Synthesis and thermophysical properties of imidazolium-based bronsted acidic ionic liquids

Article abstract of DOI:10.1021/je400243j

A range of imidazolium-based Bronsted acidic ionic liquids (ILs) have been synthesized by varying the side chain length of the alkyl and alkyl sulfonic group incorporated to the C1 and C3 positions in the imidazolium ring respectively with a fixed HSO₄^- anion. The synthesized ILs were characterized using NMR for structure confirmation and CHNS for elemental analysis. The thermal properties such as thermal decomposition temperature were determined using thermogravimetric analysis, and several key physical properties such as viscosity and density were measured within a temperature range of 20 C to 80 C using an Anton Paar viscometer and densitometer. The density measurement results and established equations were used to calculate the molecular volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients of the synthesized ILs.

Full text of DOI:10.1021/je400243j

Article
pubs.acs.org/jced
Synthesis and Thermophysical Properties of Imidazolium-Based
Bronsted Acidic Ionic Liquids
Nawshad Muhammad,* Zakaria Man, Yasir A. Elsheikh, M. Azmi Bustam, and M.I. Abdul Mutalib
,
†
‡
§
‡
‡
†
Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan
‡
PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh,
Perak, Malaysia
§
Department of Chemical and Materials Engineering Faculty of Engineering, King Abdulaziz University, P.O. Box 344, Rabigh, 21911
Saudi Arabia
ABSTRACT: A range of imidazolium-based Bronsted acidic
ionic liquids (ILs) have been synthesized by varying the side
chain length of the alkyl and alkyl sulfonic group incorporated
to the C1 and C3 positions in the imidazolium ring
−
respectively with a fixed HSO4 anion. The synthesized ILs
were characterized using NMR for structure confirmation and
CHNS for elemental analysis. The thermal properties such as
thermal decomposition temperature were determined using
thermogravimetric analysis, and several key physical properties
such as viscosity and density were measured within a
temperature range of 20 °C to 80 °C using an Anton Paar
viscometer and densitometer. The density measurement results and established equations were used to calculate the molecular
volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients of the synthesized ILs.
INTRODUCTION
In this study a number of Bronsted acidic ILs namely 1-
methyl-3-(3-sulfopropyl) imidazolium hydrogensulfate
■
Room temperature ionic liquids (ILs) are typically salts that
contain at least one organic cation and organic or inorganic
anion, and have melting points below 100 °C. Such materials
are in demand as alternatives to traditional molecular solvents
owing to their desirable properties such as high chemical and
thermal stabilities and extremely low flammabilities and vapor
(
MSPIMHSO4), 1-butyl-3-(3-sulfopropyl) imidazolium hydro-
gensulfate (BSPIMHSO4), 1-methyl-3-(3-sulfobutyl) imidazo-
lium hydrogensulfate (MSBIMHSO4) and 1-butyl-3-(3-sulfo-
butyl) imidazolium hydrogensulfate (BSBIMHSO4) were
prepared by incorporating a methyl or butyl group at the
position of C1 and a propyl or butyl attached with a sulfonate
group at the position of C3 within the imidazolium ring with
fixed HSO4− anion. The structures of the synthesized ILs were
confirmed using NMR, and the elemental analysis was
conducted using CHNS. A number of thermophysical proper-
ties of the synthesized ILs such as thermal stability, viscosity,
and density were measured with respect to temperature for a
range between 20 and 80 °C. The effects of the alkyl chain
length attached to the imidazolium cation were investigated for
thermal stability, viscosity, and density. The values such as
molecular volumes, standard entropies, crystal lattice energies,
and thermal expansion coefficients were then calculated for all
the synthesized ILs.
1
,2
pressures.
The wide range of possible cation−anion
combinations has enabled ILs to be developed with a specific
set of desired physicochemical properties or for specific
applications. The present applications of ILs in the field of
catalysis are found to be numerous and cover many areas. One
of the main areas of interest is to replace the use of traditional
3
liquid acids in catalytic reactions.
