trend in the
literature toward the estimation of the physicochemical proper-
ties for compounds by semiempirical methods, in particular, for
ILs.6ꢀ8 Although the estimated result cannot be regarded as
accurate physicochemical data, it is to be commended because it
provides valuable insight into the origins of the behavior of the
materials. Among all the empirical methods, parachor is the
simplest.9ꢀ11 As a continuation of our previous investigation,12,13
this article reports the following: (1) five acetic acid ionic liquids
[Cnmim][OAc] (n = 2,3,4,5,6) (1-alkyl-3-methylimidazolium
acetate) were prepared by the neutralization method; (2) the
values of the density and surface tension for [Cnmim][OAc] (n =
2,3,4,5,6) were measured at 298.15 ( 0.05 K. Since AcAILs can
2. EXPERIMENTAL SECTION
2.1. Chemicals. Deionized water was distilled in a quartz still,
and its conductance was 0.8ꢀ1.2 ꢁ 10ꢀ4 S mꢀ1. Acetic acid was
3
distilled and dried under reduced pressure. N-Methylimidazole
(AR-grade reagent) was vacuum-distilled prior to use. 1-Bro-
moethane, 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
Received: August 16, 2011
Revised:
September 15, 2011
Published: October 06, 2011
r
2011 American Chemical Society
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|

The Journal of Physical Chemistry B
ARTICLE
Table 1. Density Values of Ionic Liquids Containing Various
Amounts of Water at 298.15 K (G/g cmꢀ3 a
)
3
density F for [Cnmim][OAc] (n = 2,3,4,5,6)
Figure 1. Preparation of AcAILs by the neutralization method. 1,
[Cnmim][Br]; 2, [Cnmim][OH]; and 3, [Cnmim][OAc].
[C2mim][OAc]
103w2
6.79 8.03 9.58 10.34 11.87
0
r2 s ꢁ 105
F (g cmꢀ3) 1.1471 1.1478 1.1485 1.149 1.1497 1.1437 0.99 5.5
3
over molecular sieves in tightly sealed glass bottles. Anion-ex-
change resin (type 717) was purchased from Shanghai Chemical
Reagent Co. Ltd. and activated by the regular method before use.
2.2. Preparation of the AcAILs. The AcAILs [Cnmim][OAc]
(n = 2,3,4,5,6) were prepared by a neutralization method
according to Fukumoto et al.15 Figure 1 is a schematic of this
synthetic route. First, [Cnmim]Br (n = 2,3,4,5,6) was synthesized
according to the literature.16,17 Then, aqueous 1-alkyl-3- methy-
limidazolium hydroxide ([Cnmim][OH]) (n = 2,3,4,5,6) was
prepared from [Cnmim]Br (n = 2,3,4,5,6) by use of the activated
anion-exchange resin over a 100 cm column. However,
[Cnmim][OH] (n = 2,3,4,5,6) is not particularly stable and
should be used immediately after preparation. The onium hydro-
xide aqueous solution was added dropwise to a slight excess of
acetic acid aqueous solution. The mixture was stirred under
cooling for 24 h. Then, water was evaporated under reduced
pressure at 40ꢀ50 °C. To this reaction mixture was added the
mixed solvent acetonitrile/methanol (volumetric ratio = 9/1), and
it was stirred vigorously. The mixture was then filtered to remove
the slight excess of acetic acid. The filtrate was evaporated to re-
move the solvents. The product of [Cnmim][OAc] (n = 2,3,4,5,6)
was dried in vacuo for 2 days at 85 °C. Structures of the resulting
acetic acid ionic liquids were confirmed by 1HNMR spectroscopy
(see Figure A of the Supporting Information).
[C3mim][OAc]
103w2
6.31 7.84 9.51 10.31 11.87
F (g cmꢀ3) 1.1219 1.1226 1.1232 1.1238 1.1244 1.1190 0.99 9.9
0
r2 s ꢁ 105
3
[C4mim][OAc]
103w2
5.88 7.18 8.43 9.72 11.06
F (g cmꢀ3) 1.0995 1.1001 1.1006 1.1012 1.1019 1.0968 0.99 4.8
0
r2 s ꢁ 105
3
[C5mim][OAc]
103w2
6.13 7.35 8.69 9.95 11.23
F (g cmꢀ3) 1.0803 1.0809 1.0814 1.0821 1.0828 1.0773 0.99 7.9
0
r2 s ꢁ 105
3
[C6mim][OAc]
103w2
5.74 7.02 8.28 9.52 10.74
0
r2 s ꢁ 105
F (g cmꢀ3) 1.0633 1.0638 1.0643 1.0649 1.0656 1.0606 0.99 8.9
a w2 is the water content; r2 is the squared correlation coefficient; and s is
3
the standard deviation.
