Thermodynamic Properties and Intermolecular Interactions of a Series of N-Butylammonium Carboxylate Ionic Liquids

Article abstract of DOI:10.1021/acs.jced.8b00583

A series of new N-butylammonium carboxylate ionic liquids (N-butylammonium formate, N-butylammonium acetate, N-butylammonium propionate, and N-butylammonium butyrate) were synthesized by a one-step method and characterized. The thermodynamic properties such as surface tension, density, electrical conductivity, and dynamic viscosity of the ILs were measured as functions of temperature. Important parameters such as molecular volume, crystal energy, thermal expansion coefficient, standard molar entropy, and the surface properties are estimated by the empirical equations. The Vogel-Fulcher-Tamman (VFT) equation is used to explore the dependence of electrical conductivities and dynamic viscosities on temperature. The relationship between the molar conductivity and the fluidity of the ILs was examined through the use of Walden plots. To describe intermolecular interactions of the N-butylammonium carboxylate ILs, the optimized structures and energetics of ILs are calculated by DFT calculations. The calculated energies showed that the increase in the interaction energies between the ion clusters is the same as the trend of experimental viscosities.

Full text of DOI:10.1021/acs.jced.8b00583

Article
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
pubs.acs.org/jced
Thermodynamic Properties and Intermolecular Interactions of a
Series of N‑Butylammonium Carboxylate Ionic Liquids
†
†
†
†
,‡
Ying Wei, Tongtong Xu, Xinyuan Zhang, Yang Di, and Qingguo Zhang*
†
College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121013, Liaoning Province, China
College of New Energy, Bohai University, Jinzhou 121013, Liaoning Province, China
‡
*
S Supporting Information
ABSTRACT: A series of new N-butylammonium carboxylate
ionic liquids (N-butylammonium formate, N-butylammonium
acetate, N-butylammonium propionate, and N-butylammonium
butyrate) were synthesized by a one-step method and
characterized. The thermodynamic properties such as surface
tension, density, electrical conductivity, and dynamic viscosity of
the ILs were measured as functions of temperature. Important
parameters such as molecular volume, crystal energy, thermal
expansion coefficient, standard molar entropy, and the surface
properties are estimated by the empirical equations. The Vogel−Fulcher−Tamman (VFT) equation is used to explore the
dependence of electrical conductivities and dynamic viscosities on temperature. The relationship between the molar
conductivity and the fluidity of the ILs was examined through the use of Walden plots. To describe intermolecular interactions
of the N-butylammonium carboxylate ILs, the optimized structures and energetics of ILs are calculated by DFT calculations.
The calculated energies showed that the increase in the interaction energies between the ion clusters is the same as the trend of
experimental viscosities.
1
. INTRODUCTION
researched the properties of the N-alkyl ethylenediammonium
with chelate amine such as thermal stability, viscosity, glass
Ionic liquids (ILs) have been favored by researchers from
many fields over the past decade, due to their unusual
properties and potential environmentally friendly behaviors.
In recent years, ILs have shown great promise in many respects
such as catalytic reactions, organic synthesis, CO capture,
electrochemistry, extraction, and multiphase separations.
The investigations on physicochemical properties of ILs are
also very necessary not only for designing chemical industry
separation processes and transport equipment but also for
predicting the properties and characteristics of ILs for the
application in various fields.
Protic ionic liquids (PILs), generally more hydrophilic than
aprotic ionic liquids (AILs), are a type of ILs with active
protons. Simple ammonium ILs from amines and acids by the
neutralizing reaction belong to protic ionic liquids and are
expected to be widely used in the extraction of metal ions,
electrolyte of fuel cells, and absorption of CO2, etc. Due to
its low toxicity, convenient preparation, and low cost, its
physical and chemical properties have also received extensive
17
transition, conductivity, and density.
