Effect of Structural Variations on the Thermophysical Properties of Protic Ionic Liquids: Insights from Experimental and Computational Studies

Article abstract of DOI:10.1021/acs.jced.6b00450

In this work, new protic ionic liquids (PILs) based on 1-methylimidazolium/1-propanenitrileimidazolium cations with hydrogen sulfate, methanesulfonate, trifluoromethanesulfonate, para-toluenesulfonate, trifluoroacetate, and acetate anions were synthesized. The structures and purity of the products were confirmed by using ¹H and ¹³C NMR and CHNS elemental analysis. The effect of structural variations on the thermophysical properties, namely, refractive index, density, and viscosity, was evaluated in a wide temperature range. The viscosity and density values were measured within the temperature range of 293.15-373.15 K. The density values were used further to calculate more properties like thermal expansion coefficient, molecular volume, standard entropy, and the lattice energy. Moreover, the experimental values of viscosity were used for the calculation of activation energy. Refractive indices were measured within the temperature range of 293.15-323.15 K, and these values were also used in the calculation of electronic polarizability. Acid numbers of the prepared PILs were measured and correlated with their structure moiety. In addition, the density functional theory (DFT) calculations were performed to get a deeper insight into the effect of structural variations of the ion pairs on their physical properties.

Full text of DOI:10.1021/acs.jced.6b00450

Article
pubs.acs.org/jced
Effect of Structural Variations on the Thermophysical Properties of
Protic Ionic Liquids: Insights from Experimental and Computational
Studies
Amir Sada Khan,*,†,‡ Zakaria Man,† Mohamad Azmi Bustam,† Girma Gonfa,†,§ Fai Kait Chong,⊥
Zahoor Ullah,†,∥ Asma Nasrullah,⊥ Ariyanti Sarwono,† Pervaiz Ahmad,# and Nawshad Muhammad*,∇
†Centr of Research in Ionic Liquids (CORIL), Department of Chemical Engineering, Universiti Teknologi PETRONAS, 31750
Tronoh, Perak, Malaysia
‡Department of Chemistry, University of Science and Technology, Bannu 28100, Khyber Pakhtunkhwa, Pakistan
§College of Biological and Chemical Engineering, Addis Ababa Science and Technology University, 16417 Addis Ababa, Ethiopia
∥Department of Chemistry, The Balochistan University of IT, Engineering and Management Sciences (BUITEMS), Takatu Campus,
Quetta 87100, Pakistan
⊥Fundamental and Applied Science Department, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia
#Department of Physics, Abbottabad University of Science and Technology, Havelian, KP, Pakistan
∇Interdisciplinary Research Centre in Biomedical Materials, COMSAT Institute of Information Technology, Lahore, Pakistan
S
* Supporting Information
ABSTRACT: In this work, new protic ionic liquids (PILs)
based on 1-methylimidazolium/1-propanenitrileimidazolium
cations with hydrogen sulfate, methanesulfonate, trifluoro-
methanesulfonate, para-toluenesulfonate, trifluoroacetate, and
acetate anions were synthesized. The structures and purity of
1
the products were confirmed by using H and 13C NMR and
CHNS elemental analysis. The effect of structural variations on
the thermophysical properties, namely, refractive index,
density, and viscosity, was evaluated in a wide temperature range. The viscosity and density values were measured within the
temperature range of 293.15−373.15 K. The density values were used further to calculate more properties like thermal expansion
coefficient, molecular volume, standard entropy, and the lattice energy. Moreover, the experimental values of viscosity were used
for the calculation of activation energy. Refractive indices were measured within the temperature range of 293.15−323.15 K, and
these values were also used in the calculation of electronic polarizability. Acid numbers of the prepared PILs were measured and
correlated with their structure moiety. In addition, the density functional theory (DFT) calculations were performed to get a
deeper insight into the effect of structural variations of the ion pairs on their physical properties.
techniques, electrolytes, catalyst, and solvent. In this scenario,
PILs is currently extensively known as a new class of advanced
acidic catalysts for many organic transformations.7−9
INTRODUCTION
■
Ionic liquids (ILs) are organic molten salts having melting
point below 100 °C at atmospheric pressure. Unlike common
organic solvents, they have unique and superior physical
properties such as very low vapor pressure, high thermal and
chemical stabilities, nonflammability, distinctive solvation
potential, low melting point, high ionic conductivities, and
wide liquid temperature range making them distinct candidates
for various applications.1 Owing to large possibility of combing
the various cations and anions, ILs have received great attention
in science and technology.2−5 Recently, protic ILs have been
the principle focus of numerous studies because of their simple
synthesis methods, low cost of synthesis and purification, and
their biodegradable nature. The most important property of
protic ILs which differentiate them from other ILs is the easy
transfer of a proton from the acid to base, to make proton
donor and acceptor sites.1,6 PILs have extensive applications
and been applied in organic synthesis, chromatography
The properties of PILs can be further enhanced by
incorporating functional group to a cation, anion, or on both.
There are many reports available on synthesis and applications
of functionalized ILs. Among functionalized ILs, cyano-based
ILs are attracting considerable attention for industrial
applications because of their higher thermal stability and
lower melting points and viscosities compared to most of other
reported ILs.10,11 Cyano-based ILs contain one or more cyano
(CN) groups in their structural moieties. The high electro-
negativity, triple bond, and delocalized charges of the cyano
group offered them unique properties and applications.12
Received: June 2, 2016
Accepted: August 10, 2017
© XXXX American Chemical Society
A
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Dayson et al.13 for the first time reported nitrile functionalized
ILs and found to be good solvents and ligands for the catalysis
reactions. ILs having a CN group also have better cellulose
dissolution ability because of the electron-withdrawing nature
and small size of the CN group. Multiple bonds of CN group
induce strong polarity in ILs, which needed to create
polarization interface with cellulose molecules for effective
dissolution.14 Nitrile-functionalized ILs have the ability to
establish superior media for generation of stable and
monodispersed nanoparticles in comparison to nonfunctional-
ized ILs.15
Industrial process design and the yield of ILs-based products
depend on the thermophysical properties, namely, density,
viscosity, refractive index, thermal behavior, and surface tension
of ILs. Therefore, an accurate and systematic thermophysical
properties measurement of ILs is important in both
fundamental and applied research as it can help us in screening
to find the most suitable ILs for targeted applications.
