Study on thermophysical properties for aqueous solutions of ionic liquid N-ethyl-pyridine dicyanamide [C2py][DCA]

Article abstract of DOI:10.1016/j.molliq.2016.03.005

The IL (ionic liquid) [C₂py][DCA] (N-ethyl-pyridine dicyanamide) was prepared and confirmed by ¹H NMR (¹H nuclear magnetic resonance) spectroscopy, ¹³C NMR (¹³C nuclear magnetic resonance) spectroscopy. The values of density and surface tension of the IL [C₂py][DCA] and its aqueous solutions with various molalities were determined at T = (288.15 to 318.15) K. The values of the molar volume for aqueous solutions of [C₂py][DCA], V, were calculated and increase with the increase of temperature and molarity. On the basis of thermodynamics, a semi-empirical model-molar surface Gibbs free energy model of the ionic liquid solution was put forward. It can be used to predict the surface tension and the predicted values of the surface tension for aqueous [C₂py][DCA] are all most the same with the corresponding experimental ones. In terms of the concept of the molar surface Gibbs free energy, a new E?tv?s equation is obtained. Each parameter of the new equation has a clear physical meaning, energy dimension and is more convenient to use.

Full text of DOI:10.1016/j.molliq.2016.03.005

Journal of Molecular Liquids 219 (2016) 390–395
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Study on thermophysical properties for aqueous solutions of ionic liquid
N-ethyl-pyridine dicyanamide [C2py][DCA]
Nan Nan Xing a,c, Ben Han Fan a, Xiao Xue Bu a, Jie Wei a, Yi Pan a, Wei Guan a,b,
⁎
a
College of Chemistry, Liaoning University, Shenyang 110036, PR China
School of Environmental Science, Liaoning University, Shenyang 110036, PR China
b
c
College of Chemistry and Chemical Engineering, Huangshan University, Huangshan 245041, PR China
a r t i c l e i n f o
a b s t r a c t
The IL (ionic liquid) [C2py][DCA] (N-ethyl-pyridine dicyanamide) was prepared and confirmed by 1H NMR (1H
nuclear magnetic resonance) spectroscopy, 13C NMR (13C nuclear magnetic resonance) spectroscopy. The values
of density and surface tension of the IL [C2py][DCA] and its aqueous solutions with various molalities were deter-
mined at T = (288.15 to 318.15) K. The values of the molar volume for aqueous solutions of [C2py][DCA], V, were
calculated and increase with the increase of temperature and molarity. On the basis of thermodynamics, a semi-
empirical model-molar surface Gibbs free energy model of the ionic liquid solution was put forward. It can be
used to predict the surface tension and the predicted values of the surface tension for aqueous [C2py][DCA] are
all most the same with the corresponding experimental ones. In terms of the concept of the molar surface
Gibbs free energy, a new Eötvös equation is obtained. Each parameter of the new equation has a clear physical
meaning, energy dimension and is more convenient to use.
Article history:
Received 3 November 2015
Accepted 2 March 2016
Available online xxxx
Keywords:
Aqueous solution
Ionic liquid
Density
Surface tension
Molar surface Gibbs free energy
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Material and methods
Ionic liquids (ILs) have attracted broad attention in the past a few
years, this is because they have some practical properties such as negli-
gible vapor pressure, high heat capacity, high thermal conductivity and
a wide temperature range for liquids [1]. Due to these properties, they
have been applied in pharmaceuticals [2,3], therapeutics [4], biotech-
nology [5–7] and chemical fields [8]. Recently, the research on the prop-
erties of ILs and their mixtures is gradually increasing. Solution,
however, can be a reasonable combination of the IL with other solvents
and is able to provide better physical and chemical properties with the
changes of solution composition. In addition, the physicochemical prop-
erty data of the ionic liquid solution can provide available information
for the applications of ILs.
As our continual investigation [9–13], the followings were reported
in this work: (1) The IL [C2py][DCA] was prepared and characterized.
(2) The values of density and surface tension of the IL [C2py][DCA] and
aqueous solutions of [C2py][DCA] were determined at T = (288.15 to
318.15) K with an interval of 5 K. (3) On the basis of a new thermody-
namic function-molar surface Gibbs free energy, gs, a semi-empirical
model for the IL solutions is proposed, which can predict the surface
tension of the solutions. (4) A new Eötvös equation is obtained.
2.1. Materials
The source and purity of the materials are listed in Table S1.