1
,2
We have prepared four new simple carboxyl ammonium ILs,
N-butylammonium formate ([N ][HCOO]), N-butylammo-
4
nium acetate ([N ][CH COO]), N-butylammonium propio-
4
3
2
3
−7
nate ([N ][C H COO]), and N-butylammonium butyrate
4 2 5
(
[N ][C H COO]), by the one-step synthesis method.
4 3 7
Compared with the reported works, current work in our
laboratory is aimed at characterizing the physicochemical
properties of the ILs and exploring the intermolecular
interactions on the macroscopic thermodynamic behaviors.
In this work, the four new ILs were synthesized and
8
1
characterized with proton nuclear magnetic resonance ( H
9
NMR), infrared spectroscopy (IR), and elemental analysis
(
EA). The dynamic viscosity, surface tension, density, and
electrical conductivity of the ILs were determined from 293.15
to 333.15 K at 0.1 MPa. The thermodynamic parameters of the
ILs ([N ][HCOO], ([N ][CH COO]), ([N ][C H COO]),
10
11
4
4
3
4
2
5
and ([N ][C H COO]), such as thermal expansion coefficient,
4
3
7
1
2,13
molecular volume, standard molar entropy, and lattice energy,
are estimated according to semiempirical/empirical methods.
The temperature dependences of viscosities and electrical
conductivities for the ILs are illustrated by the VFT equation.
Then, the Walden rule is used to describe the “ionicity” of the
attention.
Xu et al. have synthesized N0butylammonium
acetate and systematically studied its physicochemical proper-
ties. The N-butylammonium acetate was used in the
treatment of coir fiber prior to acid hydrolysis by
1
4
15
Morandim−Giannetti et al. The hydroxyl ammonium-based
16
ILs are synthesized and characterized by Gardas et al., and
the intermolecular interactions are discussed by molecular
molecular radius, standard entropy, molar volume, and
intermolecular free length of cations and anions. Wang et al.
Received: July 7, 2018
Accepted: October 23, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.jced.8b00583
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Journal of Chemical & Engineering Data
Article
Table 1. Information of Samples
chemical name
N-butylamine
formic acid
acetic acid
propionic acid
CAS registry nos.
source
purification method
distillation
distillation
distillation
distillation
mass fraction purity
109-73-9
64-18-6
64-19-7
79-09-4
107-92-6
Sinopharm (China)
Sinopharm (China)
Sinopharm (China)
Sinopharm (China)
Sinopharm (China)
>0.99
>0.99
>0.99
>0.99
>0.99
>0.99
>0.99
>0.99
>0.99
butyric acid
distillation
N-butylammonium formate
N-butylammonium acetate
N-butylammonium propionate
N-butylammonium butyrate
solvent extraction, vacuum drying
solvent extraction, vacuum drying
solvent extraction, vacuum drying
solvent extraction, vacuum drying
ILs through the molar conductivities and viscosities. In
addition, the intermolecular interactions in ILs were evaluated
by using DFT calculation combining COSMO−RS method-
ology. The cation/anion of ionic liquids, optimized structures,
hydrogen bonds, charge distributions, and energies were
obtained to study the intermolecular interactions of the ILs,
and the relation between the viscosity and interactions is also
discussed.
continuously, automatically measured at 293.15−333.15 K. To
avoid entrainment of air bubbles in the closed cell full of the
sample, we carefully injected the samples into the cell to
reduce the possibility of this random error. The measurement
uncertainty is 0.001 g cm at 0.1 MPa, and the values are
listed in Table S1.
−
3
2.3. Measurement of Surface Tension. The surface
tensions of the ILs under study were tested by a tensiometer
(
DPAW type produced by Sang Li Electronic Co.) using the
2
. EXPERIMENTAL SECTIONS
.1. Chemicals and Synthesis. All materials are supplied
forced bubble method. The deionized water’s surface tension
was measured at 293.15−333.15 K, and the uncertainty is 0.1
mN m . Over the same temperature range, the surface tension
of the sample was then measured in the same manner, and the
values are listed in Table S1.
2
−
1
by Sinopharm Chemical Reagent Co., Ltd., China, and purified
before proceeding to the next test. The purities and source of
the materials are shown in Table 1.
