Thermophysical properties of a new dicationic ionic liquid

Article abstract of DOI:10.1021/je3000876

A new functionalized dicationic ionic liquid was synthesized with the final aim of studying its application in the extraction of metal ions in the mineral industry. In light of this application, the density, refractive index, and dynamic viscosities have been determined and correlated as a function of temperature. Density data were used to calculate the volumetric properties of the ionic liquid. Several empirical equations were used to evaluate the correlation between the density and refractive indices. The results show that the modified Eykman equation has the best fit. In addition, thermal behavior was measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and a high thermal stability and a wide liquid phase range were determined.

Full text of DOI:10.1021/je3000876

Article
pubs.acs.org/jced
Thermophysical Properties of a New Dicationic Ionic Liquid
Martha Claros, Hector R. Galleguillos, Ivan Brito, and Teofilo A. Graber*
́
́
́
Departamento de Ingeniería Química-CICITEM, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
ABSTRACT: A new functionalized dicationic ionic liquid was
synthesized with the final aim of studying its application in the
extraction of metal ions in the mineral industry. In light of this
application, the density, refractive index, and dynamic viscosities have
been determined and correlated as a function of temperature. Density
data were used to calculate the volumetric properties of the ionic
liquid. Several empirical equations were used to evaluate the
correlation between the density and refractive indices. The results
show that the modified Eykman equation has the best fit. In addition,
thermal behavior was measured by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC), and a high thermal
stability and a wide liquid phase range were determined.
(298.15 to 343.15) K. The coefficient of thermal expansion was
obtained from experimental density data as a function of
temperature. Moreover, the standard entropy was calculated
using the expression proposed by Glasser.14 The Lorentz−
Lorenz, Dale−Gladstone, Oster, Arago−Biot, Newton, Eykman,
and modified Eykman empirical equations were used to evaluate
the correlation between the densities and refractive indices of the
IL. In addition, the thermal behavior of this new compound was
determined with TGA and DSC analysis.
INTRODUCTION
■
Ionic liquids (ILs) are salts that are composed entirely of ions.
They are stable in a wide range of temperature, and they have
good thermal, chemical, and electrochemical stability. In
addition, most have negligible vapor pressure, so there is no
loss of solvent through evaporation, which avoids environmental
problems due to volatilization.1,2
The understanding of dicationic ionic liquids (DILs), which
consist of two head groups linked by a rigid or flexible spacer and
two monoanions, is an interesting research topic because the
numbers of possible combinations of cations and anions in
dicationic ILs are greater than in monocationic ILs. Therefore,
broader variability in the physical properties of these ILs should
be possible such that changes in their structure can be “tuned”,
controlled or altered to a greater extent than conventional ILs.3−5
These DILs have been studied in different applications such as
electrolytes,6 additives in chromatography,7 solvents for high-
temperature organic reactions,8 and agents for selectively
complexing and extracting mercury(II).9 There are several
studies on the physical properties (e.g., density and viscosity) of
DILs. Most of these properties are related to structural
changes6,10,11 rather than temperature changes, as is usually the
case in monocationic ILs.
In this work, a new DIL with a functional amino group in the
imidazolium ring and a bis(trifluoromethanesulfonyl)imide
anion was synthesized. Metallic ions can be extracted from
aqueous solution by a chelating effect by at least two points in the
extracting agent structure, as occurs with a great variety of oxime
compounds.12,13 Due to their modified substituents this IL was
designed for application in the liquid−liquid extraction of metal
ions such as ferric and cupric ions in mineral processing in
northern Chile.
EXPERIMENTAL SECTION
■
Chemicals. For the synthesis, imidazole with a purity of > 99
%, acrylonitrile with a purity of > 99 %, 1,4-dibromobutane with a
purity of > 99 %, 2-bromoacetamide with a purity of > 98 %, and
lithium bis(trifluoromethanesulfonyl)imide with a purity of > 99
% were procured from Sigma-Aldrich and used without further
purification. Milli-Q quality distilled water was used in all
experiments (κ = 0.054 μS·cm−1).