Physiochemical properties of ILs such as melting point,
viscosity, density, and miscibility with water was found to vary
with the length of the alkyl chain on the cation or the type of
anion incorporated to the ILs. It has been reported that 1-alkyl-
4
3
-methylimidazolium with tetrafluoroborates and hexafluor-
5
ophosphates anions formed liquid crystalline when the alkyl
chain length exceeded more than 12 carbon atoms. Below this
chain length, the ILs formed liquid at room temperature. In
another account, the hydrophobic property of an IL was found
to favor solvent extraction for product separation that is based
on relative solubility between phases. Generally, the cation part
of ILs appears to have bulky and uneven structures which result
in a low degree of packing symmetry. This results in reduced
lattice energy of the salt crystalline formed, thereby decreasing
METHODS
■
Materials. The following chemicals were obtained from
Sigma-Aldrich (Malaysia): 1-methylimidazole (99.5 %), 1-
butylimidazole (99.0 %), 3-propanesultone (99.0 %), toluene
Received: March 12, 2013
Accepted: February 6, 2014
Published: February 14, 2014
6
its melting point.
©
2014 American Chemical Society
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Journal of Chemical & Engineering Data
Article
(
(
AR grade), ethyl acetate (AR grade) anhydrous methanol
99.8 %); sulfuric acid (ACS grade, 95.97 %). 1,4-
checked with several ILs previously investigated and published
by our research group. The Anton Paar viscometer (model
8
Butanesultone was obtained from Fluka (Malaysia). All
chemicals were used without any drying or further purification.
Synthesis of Bronsted Acidic Ionic Liquids. The
preparations of the ILs [(MSPIMHSO4), (MSBIMHSO4),
SVM3000) and densimeter (model DMA5000) were used
accordingly to measure the viscosity and density of the ILs over
a temperature range of 20 °C to 80 °C with a temperature
−4
control of ± 0.01 °C and uncertainties of ± 0.4 % and ± 3·10
−3
9
(
BSPIMHSO4), (BSBIMHSO4)] were similar to those used in
g·cm , respectively.
the previous literature, for which a typical reaction is depicted
7
in Figure 1. First, 0.03 mol of 1,3-propane and 1,4-
RESULTS AND DISCUSSION
. Synthesis of ILs. The results obtained for the H NMR
■1
1
spectroscopy and CHNS analysis of all the synthesized ILs as
given below confirmed the structure of the respective ILs.
1
MSPIMHSO4. H NMR δ (300 MHz): 95.9 8.486 (s, 1H),
7
2
2
.147 (s, 1H), 7.134 (s, 1H), 4.830 (s, 2H), 4.158−4.186 (t,
H), 3.697 (s, 3H), 2.648−2.681 (t, 2H), 1.827−1.877 (m,
H). Anal. Calcd (measured) for C H N O S : C = 27.80
7
14
2
7 2
(
0
27.85 ± 0.047), H = 4.66 (4.58 ± 0.053), N = 9.26 (9.29 ±
.081), S = 21.21 (21.26 ± 0.046).
1
MSBIMHSO4. H NMR δ (300 MHz): 9.182 (s, 1H), 7.782
(
s, 1H), 7.723 (s, 1H), 5.007 (s, 2H), 4.177−4.158 (t, 2H),
3
1
.860 (s, 3H), 2.592−2.634 (t, 2H), 1.835−1.878 (m, 2H),
.535−1.557 (m, 2H). Anal. Calcd (measured) for
Figure 1. A typical pathway for the preparation of some Bronsted
acidic ILs.
C H N2O S : C = 30.37 (30.32 ± 0.095), H = 5.09 (5.094
8
16
7 2
±
0.024) N = 8.85 (8.99 ± 0.0164), S = 20.27 (20.26 ± 0.073).
1
butanesulfone were mixed and dissolved in anhydrous toluene
in a dry round-bottom flask under vigorous stirring followed by
dropwise addition of equal molar amounts of N-methyl/
butylimidazole, over a period of 15 min in an ice bath. Upon
completing it, the system was gradually heated to room
temperature and stirred for 2 h. The resultant mixture was
filtered and the precipitated white solid zwitterions, that is, 1-
methyl/butyl-3-(propyl/butyl-3-sulfonate) imidazolium were
collected. The precipitate was washed with ethyl acetate (3 ×
BSPIMHSO4. H NMR δ (300 MHz): 9.169 (s, 1H), 7.768
(s, 1H), 7.759 (s, 1H), 4.880 (s, 2H), 4.216−4.337 (t, 2H),
4.099−4.116 (t, 2H), 2.626−2.713 (t, 2H), 2.094−2.121 (m,
2H), 1.697−1.768 (m, 2H), 0.978−1.015 (m, 2H), 0.742−
0.761 (t, 3H). Anal. Calcd (measured) for C H N O S : C =
10
20
2
7 2
34.87 (34.82 ± 0.05), H = 5.85 (5.89 ± 0.058), N = 8.13 (8.10
± 0.03), S = 18.62 (18.66 ± 0.04).