Table 2. Surface Tension Values of Ionic Liquids Containing
Various Amounts of Water at 298.15 K(γ/mJ mꢀ2 a
)
3
surface tension γ for [Cnmim][OAc] (n = 2,3,4,5,6)
[C2mim][OAc]
Differential scanning calorimetric (DSC) measurements showed
that ILs [Cnmim][OAc] (n = 2,3,4,5,6) had no obvious melting
103w2
γ (mJ mꢀ2
6.65 8.05 9.71 11.15 12.74
0
r2
38.1 0.99 0.018
s
)
)
)
)
)
39.6 39.9 40.3 40.6
41.0
3
point, but their glass transition temperatures Tg(n=2,3,4,5,6)
=
ꢀ75.5 °C, ꢀ73.4 °C, ꢀ70.5 °C, ꢀ68.5 °C, and ꢀ63.2 °C,
respectively. All traces of DSC are listed in Figure B of the Sup-
porting Information. The water content, w2 (in mass fraction), in
the AcAILs was determined by the use of a Karl Fischer moisture
titrator (ZSD-2 type).
2.3. Determination of Density and Surface Tension of the
AcAILs. Since the AcAILs can strongly form hydrogen bonds
with water, the small amount of water in the AcAILs is difficult to
remove by common methods so that trace water becomes the
most problematic impurity. In order to eliminate the effect of the
impurity of water, the standard addition method (SAM) was
applied to these measurements. According to SAM, a series of
samples of [Cnmim][OAc] (n = 2,3,4,5,6) with different water
contents were prepared.
[C3mim][OAc]
103w2
γ (mJ mꢀ2
5.66 7.37 9.13 10.97 12.84
0
r2
36.8 0.99 0.041
s
38.2 38.7 39.1 39.6
40.0
3
[C4mim][OAc]
103w2
γ (mJ mꢀ2
7.00 8.26 9.88 11.97 14.72
0
r2
35.2 0.99 0.106
s
37.4 37.7 38.2 38.9
39.4
3
[C5mim][OAc]
103w2
γ (mJ mꢀ2
6.43 8.10 9.81 11.39 12.95
0
r2
34.1 0.99 0.119
s
35.8 36.2 36.5 37.0
37.6
3
[C6mim][OAc]
103w2
γ (mJ mꢀ2
6.25 7.91 9.58 11.18 12.83
34.6 34.8 35.3 35.8 36.1
a w2 is the water content; r2 is the squared correlation coefficient; and s is
0
r2
33.0 0.99 0.099
s
3
The density of degassed water was measured on a Westphal
balance at 293.15 ( 0.05 K and was in good agreement with the
the standard deviation.
literature18 within the experimental error of (0.0002 g cmꢀ3
Then, the densities of the samples were measured at 298.15 (
0.05 K. The sample was placed in a cell with a jacket and was
thermostatted with an accuracy of (0.05 K.
.
3
3. RESULTS AND DISCUSSION
3.1. Values of Density and Surface Tension for the Sam-
ples. The values of density and surface tension for the samples of
ILs [Cnmim][OAc] (n = 2,3,4,5,6) containing various contents
of water are listed in Tables 1 and 2, respectively. Each value in
the tables is the average of triplicate measurements. According to
SAM, the values of the density or surface tension of the samples
at a given temperature were plotted against the water content,
By use of the tensiometer of the forced bubble method (DPAW
type produced by Sang Li Electronic Co.), the surface tension of
the water was measured at 298.15 ( 0.05 K and was in good
agreement with the literature18 within the experimental error
of (0.1 mJ mꢀ2. Then, the values of surface tension of the
3
samples were measured by the same method at 298.15 ( 0.05 K .
