Estimation of Physico-Chemical Properties and Structure Characteristics of New Alkylimidazolium Salicylate Ionic Liquids

Article abstract of DOI:10.1515/zpch-2015-0576

Two new ionic liquids derived from natural product salicylate, 1-methyl-3-ethylimidazolium salicylate ([Emim][Sal]) and 1,2-dimethyl-3-ethylimidazolium salicylate ([Emmim][Sal]), were synthesized and characterized. The density, surface tension, viscosity,

Full text of DOI:10.1515/zpch-2015-0576

Z. Phys. Chem. 2015; aop
Ying Wei, Bing Wang, Zhen Zhao, Xin-yuan Zhang, Xuan-yu Wu,
and Qing-Guo Zhang*
Estimation of Physico-Chemical Properties
and Structure Characteristics of New
Alkylimidazolium Salicylate Ionic Liquids
DOI 10.1515/zpch-2015-0576
Received January 25, 2015; accepted November 7, 2015
Abstract: Two new ionic liquids derived from natural product salicylate, 1-
methyl-3-ethylimidazolium salicylate ([Emim][Sal]) and 1,2-dimethyl-3-ethyl-
imidazolium salicylate ([Emmim][Sal]), were synthesized and characterized. The
density, surface tension, viscosity, and electrical conductivity of two ionic liq-
uids were determined from 288.15 to 343.15 K. The physico−chemical proper-
ties of the ionic liquids were estimated from the experimental data by empiric
or semi-empiric methods, such as, coefficients of thermal expansion, molecular
volume, standard molar entropy, lattice energy, enthalpy of vaporization, and so
on. According to the interstice model of ionic liquids, the interstice parameters
like coefficients of thermal expansion and interstice volume etc. were estimated,
too. The experimental results and Density Functional Theory (DFT) calculation
were employed to discuss the structure characteristics caused by differences of
cations/anions for properties of two ionic liquids. And by the Vogel–Fulcher–
Tamman (VFT) equation, the temperature dependences of the ILs viscosities and
electric conductivities were graphically described. The structures, the energetics
and COSMOvolumes of allinvestigated ions areobtained by combiningDFT calcu-
lations and the COSMO–RS methodology to describe intermolecular interactions
of ionic liquids.
*Corresponding author: Qing-Guo Zhang, College of New Energy, Bohai University, Jinzhou
121013, Liaoning Province, China, e-mail: zhangqingguo@bhu.edu.cn
Ying Wei, Bing Wang, Xuan-yu Wu: College of Chemistry, Chemical Engineering and Food
Safety, Bohai University, Jinzhou 121013, Liaoning Province, China
Zhen Zhao: School of Chemistry and Life Science, Anshan Normal University, Anshan 114005,
Liaoning Province, China
Xin-yuan Zhang: College of New Energy, Bohai University, Jinzhou 121013, Liaoning Province,
China
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

2
| Y. Wei et al.
Keywords: Ionic Liquids, Physico-Chemical Properties, Interstice Model, VFT
Equation, Structure Characteristics.
1 Introduction
Ionic liquids (ILs), normally composed of bulky organic cations and inorganic an-
ions, have drawn immense attention due to their outstanding physicochemical
properties and interesting natures such as wide range of liquid status, negligi-
ble vapor pressure, high thermal stability, wide electrochemical windows, high
conductivity, and outstanding designability [1–8]. As a class of promising “green
solvent”, ILs have intriguing potential in application fields, such as synthetic
organic chemistry [5], gas separation processes [6, 7], electrochemistry/energy
fields [8–12], catalysis [13], and nanomaterial preparation [14]. To our knowledge,
the physicochemical properties of ionic liquids can vary considerably depending
on the combination of cations and anions. And ILs could be designed for particu-
lar purposes or show a specific physicochemical properties attributed to the wide
range of possible cation−anion combinations [15–18].
Recent years, some “greener ILs” derived from natural products have shown
some special characteristics [19–24] such as the good ability to dissolve cellulose
or carbohydrates, the specific viscous property, chirality, the particular hybrid ca-
pability as a surfactant, or accessory ingredients in functional material synthe-
sis. It is well known that salicylate is an important chemical raw material from
Salix and employed widely in medical and chemical industries. For example, Mu-
dring [25] reported the facile synthesis and optical characterization of highly lu-
minescent ILs based on the salicylate anion, varying the employed cation to con-
trol the resultant luminescent properties. Due to the bright future of ILs based on
natural compounds, we have designed a new class of ionic liquids from the nat-
ural product salicylic acid and dialkyl imidazolium, which may have the applied
potential in peptide syntheses of intermediates, chiral solvents, separation pro-
cesses, and biodegradable ionic liquids [26].