2.2. Preparation of IL [C2py][DCA]
[C2py][Br] was synthesized by direct reaction of pyridine with
bromoalkane in ethyl acetate and the purification was accomplished
using standard procedure [14,15]. [C2py][DCA] was synthesized using
previously reported method [10]. The pure IL [C2py][DCA] was con-
firmed by 1H NMR spectroscopy and 13C NMR spectroscopy. (Figs. S1
and S2 in Supporting Information) (Digital pre-S means Supporting In-
formation). In the NMR spectrum, no impurity peaks were found. The
purity of [C2py][DCA] in peak area fraction is more than 0.99 by high
performance liquid chromatography (HPLC) (Fig. S3). The mass fraction
of water in the pure IL [C2py][DCA] determined by a Karl Fischer mois-
ture titrator (ZSD-2 type) was 0.00233.
2.3. Measurement of the density and surface tension of the IL [C2py][DCA]
and aqueous solutions of [C2py][DCA]
The standard addition method (SAM) was applied in the measure-
ment of the density and the surface tension of the pure IL [C2py][DCA]
because [C2py][DCA] could strongly form hydrogen bonds [16] with
water, any water in it is hard to be eliminated by ordinary methods.
⁎
Corresponding author at: College of Chemistry, Liaoning University, Shenyang
110036, PR China
E-mail address: guanweiy@sina.com (W. Guan).
http://dx.doi.org/10.1016/j.molliq.2016.03.005
0167-7322/© 2016 Elsevier B.V. All rights reserved.

N.N. Xing et al. / Journal of Molecular Liquids 219 (2016) 390–395
391
Table 1
and then took an average value, which were listed in Tables S2 and S3.
The experimental expanded uncertainties of density and surface tension
are 0.00006 g cm−3 and 0.3 mJ m−2 at 0.95 confidence level. Ac-
cording to SAM, the values of density and surface tension were plotted
against the water content, w2 (w2 is the mass fraction) at T = (288.15,
293.15, 303.15, 308.15, 313.15, 318.15) K (see Figs. S4 and S5 in the
Supporting Information). The correlation coefficients, r, of all linear re-
gressions were larger than 0.99 and all values of standard deviation, s,
were within experimental error. These data show that the standard ad-
dition method is appropriate for this work. The experimental values of
density and surface tension of [C2py][DCA] without water corresponded
to the intercepts of the straight lines. At T = 298.15 K, the values of den-
sity and surface tension of the IL [C2py][DCA] had been reported in our
previous work [10]. The values of density and surface tension of IL
[C2py][DCA] within their expanded combined uncertainty at 0.95 confi-
dence level are listed in Table 1.
Values of density, ρ/g cm−3 and surface tension, γ/mJ m−2 of the ILs [C2py][DCA] at T =
(298.15 to 338.15) Ka, bp = 0.1 MPa within the expanded uncertainties cUc (0.95 level of
confidence)
T/K
ρ
Uc (ρ)/g cm−3
γ
Uc (γ)/mJ m−2
288.15
293.15
298.15
303.15
308.15
313.15
318.15
1.12473 0.00007
1.12156 0.00006
1.11849 0.00009 (ref.10)
1.11532 0.00009
1.11215 0.00009
1.10900 0.00010
1.10588 0.00010
59.9 0.3
59.5 0.3
59.2 0.3 (ref.10)
58.8 0.4
58.5 0.4
58.1 0.3
57.9 0.4
The standard uncertainty (0.68 level of confidence): au(T) = 0.03 K for density, au(T) =
0.01 K for surface tension and refractive index; bu(p) = 0.002 MPa. cThe expanded un-
certainties Uc (0.95 level of confidence): Uc(ρ) for density, Uc(γ) for surface tension and
these values were given inside the table, respectively.
All aqueous solutions of [C2py][DCA] to be measured were prepared
freshly in the molarity range from (0.01000 to 0.5000) mol·kg−1, and
performed on an electronic balance (FA-2104) accurate to 0.0001 g
with calibration of air buoyancy. Water content in the pure IL
[C2py][DCA] had been taken into account and the appropriate molality
corrections had been applied. Each sample was immediately measured
after it was mixed well by a magnetic stirrer (CL-1A). The uncertainty
3.2. The experimental values of density and surface tension of aqueous
solutions of [C2py][DCA]
The experimental values of density and surface tension for the sam-
ples of aqueous [C2py][DCA] with various molalities at 288.15 K to
318.15 K are listed in Tables 2 and 3. Each value in the tables is the av-
erage of three measurements. The uncertainties of density and surface
tension for aqueous solutions of [C2py][DCA] within their expanded
combined uncertainty at 0.95 confidence level are 0.00007 g cm−3
and 0.3 mJ m−2. From the data of Table 2, the density of aqueous
[C2py][DCA] decreases with the increase of temperature and increases
with the increase of molarity. From Table 3, the surface tension of the
solutions decreases with the increase of temperature and molarity.
