Eutectic ionic liquid mixtures and their effect on CO2 solubility and conductivity

Article abstract of DOI:10.1039/c5ra06561e

A simple binary system of compounds resembling short-chain versions of popular ionic liquids has been shown to have surprisingly complex properties. Combining methylated versions of pyridinium and pyrrolidinium bis[(trifluoromethyl)sulfonyl]imide gave desirable properties such as low viscosity and high conductivity solubility per unit volume. The binary combinations studied in this study showed that these materials were stable liquids at 50 °C and had a threefold improvement in conductivity over [C₆C₁im][Tf₂N]. Despite the high densities of these materials, 2D-IR studies indicate increased ion mobility, likely due to the lack of hindering alkyl chains.

Full text of DOI:10.1039/c5ra06561e

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Brinzer, E. Roth, V. A. Kusuma, J. D. Watkins, X. Zhou, D. R. Luebke, D. Hopkinson, N. Washburn, S.
Garrett-Roe and H. Nulwala, RSC Adv., 2015, DOI: 10.1039/C5RA06561E.
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ARTICLE
Eutectic Ionic Liquid mixtures and its effect on CO2 solubility and
conductivity
Anna S. Ivanova,a Thomas Brinzer,b Elliot A. Roth,d Victor A. Kusuma,d John D. Watkins,d Xu Zhou
d,c David Luebke,d David Hopkinson,d Newell R. Washburn,a Sean Garrett-Roe,b† Hunaid B.
Nulwalaa,d†
-Received 00th January 20xx,
Accepted 00th January 20xx
,
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple binary system of compounds resembling short-chain versions of popular ionic liquids has been shown to have
surprisingly complex properties. Combining methylated versions of pyridinium and pyrrolidinium
bis[(trifluoromethyl)sulfonyl]imide accesses desirable properties such as low viscosity, high conductivity, and high CO2
solubility per unit volume was acheived. The binary combinations studied in this study showed that these materials were
stable liquids at 50°C and had is a threefold improvement in conductivty over [C6C1im][Tf2N]. Despite their high densities,
2D-IR studies indicate increased ion mobility, likely due to the lack of hindering alkyl chains.
mixing of ILs can lead to improve liquid range and compound
various properties of ILs. Ionic materials having melting points
over 100°C have not been extensively studied and there
Introduction
Ionic liquids (ILs) have a potential advantage over existing
solvents as a green medium due to their wide liquid range,
outstanding solvation potential, negligible vapour pressure,
thermal stability, and recyclability.1–3 These properties have
prompted their use as lubricants, electrolytes for energy
storage, CO2 capture media, coatings, and fuel cells.4–7
However, the optimization of IL properties for a given
application is a challenge. Through skilful manipulation of IL
structures, one can target specific properties such as viscosity,
solubility, conductivity, melting point, density, refractive ind6,8–
remain an opportunity to further extend the useful
properties.23 The exception is work by maximo et al.23 who has
studied the melting behaviour of ionic solids mixtures.
Ionic solids are crystalline material with higher density. Like
normal salts, these organic solids show eutectic behaviour
upon mixing. In our work various compositions of mixtures
combining 1,1-dimethylpyrrolidinium and 1-methylpyridinium
and bis[(trifluoromethyl)sulfonyl]imide (Both are solids and
crystalline
materials
at
room
temperature)
12
([C1C1pyrr]x[C1pyr](1-x)[Tf2N]) (The subscripts show the mole
fraction) (Figure 1) were studied. This combination is unusual
because it lacks the bulky alkyl chain normally used to disrupt
crystal lattice packing and lower the melting points of
traditional ILs. Also the anion is not symmetrical unlike the
study done previously. The pristine ionic salts are crystalline
solids with sharp melting transitions.
principle, one can design novel ILs with the exact properties
needed, but in practice, this is a daunting goal to achieve.