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The Journal of Physical Chemistry B
ARTICLE
Figure 3. Plot of surface tension vs amount of water in [Cnmim][OAc]
(n = 2,3,4,5,6). (black square) [C2mim][OAc] γ = 38.1 + 0.223w2, s =
0.018, r2 = 0.99; (red circle) [C3mim][OAc] γ = 36.8 + 0.250w2, s =
0.041, r2 = 0.99; (blue upward facing triangle) [C4mim][OAc] γ = 35.5
+ 0.269w2, s = 0.106, r2 = 0.99; (green downward facing triangle)
[C5mim][OAc] γ = 34.1 + 0.269w2, s = 0.119, r2 = 0.99; (pink left facing
triangle) [C6mim][OAc] γ = 33.0 + 0.243w2, s = 0.099, r2 = 0.99.
Figure 2. Plot of density vs amount of water in [Cnmim][OAc] (n =
2,3,4,5,6). (black square) [C2mim][OAc] F = 1.1437 + 5.1136 ꢁ
10ꢀ4w2, s = 5.5 ꢁ 10ꢀ5, r2 = 0.99; (red circle) [C3mim][OAc] F =
1.1190 + 4.5245 ꢁ 10ꢀ4w2, s = 9.9 ꢁ 10ꢀ5, r2 = 0.99; (blue upward
facing triangle) [C4mim][OAc] F = 1.0968 + 4.5759 ꢁ 10ꢀ4w2, s = 4.8
ꢁ 10ꢀ5, r2 = 0.99; (green downward facing triangle) [C5mim][OAc] F =
1.0773 + 4.8418 ꢁ 10ꢀ4w2, s = 7.9 ꢁ 10ꢀ5, r2 = 0.99; (pink left facing
triangle) [C6mim][OAc] F = 1.0606 + 4.5564 ꢁ 10ꢀ4w2, s = 8.9 ꢁ 10ꢀ5
,
r2 = 0.99.
Table 3. Values of Molecular Volume, Standard Molar En-
tropy, and Lattice Energy of Ionic Liquids at 298.15 K
w2 (w2 is the mass percentage) so that a series of good straight
lines were obtained. The intercepts of the straight lines were the
values of the density or surface tension of [Cnmim][OAc] (n =
2,3,4,5,6) without water and were seen as experimental values,
which are included in the column of w2 = 0 in Tables 1and 2,
respectively. Figures 2 and 3 are plots of the density and surface
tension, respectively, against w2 at 298.15 ( 0.05 K. The squared
correlation coefficients, r2, of all linear regressions were larger
than 0.99, and all values of the standard deviation, s, were within
experimental error. These facts show that the standard addition
method is suitable in this work.
M
F
Vm
S0
Upot
ionic liquid (g molꢀ1) (g cmꢀ3) (nm3) (J Kꢀ1 molꢀ1) (kJ molꢀ1
)
3
3
3
3
3
[C2mim][OAc] 170.21
[C3mim][OAc] 184.24
[C4mim][OAc] 198.26
[C5mim][OAc] 212.29
[C6mim][OAc] 226.32
1.1437 0.2471
1.1190 0.2734
1.0968 0.3002
1.0773 0.3272
1.0606 0.3543
337.5
370.3
403.7
437.4
471.1
478
465
454
444
435
According to eq 2, the mean methylene volume is 0.0268 nm3,
and the fitted slope of eq 2 is 1246.5, so the mean entropy
contribution per methylene group is 33.4 J Kꢀ1 molꢀ1, and this
3.2. Volumetric Properties of [Cnmim][OAc] (n = 2,3,4,5,6).
From the experimental values of density, the molecular volume
(sum of the positive and negative ion volumes), Vm, of [Cnmim]-
[OAc] (n = 2,3,4,5,6) was calculated from the following equation:
3
3
value is in good agreement with the values of 33.9 J Kꢀ1 molꢀ1
3
3
for [Cnmim][BF4] and 35.1 J Kꢀ1 molꢀ1 for [Cnmim][NTf2].19
3
ꢀ31
The lattice energy, Upot (kJ mol ), may be estimated from the
3
following equation:19
Vm ¼ M=ðNFÞ
ð1Þ
1=3
where M is molar mass, and N is Avogadro's constant so that
Vm(n=2,3,4,5,6) = 0.2471 nm3, 0.2734 nm3, 0.3002 nm3, 0.3272 nm3,
and 0.3543 nm3 for [Cnmim][OAc] (n = 2,3,4,5,6), respectively, at
298.15 K. The average difference between the molecular volumes
of [Cnmim][OAc] and [Cnꢀ1mim][OAc] is 0.0268 nm3, which
can be seen as the contribution to the molecular volume of one
methylene (ꢀCH2ꢀ) group, and was in good agreement with a
mean contribution of 0.0272 nm3 per methylene (ꢀCH2ꢀ) group
obtained by Glasser19 from ionic liquids [Cnmim][BF4]. The values
of molecular volume, Vm, are listed in Table 3.