According to the following equation, the average molar volume of
aqueous [C2py][DCA], V, can be calculated:
of molality was 0.0003 mol kg−1
.
The densities of the IL [C2py][DCA] and aqueous solutions of
[C2py][DCA] were measured by an Anton Paar DMA 4500 oscillating
U-tube densitometer. The temperature in the cell was adjusted to
0.03 K with a solid state thermostat. Before the measurement, the ap-
paratus was calibrated once a day with dry air and double-distilled
freshly degassed water. First, the density of degassed water was mea-
sured at {(288.15 to 318.15) 0.03} K, which was in good agreement
with the literature values [17] within an experimental error of
0.00002 g cm−3. Then, densities of the samples were measured by
the same method.
V ¼ ðx1M1 þ x2M2Þ=ρ
ð1Þ
The tensiometer of the forced bubble method (DPAW type produced
by Sang Li Electronic Co.) was used to measure the surface tension of the
IL [C2py][DCA] and its aqueous solutions. First, the surface tension of
where M is the molar mass, x is mole fraction, subscripts 1 and 2
mean solvent and solute, respectively. The calculated values of V
are listed in Table S4 in the Supporting Information. From Table S4,
the values of V increase with the increase of temperature and
molarity.
degassed water was measured at {(288.15 to 318.15)
0.01} K,
which was in good agreement with the literature values [17] within
an experimental error of 0.1 mJ m−2. Then, surface tensions of the
samples were measured by the same method.
In the process of experiment, the density and surface tension of sam-
ples were measured in the glove-box which was full of argon in the pu-
rity of 99.99% (in volume fraction).
3.3. The molar surface Gibbs free energy, gs, for aqueous solutions of
[C2py][DCA]
It is generally known that data of surface tension for the solution is of
great importance in study and application of ionic liquids. In order to
predict the surface tension of solution, a new semi-empirical model -
molar surface Gibbs free energy model was proposed. The basic as-
sumptions of the model are: Firstly, there is a surface phase between
the vapor phase and the liquid phase of the solution, assuming that
the surface phase is electrically neutral, and the homogeneous concen-
tration which is different from the bulk. Secondly, the concentration in
3. Results and discussion
3.1. The experimental values of surface tension and density of the pure IL
[C2py][DCA]
The experimental values of density and surface tension of the pure IL
[C2py][DCA] with different water contents were measured three times
Table 2
The values of the density, ρ /g·cm−3, for aqueous solutions of [C2py][DCA] with various molalities at 288.15 K to 318.15 Ka, bp = 0.1 MPa
m/mol kg−1
288.15 K
293.15 K
298.15 K
303.15 K
308.15 K
313.15 K
318.15 K
0.0115
0.0277
0.0559
0.1192
0.1702
0.2822
0.3221
0.3867
0.4419
0.99938
0.99980
1.00047
1.00185
1.00289
1.00507
1.00580
1.00701
1.00798
0.99848
0.99888
0.99951
1.00082
1.00180
1.00388
1.00457
1.00572
1.00665
0.99731
0.99769
0.99829
0.99953
1.00047
1.00244
1.00310
1.00421
1.00509
0.99590
0.99626
0.99683
0.99802
0.99891
1.00080
1.00143
1.00249
1.00333
0.99427
0.99462
0.99517
0.99630
0.99715
0.99897
0.99957
1.00058
1.00140
0.99245
0.99278
0.99330
0.99439
0.99520
0.99695
0.99753
0.99851
0.99929
0.99043
0.99074
0.99123
0.99230
0.99308
0.99477
0.99533
0.99627
0.99702
The standard uncertainty (0.68 level of confidence): au(T) = 0.03 K, bu(p) = 0.002 MPa.

392
N.N. Xing et al. / Journal of Molecular Liquids 219 (2016) 390–395
Table 3
The values of the surface tension, γ/mJ m−2, for aqueous solutions of [C2py][DCA] with
various molalities at 288.15 K to 318.15 Ka, bp = 0.1 MPa
m/mol
288.15
K
293.15
K
298.15
K
303.15
K
308.15
K
313.15
K
318.15
K
kg−1
0.0115
0.0277
0.0559
0.1192
0.1702
0.2822
0.3221
0.3867
0.4419
72.9
72.4
71.6
70.4
69.4
66.7
65.8
64.5
63.3
72.2
71.8
70.8
69.8
68.7
66.0
65.3
63.9
62.7
71.4
70.9
70.1
69.1
68.1
65.2
64.7
63.1
61.8
70.6
70.0
69.3
68.3
67.1
64.6
63.8
62.4
61.2
69.7
69.4
68.5
67.5
66.3
63.8
63.2
61.9
60.6
69.0
68.7
67.7
66.6
65.8
63.2
62.5
61.3
60.1
68.1
67.5
66.8
65.9
65.0
62.6
61.9
60.5
59.6
The standard uncertainty (0.68 level of confidence): au(T) = 0.01 K, bu(p) = 0.002 MPa.
the surface phase, xF2, is proportional to the concentration of x2 in the
bulk phase, and the k2 is the proportional coefficient.