Therefore, the knowledge of thermophysical properties is
significant to decide the potential applications of the
synthesized PILs.16
Elemental analyses were performed using CHNS-932
(LECO instruments). Water content in synthesized ILs was
determined using Karl Fisher Titration (Mettler Toledo DL39).
1-Propanenitrileimidazolium Hydrogen Sulfate [C2CNIM]-
1
[HSO4]. Spectroscopic data: H NMR (500 MHz, CH3OH): δ
= 3.221−3.247 (t), 3.715 (s), 4.663−4.689 (t), 7.656 (s), 7.803
(s), 9.115 (s). 13C NMR (500 MHz, CH3OH): 16.907, 44.707,
54.438, 117.259, 120.548, 122.033, 135.525. CHNS elemental
analysis: Calculated (%): C: 29.27, H: 3.44, N: 20.48, S: 15.63.
Found (%): C: 29.11, H: 3.47, N: 20.35, S: 15.59. Elemental
analysis standard uncertainty μ(E) = 0.075%.
1-Propanenitrileimidazolium Methanesulfonate
1
[C2CNIM][CH3SO3]. Spectroscopic data: H NMR (500 MHz,
CH3OH): δ = 2.768 (s), 3.235 (s), 3.325−3.332 (t), 4.656−
4.682 (t), 7.678 (s), 7.819 (s), 9.134 (s). 13C NMR (500 MHz,
CH3OH): 18.822, 38.493, 44.698, 117.069, 120.483, 122.053.
CHNS elemental analysis: Calculated (%): C: 35.46, H: 4.46,
N: 20.68, S: 15.78. Found (%): C: 35.50, H: 4.50, N: 20.56, S:
15.62. Elemental analysis standard uncertainty μ(E) = 0.050%/
1-Propanenitrileimidazolium p-Toluenesulfonate
1
[C2CNIM][p-TSA]. Spectroscopic data: H NMR (500 MHz,
CH3OH): δ = 2.365 (s), 3.135−3.161 (t), 4.560−4.587 (t),
5.161 (s), 7.245−7.260 (d), 7.554 (t), 7.709−7.713 (t), 7.740
(s), 7.756 (s), 8.995 (s). 13C NMR (500 MHz, CH3OH):
18.864, 20.248, 44.519, 117.080, 120.889, 120.932, 121.779,
121.824, 125.550, 128.875, 135.632, 140.901, 141.755. CHNS
elemental analysis: Calculated (%): C: 51.60, H: 4.69, N: 15.04,
S: 11.48. Found (%): C: 51.40, H: 5.10, N: 15.10, S: 11.42.
Elemental analysis standard uncertainty μ(E) = 0.052%.
1-Propanenitrileimidazolium Trifluoromethanesulfonate
In this study, we prepared a novel series of PILs using 1-
propyronitrile imidazolium/1-methyl imidazolium cations, with
acids of widely varying strength such as sulfuric acid (H2SO4),
methanesulfonic acid (CH3SO3H), trifluoromethanesulfonic
acid (CF3SO3H), para-toluenesulfonic acid (p-TSA), trifluoro-
acetic acid (CF3CO2H), and acetic acid (CH3CO2H). The
structures and purity of the synthesized PILs were confirmed
using NMR and elemental analysis (CHNS). To understand
the nature of these new synthesized PILs, physicochemical
properties such as density, viscosity, refractive index, and
thermal behavior were studied over a wide temperature range.
The acidity (acid number) was measured and correlated with
structure moiety especially anions structure. Furthermore,
density functional theory (DFT) calculations were carried out
to investigate the effect of structural variations of the cations
and anions on the properties of the synthesized PILs.
1
[C2CNIM][CF3SO3]. Spectroscopic data: H NMR (500 MHz,
CH3OH): δ = 3.164−3.190 (t), 4.591−4.617 (t), 4.950 (s),
7.550 (t), 7.642 (s), 7.760 (s), 9.055 (s). 13C NMR (500 MHz,
CH3OH):18.694, 44.671, 116.747, 119.04, 120.449, 121.950,
135.501. CHNS elemental analysis: Calculated (%): C: 29.20,
H: 2.75, N: 16.34, S: 12.47. Found (%): C: 29.10, H: 2.87, N:
16.20, S:12.30. Elemental analysis standard uncertainty μ(E) =
0.072%.
1-Propanenitrileimidazolium Acetate [C2CNIM][CH3COO].
Spectroscopic data: H NMR (500 MHz, CH3OH): δ = 1.665
EXPERIMENTAL SECTION
1
■
Materials. All of the chemicals such as imidazole, 1-
methylimidazole, acrylonitrile, sulfuric acid, trifluoromethane-
sulfonic acid, para-toluenesulfonic acid, trifluoroacetic acid,
acetic acid, methanol, diethyl ether, and ethyl acetate of
analytical grade were purchased from Merck and used without
any further purification.
(s), 2.843−2.868 (t), 4.243−4.268 (t), 7.119 (s), 7.242 (s),
8.303 (s). 13C NMR (500 MHz, D2O): 19.261, 23.014, 43.668,
118.142, 121.090, 122.689, 135.826, 180.356. CHNS elemental
analysis: Calculated (%): C; 50.29, H: 5.43, N: 25.14, O: 19.14.
Found (%): C: 50.10, H: 5.35, N: 25.20, O: 18.90. Elemental
analysis standard uncertainty μ(E) = 0.112%.
Synthesis and Characterization. The present PILs were
synthesized in two steps; in the first step, imidazole (0.2 mol)
and methanol were added into the three-necked flask and
stirred until imidazole completely dissolved. Acrylonitrile (0.23
mol) was added dropwise, and the reaction mixture was stirred
at 55 °C for 24 h. The unreacted material was removed by
vacuum rotary at 70 °C. In the second step, 1-propyronitrile
imidazolium was dissolved in acetonitrile, and respective acids
were added dropwise followed by a continuous stirring for 12 h.