ILs are complex solvents, whose properties ultimately depend
on multiple intermolecular forces, including hydrogen bonding,
ionic/charge–charge, dipolar, π–π, n–π, and van der Waals
interactions. These interactions are large and quite complex
when compared to simple solvents. Thus, it is difficult to tune
the properties of ILs without extensive synthetic manipulations
and careful characterization. One simple, alternate route to
access specific properties is by creating mixtures of ILs.13,14 The
properties of IL mixtures have been shown to be distinct from
those of the parent compounds and researchers have started
to evaluate the mixture of binary and ternary mixture.15–24 The
Typically, ILs rely on molecular asymmetry to disrupt the
crystalline ordering of the ions and widen the liquid range.25
With symmetric cations, this effect must be achieved by other
means. By combining two cations with different electronics or
crystal phase, we aimed to introduce defects into the
molecular ordering and induce a eutectic effect, resulting in a
lower melting point.14 Additionally, the shortened alkyl chains
should eliminate the problem of tail group domains causing
spatial heterogeneity,26 potentially increasing the rate of
diffusion through the IL and, thus, improving the kinetics of gas
adsorption.27 Though gas solubility is known to generally
increase with the size and flexibility of ionic structures, for CO2
the dominant effect is gas–anion interaction.28 Finally, the
decreased cation size should increase the charge density of the
mixture, possibly improving conductivity. Thus, the
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combination [C1C1pyrr]x[C1pyr](1-x)[Tf2N] should result in a low- batch of precipitate was collected from the filtrate and
melting, conductive mixture with good gas solubility, making it combined with the first. This solid was then dried in vacuo,
widely applicable.
yielding a white powder with a yellow tinge (20.73 g, 91.3
1
mmol, 91.5%). H NMR (300 MHz, (CD3)2SO): δ 3.47 (m, 4H),
3.10 (s, 6H), 2.10 (m, 4H).
[C1pyr][Tf2N]
A solution of lithium bis[(trifluoromethyl)sulfonyl]imide (31.06
g, 108 mmol) in water (70 mL) was added to a solution of 1,1-
methylpyrrolidinium iodide (22.33 g, 98.3 mmol) in water (20
mL) at room temperature in air. The resulting mixture of
colorless liquid and white precipitate was stirred for 2 hours,
then the solid product was collected on a fine sintered glass
frit and rinsed three times with water. This white powder was
then dissolved in dichloromethane and washed with an
equivalent amount of water, shaking gently to avoid emulsion.
The organic layer was collected and dried with MgSO4, then
solvent was removed by rotary evaporation, yielding a white
powder (24.85 g, 65.3 mmol, 66.5%).
Figure 1. Components of synthesized compounds: a) 1-methylpyridinium cation
cation
[C1pyr];
bis((trifluoromethyl)sulfonyl)imide anion [Tf2N].
b)
1,1-dimethylpyrrolidinium
[C1C1pyrr];
c)
Experimental
Synthetic Details
Note: All compounds were tested with Aq silver nitrate (1 M)
solution. No visible silver halide formation was observed.
Experimental details are provided in the supporting material.
[C1C1pyrr][I]
Pyridine (8.2 mL, 101 mmol) was stirred into acetonitrile (50
mL) at room temperature in air. The solution was cooled in an
ice/water bath while slowly adding methyl iodide (7.6 mL, 121
mmol). The resulting clear yellow solution was allowed to
warm to room temperature and stirred for 7.5 hours. Diethyl
ether (~40 mL) was added, resulting in precipitation of a white
powder. This product was collected on a fine sintered glass frit
and rinsed three times with diethyl ether. A second batch of
precipitate was collected from the filtrate and combined with
the first. This solid was then dried in vacuo, yielding a white
powder with a faint yellow tinge as a crude product (22.16 g,
100 mmol, 99.3%). 1H NMR (300 MHz, (CD3)2SO): δ 9.00 (d,
2H), 8.59 (t, 1H), 8.14 (t, 2H), 4.36 (s, 3H).