U
pot=kJ molꢀ1 ¼ 1981:2ðF=MÞ þ 103:8
ð3Þ
3
The estimated values of Upot for [Cnmim][OAc] are listed in
Table 3. The largrest value, Upot = 478 kJ molꢀ1 for [Cnmim]-
3
[OAc], is much less than that of fused salts; for example, Upot
=
613 kJ molꢀ1 for fused CsI,18 which is the lowest lattice energy
3
among alkali halides. The lower lattice energy is the underlying
reason for forming an ionic liquid at room temperature.
3.3. Ionic Parachor. The parachor, P, is a relatively old con-
cept that is available as a link between the structure, density, and
surface tension of liquids.9,10 The parachor as a tool to pre-
dict physical properties of substances was developed in 1924 by
Sugden11 and was defined using the following equation:
According to Glasser’s theory,19 the standard molar entropy
for IL, S0, is given by the following equation, and the values of S0
(J Kꢀ1 molꢀ1) for [Cnmim][OAc] (n = 2,3,4,5,6) are listed in
3
3
Table 3.
P ¼ ðMγ1=4Þ=F
ð4Þ
S0ð298Þ=ðJ Kꢀ1 molꢀ1Þ ¼ 1246:5ðVm=nm3Þ
3
3
where γ is the surface tension, M is the molar mass, and F is the
density of the substance. However, the vast majority of parachor
þ 29:5
ð2Þ
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ARTICLE
Table 4. Values of the Ionic Parachor for the ILs According to
the Second Extrathermodynamic Assumption
M+
P
V+ (crystal) V (crystal)
ꢀ
ionic liquid
(g molꢀ1) (exp) (nm3)20
(nm3)21
P+
P
ꢀ
3
[C2mim] [OAc] 111.168 369.1
[C3mim] [OAc] 125.194 405.0
[C4mim] [OAc] 139.220 441.1
[C5mim] [OAc] 153.246 476.9
[C6mim] [OAc] 167.272 512.1
0.156
0.178
0.196
0.219
0.242
0.048
0.048
0.048
0.048
0.048
283.1 86.0
319.0 86.0
355.0 86.1
390.9 86.0
426.1 86.0
Table 5. Values of Ionic Parachor of ILs at 298.15 K
[C2mim]+ [C3mim]+ [C4mim]+ [C5mim]+ [C6mim]+
cation parachor 285.2
321.1
357.2
393.0
428.2
[NTf2]ꢀ 22
[Ala]ꢀ 12
[AlCl4]ꢀ 23
[GaCl4]ꢀ 24
Figure 4. Plot of parachor P vs M+. Fitting equation: P = 81.66 + 2.577M+.
anion parachor
335.9
197.1
287.1
283.1
studies have focused on uncharged compounds. The data of the
individual group contribution of parachor determined by Rowley
and co-workers10 are from the neutral molecule and difficult to
use in the application of an ionic liquid because there is no con-
sideration of the Coulomb force between the ions. Although a
number of early studies attempted to determine parachor for
ions, these studies were hampered by the experimental difficul-
ties encountered in determining the surface tensions and den-
sities of high melting salts, and no related investigations followed.
However, since numerous ionic liquids are fluid at room tem-
perature, they offer a solution to determining experimental parachor
of ionic compounds. Therefore, ionic parachor was proposed as a
new concept, that is, ions composed of ionic liquids can be seen
as independent descriptors of parachor. Ionic parachor, Pi, can be
defined by the following equation:
where V+(crystal) and V (crystal) are the corresponding catio-
nic and anionic volumes derived from crystal structures, respec-
ꢀ
tively; Vmꢀ (IL) is the ionic volume of the corresponding anion
in an IL, Vm is the 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. If the values of V+(crystal) and
V (crystal) may be obtained, the ionic parachor of the cation and
ꢀ
anion can be determined with eq 8.