The resultant final product was washed with diethyl ether and
ethyl acetate to remove the unreacted materials. The
synthesized PILs were subjected to drying using vacuum oven
at 60 °C for at least 24 h.
1-Propanenitrileimidazolium Trifluoroacetate [C2CNIM]-
[CF3COO]. Spectroscopic data: 1H NMR (500 MHz,
CH3OH): δ = 3.164−3.190 (t), 4.591−4.617 (t), 7.611 (s),
7.735 (s), 9.016 (s). 13C NMR (500 MHz, CH3OH): 18.774,
44.639, 115.613, 116.872, 117.941, 120.605, 121.902, 135.525.
CHNS elemental analysis: Calculated (%): C: 38.02, H: 2.73,
N: 19.00, O: 14.47. Found (%): C, 38.17, H, 2.84; N, 18.96; O,
14.20. Elemental analysis standard uncertainty μ(E) = 0.012%.
1-Methylimidazolium Hydrogen Sulfate [MIM][HSO4].
1
Spectroscopic data: H NMR (500 MHz, DMSO): δ = 3.86
(s), 5.72 (s), 7.59 (s), 7.66 (s), 8.98 (s). 13C NMR (500 MHz,
DMSO): 35.234, 119.270, 123.057, 135.809. CHNS elemental
analysis for C4H8N2SO4. Found (%): C: 26.68, H: 4.45, N:
15.56, S: 17.86. Calculated (%): C: 26.66, H: 4.48, N: 15.55, S:
17.80. Elemental analysis standard uncertainty μ(E) = 0.015%.
Physical Characterization. Thermophysical properties
such as refractive index, density, viscosity, and thermal stability
1H NMR and 13C NMR spectra of PILs were recorded on a
Bruker Avance 500 MHz using tetramethylsilane (TMS) as an
internal standard solvent. NMR spectra (1H and 13C) is
provided as Supporting Information (SI, Figures S1−S14).
B
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Figure 1. Scheme of the synthesis of protic ionic liquids.
Table 1. Name, Abbreviation, and Chemical Structure of the Synthesized PILs
of the synthesized PILs were discussed subsequently below.
The ILs that were obtained as solids are ([C2CNIM]-
[CH3SO3], [C2CNIM][CF3SO3], and [C2CNIM][CF3COO])
at room temperature and are not evaluated for their properties
such as viscosity and density. However, the PILs, which are
liquid at room temperature, were subjected to measure their
detail physiochemical properties.
The refractive index values of synthesized PILs were
measured using an ATAGO digital refractometer (RX-5000α)
with the accuracy of 4.5 × 10−5 in temperature ranging from
293.15 to 323.15 K with five-degree intervals.
Densities and viscosities of synthesized PILs were measured
using Anton-Parr viscometer (model SVM3000) at atmospheric
pressure in temperature ranging from 293.15 to 373.15 K with
ten-degree intervals. Before measurement of density and
viscosity, the instrument was calibrated using ultrapure
Millipore quality water followed by ILs for which the data
already reported in the literature.1,6 The measurement was
performed in triplicate to obtain the average value.
An acid−base titration method was used for the determi-
nation of the acid value of PILs.17 Approximately 100 mg of
PIL sample was dissolved in 10 mL of 2-propanol, and some
drop of phenolphthalein was added as an indicator. The
mixture for each PILs were titrated against 0.1 N NaOH
solution, and the end point was noted. The same procedure was
used for a blank titration without addition of IL. The acid
values for the PILs were determined using the following
equation.
(B − A)M × 40
acid value =
(1)
W
C
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where A is the volume of NaOH solution needed for the
titration of blank (mL), B is the volume of NaOH solution
needed for the titration of the sample (mL), M is the molarity
of NaOH solution, and W is the mass of sample in gram.
The thermal degradation temperature of PILs in the nitrogen
atmosphere was measured using PerkinElmer TGA, Pyris-1, V-
3.81 in the temperature range of 323−773 K with the heating
rate of 10 K/min.
A Mettler-Toledo differential scanning calorimeter
(DSC822e) with Mettler-Toledo STARe software was used
for the measurement of glass transition temperature (Tg) and
melting point (Tm). The glass transition temperature is the
midpoint of a small heat capacity change on heating from the
amorphous glass state to a liquid state. The melting point is the
onset of an endothermic peak on heating.18 The instrument
was calibrated with indium (calibration standard, purity
>99.999%) by heating from 120 to 180 °C at 10 °C/min
with atmosphere nitrogen, 50 cm3/min.
temperature, Ea is the energy of activation (J/mol), and A0, A1,
A2, and A3 are adjustable parameters. The standard deviation
(SD) was determined using the following expression:
∑N (Zexp − Zcalc
)
2
i
SD =
(5)
N
Refractive Index. The measured refractive index (nD) data
for the synthesized PILs as a function of temperature (293.15−
323.15 K) are shown in Figure 2, and the values are listed in
The sample was weighted in aluminum pans and subjected to
thermocycles in which initially the sample was heated in a
nitrogen atmosphere with the heating rate of 10 °C·min−1 from
0 to 110 °C, cooled from 110 °C to −150 °C, and then heated
again to 110 °C. The glass transition temperatures were
determined with a temperature accuracy of 1 °C.
Computational Methods. Density functional theory
(DFT) calculations were performed to understand the effect
of the structural variations of the cations and anions on the
molecular properties of the ILs. Turbomole 6.2 software
package was used for the structural optimizations and quantum
calculations.The geometry optimizations were performed using
the dispersion corrected density functional theory (DFT-dsp)
method, utilizing the Becke-Perdew-86 (BP86) functional19,20
with the resolution of identity (RI) approximation and a triple-
ξ valence polarized basis set (TZVP).21,22 Natural bond orbital
(NBO) population analyses for the species were performed at
the same level and basis set. The ideal screening charges on the
molecular surface of the species were calculated using
COSMOthermX program (version C2.1) with
BP_TZVP_C21_0111.ctd parametrization.23
Figure 2. Refractive indices (nD) as a function of temperature for the
protic ionic liquids.