However, the systematic data regarding the basic physicochemical proper-
ties of ILs are still not complete. In this context, the study of physicochemical
properties, intermolecular interactions, structure characteristics, and the estima-
tion of thermodynamic properties of ILs become necessary in any of their ap-
plications [27–29]. Herein, two new types of ionic liquids from natural salicy-
late, 1-methyl-3-ethylimidazolium salicylate ([Emim][Sal]) and 1,2-dimethyl-3-
ethylimidazolium salicylate ([Emmim][Sal]), were successfully synthesized by an-
ion exchange method and characterized by infrared spectroscopy (IR) and proton
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 3
nuclear magnetic resonance (1H NMR). The physicochemicalproperties of the ob-
tained ILs such as density, surface tension, viscosity, and electrical conductivity
were determined at the temperature range of 288.15–343.15 K. The thermody-
namic parameters of ILs were estimated from the experimental data by empiric
or semi-empiric methods like coefficients of thermal expansion, molecular vol-
ume, standard molar entropy, lattice energy, and enthalpy of vaporization etc.
By interstice model [30, 31] of ionic liquids, the interstice parameters of two ILs
were also estimated. According to the Vogel–Fulcher–Tamman (VFT) equation,
the temperature dependences of the ILs viscosities and electric conductivities
have gotten a proper graphical description. Further, the structure characteristics
of ILs, which are resulted from the cation/anion size and position change of the
alkyl substituent in the imidazolium ring, were discussed by DFT calculations and
COSMO–RS methodology, a thermodynamic model based on quantum chemistry
calculations [32]. Then, the optimized energy model and the predicted molar liq-
uid volumes of the ILs were obtained by the calculation to describe intermolecular
interactions of pure ionic liquids and the variation of IL properties.
2 Experimental
2.1 Materials
The 1-methylimidazole, 1,2-dimethylimidazolium (Acros, analytical reagent) and
bromoethane (Beijing Co. Chemicals, analytical reagent) were distilled under ni-
trogen atmosphere respectively. Sodium salicylate (AR grade Sinopharm Chemi-
cal Reagent Co., Ltd.), was recrystallized twice. Methanol, ethyl aceta te, and ace-
tonitrile were distilled and then stored over molecular sieves of type 4 Å, respec-
tively.
2.2 Preparation and characterization
[Emim][Sal] and [Emmim][Sal] were prepared by a neutralization method
(Scheme 1) according to the previous literatures [33, 34], and then the viscous
ionic liquid was obtained. The synthesis method of [Emmim][Sal] is the same as
[Emim][Sal]. The reaction routes follow the Scheme 1.
The viscous ILs product [Emim][Sal] and [Emmim][Sal] were characterized by
IR and 1H NMR (see Tables 1 and 2), the water contents of the ILs were measured
using a Karl Fischer moisture titrator (ET08, METTLER EasyPlus) at atmospheric
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

4
|
Y. Wei et al.
OH
ONa
C2H5
N
C2H5
N
a
N
OH
O
O
C2H5
Br
NaBr
Br
C2H5OH
N
N
N
O
CH3
CH3
CH3
OH
ONa
C2H5
CH3
b
N
N
O
OH
O
C2H5
CH2
H3C
C2H5
Br
CH2
N
N
NaBr
C2H5OH
Br
N
N
CH3
O
CH3
Scheme 1: Synthesis Route of IL; a, [Emim][Sal]; b, [Emmim][Sal].
Table 1: IR assignment of synthesized [Emim][Sal] and [Emmim][Sal] ILs.
−1
Wavenumbers [cm
]
Assignments
3100–3000
benzene ring of C–H stretching vibration
770–730
benzene ring of ortho C–H plane bending vibration
3412
O–H bond stretching vibration
1352
O–H bond in-plane bending vibration
1735
C=O stretching vibration
3210, 3186
C–H bond stretching vibration
1573, 1466
imidazolium ring C=C stretching vibration
imidazolium ring C–H deformation vibration
1170
Table 2: 1H NMR (CDCl3, 400 MHZ) (ꢀ, ppm) of [Emim][Sal] and [Emmim][Sal] ILs.
ꢀ, ppm
[Emim][Sal] [Emmim][Sal]
1H, s, NCHN
9.25
9.28
1H, m, CH3NCHCHN
7.88
7.91
1H, m, CH3NCHCHN
7.79
7.83
1H, m, C(6)H
7.36
7.34
1H, m, OH
5.56
5.52
1H, s, CH
7.36
–
2H, t, NCH2(CH2)3CH3
4.53
4.54
3H, s, NCH3
4.37
4.40
2H, m, NCH2CH2(CH2)2CH3
1.97
2.01
2H, m, N(CH2)2CH2CH2CH
1.46
1.43
2H, m, N(CH2)3CH2CH3
3H, t, N(CH2)4CH3
1.33
0.84
1.36
0.89
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 5
−6
pressure and were found to be <600 × 10−6 and 850 × 10 wt % before and after
characterization and determination of properties, respectively.
2.3 Measurements
Density: The densities of ILs samples were measured using a Mettler Toledo DM 45
density meter at atmospheric pressure which utilizes Mettler Toledo’s vibrating
−5
−3
U-tube technology, and it provides measurement reproducibility of 10 g ⋅ cm .
The values are listed in Table 3.
Surface tensions: The surface tensions of the ILs were measured using a ten-
siometer by the platinum plate method (BZY–101 type produced by Shanghai
Fangrui instrument Co Ltd.) in a glove box. The samples were thermostatted
at each temperature with accuracy of 0.02 K. First, the surface tension of
degassed water (boiling and cooling) was measured in the temperature range
293.15–343.15 K and was in good agreement with the literature [35] within the
−1
experimental error of 0.1 mN ⋅ m . Then, the surface tensions of ILs were mea-
sured by the same method in the same temperature range and listed in Table 3.