According to the first extrathermodynamic assumption, plot-
ting the experimental values of P against the molar mass of the
cations, M+, of ILs [Cnmim][OAc], we obtained a good straight
line (see Figure 4). The regression eq is P = 81.66 + 2.577M+
with a correlation coefficient of r = 0.999 and standard deviation
of s = 0.85. The value of the intercept 81.66 means a contribution
of the anion to P; that is P = 81.7, ionic parachor of [OAc]ꢀ.
ꢀ
Pi ¼ γ1=4Vi
ð5Þ
The values of V+(crystal) and V (crystal) for [Cnmim][OAc]
ꢀ
obtained from the literature20,21 are listed in Table 4. According
to the second extrathermodynamic assumption, the values of ionic
parachor of the cation and anion for the ILs can be calculated by
eq 8 and are also listed in Table 4. From Table 4, the average of
where Vi is the molar volume of an ion in IL. According to the
additivity, the parachor value of an ionic liquid is equal to the sum
of ionic parachors of the corresponding cation and anion:
the ionic parachor, P , can be seen, and for anion [OAc]ꢀ, it
ꢀ
P ¼ P þ P
ð6Þ
þ
ꢀ
is 86.0.
If [OAc]ꢀ was used as the reference ion, the reference value of
where P+ and P are the ionic parachors of the cation and anion,
ꢀ
respectively. Now, the key question is how the experimental value
of parachor for an ionic liquid was divided into ionic parachors of the
corresponding cation and anion. Two extrathermodynamic assump-
tions were recommended for the evaluation of the individual
ionic parachor. The first of them is the extrapolation method. For
the acetic acid ionic liquid homologue 1-alkyl-3-methylimidazo-
lium acetate ([Cnmim][OAc] (n = 2,3,4,5,6)), according to eq 7,
when the molar mass of the cation, M+, approaches zero, the
P
= 83.9, which is the average of that obtained from two ex-
ꢀ
trathermodynamic assumptions, values of the ionic parachor for
all corresponding imidazolium cations, [Cnmim]+, were obtained
and are listed in Table 5. Then, using data in the literature and P+
of [Cnmim]+, the ionic parachors of [NTf2]ꢀ, [Ala]ꢀ, [AlCl4]ꢀ,
and [GaCl4]ꢀ were obtained and are also listed in Table 5.
3.4. Predicting Parachor and Surface Tension. In order to
verify the reliability of the ionic parachors for [Cnmim]+ and the
anions in Table 5, the parachors for the ILs ([Cnmim][NTf2],
[Cnmim][Ala], [Cnmim][AlCl4], and [Cnmim][GaCl4]) were
estimated. The estimated values, PEst, and the experimental
values, PEx, determined by eq 4 are listed in Table 6, where ΔP
means the difference between the experimental value and cor-
responding estimated value of the parachor for the ILs, that is, ΔP =
PEx ꢀ PEst. As can be seen from Table 6, only ΔP for [C2mim]-
[AlCl4] is relatively large (relative deviation is close to 2%). This
may be due to the use of a different author’s data in the calculation of
the experimental parachor for [Cnmim][AlCl4].10 For the remaining
ionic parachor of the anion, P , could be obtained.