Table S1 in the SI for which the standard uncertainty values are
u(nD) = 3.5 × 10−3. As the experimental results shows that the
refractive indices decrease linearly with the increase of
temperature, as reported elsewhere.24 The values of refractive
indices are in the range of 1.47508−1.54509. The decreasing
order of refractive indices of the investigated PILs is
[C2CNIM][p-TSA] > [C2CNIM][CH3SO4] > [C2CNIM]-
[HSO4] > [MIM][HSO4] > [C2CNIM][CH3COO]. The
result indicates that there is a strong effect of anion structure
on the refractive index value. The lower values are noted for the
[C2CNIM][CH3COO], while the higher values are observed
for the PIL [C2CNIM][p-TSA] owing to more aromaticity
(electron mobility) in its structure.24 Furthermore, for the PIL
with the same anion, that is, [HSO4], the refractive index value
of the nitrile functionalized ionic liquid, [C2CNIM][HSO4] is
higher than the methyl functionalized PIL, [MIM][HSO4].1,6
The high refractive index value of the nitrile functionalized ILs
may be due to the additional electron mobility in the nitrile
functionalized side chain compared to nonfunctionalized
imidazolium alkyl chain.12,25 Generally, the refractive indices
increase with the increase of alkyl chain length of functional
group attached to the cation as the mobility of the electrons
was suggested to increase.26 Zahoor et al.4 reported refractive
index values for [MIM][HSO4] which have differences with the
current work are about 0.006. The fitting parameters and
standard deviation of refractive indices for PILs are summarized
in Table S2 in the SI.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The PILs were prepared
in two steps. In the first step, acrylonitrile was added dropwise
to an imidazole solution in methanol. The reaction was stirred
at 55 °C for 24 h to get the product. In the second step, 1-
propyronitrile imidazolium was dissolved in acetonitrile, and
the respective acid was added dropwise followed by continuous
stirring for 12 h. The synthesized PILs were dried in a vacuum
oven at 60 °C for at least 24 h. The scheme of synthesis of PILs
is shown in Figure 1. The names, abbreviations, and chemical
structures of the synthesized PILs are given in Table 1.
The dependence of the values of refractive index, density,
and viscosity on temperature were fitted by the least-squares
method using the following equations:
ρ = A0 + A1T
(2)
nD = A2 + A3T
(3)
Density. The experimental measured values of density for
the PILs at nine selected temperatures between 293.15 and
373.15 K are shown in Figure 3, and the values are given in
Table S3 in the SI. The decreasing order of the density for
synthesized PILs are [MIM][HSO4] > [C2CNIM][HSO3] >
Ea
RT
ln η = ln η∞ +
(4)
where T is the absolute temperature in Kelvin, R is the universal
gas constant (8.314 J/K·mol), η∞ is the viscosity at infinite
D
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ranging from 4.21 × 10−4 to 7.53 × 10−4 K−1, are typical values
of thermal expansion coefficient for ILs. Among these
synthesized PILs, [C2CNIM][CH3COO] has the highest α
value, while [MIM][HSO4] has the lowest value. PILs
containing a methyl group on the imidazole ring such as
[MIM][HSO4] have small thermal expansion coefficients as
compared to those having a propyronitrile group on the
imidazole ring, [C2CNIM][HSO4]. This increase in thermal
expansion could be due to increase in steric hindrance, which
causes the decrease in the interactions between cation and
anion of ionic liquid and thereby results in higher values are
measured for propyronitrile containing side chain ionic liquids.
The effect of alkyl chain length on thermal expansion
coefficient for amino acid based ILs was also observed by
Santis et al.18,24 Our values are slightly lower than those
measured by De Santis et al. for sulfate based ILs.24 The lower
values of α and less expansion with temperature for
[MIM][HSO4] and [C2CNIM][HSO4] is due to the additional
capability to form hydrogen bonds.31 The α values of the PILs
are considerably lower than that of most molecular organic
liquids (10−3 K−1) while greater than the α value of high
temperature classical molten salt (1−2 × 10−4 K−1). In this
study, a considerable change was noted in thermal expansion
coefficient value with respect to temperature. This increase in
thermal expansion coefficient values with increase of temper-
ature is due to decrease in ordering of PILs.28
Figure 3. Density (ρ) as a function of temperature for the protic ionic
liquids.
[C2CNIM][p-TSA] > [C2CNIM][CH3COO]. The [MIM]-
[HSO4] has higher density values, while [C2CNIM]-
[CH3COO] have lower values under same experimental
conditions. The higher values of density for PILs containing
the sulfate anion is due to the high molecular weight of anion
compared to lower molecular weight anion such as acetate.3
Likewise, the PILs containing 1-ethyl-3-methylimidazolium
cation with the different anions such as acetate and tosylate
showed the same trend as observed in this study.3 In general,
the density of ILs increasing with the increase of anion
molecular weight; however, sometimes density decreases with
increasing the anion molecular weight.24,27,28 To understand
the relationship between the densities of ILs with structure of
anion is unclear, probably due to the large number of factors,
namely, ion shape, size, geometry, charge density, and
molecular weight. A decrease in the values of densities was
observed for the PILs by incorporating propyronitrile func-
tional group on the cation, referring to density values of
[MIM][HSO4] and [C2CNIM][HSO4] containing same anion
but different cations. This decrease in density of PILs is due to
an increase of volume occupied by the propyronitrile functional
group as compared to the methyl group. Gardas et al. reported
the decrease in density of sulfate based ILs with the increase of
alkyl chain length on imidazole.29 With an increase of
temperature, the decrease in density of ILs were observed;
this decrease in density values with temperature is due to the
decrease in the van der Waal forces of interactions.30 The fitting
parameter values with R2 and standard deviation (SD) for
empirical correlation of density of the measured ILs are
reported in Table S4 in SI.