Synthesis. The new compound 1,1′-butane-1,4-di-yl-bis[3-
(2-amino-2-oxoethyl)-1H-imidazol-3-ium] dibis-
−
(trifluoromethanesulfonyl)imide ([C4(amim)2]TFSI2 ) was
synthesized as follows. Imidazole (15.49 g, 0.227 mol) was
dissolved in methanol in a round-bottom flask placed in a heating
jacket connected to a thermo-regulated Lauda RM-B bath, with
an accuracy of 0.1 K. An excess of acrylonitrile (21.09 g, 0.397
mol) was added dropwise. The system was degassed several
times with dry argon, heated at 333.15 K, and stirred for 6 h.
Methanol and unreacted acrylonitrile were removed under
reduced pressure, and the temperature was increased to 353.15
K, resulting in compound A (Figure 1). After cooling to 278.15
K, 2-bromoacetamide dissolved in acetonitrile was carefully
added dropwise, and an exothermic reaction was observed. The
Since this is a new compound, the knowledge of physical
properties is of great importance for process engineering and
equipment design, and thus, the density, dynamic viscosity, and
refractive index were measured as a function of temperature from
Received: December 20, 2011
Accepted: June 20, 2012
© XXXX American Chemical Society
A
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Journal of Chemical & Engineering Data
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Figure 1. Route of synthesis of the DIL 1,1′-butane-1,4-di-yl-bis[3-(2-amino-2-oxoethyl)-1H-imidazol-3-ium] dibis(trifluoromethanesulfonyl)imide
dibis(trifluoromethanesulfonyl)imide.
mixture was stirred for 13 h, increasing the temperature gradually
until reaching 323.15 K. After the reaction was completed, an
excess of 15 wt % NaOH and chloroform were added, and this
mixture was left to stir at room temperature for 1 h, until an
organic and aqueous phase was observed. The viscous brown
upper phase was separated, and residual solvent was removed
under reduced pressure at 353.15 K, yielding 27.53 g (0.22 mol)
of compound B (Figure 1). Afterward, 1,4-dibromobutane (24.5
g, 0.11 mol) was added dropwise to the mixture of methanol and
compound B under dry argon atmosphere, resulting in an
homogeneous mixture that was left to stir at 323.15 K for 96 h
under reflux. Methanol was removed under reduced pressure and
heated to 353.15 K. The product C (Figure 1) was washed
several times with 20 cm3 of ethyl acetate to eliminate the
unreacted reagents, and the remaining ethyl acetate (b.p.: 350 K)
was removed under reduced pressure at 343.15 K. The bromide
salt was finally dried in vacuo at 343.15 K. The obtained product
was a very viscous light brown liquid (51.21 g, 0.11 mol, 93 %
yield).
For the metathesis reaction, compound C (Figure 1) was
dissolved in deionized water. The lithium bis-
(trifluoromethanesulfonyl)imide salt (66.8 g, 0.23 mol) was
added and left to stir for 1 h at room temperature. Two liquid
phases were observed; the bottom phase was washed several
times with deionized water. The bromide salt in the residual
water was detected with AgNO3 0.1 N. By the third wash, there
was no presence of bromide salt. The product was dried in vacuo
at 353.15 K. The water content was determined using a Karl
Fischer titrator and was found to be 0.0034 mass fraction. The
final obtained product D (Figure 1) was a very viscous yellow
liquid. Yield 80.5 %. 1H NMR (MeOD, δ ppm): 3.48 (m, 2H),
3.98 (t, 2H), 5.02 (s, 2H), 7.81 (s, 1H), 7.89 (s, 1H), 9.24 (s, 1H).
FT-IR (400 to 4000 cm−1). 3453, 3361 [υ(N−H)]. 3157, 2974
[υ(Ar−C−H)]. 1754, 1701 [υ(CO)]. 1614, 1459 [υ(CC)].
1568 [υ(NH2)]. CHN analysis: % calcd, (% found): C 24.95
(26.30), H 2.56 (1.84), N 12.93 (11.91), S 14.80 (13.18).
Apparatus and Procedure. The water content has been
determinate by a Karl Fischer titrator Mettler Toledo DL31, with
an uncertainty of 0.0005 mass fraction (Combititrant 5, one
component 1 mL = ca. 5 mg of H2O). After drying, samples were
stored under dry atmosphere in vacuum desiccators.
refractometer with an uncertainty of
1·10−4 nD units. The
densimeter and refractometer had self-contained Peltier systems
for temperature control with an uncertainty of 0.1 K. Dynamic
measurements of viscosity were implemented using a Brookfield
model DV-III cone/planar rheometer with a CP-40 spindle. A
Lauda model RM-6 water circulator bath was used to maintain
the temperature of the measurements within
0.1 K. The
average reproducibility of the viscosity measurements was
0.00125 Pa·s. All viscosity measurements were carried out at a
constant share rate of 0.225 s−1.