F
Fig. 1. Plot of gs(exp.) vs m for aqueous [C2py][DCA] at 288.15 K to 318.15 K. From top to
bottom: (black square, ■) 288.15 K; (red circle, ) 293.15 K; (blue inverted triangle,
298.15; (navy hollow circle, ) 303.15 K; (cyan tilted hollow square ) 308.15 K; (dark
cyan inverted triangle 313.15 K; (magenta tilted square 313.18 K. (For
interpretation of the references to color in this figure legend, the reader is referred to
the online version of this chapter.)
x2 ¼ k2 x2
ð2Þ
)
Similarly, the solvent concentration in the surface phase, xF1, is
proportional to the concentration, x1, in the bulk phase, superscript, F,
means surface phase and k1 is the proportional coefficient.
)
)
F
x1 ¼ k1 x1
ð3Þ
While 1 mol solution, (x1+x2), was moved from the bulk to the
surface, the Gibbs free energy change, ΔG, is:
In a solution of 1–1 type ILs, the chemical potential of solute and sol-
vent are expressed as follows. For the bulk phase we have:
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
ΔG ¼ x1 μo F−μo þ x1 RT lnaF1−RT lna1 þ x2 μo F−μo
1
1
2
2
ꢀ
ꢁ
þ x2 RT lnaF2−RT lna2 −γðx1S1 þ x2 S2Þ
ð8Þ
μ2 ¼ μo2 þ RT lna2
μ1 ¼ μo1 þ RT lna1
ð4Þ
ð5Þ
In the equilibrium, ΔG = 0, so Eq. (8) can be changed to:
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
γðx1S1 þ x2S2Þ ¼ x1 μo F−μo þ x2 μo F−μo
where μo2 and μo1 mean standard chemical potential of solute and solvent
in the bulk phase, a2 and a1 mean activity of solute and solvent, respec-
tively; R is the gas constant, T is the absolute temperature. For the surface
phase, the equations of chemical potentials are changed as follows:
1
1
2
2
h
i
ꢀ
ꢁ
ꢀ
ꢁ
x1
x2
þ RT ln aF1=a1
aF2=a2
ð9Þ
where (x1S1 + x2S2) is the average molar area of the solution,
ðx1S1 þ x2S2Þ ¼ V2=3N1=3
μ2 ¼ μo þ RT lnaF2−γ S2
ð6Þ
ð7Þ
F
F
2
ð10Þ
μ1 ¼ μo þ RT lnaFw−γ S1
F
F
1
where V is the average molar volume of the solution, N is Avogadro
constant. Therefore, a new thermodynamic function -molar surface
Gibbs free energy, gs, is obtained and its definition is:
where μo1F and μo2F mean the standard chemical potential, aF1 and aF
2
mean the activity in the surface phase, respectively. γ is the surface ten-
sion of the solution, S1 and S2 mean the partial molar area of solvent and
solute, respectively.
gs ¼ γV2=3N1=3
ð11Þ
Table 4
The molar surface Gibbs free energy, gs(exp.)/J·mol−1 and the parameters values of B0 and B1, the correlation coefficient, r, and the standard deviation, s, obtained by fitting gs vs m for
aqueous solutions of [C2py][DCA]
m/mol·kg−1
288.15 K
293.15 K
298.15 K
303.15 K
308.15 K
313.15 K
318.15 K
0.0115
0.0277
0.0559
0.1192
0.1702
0.2822
0.3221
0.3867
0.4419
B0
4235
4212
4176
4129
4088
3968
3928
3871
3817
4241
−959
−0.9992
6.5
4197
4180
4132
4096
4050
3929
3901
3838
3784
4203
−945
−0.9986
8.6
4154
4130
4094
4059
4018
3885
3869
3794
3734
4163
−950
−0.9974
12
4111
4082
4051
4016
3963
3854
3819
3756
3702
4115
−926
−0.9988
7.8
4063
4051
4009
3973
3921
3810
3788
3731
3670
4072
−898
−0.9988
7.7
4027
4015
3967
3925
3896
3780
3751
3700
3645
4032
−872
−0.9982
9.0
3980
3951
3920
3889
3854
3749
3721
3657
3620
3981
−818
−0.9978
9.3
B1
r
s/J·mol−1

N.N. Xing et al. / Journal of Molecular Liquids 219 (2016) 390–395
393
Table 6
The molar surface Gibbs free energy, gs(exp.)/J·mol−1 and the parameters values of B0 and
B1, the correlation coefficient, r, and the standard deviation, s, obtained by fitting gs vs m for
aqueous solutions of [C4mim][Pro] and [C6mim][Pro] at 298.15 K.