Viscosity: The viscosity was measured at atmospheric pressure by a viscome-
ter (Stabinger Anton Paar SVM 3000) in the temperature range 293.15–343.15 K
for ILs. The reproducibility of the viscosity measurements is 0.35%, respectively.
High–precision thermostat was employed with stability of 0.005 K. The viscosi-
ties of the ILs are listed in Table 3.
Electrical conductivities: The electrical conductivities of the ILs were mea-
sured using a SG3 conductivity meter (Mettler–Toledo, DC 6 V), operating with
−1
InLab® 738 conductivity probe (nominal 0.57 cm
20% cell constant) under at-
mosphere pressure in the temperature range 293.15–343.15 0.02 K, and the cell
was calibrated with the standard aqueous KCl solution prior to use. Each value of
the electrical conductivity was the average of three determinations for high preci-
sion. The uncertainty in the measurements was 5%, and the electrical conduc-
tivities of the ILs are listed in Table 3.
2.4 Calculations
The DFT calculations with the density functional B3LYP were performed for iso-
lated ionic liquid complexes using Turbomole [36], the TZVP basis set from the
Turbomole library is employed throughout [37]. It was used to optimize the 3D
structures and energies, and to evaluate the H-bond interaction. All interaction
energies are calculated with the supermolecular ansatz. Some thermodynamic
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

6
| Y. Wei et al.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 7
Figure 1: ln ꢂ of ionic liquids against ꢁ,
[Emim][Sal] (
); [Emmim][Sal] (
).
parameters were calculated by COSMO [38]. The 3D structures and COSMO vol-
umes of all investigated ions are obtained by combining DFT calculations and the
COSMO–RS methodology [39].
3 Results and discussion
3.1 Estimations of volumetric properties and surface
properties
From the density values in Table 3, it is found that the density of [Emmim][Sal]
is smaller than that of [Emim][Sal]. According to the previous conclusions [40],
the extra methyl on imidazolium ring of [Emmim][Sal], which causes more com-
plicated alkyl substituent of cation, is one of the underlying reasons. Compared
to the similar ILs based on amino acid like [Emim][Glu] [41], the density of
[Emim][Sal] is slightly larger, and this implies the density change is related to the
steric effect from the size of the anion.
Figure 1 shows the experimental values of ln ꢀ against ꢁ. The variation of the
density with temperature is linear. The experimental values of ln ꢀ against ꢁ can
be fitted by the method of least squares as the following empiric equation:
−3
ln[ꢀ/(g ⋅ cm )] = ꢂ − ꢃꢁ/K
(1)
The correlation coefficient of [Emim][Sal] and [Emmim][Sal] are 0.99994 and
0.99996. The coefficient of thermal expansion of IL studied can be defined by the
following equation [41]:
ꢆ ln ꢀ
1
ꢆꢅ
ꢄ =
(
) = (
)
(2)
ꢅ
ꢆꢁ
ꢆꢁ
ꢆ
ꢆ
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

8
| Y. Wei et al.
where ꢄ is the coefficient of thermal expansion, ꢅ is the volume of the IL and ꢀ
is the density of the IL. The value of ꢄ obtained from the slope of the linear fit
−4 −1
−4 −1
of Equation (1), then, ꢄ = 5.21 × 10
K
and 5.20 × 10
K
for [Emim][Sal]
and [Emmim][Sal], respectively. The values have the same order to those of
−4 −1
other common alkylimidazoulium ILs like [Bmim][Zn3Cl7] (8.7 × 10 K ),
−4 −1
−4 −1
[Omim][PF6](7.95 × 10 K ) [42] and [Emim][Glu] (3.03 × 10 K ) [41].
From the experimental densities, the IL molecular volume at 298.15 K, ꢅ ,
m
can be predicted using the following equation:
ꢅ = ꢇ/(ꢈꢀ)
(3)
m
where ꢇ is the molar mass, ꢈ is the Avogadro’s constant, ꢀ is the density, and
ꢅ is the molecular volume of the IL. The predicted value of ꢅ based on previous
m
m
literatures [43] for [Emim][Sal] and [Emmim][Sal] are listed in Table 4.
+
Using the COSMO software, we can get the volume ꢅ of 1-methyl-3-ethyl-
imidazolium cation, 0.1491 nm ; the ꢅ of 1,2-dimethmyl-3-ethylimidazolium
3
+
m
3
−
3
cation, 0.1721 nm ; the ꢅ of salicylic acid anion, 0.1652 nm ; then the ꢅ of
m
m
3
3
[Emim][Sal], 0.3143 nm ; ꢅ of [Emmim][Sal], 0.3373 nm , and the calculation
m
results are close to the corresponding predicted values in Table 4. The calculation
results are listed in Table 6. The comparison shows the convenient prediction for
molecular volume of IL by Equation (3) is a dependable method.