Molar Volume. Molar volume (Vm) is the volume occupied
by one mole of a substance at standard temperature and
pressure. The Vm of PILs at various temperature and
atmospheric pressure can be calculated using the following
equation:
Vm = M/ρ
(7)
where Vm is the molar volume in cm3·mol−1, M is the molecular
weight in g·mol−1, and ρ is the density in g·cm−3 at 298.15 K.
The values of Vm calculated for the PILs at various temperature
are listed in Table S6 in SI. The Vm of the studied PILs follows
the order of [C2CNIM][p-TSA] > [C2CNIM][CH3COO] >
[C2CNIM][HSO4] > [MIM][HSO4]. The molar volumes of
the studied PILs are higher than their corresponding chloride
based ILs reported by Muhammad.32 Among all PILs,
[C2CNIM][p-TSA] shows a higher molar volume (232.96
cm3·mol−1), and [MIM][HSO4] shows the lower molar volume
(123.10 cm3·mol−1) at 303.15 K. The higher molar volume of
the [C2CNIM][p-TSA] is due to the large size of the anion
which leads to weak interaction with the cation.33 The lower Vm
value of [MIM][HSO4] is due to the small size of the ions
which leads to the strong interaction between cation and anion.
The molar volume of ionic liquid increases with the increase in
chain length of the functional group attached to the cation or
anion.24
Thermal Expansion Coefficient. The values of thermal
expansion coefficient (α) or cubical expansion can be calculated
using the relation between the temperature and density by the
following expression:
Standard Entropy. The values of standard entropy (So) for
the prepared PILs were calculated from their molar volume
using the relationship established by Glasser:34
So(J·K−1·mol−1) = 1246.5 V (nm3) + 29.5
(8)
⎛
⎞
⎟
A1
∂ρ
1
⎜
αp = −
= −
where V is the molar volume of the PILs. The values of the
standard entropy calculated for the PILs are listed in Table S6
in SI. The standard entropies measured for [C2CNIM][p-
TSA], [C2CNIM][CH3COO], [C2CNIM][HSO4], and
[MIM][HSO4] are 511.89, 462.03, 347.35, and 283.78 J·K−1·
mol−1, respectively. The lower value of standard entropy for
[MIM][HSO4] is due to more symmetry and fewer resonance
⎝
⎠
ρ ∂T
A0 + A1T
p
(6)
where ρ and T are the density and temperature, respectively.
The values of thermal expansion coefficients as a function of
temperature are summarized in Table S5 in the SI. The
variations of the thermal expansion coefficient value with
temperature are significant for the present PILs. The values,
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structures for its cation and anion. Therefore, a fewer number
of conformational structures will be predicated, and ultimately
the standard entropy will be lower. In this regard, the higher
standard entropy values for [C2CNIM][p-TSA] are attributed
to its more possible conformational structures. The values of
standard entropy of these PILs were measured in the same
range of other ILs like [CnMim]alaninate and [CnMim]-
glycinate (where n = 2−6), ranging from 421−456 and 429−
469 kJ·mol−1, respectively.18,35,36
Lattice Energy. The lattice energy (UPOT) express
information about the interaction of the cation and anion.
The lattice energy at 298.15 K were calculated using Glasser’s
theory34 as follows:
TSA] is due to the large size of an anion. The free volumes of
the synthesized PILs, that is, C2CNIM][HSO4], [C2CNIM]-
[CH3COO], and [MIM][HSO4], are quite similar to the protic
ionic liquids reported by Gardas et al.39
Electronic Polarizability. The electronic polarizability
(Rm) for a substance can be calculated from the refractive
index value and can provide significant information concerning
the force between molecules or their behaviors in solution.41,42
The polarizabilities blur the permanent partial charge
distribution and thus act as lubricant between the ions.18 The
electronic polarizability has been calculated using the
experimental values of refractive index by the Lorenz−Lorentz
equation given as below:43
UPOT (kJ·mol−1) = 1981.2(ρ/M)1/3 + 103.8
2
2
⎡
⎢
⎣
⎤
⎥
⎦
⎡
⎢
⎣
⎤
⎥
⎦
(9)
nD − 1
nD − 1
M
ρ
Rm
=
=
Vm
n2 + 2
n2 + 2
where, ρ and M are the density (g·cm−3) and molecular mass
(g·mol−1), respectively. The estimated lattice energies are
shown in Table S6 in SI. As expected the UPOT of the studied
ILs is much lower than those of inorganic fused salts. The UPOT
for cesium iodide is 613 kJ·mol−1,37 which has the lowest UPOT
among alkali halides. The UPOT of the present PILs are in the
order as [MIM][HSO4] > [C2CNIM][HSO4] > [C2CNIM]-
[CH3COO] > [C2CNIM][p-TSA]. The [MIM][HSO4] has
the highest UPOT energy value, while [C2CNIM][p-TSA] has
the lowest value. Moreover, the UPOT of these prepared PILs is
in similar range as reported for chloride based ILs. The
calculated UPOT for 3-(3-butyl-1H-imidazol-3-ium-1-yl)-
propanenitrile chloride ([C2CNBIM]Cl), 3-(3-allyl-1H-imida-
zol-3-ium-1-yl)propanenitrile chloride [C2CNAIM]Cl, 3-(3-
benzyl-1H-imidazol-3-ium-1-yl)propanenitrile chloride
([C2CNBzIM]Cl), and 3-[3-(2-hydroxyethyl)-1H-imidazol-3-
ium-1-yl]propanenitrile chloride ([C2CNHeIM]Cl) are 448.9,
462.3, 439.3, and 468.2 kJ·mol−1, respectively.32 The lower
value of UPOT for [C2CNIM][p-TSA] may be due to the large
size of p-toluenesulfonate anion which leads to lose packing of
ions than their corresponding chloride based ILs. The higher
UPOT value for [MIM][HSO4] is due to the smaller size of
anion and cation and strong hydrogen bonding interactions
between respective ions which cause close compactness of
ions.38
(11)
D
D
where nD is the refractive index, M is the molar mass, ρ is the
density, and Vm is the molar volume. The values of Rm
calculated for PILs at various temperatures are listed in Table
S7 in the SI. The values of Rm are found to be in the order of
[C2CNIM][HSO4] < [C2CNIM][CH3COO] < [C2CNIM][p-
TSA] at 303.15 K are 45.281 < 50.913 < 73.426, respectively.