Results and Discussion
Melting behaviour
Melting point and thermal stability were used as an initial
screening methodology. Thermal stability was determined by
using thermogravimetric analysis (TGA) to identify the onset
temperature of decomposition for each sample. In all cases,
the onset point was above 415°C with a ramp rate of 10°C/min
representing
a
~65°C improvement over typical IL
decomposition temperatures. This improvement is due to the
removal of alky chain length which would decompose first
compared to the rest of the molecule mainly due to the
presence of alpha and beta protons. The absence of the alkyl
chain lengths would also result in higher ionicity and
conductivity thus improving the overall stability. The melting
points were obtained by differential scanning calorimetry
(DSC) using a 10°C/min ramp rate and were plotted to observe
the effect of sample composition on melting temperature
(Figure 2). We also performed modulated DSC experiments on
3 samples found no deviation from normal DSC in this case. In
cases where multiple peaks were observed, the highest-
temperature peak was assumed to be the melting point,
ensuring a fully liquid sample. Nevertheless, the multiphasic
behaviour of such samples indicates potential plastic29 or
liquid crystal30 properties and is worth further exploration for
electrolytic applications. The lowest melting point of the
combinations is observed for [C1C1pyrr]0.3[C1pyr]0.7[Tf2N],
which exhibits a shallow eutectic point of 33.0°C. In total, four
samples ranging from 0–30% [C1C1pyrr][Tf2N] were completely
liquid below 50 °C. We focused on analysing the CO2 capture
properties of these low-melting candidates.
[
C1C1pyrr][Tf2N]
A solution of lithium bis[(trifluoromethyl)sulfonyl]imide (31.79
g, 111 mmol) in water (50 mL) was added to a solution of 1-
methylpyridinium iodide (22.16 g, 100 mmol) in water (35 mL)
at room temperature in air. The resulting mixture of colorless
liquid and white precipitate was stirred for 2 hours, then the
solid product was collected on a fine sintered glass frit and
rinsed three times with water. This white powder was then
dissolved in ethyl acetate (~50 mL) and washed with an
equivalent amount of water, shaking gently to avoid emulsion.
The organic layer was collected and dried with MgSO4, then
solvent was removed by rotary evaporation, yielding a white
powder (24.41 g, 65.2 mmol, 65.0%). 1H NMR (300 MHz,
(CD3)2SO): δ 8.98(d, 2H), 8.57 (t, 1H), 8.13 (t, 2H), 4.35 (s, 3H).
13C NMR (300 MHz, (CD3)2SO): δ 146.0, 145.5, 128.1, 119.9 (q),
48.4. 19F NMR (300 MHz, (CD3)2SO): δ 78.7.
[C1pyr][I]
1-methylpyrrolidine (10.4 mL, 99.8 mmol) was stirred into
acetonitrile (50 mL) at room temperature in air. The solution
was cooled in an ice/water bath while slowly adding methyl
iodide (7.5 mL, 120 mmol). The resulting cloudy white mixture
was allowed to warm to room temperature and stirred for 7.5
hours. Diethyl ether (~50 mL) was added to complete
precipitation. This product was collected on a fine sintered
glass frit and rinsed three times with diethyl ether. A second
Interestingly, the pyridinium parent compound and several of
the binary combinations exhibit a strong supercooling effect,
with crystallization occurring as much as 60 °C below the
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melting temperature. These samples were meta-stable liquids [C1C1pyrr]0.2[C1pyr]0.8[Tf2N] showed comparable absorbance of
at room temperature unless crystallization was seeded or CO2 to [C6C1im][Tf2N] at low pressures and up to 8% higher at
initiated through mechanical forces, at which point rapid elevated pressures (Figure 3) and makes it more applicable for
crystallization was observed. Such behaviour suggests
a
warm gas purification. It is suspected that not having long alkyl
kinetically hindered nucleation step in the pyridinium chain increases the overall ionic nature of the system
compound, possibly exacerbated in the mixtures due to the improving overall CO2 solubility per unit volume of liquid. Such
complex stacking interactions of multiple cations.
increased CO2 solubility is promising for future trials with
mixed systems of truncated cations.