m/mol kg−1
gs(exp.)/J mol−1
m/mol kg−1
gs(exp.)/J mol−1
[C4mim][Pro]
[C6mim][Pro]
0.0111
0.0278
0.0556
0.1112
0.1670
0.2789
0.3351
0.3912
0.4470
B0
4175
4166
4139
4120
4075
3983
3930
3869
3820
4195
−810
−0.9957
13.3
0.0115
0.0277
0.0547
0.1112
0.1673
0.2789
0.3350
0.3910
0.4475
B0
4171
4157
4120
4093
4047
3957
3906
3836
3765
4185
−884
−0.9951
15.5
B1
r
B1
r
s/J mol−1
s/J·mol−1
Fig. 2. Plot of γ(est.) vs corresponding experimental γ(exp.) γ(est.) = 0.083+
0.999γ(exp.) r = 0.999, s = 0.137.
and in terms of the relation, gs(est.) = γV 2/3N1/3, the values of the sur-
face tension of the solution, γ(est.), can be estimated. The estimated
values of gs(est.) and the surface tension, γ(est.), are listed in
Tables S5 and S6 of the Supporting Information.
Plotting values of the surface tension estimated γ(est.) against ones
of γ(exp.), a good straight line was obtained (see Fig. 2). The gradient of
the fitting straight line is close to 1 (gradient = 0.999) and intercept
close up to zero (intercept = 0.083) with the fitting correlation coeffi-
cient r = 0.999, standard deviation, s = 0.137. These facts show that
values of γ(est.) are extremely similar and highly correlated with that
of γ(exp.).
According to Eq. (10), Eq. (11) and Eq. (9) can be converted to:
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
g ¼ x1 μo F−μo þ x2 μo F−μo
1
1
2
2
s
h
i
ꢀ
ꢁ
ꢀ
ꢁ
x1
x2
þ RT ln aF1=a1
aF2=a2
ð12Þ
It can be predicted that the right of Eq. (12) is a function of concen-
tration, but it is difficult to do the theoretical calculation. It is found that
an empirical equation can replace Eq. (12):
In order to use the experimental data to prove the reliability of the
above semi-empirical method, we calculated the molar surface Gibbs
free energy, gs(exp.), for aqueous [C5mim][Pro] [18] at 288.15 K to
318.15 K, [C4mim][Pro] [19] and [C6mim][Pro] [19] at 298.15 K (in the
following, these systems are called other aqueous ILs) using the litera-
ture data [18,19] and listed the results in Tables 5 and 6. Fitting gs vs
m according to Eq. (13), the parameters values of B0 and B1, the correla-
tion coefficient, r, and the standard deviation, s, were obtained for the
other aqueous ILs and are listed in Tables 5 and 6. From them it can be
seen that all values of r are larger than 0.99 and all values of the standard
deviation, s, are within experimental error. The values of gs(est.) were
estimated by using the values of parameters (B0 and B1) and listed in
Tables S7 and S8. According to Eq. (11), the values of γ(est.) can be pre-
dicted for the other aqueous ILs. Plotting the values of γ(est.) vs the cor-
responding experimental values, γ(exp.), a good straight line was
obtained (see Fig. S6). From Fig. S6, it can be seen that estimated surface
tensions for the other aqueous ILs and the corresponding experimental
values are highly correlated and extremely similar.
gs ¼ B0 þ B1
m
ð13Þ
where B0 and B1 are empirical parameters, m is the molarity of the
solution. In the work Eq. (11) was applied to calculated gs(exp.) of aque-
ous solutions [C2py][DCA] using the suface tension and the average
molar volume of aqueous [C2py][DCA]. The values of gs(exp.) are listed
in Table 4. Plotting the values of gs(exp.) vs m, a series of fine straight
lines were obtained (see Fig. 1) at 288.15 K to 318.15 K. The values of
B0, B1 with the standard deviation, s, and correlation coefficient, r,
were obtained and are listed in Table 4. All linear regressions were larg-
er than 0.99 and all values of standard deviation, s, were within experi-
mental error.
