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The Association of Water in Ionic Liquids A Reliable Measure of Polarity.

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Angewandte
Chemie
Ionic Liquids
DOI: 10.1002/anie.200504471
The Association of Water in Ionic Liquids:
A Reliable Measure of Polarity**
Thorsten K
ddermann, Christiane Wertz,
Andreas Heintz, and Ralf Ludwig*
Ionic liquids (ILs) are liquids formed solely of ions. Unlike
inorganic molten salts such as NaCl, they are fluid at room
temperature, or at least within the typical temperature range
[*] T. Kddermann, R. Ludwig
Institut f#r Chemie
Abteilung Physikalische Chemie
Universit/t Rostock
Dr.-Lorenz-Weg 1, 18051 Rostock (Germany)
Fax: (+ 49) 381-498-6524
E-mail: ralf.ludwig@uni-rostock.de
C. Wertz, A. Heintz
Institut f#r Chemie
Abteilung Physikalische Chemie
Universit/t Rostock
Hermannstrasse 14, 18051 Rostock (Germany)
[**] This research was supported by the State of Mecklenburg-Western
Pomerania (Hochschulwissenschaftsprogramm) and the German
Science Foundation (DFG, FOR 436).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
of most organic chemical reactions.[1, 2] A number of properties make ionic liquids attractive in different areas of
chemistry. They exhibit an almost vanishing vapor pressure,
the characteristic electrical conductivity of an ionic conductor, and a considerable gap between the melting point and the
temperature of decomposition.[1, 3] The possibility of directing
the course of chemical reactions along with the low vapor
pressure make ILs an attractive alternative to traditional
organic solvents for both laboratory and industrial purposes.
Today ILs are regarded as “green solvents” and “designer
media” for chemical reactions.[4–6] To expand the utility of
ionic liquids, IL-based mixed solvents have come into focus.[7]
In particular the addition of water can strongly affect the
physical and chemical properties of ionic liquids such as
viscosity,[8, 9] electrical conductivity,[10] and reactivity,[11] as well
as solvation and solubility properties.[12–16] Water is also an
impurity commonly present in ILs since all known ILs are
hygroscopic; traces of water are thought to be ubiquitous in
these materials.
Interactions or molecular states of water dissolved in
various solvents and absorbed in many materials, such as
polymers, have been extensively studied using FTIR spectroscopy.[17] In particular, the vibrational modes of water that
result in bands in the OH-stretching region (3000–3800 cm 1)
are sensitive to the environment and intermolecular interactions.[18, 19] These modes have been used to understand the
type of bonding between water molecules in the liquid, solid,
and gas phases, as well as interactions between water and
many chemical substances, including ILs. Cammarata et al.[20]
studied interactions between ILs and water absorbed from the
atmosphere by attenuated total reflectance (ATR) infrared
spectroscopy. They concluded that anions are responsible for
the interaction of ILs and water. No change in band positions
and shapes was observed upon increasing water concentration. It was concluded that only one molecular state of water
was present. In addition, they suggested that in ionic liquids
containing [BF4] and [PF6] anions, water forms a symmetric
complex in which the two water protons are bonded to two
distinct anions. Tran et al.[21] reported differences in the nearinfrared (NIR) spectrum of water dissolved in different ionic
liquids, but their study focused on the quantitative determination of absorbed water in ionic liquids rather than on the
investigation of its molecular state.
Herein we describe our investigation of the association of
protonated and deuterated water molecules in two ILs
presumed to have completely different polarities. A combination of FTIR spectroscopy and density functional calculations (DFT) clearly shows that water molecules are mainly
H-bonded to the IL anions. The exact association of water
molecules to the anions is a reliable indicator for the
miscibility of water with ILs. Furthermore, we could demonstrate that the vibrational frequencies for single water
molecules associated in ILs can be used as a good measure
of the polarity of ILs. For this purpose we chose two
fundamentally different ILs, 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([C2MIM][NTf2]) and 1ethyl-3-methylimidazolium ethylsulfate ([C2MIM][EtSO4]).
Water is miscible with [C2MIM][EtSO4] over the whole
concentration range, whereas its solubility in [C2MIM][NTf2]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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is only 1.8 wt % (according to Ref. [22]) or 1.9 wt % (according to Ref. [14]). We recorded FTIR spectra for several water/
IL mixtures by using pure ILs as background. At water
concentrations of below 1 wt % the spectra did not show
further changes with concentration and temperature. Thus we
were confident that at these concentrations single water
molecules are embedded in the IL environment and that no
water clusters exist under these conditions. Water molecules
are isolated from each other and interact with the ions of the
ionic liquid by H-bonding.