Heat capacity
Viscosity
Low viscosity of the IL is responsible for enhancing kinetics in
gas capture and reaction media. Viscosities were measured at For applications involving heating the ionic liquid, heat
50°C for the low-melting samples (Table 1). 1-hexyl-3- capacity determines how much energy will be required. Heat
methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, or capacity for these samples was measured by DSC at three
[C6C1im][Tf2N] is used as a baseline standard for efficacy of different temperatures (Figure 4). The heat capacities range
other ILs20,31,32 and has been selected as a reference ionic from 430 to 621 J·mol-1·K-1, with some fluctuation in the
liquid for an IUPAC experimental validation project. It has a
1.20E-03
1.00E-03
[C₆C₁im][Tf₂N]
8.00E-04
[C₁pyr][Tf₂N]
[C₁C₁pyrr]₀.₂[C₁pyr]₀.₈[Tf₂N]
6.00E-04
4.00E-04
2.00E-04
Figure 2. a) DSC heating curves of [C1C1pyrr]0.3[C1pyr]0.7[Tf2N] (solid),
[C1C1pyrr]0.5[C1pyr]0.5[Tf2N] (short dashes), and [C1C1pyrr]0.7[C1pyr]0.3[Tf2N] (long
dashes) b) Melting points of binary combinations of ILs measured by DSC with a
10°C heating rate.
0.00E+00
1
2
4
6
8
10
12
13
P / bar
melting point of -7°C.18 At 50°C, the viscosity of [C6C1im][Tf2N]
is 26 mPa·s. Which is higher than most of our low-melting
candidates at 50°C. Though the melting temperature steadily
decreases from 0–30% [C1C1pyrr][Tf2N], the viscosity increases
along the series. This unexpected trend suggests that the
individual components are more important in determining
viscosity than proximity of measuring temperature to melting
point.
Figure 3. CO2 sorption of [C6C1im][Tf2N]34, [C1pyr][Tf2N], and the best-absorbing
binary mixture measured at 50°C and various pressures, interpolated from
experimental data. Sorption is shown as moles CO2 absorbed by a given starting
volume of ionic liquid.
CO2 uptake and density
The densities of the samples at 50°C are all significantly above
those of [C6C1im][Tf2N], as expected for a system involving
small, symmetrical cations and the lack of alkyl chains
(Table 1). A slight decrease in density as the fraction of
[C1C1pyrr][Tf2N] increases was noted, which can be explained
by the increased proportion of pyrrolidinium cations
interfering with orderly stacking.
Intuitively, high density should cause less free volume and
reduce CO2 uptake.33 We find, however, that the increase in
ionic concentration improves CO2 solubility of the overall
system per unit volume. The CO2 solubility of the IL mixtures
was evaluated by equilibrating each liquid under varying
pressures of CO2 gas at 50°C and measuring the amount
absorbed. These results were then fitted to a polynomial
curve, and interpolated to account for variance in pressure
during measurement. The results indicate that on the practical
basis of moles of CO2 per volume of IL mixture, the
Figure 4. Heat capacities of all samples measured at 25, 50, and 75°C. Data for
[C6C1im][Tf2N] at 25 and 50°C is shown for comparison.18 Error bars are omitted
for clarity (Error <1%).
general upwards trend with increasing [C1C1pyrr][Tf2N]
content. Overall, the heat capacities are lower than that of
[C6C1im][Tf2N]. The lower heat capacity compared to
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[C6C1im][Tf2N] is most likely due to the fewer intramolecular intermolecular interactions such as electrostatics, charge
degrees of freedom in the smaller ionic mixtures.
transfer, and hydrogen bonding.