1
BSBIMHSO4: H NMR δ (300 MHz): 9.156 (s, 1H), 7.775
(s, 1H), 7.704 (s, 1H), 4.975 (s, 2H), 4.131−4.167 (t, 2H),
3.969−3.997 (t, 2H), 2.897−2.913 (t, 2H), 2.346−2.420 (m,
2H), 1.759−1.777 (m, 2H), 1.548−1.580 (m, 2H), 1.198−
1.240 (m, 2H), 0.814−0.823 (t, 3H). Anal. Calcd (measured)
for C H N O S : C = 36.85 (36.82 ± 0.05), H = 6.18 (6.17 ±
3
0 mL) and then dried in a rotary evaporator at 100 °C for 12
h. The dry sample was then kept under vacuum overnight at
1
00 °C for further drying.
To obtain the desired product of ILs, 0.02 mols of the
11
22
2
7 2
zwitterions (1-methyl/butyl-3-(propyl/butyl-3-sulfonate) imi-
dazolium) obtained were first dissolved in deionized water
under vigorous stirring and then a corresponding stoichiometric
amount of concentrated sulfuric acid was added dropwise over
a period of 15 min. After completing the addition of acid, the
reaction mixture was slowly heated up to 80 °C for 6 h. Then a
viscous yellow IL was formed. The IL was then washed with
deionized water (3 × 40 mL) followed by overnight drying
under vacuum.
0.058), N = 7.81 (7.91 ± 0.086), S = 17.89 (18.11 ± 0.03)
In addition, the results also showed that the purity of the ILs
was higher than 99 %.
Thermalgravimetric Analysis of ILs. Studies on thermal
decomposition (T ) and effects of side chain of imidazolium-
d
based cation on thermal behavior were carried out on all the
synthesized ILs (Figure 2). According to literature, increasing
the side chain of the cation will result in decreasing the ILs
thermal stability. The reduction in the decomposition temper-
ature could be due either to the decrease in intermolecular
interaction or to the initial decomposition of the alkyl
Characterization. The structures of the ILs were confirmed
1
using H NMR (Bruker Avance 300 MHz) spectroscopy in
11
DMSO-d while a CHNS-932 (LECO) apparatus was used for
substituent. However, there were exceptions as reported by
many researchers. The finding from this work shows that
increasing the side chain generally reduced the thermal stability
of the ILs except for a few cases.
6
the elemental analysis. The water content of the synthesized ILs
was determined by coulometric Karl Fischer titration (Mettler
Toledo DL 39) with Hydranal Coulomat AG reagent (Riedel-
de Haen).
Thermal Properties. Thermogravimetric measurements
were conducted using Perkin-Elmer TGA, Pyris I to investigate
the thermal stability of the prepared ILs. A sample of ≤ 10 mg
held in a capped Al pan was heated at a heating rate of 10 °C/
min from 50 °C to 600 °C under nitrogen flow. The
experimental decomposition temperatures were presented in
terms of weight loss (%) and TG (°C).
All the synthesized ILs exhibited good thermal stability with
high decomposition temperature. The ILs with alkyl sulfonic
functional group as the anions (zwitterions) showed higher
thermal stabilities compared to the ones with alkyl sulfonic
group side chain with HSO as its anion (Table 1).
4
Studies on the effects of the alkyl chain length and the alkyl
sulfonic group side chain on decomposition temperature
showed that for the imidazolium case with the shortest alkyl
Physical Properties. All instruments used for physical
property measurements were calibrated using Millipore-quality
chain, that is, CH and propyl sulfonic chain in the cation, the
3
decomposition temperature (T ) reduces from 315 °C to 306
d
8
−10
water as described elsewhere.
The instruments were also
°C when the sulfonic chain length was increased from propyl to
5
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Journal of Chemical & Engineering Data
Article
temperatures, ca. 270 and 240 °C, respectively. Figures 2
panels a and b show the T traces of the four imidazolium salts.
d
2
. Viscosity Analysis. From an engineering point of view,
the viscosity of ILs is of prime importance as it has a major
impact on the design of mixing and flow system. In addition, it
12
also affects transport properties such as the diffusion rate. At
0 °C, the ILs MSPIMHSO , MSBIMHSO , BSPIMHSO , and
2
4
4
4
BSBIMHSO showed absolute viscosities of (1296, 1427, 1000,
4
1
592) mPa·s, respectively. These values are mainly located
outside the typical viscosity range reported by earlier researches
which is between 66 mPa·s and 1110 mPa·s.