According to the rigorous thermodynamics, Eq. (12) is derived, and
the empirical Eq. (13) is used to calculate, so that the model is called
semi-empirical.
Substituting the values of B0 and B1 to Eq. (13), the molar surface
Gibbs free energy, gs(est.), of aqueous [C2py][DCA] could be estimated
Table 5
The molar surface Gibbs free energy, gs(exp.)/J mol−1 and the parameters values of B0 and B1, the correlation coefficient, r, and the standard deviation, s, obtained by fitting gs vs m for
aqueous solutions of [C5mim][Pro]
m/mol kg−1
288.15 K
293.15 K
298.15 K
303.15 K
308.15 K
313.15 K
318.15 K
0.0111
0.0278
0.0557
0.1112
0.167
0.2774
0.335
0.3913
0.4477
B0
4240
4232
4206
4183
4141
4047
3996
3925
3877
4262
−828
4202
4194
4174
4156
4096
4002
3957
3886
3844
4225
−834
4176
4173
4142
4118
4070
3975
3906
3853
3805
4200
−865
4133
4131
4105
4081
4027
3931
3874
3808
3754
4160
−877
4103
4094
4068
4044
3978
3888
3836
3758
3715
4125
−897
4073
4064
4020
4002
3947
3850
3786
3714
3683
4089
−909
4031
4023
3985
3960
3917
3814
3743
3676
3627
4052
−928
B1
r
−0.9950
14.6
−0.9956
13.9
−0.9967
12.5
−0.9958
14.1
−0.9966
13.0
−0.9966
13.2
−0.9960
14.7
s/J·mol−1

394
N.N. Xing et al. / Journal of Molecular Liquids 219 (2016) 390–395
Table 7
Values of parameters of the empirical Eq. (15), Tc/ K and kE/ J·mol−2/3·K−1 for aqueous solutions of [C2py][DCA]
m/mol·kg−1
C0/ J·mol−1
C1/ J·mol−1·K−1
r
s/J·mol−1
Tc/K
10−7kE/ J·mol−2/3·K−1
0.0115
0.0277
0.0559
0.1192
0.1702
0.2822
0.3221
0.3867
0.4419
6699
6670
6611
6498
6367
6084
5995
5888
5731
−8.543
−8.514
−8.450
−8.200
−7.907
−7.357
−7.157
−7.007
−6.664
−0.9996
−0.9965
−0.9998
−0.9987
−0.9972
−0.9983
−0.9978
−0.9978
−0.9936
2.878
8.465
2.241
4.893
6.985
5.068
5.660
5.529
8.950
784.2
783.4
782.4
792.5
805.2
826.9
837.6
840.3
859.9
1.011
1.007
0.999
0.968
0.937
0.869
0.847
0.831
0.789
3.4. A new Eötvös equation
This is a new Eӧtvӧs equation, the relationship between parameter
1 and kE of traditional Eӧtvӧs equation is C1= N1/3kE, the dimension
C
For most of the liquids, the values of surface tension decrease with
increasing the temperature. It can be expressed by the following tradi-
tional Eӧtvӧs equation [20,21]:
is J·mol−1·K−1. Although both Eq. (16) and traditional Eӧtvӧs equation
are empirical equations, but in the Eq. (16) each item has a clearer phys-
ical meaning, energy dimension and is more convenient to use. During
this investigation, we noted that the molar surface Gibbs free energy
can be connected with the volume, density and surface tension of the
liquid, it can be developed as a tool to predict physical properties of
substances.
γV2=3 ¼ kE ðTc−TÞ
ð14Þ
where V is molar volume, γ is surface tension, Tc is critical temperature.
kE is Eӧtvӧs empirical and related to polarity of the liquid. For example,
organic solvents have small polarity, kE ≈ 2.2×10−7 J·mol−2/3·K−1
4. Conclusions
and NaCl molten salt has high polarity, kE≈0.4×10−7 J·mol−2/3·K−1
.
(1) The values of density and surface tension of the pure IL
[C2py][DCA] were determined at 288.15 K to 318.15 K with an interval
of 5 K by using the standard addition method. (2) The values of density
and surface tension for aqueous solutions of [C2py][DCA] were deter-
mined at 288.15 K to 318.15 K with an interval of 5 K. The values of
the molar volume for aqueous solutions of [C2py][DCA] increase with
the increase of temperature and molarity. (3) On the basis of thermody-
namics, a semi-empirical model-molar surface Gibbs free energy model
of the ionic liquid solution was put forward and was used to predict the
surface tension. The predicted values of surface tensions for aqueous
[C2py][DCA] are in good agreement with the corresponding experiment
ones. (4) A new Eötvös equation is obtained. Each parameter of the new
equation has a clear physical meaning, energy dimension and is more
convenient to use.