For standard entropy, a remarkably simple expression was proposed by
Glasser [44] in terms of molecular−v1olume as:
ꢇ
−1
3
ꢉ /J ⋅ mol ⋅ K = 1246.5(ꢅ /nm ) + 29.5
(4)
m
the value of the standard entropy estimated by Equation (4) for [Emim][Sal]
and [Emmim][Sal] are listed in Table 4 , which is similar to the value of
360.2 J ⋅ mol ⋅ K−1 for amino acid ionic liquid [Emim][Gly] [45].
−1
Density is also critically needed for the estimation of lattice energy of
[Emim][Sal] and [Emmim][Sal]. By Glasser’s theory [44], lattice energy of ILs,
ꢊ
POT, may be estimated using the following equation:
−1
1/3
ꢊ
/kJ ⋅ mol = 1981.2(ꢀ/M) + 103.8
(5)
POT
The estimated lattice energies of [Emim][Sal] and [Emmim][Sal] at 298.15 K are
listed in Table 4. The results are close to the lattice energy of common ILs
−1
[Cnmim][BF4](ꢋ = 2, 4, 6) [44] in the range of 432–470 kJ ⋅ mol . It is shown that
the lattice energies of the studied ILs are much less than that of fused salts, for
−1
example, the fused CsI is 613 kJ ⋅ mol (the lowest lattice energy among all the
alkali halides) [46]. The low lattice energy may explain the low liquid-state tem-
perature of room temperature ILs [47].
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 9
The variation of the surface tension of the studied ILs with temperature is
linear as shown in Figure 2. The temperature dependence of the surface tension
is presented as:
−1
ꢌ/(mN ⋅ m ) = ꢂ − ꢃꢁ/K
(6)
From the experimental surface tension, the surface entropy and surface energy
can be estimated. The surface entropy values are obtained from the slopes of the
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

10
| Y. Wei et al.
Figure 2: Surface tension as a function of
temperature for [Emim][Sal] and
[Emmim][Sal] ILs: [Emim][Sal] (
);
[Emmim][Sal] (
).
temperature plots of the surface tension.
ꢉ = −ꢃ = −(ꢆꢌ/ꢆꢁ)
(7)
(8)
a
p
ꢍ = ꢌ − ꢁ(ꢆꢌ/ꢆꢁ)
a
p
The surface entropy and the surface energy for the [Emim][Sal] are
−2
−2
0.1096 mJ ⋅ K ⋅ m and 79.6 mJ ⋅ m , respectively. The surface entropy of
−2
[Emim][Sal] is much larger than that of [Emim][Gly], 0.0245 mJ ⋅ K ⋅ m ,
containing the same cation [45]. The surface energy value of [Emim][Gly] is
57.12 mJ ⋅ m−2 [45]. The increment of surface energy is 29.86 mJ ⋅ m−2 by replac-
ing the anion [Sal]− with [Gly]−. Compared to the [Emmim][Sal], with the same
anion, the changes of surface entropy and surface energy are not so obvious. Due
to the surface energy is dependent on intermolecular interactions, it implies that
the type and bigger structure of anion [Sal]− can largely affect the interactions
between anions and cations of ILs.
The critical temperature (ꢁ ) of IL is one of the most relevant thermo-physical
properties since it can be used icn many corresponding states correlations for equi-
librium and transport properties of fluids [48]. However, reliable data are rela-
tively hard to acquire due to the intrinsic nature of ILs. Some estimation methods
are usually used to predict the critical temperature value of ILs [49–51]. In this
work, prediction of the critical temperature is carried out by Eötvos equation (Fig-
ure 3) [49]:
2/3
ꢌꢅ = ꢎ(ꢁ − ꢁ)
(9)
c
where ꢌ is the surface tension, ꢁ is critical temperature, ꢎ is an empirical con-
c
stant, ꢅ is the molar volume of the IL. The equation reflects the fact that ꢌ be-
comes null at the critical point [47, 50]. The estimated critical temperature values
of [Emim][Sal] and [Emmim][Sal] are shown in Table 4. For majority of organic
−7
−1
liquids, ꢎ is about 2.1 × 10 J ⋅ K [52], but for fused salts with large polarity is
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 11
Figure 3: Plot of ꢃꢈ2/3 against temperature
temperature for [Emim][Sal] and
[Emmim][Sal] ILs: [Emim][Sal] (
);
[Emmim][Sal] (
).
−7
−1
rather small, for example, ꢎ = 0.6 × 10 J ⋅ K for fused NaCl [52]. The values
of ꢎ are 3.15 × 10−7 ([Emim][Sal]) and 3.67 × 10−7 ([Emmim][Sal]), so the magni-
tude of ꢎ can represent the polarity of ionic liquids [53], which means the polar-
ities of [Emim][Sal] and [Emmim][Sal] are similar to organic liquids. That shows
the alkylimidazolium salicylate ILs have capability to form special hybrid solvents
with organic liquids for gas capture or intermediate for medicine transport [54].