According to the reported procedures, carboxylate-based
2.4. Measurement of Viscosity. The temperature was
controlled by the Peltier thermostat at 293.15−333.15 K, and
the viscosity is measured by the Ostwald viscometer. Three
different diameter capillaries/balls were used to measure
viscosities to allow for a wider range of viscosity tests. The
uncertainty of the viscosity measurement is 0.05.
2.5. Measurement of Electrical Conductivity. To
measure the electrical conductivity of ILs, an SG3 conductivity
meter (Mettler-Toledo, DC 6 V) operating with InLab738
conductivity probe was used, and the entire measuring process
was completed under high purity argon at 293.15−333.15 K.
Since the measurement is temperature dependent, a constant
temperature bath was used to keep the temperature and place
the electrode in the samples at a predetermined temperature.
We tested three sets of electrical conductivity for each
temperature and selected the average value of these three
sets as the final result. The uncertainty in the measurement is
ionic liquids [N ][XCOO] were obtained by the neutralization
4
15
reaction of N-butylamine and carboxylic acid (see Scheme
). The temperature was controlled to 298.15 K with
1
Scheme 1. Synthetic Route and Molecular Structures of
N ][XCOOH]
[
4
continuous stirring; 0.55 mol of N-butylamine was put in a
00 mL three-necked flask; and then 0.50 mol of a carboxylic
acid (HCOOH, CH COOH, C H COOH, and C H COOH)
1
3
2
5
3
7
was slowly added dropwise. After 4 h of neutralization reaction,
redundant N-butylamine was isolated, and the obtained
[N ][HCOO], [N ][CH COO], [N ][C H COO], and [N ]-
4 4 3 4 2 5 4
[
C H COO] were subjected to vacuum drying for at least 48 h
0.05. The results are listed in Table S1. 1
3
7
8,19
at 343.15 K.
2.6. Calculations. Using Turbomole,
the DFT of the
1
The four new ILs were characterized by IR, EA, and H
NMR spectra (see the Supporting Information). The purities
of ILs were >99% by H NMR spectroscopy. A coulometric
Karl Fischer titrator (Mettler Toledo, model ET−08) was used
to measure the water content of ILs and was found to be
density functional B3LYP was calculated for the separated
ionic liquid complex, and the TZVP base group from
Turbomole library was used throughout the process. We
obtained optimized 3D structures and energies, and it can be
used to evaluate H-bond interactions. The TURBOMOLE 6.5
program package, equipped with the B3LYP functional with
the def-SVP basis, was used to compute all the interaction
energies under the supramolecular ansatz.
1
20
<
0.06%. The thermal characteristics of the four ILs were
measured using a differential scanning calorimetry (DSC)
technique (TA Instruments, 910S). The heating rate is 5 K
−
1
min , and the temperature range was (−100 to 100) °C. The
thermogravimetry/differential thermal analysis (TG/DTA)
values were measured using PerkinElmer’s thermogravimetric
analysis TGA/SDTA851. Heating was carried out at a rate of 5
K min under a dynamic nitrogen atmosphere (flow rate 25
dm h ). All the results are listed in the Supporting
Information.
3. RESULTS AND DISCUSSION
3.1. Estimation of Volumetric Properties. Trends of
−
1
densities of the four new ILs were measured and compared
with the similar type of ILs to confirm the accuracy and
reproducibility of our measured data. The comparisons
3
−1
14,21,22
2
.2. Measurement of Density. The density was tested
between the densities from this work and the literature
using a Mettler Toledo DM 45 densitometer, and they were
are shown in Figure 1. There are slight deviations in the final
B
DOI: 10.1021/acs.jced.8b00583
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Journal of Chemical & Engineering Data
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In general, the increase in liquid density is due to a variation
in the management of ions in a unit volume. The more ions
23
that can be arranged in a unit volume, the higher the density.