1
2,13
For all the synthesized ILs except for BSPIMHSO4, the
increase in the chain length of one or both sides of the alkyl
chain with or without the sulfonate link on the cation side will
cause an increase in the viscosity. This can be observed by the
significant increase in viscosity of the ILs from 1296 mPa·s to
1
427 mPa·s when the propylsulfonate chain in MSPIMHSO is
4
replaced with a butylsulfonate chain to form MSBIMHSO4.
Similarly, changing the propylsulfonate in BSPIMHSO to a
4
butylsulfonate to form BSBIMHSO resulted in the increase of
4
the ILs viscosity from 1000 mPa·s to 1592 mPa·s respectively.
In another observation, the absolute viscosity of the ILs
increases from 1427 mPa·s to 1592 mPa·s when the methyl
chain of MSBIMHSO was changed to a butyl chain thus giving
4
BSBIMHSO . Nevertheless, when the methyl chain in
4
MSPIMHSO4 was replaced with a butyl chain to form
BSPIMHSO , a considerable decrease in the viscosity, that is,
4
from 1296 mPa·s to 1000 mPa·s, was observed instead. This
shows that the IL BSPIMHSO4 demonstrated different
viscosity behavior compared to the earlier ILs. This deviation
in the viscosity trend indicates that the viscosity behavior of ILs
does not necessarily follow a single trend. As for the sulfonate
side chain, the viscosity behavior was consistent with the
general trend as discussed earlier. This has led to the conclusion
that an increase in the sulfonate side chain length could have a
great effect in increasing the viscosity of the ILs.
Figure 2. Thermogravimetric analysis (TGA) curves of the thermal
decomposition of imidazolium ILs: (a) MSPIMHSO4 and BSPIMH-
SO4; (b) MSBIMHSO4 and BSBIMHSO4.
Table 1. Comparisons on the Thermal Decomposition
Temperature of the Synthesized Zwitterions and Their ILs
Figures 3 and 4 show the effect of temperature on the
viscosity and density of the synthesized ILs, respectively. From
zwitterions/ILs
thermal decomposition temp (T ), °C
d
MPSIM
370
315
367
306
372
323
360
311
MSPIMHSO4
MBSIM
MSBIMHSO4
BPSIM
BSPIMHSO4
BBSIM
BSBIMHSO4
butyl. On the other hand, changing the alkyl chain from CH to
3
C H , with propyl sulfonic chain in the cation, showed an
4
9
increasing thermal decomposition temperature from 315 °C to
3
23 °C. On another account, varying the sulfonic chain from
propyl to butyl decreases the T to 306 °C and 311 °C for
MSBIMHSO4 and BSBIMHSO4, respectively.
d
In general, the rate of mass loss for imidazolium type ILs
decreases with the increase in the alkyl chain length from
methyl to butyl. Apparently, the mass loss of the imidazolium
ILs increases when the length of the functional alkyl chain
increases from propyl to butyl. Similar observations were
reported here: varying the sulfonic chain from propyl to butyl
causes the weight loss of the ILs to increase, and thereby
making MSBIMHSO and BSBIMHSO less stable. MSBIMH-
Figure 3. Viscosities as a function of temperature for functionalized
imidazolium-based ILs.
the two figures, it can be clearly seen that both the viscosity and
density reduces with an increase in temperature. This is
13
consistent with the earlier findings based on other ILs. The
influence on viscosity was observed to be more pronounced
compared to density particularly at low temperature near the
4
4
SO4 and BSBIMHSO4 started to decompose at lower
5
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effect on density was observed to be very small with only about
2
% to 3 % changes over the 60 °C temperature range tested.
For example, as presented in Figure 4, the BSBIMHSO
exhibited a small density drop from 1.3642 g/cm to 1.3275
g/cm when the temperature was raised from 20 °C to 80 °C, a
reduction of only 0.0367 g/cm . These results indicate that
4
3
3
3
temperature variation has little effect on the IL densities. The
above results are found to be consistent with the observation
14
made by in their study on different types of ILs.
. Molecular Volume (V ). From the density value
4
m
measured for the ILs, the molecular volume (V ) for each of
m
them which is the combined volume of the cation and anion
can be estimated. The molecular volume (V ) can be calculated
m
using the eq 1 below at standard temperature and pressure
condition.
Figure 4. Densities as a function of temperature for functionalized
imidazolium-based ILs.