For ionic liquids, the molar enthalpy is an important parameter to evaluate
the stability, and it’s hard to be measured directly. By Kabo’s empirical equa-
tion [55], the molar enthalpy of vaporization can be estimated as following equa-
tion:
ꢒ
0
2/3 1/3
Δ ꢏꢏ = ꢐ(ꢌꢅ
ꢈ
) + ꢑ
(10)
m
ꢓ
where ꢈ is Avogadro’s constant, ꢅ is molar volume, ꢐ and ꢑ are empirical pa-
m
−1
rameters and their values are 0.01121 and 2.4 kJ ⋅ mol , respectively. The molar
enthalpies of vaporization for [Emim][Sal] and [Emmim][Sal] estimated are found
−1
to be 146.78 and 159.47 kJ ⋅ mol , which implies the introduction of a vicinal
methyl group in [Emmim] cation increases the enthalpy of vaporization because
of the enhanced intermolecular interactions.
3.2 Interstice model
Several theories [30, 56, 57] have been put forward to describe the relationships
between the internal structure and properties of ionic liquids or ionic solutions,
like Varela et al. proposed the pseudolattice theory for ionic fluids solution from
the infinite dilute solutions up to the pure IL [57, 58]. From them, for the studied
salicylate ILs, a interstice model that can succinctly describe the internal structure
change for pure IL is employeed.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

12
| Y. Wei et al.
According to the model [30, 31, 59], the inherent interstices exist in the ionic
liquid resulted from the large size and great asymmetry of ions and they can not
vanish during thermal movement. Then, the interstice volume ꢒ can be repre-
sented based on the classical statistical mechanics like following:
3/2
ꢓ = 0.6791(ꢎ ꢁ/ꢌ)
(11)
B
where ꢎ is the Boltzmann’s constant, ꢁ is the thermodynamic temperature, and
ꢌ is theBsurface tension of the IL. The values of ꢒ, the total volume of all inter-
stices ꢔꢒ = 2ꢈꢒ, and the thermal expansion coefficient ꢄ of the ILs at 298.15 K
are estimated and listed in Table 4. Typically, the ∑ ꢒ/ꢅ (the molar volume ꢅ)
are about 10% to 15% for volume change from ion solid to melt, and the results of
the salicylate ILs are very closed. Compared to the thermal expansion coefficients
ꢄ(exp) and ꢄ(cal) obtained from experimental and calculated data respectively,
the deviations are within 10%, which shows the interstice model were creditable
for these ILs.
3.3 Characteristic of electrical conductivity and dynamic
viscosity
The temperature dependence of electrical conductivity and dynamic viscosity val-
ues of the ILs (Figure 4 and Figure 5) can also be fitted using the VFT equation [60]:
ꢕ = ꢕ0 exp(−ꢑ/(ꢁ − ꢁ0))
(12)
(13)
ꢖ = ꢖ0 exp(−ꢑ/(ꢁ − ꢁ0))
here ꢕ is the electrical conductivity, ꢖ is the dynamic viscosity; ꢖ0, ꢕ0, ꢑ, and ꢁ0
are fitting parameters. The fitted parameters of ꢖ0, ꢕ0, ꢑ, and ꢁ0 and the corre-
2
sponding correlation coefficient, ꢗ , are listed in Table 5. From Table 5, the values
2
of the correlation coefficient, ꢗ , are more than 0.9999, which indicates that the
VFT equation has a well performance fitting the experimental electrical conduc-
tivity.
Sometimes, the Arrhenius equation (ꢕ = ꢕ∞ exp(−ꢍꢅ/(ꢎ ꢁ)), ꢖ =
B
ꢖ∞ exp(−ꢍꢅ/(ꢎ ꢁ))) was also used to fit the electrical conductivity and dy-
namic viscosityB. Vila et al. [61] have established the fitting parameters of VFT
equation with Arrhenius equation: ꢕ0 = ꢕ∞, ꢖ0 = ꢖ∞, ꢑ = ꢍ/ꢎ . The final version
B
of the VFT equation can be expressed as follows:
ꢕ = ꢕ∞ exp(−ꢍꢅ/(ꢎ (ꢁ − ꢁ0)))
(14)
(15)
B
ꢖ = ꢖ∞ exp(−ꢍꢄ/(ꢎ (ꢁ − ꢁ0)))
B
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 13
Figure 4: Electrical conductivity as a function
of temperature for [Emim][Sal] and
[Emmim][Sal] ILs: [Emim][Sal] (
);
[Emmim][Sal] (
).
Figure 5: Dynamic viscosity as a function of
temperature for [Emim][Sal] and
[Emmim][Sal] ILs: [Emim][Sal] (
);
[Emmim][Sal] (
).
Table 5: Adjustable parameters of dynamic viscosity ꢄ and electrical conductivity ꢅ for
[Emmim][Sal] and [Emim][Sal] ILs according to the VFT equation.
ꢄ [mPa ⋅ s]
3
2
ꢄ0 [mPa ⋅ s]
ꢔ
ꢁ0 10 ꢋꢄ/(V
ꢕ
[Emim][Sal]
0.1 −895.0
1.5 −417.4
186.4
227.1
77.0
35.9
0.99996
0.99998
[Emmim][Sal]
−1
ꢅ [mS ⋅ cm
]
−1
3
2
ꢅ0 [mS ⋅ cm
]
ꢔ
ꢁ0 10 ꢋꢅ/(V
ꢕ
[Emim][Sal]
[Emmim][Sal]
82.3
43.0
288.3
174.8
222.4
249.6
24.8
15.0
0.99998
0.99995
where ꢍꢅ, ꢍꢄ is the activation energy, which indicates the energy needed for the
ion to hop into a free hole, ꢕ∞, ꢖ∞ is the maximum electrical conductivity and
dynamic viscosity, ꢎ is the Boltzmann constant. The activation energies of elec-
B
trical conductivity for the ILs were calculated and listed in Table 5.