Figure 1 a,b shows the FTIR spectra for H2O and D2O
dissolved in ILs in the OH/OD-stretching region, respectively.
The spectrum of water dissolved in [C2MIM][EtSO4] appears
Figure 2. FTIR spectrum of 1 wt % D2O in a) [C2MIM][NTf2] and in
b) [C2MIM][EtSO4] showing the OD stretching vibrational region. The
spectrum could be deconvoluted into five and six bands, respectively.
See text for the assignment.
Figure 1. Infrared spectra of a) the OH-stretching region of 1 wt %
H2O solutions and b) the OD-stretching region of 1 wt % D2O solutions in [C2MIM][NTf2] (bands B) and [C2MIM][EtSO4] (bands A). The
vibrational bands are red-shifted for the solutions in [C2MIM][EtSO4].
at lower wavenumbers than that of water in [C2MIM][NTf2].
Apparently water is more strongly bonded to [C2MIM][EtSO4], indicating that this IL has a higher polarity. Because
the two ILs include the same cation, this finding also suggests
that the association of water is mostly determined by the
anion. In the OH/OD-stretching region the two spectra have a
similar shape and can be deconvoluted into four/five and five/
six Voigt bands, respectively. As a result of the heavier isotope
the D2O spectra are better resolved. Thus the deconvoluted
D2O spectra are referred to in the following discussion
(Figure 2). The bands for D2O in [C2MIM][NTf2] are found at
2573, 2605, 2663, 2701, and 2727 cm 1 (Figure 2 a). The D2O
bands in [C2MIM][EtSO4] lie at 2374, 2508, 2545, 2590, 2630,
and 2688 cm 1 (Figure 2 b).
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The two most intensive bands are assigned to the
asymmetric and symmetric stretching vibrations, n3 and n1,
respectively, of the D2O molecules. The small central band
comes from OD-stretching vibrations of HDO, which is
formed by H/D isotope exchange with trace amounts of
residual water H2O. However, here we are interested in the
molecular state of water present in ILs. Therefore, this band
does not play a further role in our discussion. The two bands
left and right of the symmetric and asymmetric stretches are
labeled nb and nf. The band at higher wavenumbers indicates
that not all OD groups of single water molecules are involved
in D-bonding. We assign the bands at 2727 and 2688 cm 1 to
OD groups that are “quasi free” (nf). The other OD groups of
these water molecules are strongly D-bonded to an anion (nb).
The additional band in the FTIR spectrum of D2O in
[C2MIM][EtSO4] with the lowest wavenumber represents
the overtone of the water bending modes, 2d. This water
overtone is typically missing in apolar solvents[23, 24] and occurs
only in strongly confined geometries. The fact that it occurs in
[C2MIM][EtSO4] and is missing in [C2MIM][NTf2] is a first
hint of the significantly different polarities of the two ILs.
We assigned our experimental data by comparing the
observed with the calculated Dn shifts, defined as the differences of the stretching vibrations n3 and n1, and nb and nf. The
frequencies are presented in Table 1. In Figure 3 most
representative associates of water molecules with ions of the
two ILs are shown. In principle four structure types are
possible: In a DD (double donor) structure the water
molecule forms two H-bonds either to two anions (EtSO4 )
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
Angewandte
Chemie
Table 1: Measured and calculated vibrational modes for H2O and D2O associated in ionic liquids [C2MIM][NTf2] and [C2MIM][EtSO4].[a]
[C2MIM][NTf2]
H2O
D2O
[C2MIM][EtSO4]
H2O
D2O
[C2MIM][NTf2]
H2O
D2O
[C2MIM][EtSO4]
H2O
D2O
n3
DD
n1
nb
SD
nf
Dn
3638
2701
3561
2605
77
96
3505
2573
3689
2727
184
154
3540
2629
3468
2545
72
84
3419
2696
3617
2488
198
208
3663
2682
3572
2577
91
105
3470
2691
3668
2522
3587
2612
3498
2529
89
92
3417
2649
3633
2477
Dn
nb
AD
nf
Dn
198
169
3657
2384
3284
2664
373
280
216
172
3290
2387
3668
2674
378
287
[a] Frequencies n and frequency differences Dn for water in structures DD, SD, and AD are given in wavenumbers [cm 1]. Experimentally no large Dn
values assigned to AD structures could be found.
a)
D2O
H2O
0
DD
SD
100
200
AD
300
400
500
∆ν
b)
D2O
Figure 3. The experimentally possible molecular states of a water
molecule in [C2MIM][NTf2] (top) and in [C2MIM][EtSO4] (bottom)
obtained by DFT calculations. O red, H white, N blue, C black, S
yellow, F shaded yellow. See text for the assignment.
or to one anion (EtSO4 , NTf2 ) only. In a SD (single donor)
structure the water molecule forms one strong single H-bond
to an anion. In an AD (acceptor and donor) structure water is
H-bonded between the cation and the anion, and acts as both
donor and acceptor simultaneously.