For the thermogravimetric analysis, a TGA/DSC-1 from
Mettler Toledo was used. The instrument was calibrated for
temperature and heat flow with an indium reference sample
provided by Mettler Toledo. The measurement was repeated
three times with an approximate mass of 13 mg, which was placed
in a 70 μL alumina crucible with a lid and small hole, in the
temperature range of (298.15 to 823.15) K at a heating rate of 10
K·min−1. The Mettler Toledo STARe software version 10.0 was
used to determine the onset and start temperature. The start
temperature (Tstart) is the temperature at which the decom-
position of the sample begins. The onset temperature (Tonset) is
the intersection of the baseline weight, either from the beginning
of the experiment or after the drying step, and the tangent of the
weight versus the temperature curve as decomposition occurs.
Differential scanning calorimetry (DSC) was performed using
a Mettler Toledo DSC 821 calorimetric system using the STARe
program. Dry nitrogen was used as a purge gas and measured in
the temperature range of (120.18 to 360.18) K at a scan rate of 20
K·min−1.
RESULTS AND DISCUSSION
■
The physical properties were measured experimentally from
(298.15 to 343.15) K at atmospheric pressure using the above-
mentioned techniques and are shown in Table 1.
The density of the DIL was found to be denser than most
monocationic ILs and similar to other DILs with short alkyl
chains as a spacer between cations.10 As expected, the density of
the IL decreased with an increase in temperature, which is usually
the case for other ILs.15−17 The variation in density with respect
to temperature for the IL is illustrated in Figure 2. The density
values were fitted using the following quadratic equation:
The physical properties were measured in triplicate at the
specified temperature. The densities were measured with a
Mettler Toledo DE-50 vibrating tube densimeter, with an
uncertainty of less than 5·10−2 kg·m−3. The densimeter was
calibrated at atmospheric pressure using air and distilled
deionized water as a reference substance prior to the initiation
of each run of measurements at a given temperature. The
refractive index was measured using a Mettler Toledo RE50
ρ = A0 + A1T + A2T2
(1)
where T is the absolute temperature and A0, A1, and A2 are
adjustable parameters. These correlation parameters and the
standard relative deviations (SRDs) are given in Table 2.
Density data were used to derive other thermodynamic
properties, such as the thermal expansion coefficient. This
B
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Journal of Chemical & Engineering Data
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literature20−23 ranging from (5·10−4 to 7.7·10−4) K, which is
lower than the value of α typical for conventional molecular
solvents.
Table 1. Density, ρ, Refractive Index, nD, Thermal Expansion
Coefficient, α, and Dynamic Viscosity, η, as a Function of
Temperature
From the experimental densities, the IL molecular volume
(Vmolec) was calculated at T = 298.15 K using the following
equation:17,24
T/K
ρ/kg·m−3
α/K−1
nD
η/Pa·s
298.15
303.15
308.15
313.15
318.15
323.15
328.15
333.15
338.15
343.15
1590.17
1585.41
1580.62
1575.98
1572.04
1567.22
1562.31
1557.66
1552.61
1548.03
0.00057
0.00057
0.00058
0.00059
0.00059
0.00060
0.00060
0.00061
0.00062
0.00062
1.4546
1.4532
1.4518
1.4504
1.4491
1.4477
1.4464
1.4451
1.4439
1.4430
8.689
4.730
3.285
2.365
1.824
1.517
1.323
1.221
1.129
1.098
M
N·ρ
V
molec/nm3 =
(3)
where M is the molar mass (866.65 g·mol−1), and N is
Avogadro’s number. The value obtained from this equation is
0.9049 nm3.
Glasser and Jenkins25,26 have established a linear relationship
between entropy (S0) and the molecular volume for ionic solids
and organic solids. These equations have been adapted for
ILs.14,17,24 Since ILs with large organic cations are defined
between ionic solids and organic liquids, the average values of the
linear correlation constants are used:
S0/(J·K−1·mol−1) = 1246.58(Vmolec/nm3) + 29.5
(4)
Thus, S0/(J·K−1·mol−1) = 1157.5.