The difference in the Rm values of ILs comes from the various
anions used in their synthesis. The Rm values increase slightly
with the increasing of anion size. The higher polarizability for
[C2CNIM][p-TSA] is due to the large size of anion. Usually,
large ions have great polarizabilities.44 This increase in the Rm
values with the increase of molecular weight of anion and chain
length of the cation is also reported by Ziyada and Cecilia.43
The higher value of Rm for [C2CNIM][HSO4] as compared to
[MIM][HSO4] is due to the replacing of the methyl group of
cation with the propyronitrile functional group. This increase in
polarizability of ILs with the increase of alky chain length of
alkyl group on cation is also observed in the literature.43,45 As
seen from the experimental results, Rm increases linearly with
the increase of temperature as reported elsewhere.30,43
Viscosity. Viscosity is an important property of ILs as it can
impact on mass transport phenomena, thereby synthesis or
choosing of specific ILs for particular applications; viscosity is a
key and an important property. The dynamic viscosities for
synthesized PILs are measured at temperatures ranging of
293.15−373.15 K with atmospheric pressure are reported in
Table S8 in the SI. The dynamic viscosity of PILs decreases
with the increase of temperature (Figure 4). The decreasing
order in dynamic viscosities of prepared PILs is as follows:
[C2CNIM][p-TSA] > [C2CNIM][HSO4] > [MIM][HSO4] >
[C2CNIM][CH3COO]. The result demonstrates that the
viscosities of ILs depend on the molecular weight, structure,
and symmetry of cation and, particularly, anions. The viscosity
of [C2CNIM][p-TSA] is significantly higher than the viscosities
of these synthesized PILs and other common ILs. The high
viscosity for [C2CNIM][p-TSA] is due to a bulky anion which
has higher molecular weight and great steric hindrance and
offers more resistance to flow.24 The ILs with the HSO4− anion
shows higher viscosities compared to the CH3COO−, likewise
in this study, [C2CNIM][HSO4] has a higher viscosity than
[C2CNIM][CH3COO]. This high viscosity for sulfate based
PILs is due to the higher molecular weight and hydrogen
bonding between anion and cation which leads to strong
interaction between ions. The protic ionic liquid containing the
Free Volume (Vf). The refractive index values of PILs are
useful to calculate the free volume (Vf), which is consider an
important parameter, helpful in understanding the transport
phenomena in ILs. The free volume of PILs can be correlated
to the solubility of the low molecular weight gases such as CO2,
CH4, and C2H6.30,39 With the increase in the free volume of ILs
an increase in solubility of these gases was observed earlier.39
However, it is important to mention here that the solvation of
the nonpolar gases is more likely controlled by electrostatic
specific interaction and to small extent by free volume effect.40
The free volume of ILs is calculated by using following
equation:
V = Vm − R
(10)
f
where R is the molar refraction and Vm is the molar volume.
The free volumes determined for the PILs are listed in Table S7
in the SI. The above formula can be used to calculate the free
volume of spherical molecules as well as nonspherical ions. The
order of decreasing the free volume of the synthesized PILs are
[C2CNIM][p-TSA] > [C2CNIM][CH3COO] > [C2CNIM]-
[HSO4] > [MIM][HSO4]. It has been observed that the free
volumes are increasing with the increase of the anion size.
Thus, the higher value of the free volume for [C2CNIM][p-
−
propyronitrile ([C2CNIM]) cation with the HSO4 anion
shows a higher viscosity value as compared to PILs containing
F
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from the result, the magnitude of activation energy increases
with increasing ion size. The higher value of Ea for
[C2CNIM][p-TSA] is attributed to their high molecular weight
and larger size of anion which results in the decrease of the
mobility of ions. The value of Ea for [C2CNIM][CH3COO] is
comparable to the value of caprolactam acetate reported in the
literature. The change in Ea value with anion is assigned to its
geometry and interaction with cation.49,50
Acidic Properties. The acidic properties for PILs
determined by using acid−base titration method are listed in
Table S10 in the SI. The acid value for [C2CNIM][CF3SO3],
[C2CNIM][p-TSA], [C2CNIM][CH3SO3], [C2CNIM]-
[HSO4], [C2CNIM][CH3COO], and [C2CNIM][CF3COO]
are 0.104, 0.144, 0.152, 0.16, 0.188, and 0.204 g/g, respectively.
The result indicates that the acid value is affected by the nature
of anions. Among the synthesize PILs, [C2CNIM][CF3COO]
has a higher acidic value in comparison to other PILs. The
higher acid number for PILs [C2CNIM][CH3COO] and
[C2CNIM][CF3COO] can be assigned to acetate ion. Based
on the electronegativity, the higher the electronegativity of
conjugate base of acid more it is acidic. Here in the PILs the
anions of ionic liquids are considered as conjugate bases of the
respective acids. The [CF3COO] being more electronegative
renders the [C2CNIM][CF3COO] more acidic, while the
HSO4 anion is less electronegative which import less acidity to
[C2CNIM][HSO4].17
Figure 4. Viscosity (η) as a function of temperature for protic ionic
liquids.