The density values of the four new ILs decreased with
increasing temperature and carbon chain length, and the
sequence is [N ][HCOO] > [N ][CH COO] > [N ]-
4
4
3
4
[
C H COO] > [N ][C H COO]. As shown in Figure 1, the
2 5 4 3 7
densities of ILs decrease as the length of the alkyl chain in the
carboxylate anion increases, indicating that the anionic
structure has a certain effect on the densities of these ILs.
The variation in temperature and impact of carbon chain
2
4−26
length are also consistent with previously reported trends.
27
21
Compared with Greaves’ and Torres’ work, the addition of
methylene groups to anions or cations leads to an increase in
density, which was attributed to the increase in chain length,
making the steric hindrance larger.
Figure 1. Plot of density ρ vs temperature T of the ionic liquids:
[
[
N ][XCOO], black filled square, [N ][HCOO]; red filled circle,
4
4
N ][CH COO]; blue filled triangle, [N ][C H COO]; pink filled
4
3
4
2
5
The relevant thermodynamic parameters calculated from the
upside down triangle, [N ][C H COO]; red open circle, [N ]-
4
3
7
4
experimental density, such as molar volume (V ), thermal
[
CH COO] in ref 14 ; green open circle, [N ][CH COO] and blue
m
3
4
3
open circle, [N ][C H COO] in ref 21; pink open circle, [N ]-
expansion coefficient (α), crystal energy (UPOT), and standard
4
2
5
4
0
[
C H COO] in ref 22.
entropy (S ) are important for the understanding of
3
7
intermolecular interactions.
The thermal expansion coefficient of the IL is calculated by
the following expression
Table 2. Volume Properties of the Ionic Liquids
N ][XCOO] (X = H, CH , CH CH , CH CH CH )
Estimated Above at T = 298.15 K and p = 0.1 MPa
[
4
3
3
2
3
2
2
α = 1
i∂V y
1 i ∂ρ y
j
z
j
z
α × 10−4, Vm × 10−21,
j
j
z = − j
j
z
z
z
S°,
U
POT,
−
1
3
J mol−1 ·K−1
−1
V k∂T {
ρk∂T {
(1)
K
cm
kJ·mol
P
P
[
[
[
[
[
N ][HCOO]
6.5212
6.4085
6.577
6.4636
−
0.2033
0.2314
0.2627
0.296
282.9
318
357
398.5
285.4
597.9
585.9
574.7
564.6
493.5
492.4
where α is the thermal expansion coefficient; V is the volume
of the ILs; and ρ is the density of the ILs. The values of
thermal expansion coefficient are presented in Table 2.
4
N ][CH COO]
4
3
N ][C H COO]
4
2
5
N ][C H COO]
The formula of the molecular volume, V , of IL is as follows
4
3
7
m
1
4
14
14
N ][NO ]
0.2053
when T = 298.15 K
4
3
3
3
33
33
33
BTAc
8.12
0.22
303.7
Vm = M/(Nρ)
(2)
where M is the molar mass; N is the Avogadro’s constant; and
Vm is the molecular volume of the IL. The calculated values of
V for [N ][XCOO] (X = H, CH , CH CH , CH CH CH )
m
4
3
3
2
3
2
2
are presented in Table 2.
The standard entropy expression as a function of molecular
28
volume proposed by Glasser is shown as follows:
−
1
−1
3
S°(298 K)/J·mol ·K = 1246.5(V /nm ) + 29.5
(3)
m
The results calculated by eq 3 for [N ][XCOO] (X = H,
4
CH , CH CH , CH CH CH ) are presented in Table 2.
3
3
2
3
2
2
The crystal energies UPOT of [N ][XCOO] (X = H, CH ,
4
3
29,30
CH CH , CH CH CH ) are given by
3
2
3
2
2
UPOT/kJ·mol− ≈ 2I{α(V(nm ))
1
3 −1/3
+ β}
Figure 2. Plot of surface tension γ vs temperature T of the ionic
liquids: [N ][XCOO], black filled square, [N ][HCOO]; red filled
(4)
4
4
where V is the molecular volume; I is the ionic strength; and α
and β are fitted coefficients of the salts. The β and α coefficient
circle, [N ][CH COO]; blue filled triangle, [N ][C H COO]; pink
4
3
4
2
5
filled upside down triangle, [N ][C H COO].