If an aqueous IL with different concentrations is considered as a differ-
ent liquid, the Eötvös equation can also be applied. Fitting γV 2/3 vs T
for aqueous [C2py][DCA] under the condition of constant molality, the
values of the correlation coefficient, r of the linear fitting are larger
than 0.99. The slopes of the linear fitting are kE which are listed in the
Table 7. From Table 7, the kE values of aqueous [C2py][DCA] are between
2.2 × 10−7 J mol−2/3·K−1 and 0.4 × 10−7 J mol−2/3 K−1, so this fact
means that the polarity of aqueous [C2py][DCA] is smaller than NaCl
molten salt but bigger than organic solvents. In addition, the values of
kE decreases slowly as the concentration of solution increases, that is,
the polarity of the aqueous [C2py][DCA] increases slowly with the in-
crease of concentration.
Although the traditional Eӧtvӧs empirical equation can well explain
the relationship between surface tension and temperature, in which
2/3
the physical significance of γV
is not clear and the dimension,
List of symbols
J·mol−2/3, is not easy to use.
This equation was slightly improved in this work in terms of the new
thermodynamic function, the molar surface Gibbs free energy, gs, the
dependence of the molar surface Gibbs free energy on the temperature
could be expressed as the following the empirical equation:
gs
the molar surface Gibbs free energy
nuclear magnetic resonance
Eӧtvӧs empirical parameter
molar mass
NMR
kE
M
gs ¼ kEN1=3ðTc−TÞ ¼ kEN1=3Tc−kEN1=3T ¼ C0−C1T
ð15Þ
m
N
SAM
T
Tc
molarity
Avogadro constant
where Ci is an empirical parameter, C1 = kEN1/3, C0 = kEN1/3Tc and Tc =
C0/C1. Under the conditions of constant concentration, the values of
gs(exp.) of the aqueous [C2py][DCA] were plotted against T, so that a se-
ries of fine straight lines were obtained. The values of parameters, Ci,
with the standard deviation, s, and correlation coefficient, r, were ob-
tained and are listed in Table 7. All linear regressions were larger than
0.99 and all values of standard deviation, s, were within experimental
error. That is to say the relationship between the molar surface Gibbs
free energy, gs/J·mol−1, of the solution and temperature can be well
expressed by Eq. (15). Critical temperatures of the aqueous
[C2py][DCA] with various molalities were also calculated and listed in
Table 7.
standard addition method
absolute temperature
the critical temperature
glass transition temperature
molar volume
Tg
V
Greek letters
Γ
Ρ
surface tension
density
When the temperature is increased to the critical temperature, Tc,
the molar surface Gibbs free energy can be reduced to zero, a new Eӧtvӧs
equation is derived from Eq. (15):
Acknowledgements
This project was supported by NSFC (21173107) and LNET
(LR2015025), Peoples Republic of China.
gs ¼ C1ðTc−TÞ
ð16Þ

N.N. Xing et al. / Journal of Molecular Liquids 219 (2016) 390–395
395
[10] X.X. Ma, J. Wei, W. Guan, Y. Pan, L. Zheng, Y. Wu, J.Z. Yang, Ionic parachor and its ap-
plication to pyridinium-based ionic liquids of {[Cnpy][DCA] (n = 2, 3, 4, 5, 6)}, J.
Chem. Thermodyn. 89 (2015) 51–59.
[11] W. Guan, X.X. Ma, L. Li, J. Tong, D.W. Fang, J.Z. Yang, Ionic parachor and its applica-
tion in acetic acid ionic liquid homologue 1-alkyl-3-methylimidazolium acetate
{[Cnmim][OAc] (n = 2,3,4,5,6)}, J. Phys. Chem. B. 115 (2011) 12915–12920.
[12] J. Wei, T.Y. Ma, X.X. Ma, W. Guan, Q.S. Liu, J.Z. Yang, Study on thermodynamic prop-
erties and estimation of polarity of ionic liquids {[Cnmmim][NTf2] (n = 2, 4)}, RSC
Adv. 4 (2014) 30725–30732.
[13] J. Wei, X.X. Bu, W. Guan, N.N. Xing, D.W. Fang, Y. Wu, Measurement of vaporization
enthalpy by isothermogravimetrical method and prediction of the polarity for 1-
alkyl-3-methylimidazolium acetate {[Cnmim][OAc] (n = 4, 6)} ionic liquids, RSC
Adv. 5 (2015) 70333–70338.