From the viscosity data, we have found the viscosities of [Emmim][Sal] and
[Emim][Sal] are higher than most of common ILs [62] due to their strong inner
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

14
| Y. Wei et al.
Figure 6: Optimized structures and hydrogen bonds of ionic liquids: a, structure of [Sal] anion;
b, [Emim] cation and hydrogen position; c, [Emmim] cation and hydrogen position;
d, [Emim][sal] structures and hydrogen bonds; e, [Emim][sal] structures and hydrogen bonds.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 15
Figure 7: The interaction
of ionic liquids and the
situation of hydrogen
bonds, a, [Emmim][Sal];
b, [Emim][Sal].
Table 6: Structure, abbreviation and Cosmo volume by the CosmothermX interface of studied
ions and ILs.
Structure and abbreviation Cosmo volume
Structure and abbreviation Cosmo volume
3
3
[nm ]
[nm ]
[Sal]–
[Emim][Sal]
0.1652
0.3143
[Emim]+
[Emim][Sal]
0.1491
0.3372
[Emmim]+
0.1720
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
Polarisation Charge Density σ
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

16
|
Y. Wei et al.
hydrogen bond net [62], which provide the ILs an ability to capture CO2 gas but
limittheindustrializationof them. A worthwhilesolutionistoblend ILs withother
organic solvents to form specialized mixed systems in future [63].
3.4 The structure characteristics and interactions
The DFT calculations are carried out to learn about the optimized structure, en-
ergy, charge distribution, and hydrogen bonds of two new ionic liquids (see Figs. 6
and 7, Table 6). The main difference of structure between two ILs is the vicinal
methyl on the imidazolium ring.
From the charge distribution (Table 6), it appears that the highly localized
negative charge in the [Sal] anion can form relatively strongcation–anion Coulom-
bic interactions [39]. The cation–anion interaction should be strongly affected by
the labile hydrogen on the anion [Sal]− structure which causes the smaller inter-
action energy than previous studied ILs [64].
The sum of van der Waals atomic radii of hydrogen and oxygen (2.5 Å) is used
as a critical value for judging if a hydrogen bond is present between hydrogen
and oxygen atoms [64, 65]. Hydrogen bonds are denoted by dashed lines, and
the corresponding H...O distances are labeled in Figure 6. It is found that hy-
drogen bonds can exist between C2–H or methyl C–H of cations and O atom
of anions. The same number of H-bonds between the ion pairs, however, has no
−1
evident impact on the interaction energy, −72.89 kJ ⋅ mol for [Emim][Sal] and
−55.56 kJ ⋅ mol−1 for [Emmim][Sal], indicating that the Coulombic interaction be-
tween the cation and the anion is dominant.
From the interaction energy and structure (Figure 6c), it appears the methy-
lation at C(2) can decreases interaction between cation and anion, and the pro-
truding methyl may form H-bonding interactions with other ions, which is one of
the reasons of [Emmim][Sal] having higher viscosity and lower conductivity.
4 Conclusions
In this work, two new ionic liquids [Emim][Sal] and Emmim][Sal] from natural
salicylate were prepared and characterized. The physico–chemical properties of
the ionic liquids like density, surface tension, viscosity and electrical conductivity
were measured and thermodynamic parameters were estimated from the experi-
mental data by empiric or semi-empiric methods and COSMO–RS method. The
COSMO volumes are close to the corresponding estimated values, which indicated
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 17
the molecular volume of IL can be dependably predicted. The interstice model of
ILs was employed to describe internal structure change and estimate the inter-
stice parameters. The small deviations (within 10%) of the thermal expansion co-
efficients obtained respectively from experimental data and calculation showed
the interstice model was creditable for the salicylate ionic liquids. Then, accord-
ing to the VFT equation, the temperature dependences of the ILs viscosities and
electric conductivities have been described graphically. Furthermore, the effects
on preperties resulted from the structural change of vicinal methyl for the two ILs,
the interactions between cations and anions of ILs, were discussed from the es-
timated values and DFT calculations. It is implied that the methylation at C(2)
position of cation may be one of the reasons of viscosity and conductivity differ-
ences for the salicylate ILs. These studies can provide a deeper understanding of
the physical properties of the salicylate–based ILs and may shed light on the ap-
plication of this family of ionic liquids.
Acknowledgement: This work was financially supported by the National Na-
ture Science Foundation of China under Grant: NSFC No. 21003081, 21373002,
21503020; Supported by Program for Liaoning Excellent Talents in University, No.
LJQ2013116.