4
3
7
−
1
−1
31
are 157.3 kJ mol nm and 83.3 kJ mol , respectively. The
lattice energies of the four ILs and other ILs with similar
structures are presented in Table 2. Visible from the results,
the lattice energy of the four new ILs is far less than that of
molten salt. Among all alkali metal halides, the fused CsI has
Table 3. Surface Entropy (S ), Surface Energy (E ), and
a
a
g
Molar Enthalpies of Vaporization (Δ H° ) of the Ionic
1
m
Liquids [N ][XCOO] (X = H, CH , CH CH , CH CH CH )
4
3
3
2
3
2
2
Sa (mJ·K m−2) Ea (mJ·m−2) Δ H° (kJ·mol−1
g
)
−1
1
m
the lowest lattice energy, 613 kJ·mol . The lower lattice
energy also explains why IL is liquid at room temperature.
32
[
[
[
[
N ][HCOO]
0.0897
0.067
0.050
60.1613
51.1144
46.5618
42.8826
80.0928
81.5145
79.3562
78.5304
4
N ][CH COO]
4
3
3.2. Estimation of Surface Properties. Graphically
presented in Figure 2, the surface tension of the four ILs
varies linearly with temperature. The values of surface tension
decrease with increasing temperature. At the same temper-
ature, as the length of the alkyl chain on the anion increases, its
value decreases from formic acid to butyric acid, which is
N ][C H COO]
4
2
5
N ][C H COO]
0.0547
4
3
7
measurement results, which may be resulted from the
differences in testing instruments and experiences.
C
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Figure 3. Plot of viscosity (a) or conductivity (b) vs temperature T of the ionic liquids black filled sqaure, [N ][HCOO]; red filled circle,
4
[
N ][CH COO]; blue filled triangle, [N ][C H COO]; pink filled upside downt triangle, [N ][C H COO]; black open circle, [N ][HCOO] in
4 3 4 2 5 4 3 7 4
ref 27; red open circle, [N ][CH COO] in ref 14; green open circle, blue open circle [N ][C H COO] in ref 21; pink open circle,
4
3
4
2
5
[
N ][CH COO] in ref 43.
4 3
Table 4. VFT Parameters of the Dynamic Viscosity and
Electrical Conductivity for [N ][HCOO], [N ][CH COO],
4
4
3
[
N ][C H COO], and [N ][C H COO]
4
2
5
4
3
7
σ/mS·cm−
1
σ0/mS·cm−
1
B/K
T0/K
R2
6
[
[
[
[
N ][HCOO]
5.8 × 10
1.0 × 10
2.7 × 10
3.8 × 10
−7010.0
−5701.2
−3891.4
−4191.6
189.3
45.19
1.0832
5.0096
0.9999
0.9999
0.9999
0.9999
4
7
N ][CH COO]
4
3
5
N ][C H COO]
4
2
5
5
N ][C H COO]
4
3
7
η/mPa·s
η0/mPa·s
B/K
T0/K
R2
[
N ][HCOO]
0.03
1.8 × 10
1.9 × 10
6.8 × 10
−1276.6
−3374.0
−3683.4
−3159.3
145.6
72.3
54.1
72.3
0.9999
0.9999
0.9999
0.9999
4
Figure 4. Plot of molar electrical conductivity Λ of the four ILs from
−
4
4
4
m
[
[
[
N ][CH COO]
4
3
2
93.15 to 333.15 K. Filled black square, [N ][HCOO]; filled red
4
−
N ][C H COO]
4
2
5
circle, [N ][CH COO]; filled blue triangle, [N ][C H COO]; filled
4 3 4 2 5
−
N ][C H COO]
4
3
7
upside down green triangle, [N ][C H COO].