Conductivity
We used the CO2 asymmetric stretch as a probe for 2D-IR
spectroscopy to investigate the differences in the local
The electrical conductivity of IL compounds determines their
usefulness in electrochemical applications. The three binary
combinations (10–30% [C1C1pyrr][Tf2N]) that were stable
liquids at 50°C were measured for conductivity at this
temperature. The results ranged from 11.2 to 12.2 mS·cm-1,
environment
experienced
by
CO2
in
both
[C1C1pyrr]0.3[C1pyr]0.7[Tf2N] and [C6C1im][Tf2N] (Figure 5). While
this experiment has direct motivation from the potential use of
these novel ionic liquids for carbon capture, this molecular
probe also allowed us to investigate the local motions of the
cation and anion, which are important for understanding the
differences in conductivity between the ILs, and which CO2
does not greatly disrupt.The microscopic dynamics in the two
ILs are markedly different. Spectra of the CO2 asymmetric
stretch in both ILs start elongated in the diagonal direction
which is
a threefold improvement over [C6C1im][Tf2N]’s
conductivity of 4.1 mS·cm-1 (Table 1). This could be partially
due to the increased charge per unit volume that results from
smaller cations, but the dramatic boost suggests other
mechanisms are at work as well. The lack of alkyl chains on the
cations could reduce aggregation, possibly improving
conductivity due to increased mobility. An alternate
explanation for the increase in conductivity may be the
inability of this system to form any ionic liquid aggregates and
nano-domains25,35 due to the lack of a hydrocarbon chain.
Their high conductivity makes these binary mixtures potential
candidates for electrochemical applications.
(Figure 5A,D), which indicates
a distribution of local
environments. At 5 ps, the 2D-IR spectrum of the
[C1C1pyrr]0.3[C1pyr]0.7[Tf2N] has become round, indicating that
the environment around the CO2 molecule has randomized
(Figure 5B); whereas, the 2D-IR spectrum of [C6C1im][Tf2N]
retains some diagonal character (Figure 5E), indicating that the
environment around the CO2 molecules remains similar. In
[C6C1im][Tf2N], the spectrum becomes round on a 50 ps time
scale (Figure 5F).
Two properties of these novel ionic liquid mixtures are
particularly interesting. First, the conductivity is nearly three
times that of [C6C1im][Tf2N] even though the dynamic viscosity
is the same within 5%. Second, the solubility of CO2 is This qualitative picture can be quantified by nonlinear least
comparable between the mixtures and [C6C1im][Tf2N] with
15% higher density.
squares fitting of the spectra to third-order response
functions.36 The three important parameters from the analysis
are the dephasing time (T2), which controls the antidiagonal
width of the spectra; the inhomogeneous width (Δ) which
controls the diagonal width of the spectra; and the spectral
diffusion time (τc), which controls the rate of change of shape
of the spectra. T2 is related to fast local motions such as
hindered rotations (librations), Δ is related to the distribution
of different environments, and τc is the time scale for
Table 1. Data summary for eutectic IL samples compared to [C6C1im][Tf2N]
% [C1C1pyrr][Tf2N]
[C6C1im]
[Tf2N]
[C1pyr]
[Tf2N]
10
20
34
30
33
Tm, °C
-7[9]
45
39
24.
7
η, mPa[a]
26[9]
23.9
25.1 26.2
relaxation
of
those
initial
environments.