As expected, the viscosity of the DIL showed a marked
decrease with increasing temperature. The variation in the
viscosity with temperature for the IL studied here is illustrated in
Figure 2. Plot of density, ρ, against temperature, T, and fitted curves.
Table 2. Fitting Parameters and Standard Deviations for
Correlating Density and Refractive Index as a Function of
Temperature
a
physical properties
A0
A1
A2
SRD
ρ/(kg·m−3
nD
)
1820.99
1.6150
−0.6369
−0.0005
0.2179
0.0001
−7.8·10−4
8.03·10−7
a
Calculated with eq 7.
Figure 3. Plot of dynamic viscosity, η, against temperature, T, and curve
fitted with the VTF equation: , experimental data; , calculated eq 6.
coefficient is related to the temperature derivative of the density
○
through the following equation:18
⎛
⎞
⎟
⎛
⎞
⎟
∂ρ
1
1 ∂V
⎜
⎜
α =
= −
Figure 3. The viscosity data were fitted using an Arrhenius-like
law:
⎝
⎠
⎝
ρ ∂T
⎠
V ∂T
p
p
(2)
where α is the coefficient of thermal expansion, V is the molar
volume of the IL, ρ is the DIL density, and (∂ρ/∂T)p is the value
A1 + 2A2·T using A1 and A2 parameters from the fitting of eq 1.
The thermal expansion coefficient is shown in Table 1. It slightly
increases with temperature and varies between (5.4·10−4 and
6.2·10−4) K−1. The change in these values is smaller than the
estimated uncertainty, and it can thus be considered to be
independent of the temperature. Recently, Navia et al.19 have
presented experimental data on thermal expansion with the most
widely studied ILs. They compared the experimental data
obtained with calculated data from eq 2 and found deviations
within the accepted limits after taking into account the
uncertainty of the measured density data and the fitting equation.
To some extent, these results can corroborate that this method is
valid for the calculation of α of ILs. Moreover, the value obtained
here is within the common range for ILs reported in the
⎛
⎞
⎟
⎠
Ea
RT
⎜
η = η∞ exp −
⎝
(5)
where R is the universal gas constant and the characteristic
parameters are viscosity at infinite temperature (η∞) and
activation energy (Ea). However, for ILs, it has been
reported20,21,27 that a better description is provided by the
modified version of the Vogel−Fulcher−Tammann (VTF)
equation that characterizes glass-forming liquids:
⎛
⎜
⎝
⎞
⎟
⎠
B
η = AT0.5 exp
T − T
(6)
0
where A, B, and T0 are adjustable parameters. Table 3 lists the
parameters for both equations obtained using the standard
relative deviation (SRD):
C
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Table 3. Adjustable Parameters of the Arrhenius and VTF
Equations with Fit Deviation with Respect to Viscosity
A
B
K
T0
K
η∞
Ea
equation
Pa·s·K
Pa·s
kJ·mol−1
SRD
VTF
2.06·10−2
66
278
0.0359
0.8299
Arrhenius
3.61·10−1
0
−59.3
1/2
2
η
⎧
⎪
⎫
⎪
∞
⎛
⎜
⎝
⎞
⎟
⎠
zexp − zcal
⎨
⎪
⎩
⎬
⎪
⎭
SRD =
/(ndat − np)
∑
zcal
i
(7)
where zexp and zcal are the values of the experimental calculated
property, ndat is the number of experimental data points, and np is
the number of parameters.
It is clear that the VTF model provides a better fit, and the
correlated values are in good agreement with the experimental
data. This result was expected because the VTF model contains
three parameters compared to two for the Arrhenius equation.
The refractive index data nD were fitted using the following
equation:
Figure 4. Refractive index plotted against temperature and the fitted
curve: , experimental data; , calculated eq 8.
□
Table 4. Fitting Parameters and Standard Deviations for
Deriving Density Using Refractive Index Data
k
d
SD*
nD = A0 + A1T + A2T2
(8)
Eykman modified
Dale−Gladstone
Oster
0.0003
0.0003
0.0017
0.0004
0.0002
0.0013
0.0009
1.1900
0.7327
0.7550
0.9908
1.2136
2.0822
5.2597
9.3220
where T is the absolute temperature, and A0, A1, and A2 are
adjustable parameters, which are shown in Table 2 together with
the standard deviation. Variation as a function of temperature
and the fitting results are illustrated in Figure 4, and a decrease is
clearly observed when the temperature is increased.