4 3 7
3
4
−1
similar to the fraction of IL anions. This also clearly shows
γ/(mN·m ) = a − bT/k
(5)
35
the correctness of the Langmuir principle. The IL structure is
like a negative charge/positive headgroup and an uncharged
alkyl tail group. With increase of alkyl chain length, IL polar
groups are removed from the surface, resulting in a weak
surface polarity, which is reflected in the decrease of surface
tension in macroscopic properties. In Greaves et al.’s study of
the surface tensions of three ILs, methylammonium formate,
ethylammonium formate, and propylammonium formate, we
found that these trends still exist, regardless of whether the
methylene group is on the cation or anion. Futhermore, PILs
with multibranched chains such as butylammonium formate
and pentylammonium formate indicate that the branched
chains reduce surface tension, a trend consistent with the
amount of hydrocarbons per unit surface area.
The relationship between surface tension and temperature
can be expressed as
Based on the experimental surface tension data, the surface
entropy (S ) and surface energy (E ) can be calculated
according to the following equations:
a
a
36
Sa = b = − (∂γ/∂T)P
(6)
(7)
Ea = a = γ − (∂γ/∂T)P
27
Surface energy and surface entropy values are listed in Table
3, respectively. The surface energy of ILs studied in this text is
much lower than that of molten salt (e.g., the surface energy of
−
2
37
fused NaNO can be 146 mJ·m ). Since the surface energy
3
depends on the interaction energy (lattice energy), the
interaction energy in the ionic liquid is much less than that
38
of the inorganic molten salt.
Table 5. Molar electrical conductivity (Λ ) of the four ILs from 293.15K to 333.15K
m
S (cm ·mol−1)
2
T/K
293.15
298.15
303.15
308.15
313.15
318.15
323.15
328.15
333.15
[
[
[
[
N ][HCOO]
0.350
0.071
0.077
0.055
0.406
0.091
0.095
0.068
0.471
0.115
0.117
0.084
0.546
0.144
0.145
0.106
0.631
0.180
0.177
0.131
0.729
0.224
0.216
0.161
0.833
0.279
0.264
0.197
0.950
0.347
0.316
0.240
1.093
0.426
0.379
0.290
4
N ][CH COO]
4
3
N ][C H COO]
4
2
5
N ][C H COO]
4
3
7
D
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Journal of Chemical & Engineering Data
Article
may be mainly due to an increase in molecular size or an
elongation of the alkyl chain, making the van der Waals
40,41
interaction stronger, resulting in increased viscosity.
When
comparing the two ionic liquids of 2-methylpropylammonium
formate and propylammonium formate, we found that the
increased viscosity of the substituted methyl group was
significantly greater than the increase of the CH bond in
2
27
the alkyl chain due to the increase in molecular asymmetry.
The measurement of ion transmission is electrical
conductivity, and it is mainly controlled by ion dissociation
degree, viscosity, ionic mobility, ion aggregation, and ionic
charge. Figure 3b has showed the plot of electrical conductivity
for the ILs at different temperature and its comparison with
reference values. As a result of stronger intermolecular
Figure 5. Walden rule plots for [N ][XCOO] at temperatures from
4
interactions, [N ][CH COO] showed a lower conductivity
2
93.15 to 333.15 K: filled black square, [N ][HCOO]; filled red circle
4 2 4 2 5
4
3
4
42
than 1-ethyl-3-methylimidazolium acetate. The values of
conductivity increase with a rise of temperature for all ILs. For
the work in this text, at the same temperature, ILs’ conductivity
decreases monotonically with the increase of anion chain
length, and comparing the conductivity of N-butylammonium
acetate with N-butylammonium nitrate in Xu’s work, the latter
[
N ][CH COO]; filled green triange, [N ][C H COO]; filled upside
down blue triange, [N ][C H COO]. Solid straight line is the ideal
4
3
7
line for a 0.01 M aqueous KCl solution.