In
1.35[11]
46.8
4.07
1.58
66.5
-
1.57 1.55 1.54
52.3 47.6 51.7
11.2 12.2 11.9
454 462 463
ρ, g⋅cm-3[a]
kH (atm)[a]
κ (mS⋅cm-1)[a]
[C1C1pyrr]0.3[C1pyr]0.7[Tf2N], CO2 shows a marked decrease in
τc, and an increase in T2 compared with that seen in
(A)
(B)
t=5 ps
(C)
t=20 ps
t=0.2 ps
Cp (J⋅mol-1⋅K-1)[a] 60418
434
2345
2340
2335
[a] Measured for liquid at 50°C and ~1 atm
Molecular insights via 2D-IR spectroscopy
To gain a better understanding of the microscopic causes of
these macroscopic observations, we used 2D-IR spectroscopy
to probe the picosecond dynamics of the mixtures compared
ω1 (cm-1)
(E)
with [C6C1im][Tf2N]. 2D-IR spectroscopy is
a coherent
(D)
t=0.2 ps
(F)
t=20 ps
vibrational spectroscopy, which is analogous to 2D NMR
spectroscopy. Where 2D NMR correlates nuclear spins, 2D-IR
correlates molecular vibrational frequencies. The 2D-IR
lineshape encodes the rate at which the local environment
changes around a molecule.
t=5 ps
2350
2345
2340
Not all molecular vibrations are sufficiently intense to observe
in a 2D-IR spectrum; CO2, however, is an ideal vibrational
chromophore for 2D-IR in ionic liquids.27 The vibrational
spectrum of the CO2 asymmetric stretch depends on local
ω1 (cm-1)
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Figure 5. 2D-IR spectra of CO2 antisymmetric stretch in [C6C1im][Tf2N] and in
to depress the melting point to below 35°C. The melting point
and CO2 solubility can likely be improved further with
experimentation. This approach shows promise as a general
strategy to inexpensively optimize the properties of ionic
liquids by the mixing of simple salts, rather than complicated
synthesis of new ILs.
[C1C1pyrr]0.3[C1pyr]0.7[Tf2N].
[C6C1im][Tf2N]. The difference in τc we attribute to differences
in the ease of rotations of the ions of CO2’s first solvent shell
(see Discussion). The increased dephasing time, T2, likely
results from a narrower distribution of CO2 frequencies during
homogeneous motions, perhaps due to the loss of largely
uncharged hexyl chains in the first solvation shell. Δ is similar
for the two ILs (Table 2).
Acknowledgements
This research was supported by the U.S. Department of
Energy’s National Energy Technology Laboratory under the
contract DE-FE0004000. Part of this work was also supported
by ACS PRF Award #53936-DNI6.
Table 2. Comparison of best fit parameters for solvation dynamics of CO2 in
[C6C1Im][Tf2N] and [C1C1pyrr]0.3[C1pyr]0.7[Tf2N], from 2D-IR spectral fitting.
Sample
Δ (cm-1)
τc (ps)
47±9
6±2
T2 (ps)
2.45±0.03
3.0±0.2
[C6C1im][Tf2N]
1.4±0.1
Notes and references
[C1C1pyrr]0.3[C1pyr]0.7 1.5±0.1
1
2
3
4
5
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This spectroscopic finding also provides insight into a possible
molecular mechanism for the increased conductivity seen in
[C1pyr]0.7[C1C1pyrr]0.3[Tf2N] compared to [C6C1im][Tf2N]. Under
an external potential, the decreased energy barrier to solvent
reorientation, described above, should allow easier molecular
reorientation to permit ion transport, and thus conduction.
This effect, combined with decreased intermolecular friction
due to the loss of bulky alkyl chains, and increased charge
density (due to decreased IL molar volume), provide
a
plausible explanation for the increased conductivity. It may be
possible to exploit combinations of these mechanisms to guide
the design of ionic liquids with high conductivity for
electrochemical applications.
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Conclusions
By mixing two dissimilar cations with shortened alkyl chains,
useful ionic liquids properties can be accessed, and tuned.
Mixing of the two compounds increases the mobility of the
cation, which leads to greatly increased conductivity, which is
promising for electrochemical applications. The removal of the
hexyl chain also decreases the heat capacity of the IL, which
can be important for variety of thermal applications. Despite
increased density, CO2 solubility remains comparable to that of
[CC1im][Tf2N], and the mixture of two different cations is able
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