[14] C.M. Gordon, M.J. Muldoon, M. Wagner, C. Hilgers, J.H. Davis, P. Wasserscheid, Syn-
thesis and Purification, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co.
KGaA, 2008 7–55.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2016.03.005.
References
[1] K.S. Seddon, Ionic liquids for clean technology, J. Chem. Tech. Biotechnol. 68 (1997)
351–356.
[2] G.H. Tao, L. He, W.S. Liu, X. Lin, X. Wei, T. Wang, Y. Kou, Preparation, characterization
and application of mino acid-based green ionic liquids, Green Chem. 8 (2006)
639–646.
[3] I.M. Marrucho, L.C. Branco, L.P.N. Rebelo, Ionic liquids in pharmaceutical applica-
tions, Annu. Rev. Chem. Biomol. 5 (2014) 527–546.
[4] K. Fukumoto, H. Ohno, Design and synthesis of hydrophobic and chiral anions from
amino acids as precursor for functional ionic liquids, Chem. Commun. 29 (2006)
3081–3083.
[15] M.J. Earle, C.M. Gordon, N.V. Plechkova, K.R. Seddon, T. Welton, Decolorization of
ionic liquids for spectroscopy, Anal. Chem. 79 (2007) 758–764.
[16] S.G. Zhang, X.J. Qi, X.Y. Ma, L.J. Lu, Y.Q. Deng, Hydroxyl ionic liquids: the differentiat-
ing effect of hydroxyl on polarity due to ionic hydrogen bonds between hydroxyl
and anions, J. Phys. Chem. B. 114 (2010) 3912–3920.
[5] K. Fukumoto, M. Yoshizawa, H. Ohno, New generation ionic liquids: cations derived
from amino acids, J. Am. Chem. Soc. 127 (2005) 2398–2399.
[6] O.B. Ghanem, N. Papaiconomou, M.I.A. Mutalib, S. Viboud, M. El-Harbawi, Y.
Uemura, G. Gonfa, M.A. Bustam, J.M. Lévêque, Thermophysical properties and
acute toxicity towards green algae and Vibrio fischeri of amino acid-based ionic liq-
uids, J. Mol. Liq. 52 (2013) 9490–9496.
[7] C.W. Cho, J. Ranke, J. Arning, J. Thoeming, U. Preiss, C. Jungnickel, M. Diedenhofen, I.
Krossing, S. Stolte, In silico modelling for predicting the cationic hydrophobicity and
cytotoxicity of ionic liquids towards the leukemia rat cell line, Vibrio fischeri and
Scenedesmus vacuolatus based on molecular interaction potentials of ions, SAR
QSAR Environ. Res. 24 (2013) 863–882.
[17] D.R. Lide, Handbook of Chemistry and Physics 82nd, ed, CRC Press, Boca Raton, FL,
2016 (2001–2002).
[18] C. Li, H.X. Yang, R.J. Liu, Q. Yang, J. Tong, J.Z. Yang, Volume and surface properties of
aqueous [C5mim][Pro] at 288.15 to 318.15 K, Acta Phys. -Chim. Sin. 31 (2015)
11–16.
[19] W.G. Xu, C. Li, R.J. Liu, H.X. Yang, J. Tong, J.Z. Yang, Volumetric and surface properties
of aqueous 1-alkyl-3-methylimidazolium propionate {[Cnmim][Pro] (n = 4, 5, 6)}
ionic liquids at 298.15 K, Ind. Eng. Chem. Res. 53 (2014) 9959–9967.
[20] A.W. Adamson, Physical Chemistry of Surfaces, 3rd Ed.; John Wiley: New York, 1976
translated by T. R. Gu, Science Press, Beijing, (1986).
[8] H. Roohi, S. Khyrkhah, Green chemical functionalization of single-wall carbon nano-
tube with methylimidazolium dicyanamid ionic liquid: a first principle computa-
tional exploration, J. Mol. Liq. 211 (2015) 495–505.
[21] J. Tong, Q.S. Liu, P. Zhang, J.Z. Yang, Surface tension and density of 1-methyl-3-
hexylimidazolium chloroindium, J. Chem. Eng. Data. 52 (2007) 1497–1500.
[9] X.X. Ma, J. Wei, Q.B. Zhang, F. Tian, Y.Y. Feng, W. Guan, Prediction of thermophysical
properties of acetate-based ionic liquids using semiempirical methods, Ind. Eng.
Chem. Res. 52 (2013) 9490–9496.