The vaporization enthalpy is used to determine the stability
of the ionic liquid and is an important parameter. According to
43
value is also significantly higher, which means that smaller
39
44
Kabo’s empirical formula, the vaporization enthalpy can be
ions have higher mobility. Therefore, the experimental
results show that [N ][C H COO] has the lowest electrical
estimated as follows
4
3
7
Δ H° = A(γV2/3N1/3) + B
g
conductivity. A similar trend was observed in the previous
l
m
m
(8)
study of the effect of alkyl chain length on the conductivity of
25
where N is Avogadro’s constant; V is molar volume; A and B
are empirical parameters; and their values are 0.01121 and 2.4
kJ·mol , respectively. The values of molar enthalpies of
vaporization obtained from eq 8 are presented in Table 3.
ILs in cations. Since stronger van der Waals interaction will
m
increase the concentration of ions in the liquid and reduce the
−
1
45,46
ionicity of the IL,
it is reasonable that the conductivities of
ILs are reduced in the order described above.
3
.3. Transport Property. The viscosity curves of this work
The VFT equation can also be used to fit the temperature
dependence of IL’s dynamic viscosity and electrical con-
at different temperatures and the comparison with the
literature values are shown in Figure 3a. However, there are
certain deviations in the results because some of the works
were carried out years ago, and the testing instruments and
environment are different. The viscosity values decrease with a
rise of temperature for all ILs. Compared with ILs with the
same anions, at the same temperature, with the increase in
length of the alkyl chain in carboxylate anions, the viscosity
also increases monotonically. Compared with methylammo-
nium formate, ethylammonium formate, and propylammonium
formate we can conclude that for both cations and anions the
longer alkyl chains induced the higher viscosity, as did methyl
47
ductivity values:
η = η exp[−B/(T − T )]
0
(
9)
0
σ = σ exp(−B/(T − T ))
(10)
0
0
Here σ, is the electrical conductivity; η is the dynamic
viscosity; and σ0 and η0 represent the infinite electrical
conductivity and dynamic viscosity of a system at extreme
temperatures, respectively. B involves the activation energy of
25
the charge carrier motion, and T is the “ideal” glassy transition
0
temperature of the liquid. The fitting parameters for η , σ , B,
0
0
27
2
groups substituted onto the alkyl chain of the anions. This
and T and the corresponding correlation coefficient R are
0
Figure 6. Optimized structures, hydrogen bonds, and interaction energy of ionic liquids: (a) [N ][HCOO]; (b) [N ][CH COO]; (c)
4
4
3
[
N ][C H COO]; and (d) [N ][C H COO].
4 2 5 4 3 7
E
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Table 6. Charge Distribution by the CosmothermX Interface of the Four ILs
listed in Table 4. From the values of the correlation coefficient
R in Table 4 that are all greater than 0.999, it can be seen that
the VFT equation is applicable to the fitting of the
experimental conductivity and dynamic viscosity of the ILs.
conductivity is shown in Table 5 and graphically presented
in Figure 4. It can be seen from Figure 4 that the change trend
of molar conductivity and conductivity is the same for all
investigated ILs. The molar conductivity increases with
increasing temperature. The qualitative method developed by
2
3
.4. Walden Rule. The molar conductivity, Λ , is essential
m
48
in studying ion mobility. According to the electrical
conductivity and density values, the molar conductivities of
the four ILs ([N ][HCOO], [N ][CH COO], [N ]-
Walden is defined as
Λ η = constant
(12)
m
4
4
3
4
[
C H COO], and [N ][C H COO]) are determined by the
2 5 4 3 7
where Λ is the molar electrical conductivity and η is the
m
following equation
viscosity. Figure 5 presents the Walden diagram of ILs
discussed in this work. To compare the ionicity of different
ILs, an ideal line about the Walden rule is introduced in Figure
5, which is indicated by 0.01 M KCl aqueous solution to
represent complete ion dissociation. If the lines of IL remain
−
1
Λ = σ·M·ρ
(11)
m
where M is molar mass, and ρ and σ are the density and the
electrical conductivity, respectively. The calculated molar
F
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Article
Table 7. Charge Distribution of the Ion Clusters of Four ILs
by the CosmothermX Interface
Figure 7. Optimized structures and interaction energies of ion
clusters: (a) ([N ][HCOO]) ; (b) ([N ][CH COO]) ; (c) ([N ]-
4
2
4
3
2
4
[
C H COO]) ; and (d) ([N ][C H COO]) . The hydrogen bonds
2 5 2 4 3 7 2
are indicated by dashed lines.