1
5,16
M
NAρ)
ambient condition. For example, the viscosity of BSBIMHSO4
reduces by about 91 %, when the temperature is changed from
Vm = (
(1)
2
0 °C to 50 °C as shown in Figure 3. This initial significant
−1
Where M is the molar mass in g mol , N is Avogadro’s
A
reduction in viscosity as the temperature was increased from
ambient condition has made the application of ILs in catalysis
easier at higher temperature conditions in which most reactions
were performed. However, for the ILs synthesized, this
advantage is true for temperatures up to around 60 °C beyond
which no considerable reduction in viscosity was observed.
−1
−3
constant in mol , ρ is the density in g·cm and Vm is
molecular volume in cm . The calculated values of the
molecular volume (V ) for the synthesized ILs are presented
in Table 3. The values calculated were found to be within the
3
m
17,18
range reported
for other types of room temperature ILs,
−22
that is, [C2CN BIM]TFMS 4.08 × 10 , choline hexanoate
3. Density Analysis. For density analysis of the synthesized
−22
3
.58·10
.
ILs, the same cation base, imidazolium, was used, but changes
were made to the side chains that are attached to it (Table 2). It
0
Table 3. Molecular Volume V , Standard Entropy S ,
m
Thermal Expansion Coefficient (α ) and Crystal Energy
UPOT of the ILs at 20 °C
p
Table 2. Density and Viscosity Data for the Present
Synthesized ILs (at 20 °C and p = 0.1 MPa)
a
thermal expansion
S0
density (ρ)
g/cm3
viscosity (η)
mPa·s
Vm
UPOT
coefficient
nm3
kJ mol−1
α·10 /(K−1)
4
J K−1 mol−1
structure code
structure code
MSPIMHSO4
MSBIMHSO4
BSPIMHSO4
BSBIMHSO4
1.5017
1.4647
1.3469
1.3611
1296
1427
1000
1592
MSPIMHSO4
MSBIMHSO4
BSPIMHSO4
BSBIMHSO4
0.333
0.357
0.423
0.436
442.19
434.24
416.21
413.12
3.81
3.91
4.21
4.25
445.03
474.95
557.21
573.42
a
Standard uncertainty u is u(T) = 0.01 °C and the combined expanded
−4
3
uncertainty is u (ρ) = 3·10 g/cm , u (η) = 0.4 % mPa·s, (level of
c
c
0
confidence = 0.95).
5. Standard Entropy (S ). From the calculated values of
the molecular volume (V ), the standard entropy of the
m
syntehesized ILs could be calculated using the following
equation
was observed that when the methyl side chain in MSPIMHSO4
1
9,20
was replaced with a butyl side chain to form BSPIMHSO , the
4
3
3
density dropped from 1.5048 g/cm to 1.3504 g/cm ,
respectively. Similarly the density dropped when the methyl
0
S = 1246.5(V ) + 29.5
(2)
m
3
0
chain in MSBIMHSO was replaced with a butyl chain to form
where V is the molecular volume in nm and S is the standard
m
4
−1
−1
BSBIMHSO . Also a similar trend where the density dropped
entropy in J K mol . The standard entropy values calculated
for each of the synthesized ILs are presented in Table 3. Again,
the values of the standard entropy calculated for the
synthesized ILs in the study are found to be within similar
4
3
3
from 1.5048 g/cm to 1.4678 g/cm was observed when the
propylsulfonate chain in MSPIMHSO4 was replaced with a
butylsulfonate chain to form MSBIMHSO . However, when a
4
similar change was made, for example, the propylsulfonate
range as reported for [C MIM]glycinate ILs with n = 2−6 and
n
−1
−1
chain in BSPIMHSO was replaced by butylsulfonate chain to
its standard entropy ranges from 360.2 J K mol to 498.8 J
3
4
−
1
−1
form BSBIMHSO , the density value increases from 1.3504 g/
K
mol respectively. Similarly a range of standard entropy
18
4
3
3
cm to 1.3642 g/cm instead. The increase in the density from
BSPIMHSO4 to BSBIMHSO4 could be attributed to the
uniformity in spatial arrangement of alkyl groups on the
imidazole ring of BSBIMHSO4.
values have been reported for choline carboxylates ILs, that is,
371 J K− mol to 475 J K mol . Nevertheless, these values
1
−1
−1
−1
are lower compared to those calculated for [C2CN DIM]-
−
1
−1
DOSS (1398.36 J K mol ) and [C2CN DIM]DDS (1055.74
−1
−1 17
On the general effect of temperature on the ILs density,
Figure 4 shows that a linear correlation could be established.