References
1. H. Shirota, T. Mandai, H. Fukazawa, and T. Kato, J. Chem. Eng. Data 56 (2011) 2453.
2. J. P. Hallett and T. Welton, Chem. Rev. 111 (2011) 3508.
3. H. Weingärtner, Angew. Chem. Int. Edit. 47 (2008) 654.
4. D. D. Patel and J. M. Lee, Chem. Rec. 12 (2012) 329.
5. V. I. Parvulescu and C. Hardacre, Chem. Rev. 107 (2007) 2615.
6. C. I. Meloa, R. Bogel-Łukasik, E. Bogel-Łukasik, J. Supercrit. Fluid. 61 (2012) 191.
7. M. G. Freire, A. R. R. Teles, M. A. A. Rocha, B. Schröder, C. M. S. S. Neves, P. J. Carvalho, D. V.
Evtuguin, L. M. N. B. F. Santos, and J. A. P. Coutinho, J. Chem. Eng. Data 56 (2011) 4813.
8. K. Yuyama, G. Masuda, H. Yoshida, and T. Sato, J. Power Sources 162 (2006) 1401.
9. K. Kawai, K. Kaneko, and T. Yonezawa, Langmuir 27 (2011) 7353.
10. T. Nishida, Y. Tashiro, M. Yamamoto, J. Fluorine Chem. 120 (2003) 135.
11. X. Y. Liu, W. Afzal, and J. M. Prausnitz, J. Chem. Thermodyn. 70 (2014) 122.
12. M. A. A. Rocha, F. M. S. Ribeiro, A. I. M. C. L. Ferreira , J. A. P. Coutinho, and L. M. N. B. F.
Santos, J. Mol. Liq. 188 (2013) 196.
13. L. P. Jameson and S. V. Dzyuba, J. Nat. Prod. 74 (2011) 310.
14. L. Wang, L. Chang, B. Zhao, Z. Yuan, G. Shao, and W. Zheng, Inorg. Chem. 47 (2008) 1443.
15. C. W. Cho, U. Preiss, C. Jungnickel, S. Stolte, J. Arning, J. Ranke, A. Klamt, I. Krossing, and
J. Thoming, J. Phys. Chem. B 115 (2011) 6040.
16. H. Ohno, B. Chem. Soc. Jpn. 79 (2006) 1665.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

18
| Y. Wei et al.
17. P. Wasserscheid and T. Welton (Eds.), Ionic Liquids in Synthesis, Wiley-VCH Verlags GmbH
& Co. KGaA, Weinheim, Germany (2008).
18. E. D. Bates, R. D. Mayton, I. Ntai, and J. H. Davis, J. Am. Chem. Soc. 124 (2002) 926.
19. G. H.Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang, and Y. Kou, Green Chem. 8 (2006) 639.
20. H. Ohno and K. Fukumoto, Accounts Chem. Res. 40 (2007) 1122.
21. A. E.Visser, R. P. Swatloski, W. M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, J. H. Davis,
and R. D. Rogers, Chem. Commun. 36 (2001) 135.
22. M. Yoshizawa, A. Narita, and H. Ohno, Aust. J. Chem. 57 (2004) 139.
23. S. Taguchi, T. Matsumoto, T. Ichikawa, T. Kato, and H. Ohno, Chem. Commun. 47 (2011)
11342.
24. T. J. Trivedi, K. S. Rao, T. Singh, S. K. Mandal, N. Sutradhar, A. B. Panda, and A. Kumar,
ChemSusChem 4 (2011) 604.
25. P. S. Campbell, M. Yang, D. Pitz, J. Cybinska, and A.-V. Mudring, Chem. Eur. J. 20 (2014)
4704.
26. J. Z. Yang, X. M. Lu, J. S. Gui, and W. G. Xu, Green Chem. 6 (2004) 541.
27. H. Yoon, P. C. Howlett, A. S. Best, M. Forsyth, and D. R. MacFarlane, J. Electrochem. Soc.
160 (2013) A1629.
28. Z. B. Zhang, Q. Zhou, X. M. Lu, C. Z. Qiao, and S. J. Zhang, J. Chem. Eng. Data 59 (2014)
2377.
29. Z. Wojnarowska, G. Jarosz, A. Grzybowski, J. Pionteck, J. Jacquemin, and M. Paluch, Phys.
Chem. Chem. Phys. 16 (2014) 20444.
30. Q. G. Zhang, J. Z. Yang, X. M. Lua, J. S. Gui, and M. Huang, Fluid Phase Equilibr. 226 (2004)
207.
31. Q. G. Zhang and Y. Wei, J. Chem. Thermodyn. 40 (2008) 640.
32. J. Palomar, V. R. Ferro, J. S. Torrecilla, and F. Rodríguez, Ind. Eng. Chem. Res. 46 (2007)
6041.
33. Y. Wei, Q. G. Zhang, Y. Liu, X. R. Li, S. Y. Lian, and Z. H. Kang, J. Chem. Eng. Data 55 (2010)
2616.
34. Y. Wei, Y. Jin, Z. J. Wu, Y. Yang, Q. G. Zhang, and Z. H. Kang, J. Chem. Eng. Data 58 (2013)
349.
35. D. R. Lide, Handbook of Chemistry and Physics, 82nd edn., CRC Press, Boca Raton, FL
(2001–2002).