the 1-methyl imidazolium cation ([MIM]). This higher
viscosity is due to the large size of the ([C2CNIM]) cation
as compared to the ([MIM]) cation.27 Another possible reason
for the higher viscosity of [C2CNIM][HSO4] as compared to
[MIM][HSO4] might be due to the propyronitrile functional
group attached to imidazole which leads to a higher van der
Waals interaction. The result of the experimental values of
viscosity has confirmed that the incorporation of functionalized
group on the cation and increase in the molecular weight of the
anion cause the increase in viscosities of PILs.24 Generally, the
variation of viscosity of ILs with the structure of the anion is
difficult to predict due to large number of parameters such as
ion size, shape, charge density, polarizability and flexibility of
ions and planarity of molecular geometry that strongly affecting
the viscosity of ILs.1,46,47 The dependence of viscosity values of
PILs at nine selected temperatures in the range 293.15−373.15
K is shown in Figure 4. It has been confirmed from the
experimental results that the viscosity decreases rapidly with the
increase of temperature. Many researchers have already
reported this rapid decrease in viscosities of many ILs with
temperature.1,24,48 The viscosity of [C2CNIM][HSO4] (7937
mPa·s) is lower compared with 1,4-sultone-methylimidazolium
hydrogen sulfate [BSMIM][HSO4] (49850 mPa·s) having
different cations and the same anion. This higher viscosity of
[BSMIM][HSO4] is due to the sultone group.1 The viscosity of
[C2CNIM][p-TSA] is in the same range of 1-butyl-imidazolium
hydrogen sulfate [C4C0IM][HSO4]6. Similarly, the viscosity of
[C2CNIM][HSO4] (7937 mPa·s) is comparable with the
viscosity of 1-octyl-3-propanenitrile imidazolium trifluoro-
methanesulfonate (6784.2 mPa·s), measured at 293.15 K.51
The standard deviations (SDs) and fitting parameters of
viscosities for synthesized PILs are listed in Table S9 in the SI.
Activation Energy. The activation energy (Ea) calculated
for the PILs are listed in Table S9 in the SI. Ea is the minimum
quantity of energy needed for the ions to move across each
other and, therefore, can be linked with structural information
on ILs. The lower value of Ea for ILs has been attributed to the
lower electrostatic force acting between anions and cations;
thereby ions are easily able to cross over each other’s. The Ea
calculated for [C2CNIM][p-TSA], [C2CNIM][HSO4],
[MIM][HSO4], and [C2CNIM][CH3COO] are 75.48, 52.94,
34.12, and 36.315 J·K−1·mol−1, respectively. As can be seen
Thermal Stability. The thermal properties of PILs were
measured by the thermogravimetric analysis (TGA) are
summarized in Table S11 in the SI. The change in weight of
PILs with the increase of temperature at a heating rate of 10 °C
min−1 is shown in Figure 5. The results indicated that PILs
Figure 5. Thermogravimetric analysis (TG) of protic ionic liquids.
containing [C2CNIM][CF3SO3] shows higher thermal stability
comparatively. The PILs contain derived from the strong acid
such as H2SO4, CF3SO3H, CH3SO3H, and p-TSA show a high
decomposition temperature compared to PILs derived from the
weak acids such as CF3CO2H and CH3CO2H. These data are
in good agreement with the reported literature.51−53 Miran et
al.54 reported that the 1,8-diazabicyclo-[5,4,0]-undec-7-ene
(DBU) based protic ILs with different anions such as acetate,
trifluoroacetate, and methanesulfonate have the decomposition
temperature in the range of 171−451 °C. PILs are generally
believed to have lower thermal stability compared to aprotic
ionic liquids. PILs due to N−H bonding in its cations structure
are thermally more unstable as compared to aprotic ionic
G
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Figure 6. DSC curves measured for protic ionic liquids.
liquids, which have an alkyl side chain instead of the proton in
cations moiety such as 1-methyl-3-ethylimidazolium bis-
(trifluoromethanesulfonyl)amide ([C2MIM][NTf2]).55 More-
over, the acetate based PILs were found to be more unstable as
compared to other anions.
imidazolium ring contains delocalized 3-centered-4-electron
configuration across the N1−C2−N3 moiety, a double bond
between C4 and C5 on the opposite side of the ring and a weak
delocalization in the central region56 (Figure 7). Although the
Melting Point and Glass Transition. Glass transition
temperature (Tg) is an important thermal property which gives
significant information about the cohesive energy of ILs, the
low Tg value representing low cohesive energies for ILs. The
cohesive energy increased for ILs with the increase of attractive
forces such as hydrogen bonding, van der Waals, and
Coulombic interaction, while it is decreased by their pulsive
Pauli interaction due to overlapping of closed electron shells. It
is worth to mention that low Tg values indicate that the ionic
liquid probably has desirable physicochemical properties such
as low viscosity and high ionic conductivity. The DSC curves
for all the synthesized PILs are shown in Figure 6. The glass
transition and crystalline melting temperature for PILs are
listed in Table S12 in the SI. [C2CNIM][CF3COO] and
[C2CNIM][CF3SO3] PILS show only Tm, whereas [C2CNIM]-
[p-TSA], [C2CNIM][HSO4], and [C2CNIM][CH3SO3] ex-
hibit only Tg. [C2CNIM][CH3COO] and [MIM][HSO4]
shows both Tm and Tg. The trends of DSC curve for
[C2CNIM][p-TSA], [C2CNIM][HSO4], [MIM][HSO4],
[C2CNIM][CH3SO3], and [C2CNIM][CH3COO] are similar
to each other’s, but the positions of peak are different. The
glass-transition temperatures determined from DSC measure-
ments for [C2CNIM][p-TSA], [C2CNIM][HSO4], [MIM]-
[HSO4], and [C2CNIM][CH3COO] are −75, −49, −73, and
−78 K, respectively. The change in Tg was observed with the
change of the anion; this effect of the change of the anion on Tg
has been also previously reported by other research groups.16,54
Computational Study. DFT calculations were performed
to get a deeper insight into the effect of the structural variations
of the cations and anions on the structural and physical
properties of the studied PILs. The electronic structure of
Figure 7. Optimized structure and COSMO-surface charge distribu-
tions. (a) [MIM]+ structure; (b) [MIM]+ COSMO-surface charge
distributions; (c) [C2CNHIM]+ COSMO-surface charge distributions.