The experimental refractive index was used to correlate the
experimental density using several empirical equations of the
following form:
Eykman
Lorentz−Lorenz
Newton
Arago−Biot
*
SD = (∑inDAT(zexp − zadjust)2/nDAT)0.5, where nDAT is the number of
the experimental data.
f(nD) = kρ
use only one. Among the one-parameter equations, the Dale−
Gladstone and Oster equations show the best results.
2
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎫
⎪
⎪
⎪
⎪
⎪
⎪
⎪
nD − 1
(Lorentz−Lorenz)
(Dale−Gladstone)
(Eykman)
2
Figure 2 shows the results of the density calculations from
refractive index data using the modified Eykman equation
together with correlated data using eq 2. As shown in this figure,
the density correlation with the refractive index data fits well with
the present measurements, as does eq 2. Furthermore, this
correlation for density calculation using the refractive index
measurements could be very helpful because the refractive index
measurements are much simpler than the density measurements.
The thermal stability of the IL is shown in Figure 5. At first
sight, the thermal stability seems to be above 650 K (dotted line);
nD
n − 1
D
2
n
− 1
D
nD + 0.4
⎪
⎪
⎪
⎪
2
2
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎬
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
=
(nD − 1)(2nD + 1)
(Oster)
2
nD
nD
(Arago−Biot)
2
nD
(Newton)
2
nD − 1
n + d
(Eykman modified)
⎪
⎩
⎪
⎭
D
where f(nD) is a function of refractive index, such as in the
Lorentz−Lorenz, Dale−Gladstone, Eykman, Oster, Arago−Biot
and Newton equations. In these equations, k is an empirical
constant that depends on the liquid and the wavelength at which
the refractive index is measured. In this study, the modified
Eykman equation proposed by Pineiro et al.28 is tested by taking
into account the optimization of the new parameter d.
The parameters k and d along with the standard deviation from
the mathematical fittings are shown in Table 4. Based on these
results, the modified Eykman equation shows the best fit with the
experimental data; this was expected because the modified
Eykman equation uses two parameters, while the other equations
Figure 5. TGA results for the IL.
D
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Journal of Chemical & Engineering Data
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however, a detailed observation reveals a decrease in the weight
at 558.1 K, which levels off approximately 615 K and then is
followed by an abrupt decrease above 650 K. These temperatures
are the start temperature, determined from the step tangent29
where the start point of decomposition can be stated at 558.1 K
(Tonset). Thermal decomposition is strongly dependent on the IL
structure, but it also depends on the crucible material and the
sample mass.30 In addition, the IL does not completely
evaporate, since an amount of charcoal in the crucible was
observed after the measurement, as has been observed in other
studies with ILs.30 The decomposition of imidazolium salts is
attributed to decomposition of the cation, which is facilitated by
the anion, while pyrolysis of imidazolium salts yielded volatile
degradation products such as 1-alkylimidazoles.31
in a temperature range of (298.15 to 343.15) K. In addition,
results from thermal stability analysis and DSC scanning were
presented. In general, all experimental physical properties
exhibited a decrease with an increase in temperature in the
range of temperature studied. Empirical equations were used to
fit the experimental data of the measured physical properties,
resulting in good agreement. Density data were used to derive the
thermal expansion coefficient, and the molecular volume and
standard entropy of the IL were calculated at T = 298.15 K.
Several empirical equations were used to fit the experimental
density data using the refractive index data. Good agreement was
found with the modified Eykman equation; this finding can be
useful for deriving approximate density data. Thermal stability
was established at 558.1 K. Based on the DSC scan, no melting
point was found, and the glass transition temperature was 230.5
K, with ΔCp = 311.8 J·mol−1·K−1.
The DSC thermogram obtained at a rate of 20 K·min−1 is
shown in Figure 6. The heat jump observed is the calorimetric
AUTHOR INFORMATION
Corresponding Author
*E-mail: tgraber@uantof.cl.
■
Funding
The authors acknowledge Conicyt-Chile for their support
provided through the project Fondecyt 1085059. M.C. also
thanks Conicyt-Chile for a doctoral fellowship.
Notes
The authors declare no competing financial interest.
REFERENCES
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CONCLUSIONS
A new functionalized dicationic IL was prepared, and the density,
dynamic viscosity, and refractive index of the IL were measured
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Journal of Chemical & Engineering Data
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