close to the ideal line, their ionicity is considered good, and the
reverse indicates not good ionicity. As expected, all ILs are
below the ideal line through the origin and have a slope of one
in the Walden diagram. Such similarity was also observed
49
among 1-ethyl-3-methyl imidazolium n-alkyl sulfate ILs. It
can be seen from Figure 5 that the observed curves of the four
ionic liquids are approximate to straight lines which are
50
substantially consistent with the trend in the literature,
indicating that the ionic liquids obey the Walden rule. As is
shown, the ILs are superior conductors, among which the
51
proton transport Grotthuss mechanism dominates.
.5. Intermolecular Interactions. First, the cation and
3
anion structures of ILs are optimized to obtain a stabilized
conformation. Next, the structures of ion pairs or ion clusters
involving one or two pairs of cation and anion are optimized.
The most stable structure of ILs (ion pairs) is shown in Figure
6
. It is noteworthy that there may be more possible interaction
+
structures of [N ] or the carboxylic acid anion, and several
4
possible mutual orientations of the interaction complex are
investigated by calculation. The picked ones are the most
stable optimized geometries. Although all the hydrogen atoms
on cations carry positive charges, the N−H is more acidic
because it is associated with electronegative O atoms. This is
also evident from the quantum calculations in Table 5 and
Table 6.
5
4
Å. Graphically presented in Figure 6 and Figure 7, for the
four ILs, there are H···O hydrogen bonds between the N-
butylammonium and the carboxylate. Hydrogen bonds are
represented by dotted lines, and the corresponding H···O
distance is as shown in Figure 6. The charge distributions of
the four ILs calculated by the CosmothermX interface are
given in Table 6. Hydrogen bonds were found between the N−
H of the cation and the O atom of the anion. However, the
influence on the interaction energy is not dominant, which
The sum of the van der Waals atomic radii of hydrogen and
oxygen serves as a indispensable value for determining whether
a hydrogen bond exists between a hydrogen atom and an
5
2,53
oxygen atom.
Usually, if the donor groups on the distance
between the protons and the receptor are shorter than van der
Waals distances and angles greater than 90°, we can define it as
a hydrogen bond, and the H···O van der Waals distance is 2.72
G
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indicates that the Coulomb interaction between the cation and
the anion is dominant.
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■
A new series of N-butylammonium carboxylate ionic liquids
were prepared and characterized. The property measurements
show that the surface tension and density decrease linearly
with the increase of temperature. Based on the experimental
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by empirical or semiempirical methods. As the temperature
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ASSOCIATED CONTENT
■
(
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AUTHOR INFORMATION
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Corresponding Author
E-mail: zhangqingguo@bhu.edu.cn. Tel.: + 86 18604968816.
Fax: + 86 416 3400708.
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17) Er, H.; Wang, H. Properties of protic ionic liquids composed of
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ORCID
(18) Turbomole, a Development of University of Karlsruhe and
Qingguo Zhang: 0000-0002-0172-9579
Funding
This work was financially supported by the National Nature
Science Foundation of China (No. 21503020, No. 21373002),
the Doctoral Fund of Liaoning Province of China (No.
01601347), the Nature Science Foundation of Liaoning
Province (No. 201602016), and the Program for Liaoning
Excellent Talents in University, China (LJQ2015099).
Forschungszentrum Karlsruhe GmbH v6.6 2014; TURBOMOLE
GmbH: 2007; available from http://www.turbomole.com. Order
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2
Notes
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H
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