These linear equations representing the various ILs could then
be used for calculating the density of the 4 ILs analyzed, at any
temperature. Nevertheless, the magnitude of the temperature
J K mol ) which have lower densities. Hence it can be
concluded that the standard entropy has an inverse relationship
with density.
6. Lattice Energy (UPOT). The lattice energy of a salt is
reflected by the strength of the interactions between its ions.
5
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The lattice energies of the ILs could be calculated using the eq
ILs generally showed increasing values for the ILs having longer
alkyl chain attached either to the imidazoilum ring or to the
sulfonic group with an exception noted for the case where the
increase in the alkyl chain from MSPIMHSO4 to form
21
3
, which is based on the Glasser theory.
1
/3
UPOT = 1981.2(ρ/M) + 103.8
(3)
−1
BSPIMHSO resulted in decreasing value, that is, from 1296
4
where UPOT is the lattice energy in kJ mol . The calculated
values for the lattice energy for all the synthesized ILs are listed
in Table 3. Compared to the normal alkali chloride which has a
mPa·s to 1000 mPa·s. Similarly the density value was also
affected by the alky chain length where a decreasing trend was
observed with the increase in the alkyl chain length. Beside the
effect from the alkyl chain length, the effect of temperature on
both viscosity and density was also investigated. Both
properties showed reduction in values with increasing temper-
ature with the effect impacting viscosity more than density. The
findings are in line with the reported work elsewhere. The
−1
4
lattice energy value of 602.5 kJ mol , the data obtained for the
ILs show significantly lower values, i.e., in the range of 413 kJ
−1
−1
mol to 442 kJ mol . It has been reported that it is not the
low packing symmetry of IL molecules that leads to its low
lattice energy and thus to low melting points. Also, it has been
mentioned that the lattice energies of an IL are in a range that is
expected for typical salts and even higher, due to a large
molecular volume (V ) and thermal expansion coefficient (α)
m
22
were calculated using the measured values obtained for viscosity
and density of the ILs, and they were found to have a direct
relationship with density, while the lattice energy (UPOT) and
dispersive contribution. The lower values of melting point
and thus lattice energy for ILs were attributed due to the large
number of entropic degrees of freedom that is released upon
melting; that is, in the solid state, entropy is almost fixed, and
upon melting it is released which makes possible the lower
0
the standard entropy (S ) values showed inverse relationship
with density values.
23
values of melting point for the ILs. A similar range of
calculated values of lattice energy for [CnMIM]alaninate,
AUTHOR INFORMATION
Corresponding Author
*E-mail: nawshadchemist@yahoo.com.
■
[
CnMIM]glycinate (where n = 2−6), and choline carboxylates
−1
−1
ranging from (421 to 456) kJ mol , (429 to 469) kJ mol and
(
where.
for the ILs [C2CN DIM]DOSS (332 kJ mol ) and [C2CN
DIM]DDS (355 kJ mol ).
. Thermal Expansion Coefficient (α ). Using the values
−1
434 to 464) kJ mol respectively, were reported else-
18,19
Funding
And even lower calculated values were also reported
The financial assistance provided by FRGS, MOHE, and
PETRONAS Ionic Liquid Centre, Universiti Teknologi
PETRONAS (UTP) is gratefully acknowledged.
−1
−1
17
7
p
Notes
of densities measured for the synthesized ILs, an important
thermo-physical property namely the thermal expansion
coefficient (α) was calculated accordingly for each IL, and
the results are shown in Table 3. The calculation was
The authors declare no competing financial interest.
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■
9
,24
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⎜
⎝
⎟
⎠
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p
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where A and A are the fitting parameters that have been
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0
1
temperature, while α , ρ, and T are the thermal expansion
P
2
(
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P
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volume expansivity. The thermal expansion coefficient values
reflect the free volume of ionic liquid; that is, higher thermal
coefficient values correspond to higher free volume of ionic
9
,25
liquid. Similar to the other types of imidazolium-based ILs,
(
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relatively low values of thermal expansion coefficient were
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CONCLUSION
■
In this study, four different types of imidazolium-based
Bronstead acidic ILs were synthesized and characterized. The
difference in the ILs was due to the variation in chain length of
the alkyl group and alkyl sulfonic group attached to the
imidazolium ring but with a fixed HSO anion. All the ILs
−
4
1
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using CHNS, and the purity was found to be more than 99 %.
The thermal analysis using TGA showed an increasing stability
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the imidazolium ring from methyl to butyl and vice versa when
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