36. TURBOMOLE V6.6 2014, a development of University of Karlsruhe and Forschungszentrum
Karlsruhe GmbH, 1989–2007, TURBOMOLE GmbH, since 2007, available from http://www.
turbomole.com.
37. P. Nockemann, B. Thijs, S. Pittois, J. Thoen, C. Glorieux, K. V. Hecke, L. V. Meervelt,
B.Kirchner, and K. Binnemans, J. Phys. Chem. B 110 (2006) 20978.
38. H. F. Hizaddin, M. A. Hashim, and R. Anantharaj, Ind. Eng. Chem. Res. 52 (2013) 18043.
39. J. Jacquemin, J. Feder-Kubis, M. Zorebski, K. Grzybowska, M Chorazewski, S. Hensel-
Bielowka, E. Zorebski, M. Paluchd, and M. Dzida, Phys. Chem. Chem. Phys. 16 (2014) 3549.
40. Q. G. Zhang, Y. Wei, S. S. Sun, C. Wang, M. Yang, Q. S. Liu, and Y. A. Gao, J. Chem. Eng. Data
57 (2012) 2185.
41. W. Guan, J. Tong, S. P. Chen, Q. S. Liu, and S. L. Gao, J. Chem. Eng. Data 55 (2010) 4075.
42. J. Y. Wang, H. C. Jiang, Y. M. Liu, and Y. Q. Hu, J. Chem. Thermodyn. 43 (2011) 800
43. Q. G. Zhang, M. C. Li, X. Y. Zhang, and X. Y. Wu, Z. Phys. Chem. 228 (2014) 851.
44. L. Glasser, Thermochim. Acta 421 (2004) 87.
45. J. Z. Yang, Q. G. Zhang, B. Wang, and J. Tong, J. Phys. Chem. B 110 (2006) 22521.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM

Estimation of Salicylate Ionic Liquids
| 19
46. T. L. Greaves, A. Weerawardena, I. Krodkiewska, and C. J. Drummond, J. Phys. Chem. B 112
(2008) 896.
47. K. A. Kurnia, M. I. Abdul Mutalib, and B. Ariwahjoedi, J. Chem. Eng. Data 56 (2011) 2557.
48. M. G. Freire, P. J. Carvalho, A. M. Fernandes, I. M. Marrucho, A. J. Queimada, and
J. A. P.Coutinho, J. Colloid Interf. Sci. 314 (2007) 621.
49. F. Gharagheizi, A. Eslamimanesh, and A. H. Mohammadi, and D. Richon, J. Chem. Eng. Data
56 (2011) 2460.
50. J. Tong, M. Hong, W. Guan, J. B. Li, and J. Z. Yang, J. Chem. Thermodyn. 38 (2006) 1416.
51. J. Tong, Q. S. Liu, W. Guan, and J. Z. Yang, J. Phys. Chem. B 111 (2007) 3197.
52. L. Rebelo, L. Canongia, J. Esperança, and E. Filipe, J. Phys. Chem. B 109 (2005) 6040.
53. Adamson, A. W., Physical Chemistry of Surfaces, 3rd edn., John Wiley, New York (1976).
54. B. H. Lv, Y. Shi, C. Sun, N. Liu , W. Li, and S. J. Li, Chem. Eng. J. 270 (2015) 372–377
55. D. H. Zaitsau, G. J. Kabo, A. A. Strechan, and Y. U. Paulechka, J. Phys. Chem. A 110 (2006)
7303.
56. J. O’M. Bockris and A. K. N. Reddy, Modern Electrochemistry, Vol. 1 Ionics, 2nd edn.,
Plenum Press, New York (2000), Chapter 5.
57. L. M. Varela, J. Carrete, M. García, L. J. Gallego, M. Turmine, E. Rilo, and O. Cabeza, Fluid
Phase Equilibr. 298 (2010) 280.
58. L. M. Varela, J. Carrete, M. Turmine, E. Rilo, and O. Cabeza, J. Phys. Chem. B 113 (2009)
12500.
59. D. W. Fang, W. Guan, J. Tong, Z. W. Wang, and J. Z. Yang, J. Phys. Chem. B 112 (2008) 7499.
60. L. Timperman and M. Anouti, Ind. Eng. Chem. Res. 51 (2012) 3170.
61. J. Vila, P. Ginés, J. M. Pico, C. Franjo, E. Jiménez, L. M. Varela, and O. Cabeza, Fluid Phase
Equilibr. 242 (2006) 141.
62. Y. Gao, F. Zhang, K. Huang, J. W. Ma, Y. T. Wu, and Z. B. Zhang, Int. J. Greenh. Gas Con. 19
(2013) 379.
63. J. E. Bara, C. J. Gabriel, S. Lessmann, T. K. Carlisle, A. Finotello, D. L. Gin, and R. D. Noble,
Ind. Eng. Chem. Res. 46 (2007) 5380.
64. L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell University Press, New York
(1960).
65. Q. G. Zhang, N. N. Wang, and Z. W. Yu, J. Phys. Chem. B 114 (2010) 4747.
Brought to you by | University of California
Authenticated
Download Date | 12/8/15 4:21 AM