ꢀ
P ¼ P þ ðM þ γ1=4Þ=F
ð7Þ
ꢀ
The second extrathermodynamic assumption was proposed as
eq 8
V ðcrystalÞ=V ðcrystalÞ ¼ VmꢀðILÞ=½Vm ꢀ VmꢀðILÞꢂ
ꢀ
þ
¼ P ðILÞ=½P ꢀ P ðILÞꢂ
ꢀ
ꢀ
ð8Þ
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The Journal of Physical Chemistry B
ARTICLE
Table 6. Parachors and Surface Tensions of the Investigated Ionic Liquids
[Cnmim][NTf2]
[Cnmim][Ala]
[Cnmim]X
PEx
PEst
ΔP
7.7
γEx
γEst
Δγ
1.7
PEx
PEst
ΔP
γEx
γEst
Δγ
[C2mim]X
[C3mim]X
[C4mim]X
[C5mim]X
[C6mim]X
628.8
657.0
692.9
722.5
762.8
621.1
657.0
693.1
728.9
764.1
35.8
32.9
32.0
30.1
30.2
34.1
32.9
32.0
31.2
30.4
479.0
516.9
553.4
592.4
628.6
482.3
518.2
554.3
590.1
625.3
ꢀ3.3
ꢀ1.3
ꢀ0.9
2.3
52.7
50.1
47.7
46.0
43.4
54.2
50.6
48.0
45.3
42.5
ꢀ1.5
ꢀ0.5
ꢀ0.3
0.7
0.0
0.0
ꢀ0.2
ꢀ6.4
ꢀ1.3
0.0
ꢀ1.1
ꢀ0.2
3.3
0.9
[Cnmim][AlCl4]
[Cnmim][GaCl4]
[Cnmim]X
PEx
PEst
ΔP
9.5
γEx
γEst
Δγ
PEx
PEst
ΔP
γEx
γEst
Δγ
[C2mim]X
[C3mim]X
[C4mim]X
[C5mim]X
[C6mim]X
581.8
608.8
646.5
678.1
705.4
572.3
608.3
644.3
680.1
715.3
52.4
46.7
45.6
42.6
40.8
49.0
46.5
45.0
43.1
41.9
3.4
565.6
568.3
ꢀ2.7
44.6
45.5
ꢀ0.9
0.5
ꢀ0.2
0.6
2.2
641.3
679.5
709.7
640.3
676.1
711.3
1.0
45.1
43.1
40.4
44.8
42.3
40.8
0.3
ꢀ2.0
ꢀ9.9
ꢀ0.5
ꢀ1.1
3.4
0.8
ꢀ1.6
ꢀ0.4
Figure 6. Plot of the estimated surface tension for ILs [Cnmim]-
Figure 5. Plot of the estimated parachor for ILs [Cnmim]([NTf2],
[Ala], [AlCl4], and [GaCl4]) vs their experimental values. PEst = ꢀ7.907
+ 1.012PEx, s = 4.45, r2 = 0.99. (black square) [Cnmim][NTf2] (n =
2ꢀ6); (red circle) [Cnmim][Ala] (n = 2ꢀ6); (blue upward facing
triangle) [Cnmim][AlCl4] (n = 2ꢀ6); (green downward facing triangle)
[Cnmim][GaCl4] (n = 2, 4ꢀ6).
([NTf2], [AlCl4], [GaCl4], and [Ala]) vs their experimental values. γEst
=
1.127 + 0.971γEx, s = 1.14, r2 = 0.99. (black square) [Cnmim][NTf2] (n =
2ꢀ6); (red circle) [Cnmim][Ala] (n = 2ꢀ6); (blue upward facing triangle)
[Cnmim][AlCl4] (n
=
2ꢀ6); (green downward facing triangle)
[Cnmim][GaCl4] (n = 2, 4ꢀ6).
(relative deviation is close to 7%). As illustrated by Figure 6, the
predicted surface tensions of the ILs correlate quite well with
their matching experimental values (squared correlation coeffi-
cient r2 = 0.99; standard deviation s = 1.14).
ILs, ΔP values are very small, that is, the experimental values of
the parachor are in accordance with the estimated one. This
implies that the data of the ionic parachors for [Cnmim]+ and the
anions are reliable. Figure 5 is a comparative plot of estimated
parachor values as a function of corresponding experimental values
for the investigated ionic liquids and shows that the estimated
parachor and the experimental values are highly correlated (squared
correlation coefficient r2 = 0.99; standard deviation s = 4.45) and
extremely similar (gradient = 1.01; intercept = ꢀ7.91).
’ ASSOCIATED CONTENT
Supporting Information. 1HNMR spectra and DSC
S
b
traces of the five studied acetic acid ionic liquids. This material
is available free of charge via the Internet at http://pubs.acs.org.
In terms of the ionic parachor values in Table 5, the predicted
values of the surface tension for the ILs using eq 1 and their
corresponding experimental values are listed in Table 6. In
Table 6, Δγ means the difference between the experimentally
determined and corresponding estimated surface tensions for the
ILs, that is, Δγ = γEx ꢀ γEst. As can be seen from Table 6,
only the values of Δγ for [C2mim][AlCl4] are relatively large
’ ACKNOWLEDGMENT
This project was supported by the NSFC (21173107 and
20903053) and Education Bureau of Liaoning Province (LS2010068
and LS2010069), Peoples Republic of China.
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dx.doi.org/10.1021/jp207882t |J. Phys. Chem. B 2011, 115, 12915–12920

The Journal of Physical Chemistry B
ARTICLE
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