hydrogen atoms on C2−H, C4−H, and C5−H carry positive
charges, the C2−H is more acidic since it is located between
two electronegative nitrogen atoms. For most 1-alkyl-3-
methylimidazolium based ionic liquids, the acidic C2−H is
the preferential site for interaction with the anions and solutes
to interact with the cation.56,57 However, if alkyl chain is not
attached to the position 3 (N3−H) of the imidazolium cation,
the hydrogen atom attached to the nitrogen atom could be
more acid compared to C2−H. This could also be observed
from the COSMO-surface charge density obtained from the
quantum calculations (Figure 7b,c). Moreover, the introduction
of the nitrile functional group in the imidazolium alkyl spacer
increases the excess electron (basic) regions in the imidazolium
alkyl chain. This could increase the cation−anion interaction
and interactions between the ionic liquid molecules, which
affect the physical properties of the ionic liquids. For instance,
the increase in refractive index for the nitrile functionalized
[C2CNIM][HSO4] compared to its counterpart [MIM]-
H
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[HSO4] could be due to the excess electron mobility in the
alkyl chain.57
Sigma Profile of the Cation. The effect of the structural
variation of the cations on the electronic properties of the ILs
can also be observed from the sigma-profile (σ-profile) of the
cations. The σ-profile shows the screening charge densities,
which is expressed in term of relative amount of the surface
with polarity σ for a given molecule. Figure 8 shows the σ-
Figure 9. Sigma profile of the studied anions computed by COSMO-
RS.
Figure 8. Sigma profile of the studied cations computed by COSMO-
RS.
profile of the two cations, 1-propyronitrile imidazolium and 1-
methylimidazolium. The σ-profile of [MIM]+ extends from
(−2.4 to 0.3) e/nm2 with a maximum peak at about −0.8 e/
nm2, while for [C2CNHIM]+ the σ-profile has wider ranges
(−2.4 to 1.6). If the molecule or ionic species has σ-profile
peaks in the range of σ > 1.0 or/and σ < 1.0, it will have
hydrogen bond donor or/and acceptor properties.57 This
shows that [C2CNHIM]+ has a more extended σ-profile
indicating its strong hydrogen bond donor and acceptor
capacity, while [MIM]+ acts mainly as a hydrogen bond donor.
Sigma Profile of the Anion. The effect of structure
variations of the anions can also be observed from their surface
charge distributions and σ-profiles. As it can be seen from
Figure 9, the anions have two major peaks, one in the nonpolar
region (between −1 and 1 e/nm2) and the other in hydrogen
bond acceptor regions (σ > 1 e/nm2). Moreover, HSO4 shows
a third minor peak in the hydrogen bond donor region (σ < −1
e/nm2) due to the hydrogen attached to one of the four oxygen
atoms (see Figure 10e). The peaks in the nonpolar regions (−1
and 1 e/nm2) are due to the CH3, CF3, and toluene side chain
and bear nonpolar characteristics of the anions. That means
that the CF3 based anions has more nonpolar characteristics
compared to the CH3-based anions since the CF3-based anions
have a high peak in this region. Moreover, the peaks of acetate-
based anions slightly shift to the strong hydrogen bond
acceptor region (to the right) compared to the sufate/sulfite-
based anions, which shows the actate-based anions are stronger
anions compared to the sufate/sulfite-based anions. Therefore,
the acetate based anions show a higher acid number compared
to other studied anions. [p-TSO3]+ has a wider peak in the
nonpolar region due to the aromatic characteristic of the
toluene structure in the anion.
Figure 10. COSMO-surface charge distribution of the anions.
CONCLUSIONS
■
Nitrile functionalized protic ionic liquids containing [CH3SO3],
[HSO4], [CF3SO3], [p-TSA], [CH3COO], and [CF3COO]
anions have been synthesized and characterized. The elemental
analysis and NMR data confirmed the purity and structures of
the synthesized PILs. Physicochemical properties such as the
density, viscosity, and refractive index were demonstrated to be
well-correlated with structural features of the anions. Both
density and viscosity were found to decrease with increasing
temperature for the range covered in the present work. The
introduction of nitrile containing chain on cation cause the
increase in viscosity, whereas the density was noted to decrease.
The values of molar volume Vm and thermal expansion
coefficient α increased linearly with temperature. The electronic
polarizability Rm and activation energies Ea were calculated
using the measured values of refractive index and viscosity,
respectively. The present PILs were found to be thermally
stable in a wide range of temperatures. The acid number
analysis showed that the [CF3COO] is more electronegative
which render the [C2CNIM][CF3COO] more acidic while the
HSO4 anion is less electronegative which import less acidity to
[C2CNIM][HSO4]. The DFT calculations showed that the
incorporation of the functional groups in the imidazolium alkyl
chain or change in the nature of anion creates more electron-
deficient and/or electron excess regions. This could affect the
cation−anion interaction, which in turn affects their physical
properties.
I
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(13) Zhao, D.; Fei, Z.; Scopelliti, R.; Dyson, P. J. Synthesis and
characterization of ionic liquids incorporating the nitrile functionality.
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Separation and recovery of cellulose and lignin using ionic liquids: a
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.6b00450.
(PDF)
(15) Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J.
Nitrile-Functionalized Pyridinium Ionic Liquids: Synthesis, Character-
ization, and Their Application in Carbon−Carbon Coupling
Reactions. J. Am. Chem. Soc. 2004, 126 (48), 15876−15882.
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Muhammad, N. Synthesis, characterization and physicochemical
properties of dual-functional acidic ionic liquids. J. Mol. Liq. 2016,
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for efficient glucose conversion to 5-hydroxymethyl furfural and
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: aamirsada_khan@yahoo.com.
*E-mail: nawshadchemist@yahoo.com.
ORCID
Girma Gonfa: 0000-0002-1161-9517
Nawshad Muhammad: 0000-0001-6453-0658
Funding
The authors gratefully acknowledge the Ministry of Higher
Education (MOHE) for funding the research work under the
Fundamental Research Grant Scheme and the Centre of
Research in Ionic Liquids (CORIL), all of the research officers
and postgraduate students for helping in all aspects.
Notes
The authors declare no competing financial interest.
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with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys.
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