For these structure types, characteristic Dn shifts for the
stretching frequencies of the water molecule are expected as
shown in Figure 4 for [C2MIM][NTf2] and [C2MIM][EtSO4].
For the DD structure Dn values between 70–80 cm 1 are
calculated. For water molecules in the SD structure, in which
one OH group is H-bonded to the anion and the other one is
“quasi free”, the value of Dn increases up to 200 cm 1. The
largest gap of about 378 cm 1 is obtained for the AD
structure, in which one OH group of the water molecule is a
proton donor to the anion and a proton acceptor from the IL
cation (“sandwich structure”). Because the Dn values differ
significantly for the given structure types, they can be used as
a good indication of the molecular state of water in ILs. By
Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
H2O
DD
0
100
SD
AD
200
300
400
500
∆ν
Figure 4. Calculated Dn values (a) for vibrational modes of H2O
and D2O dissolved in a) [C2MIM][NTf2] and in b) [C2MIM][EtSO4].
c: measured Dn values. The Dn values are characteristic for water
associated in the DD, SD, and AD structures. It can be clearly
concluded that water is associated in the DD and SD structures only.
comparing the calculated Dn value with the measured
frequency gap, we can conclude that AD structures do not
exist and that DD and SD structures are present. Apparently
the water molecules are either doubly H-bonded to the IL
anion (dominant) or linked to the anion with one OH group
and the other one “quasi free”. The calculated and measured
Dn values for D2O suggest the same structure types. It is
interesting to note that the Dn for the symmetric and
asymmetric stretches for the DD structure is increased by
isotopic substitution. The Dn values for all other species
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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become smaller on going from H2O to D2O because the two
vibrational bands are no longer coupled. Another interesting
finding is that Dn values between 70 and 80 cm 1 do not
necessarily suggest a symmetric 2:1 complex of water
(2 ILs@1 H2O) as usually assumed in literature.[20] Water
doubly H-bonded to the anion yields comparable Dn values.
We also show here that both the asymmetric stretch, n3,
and the symmetric stretch, n1, of water are sensitive probes for
the polarity of ionic liquids, as expressed by its dielectric
constant (e). Conventional methods for measuring static
dielectric constants of ILs fail, because the samples are largely
short-circuited as a result of the high electrical conductance.
For this reason alternative polarity probes have been
introduced based on solvatochromic shifts and fluorescence
probe dyes,[22, 25] partitioning of solutes in water/IL bilayers,[26]
gas chromatography with ILs as stationary phases,[3] and
solvent effects on chemical reactions.[27] However, the
deduced polarities of ILs differ substantially. Recently, the
first values for dielectric constants e were published by Wakai
et al.[28] They determined the static dielectric constants of five
ILs by zero-frequency extrapolation of the complex dielectric
functions measured by dielectric spectroscopy in the megahertz/gigahertz regime. However, we could not consider the
ILs used by Wakai et al.[28] in our present study since four out
of the five ILs are known to undergo substantial hydrolysis to
form HF upon contact with water.[29]
The vibrational frequencies of single embedded water
molecules prove to be a good measure of the polarity of ILs.
For this purpose the stretching frequencies of H2O and D2O
dissolved in trace amounts in simple organic solvents
(Table 2) are plotted versus known dielectric constants, e.
Figure 5 shows a linear relationship between e and the
stretching frequencies. If we now plug in our measured
frequencies of H2O and D2O dissolved in [C2MIM][NTf2] and
[C2MIM][EtSO4]), we can predict the dielectric constants of
the two ILs. The e values calculated as the arithmetic mean of
the two single values for e(n3) and e(n1) are given in Table 2.
The values are 15.0 and 14.7 for H2O and D2O in [C2MIM][NTf2] and 37.3 and 38.6 for H2O and D2O in [C2MIM][EtSO4], respectively. Whereas [C2MIM][EtSO4] has a dielectric constant close to that of dimethyl sulfoxide and thus
indicating strong polarity, [C2MIM][NTf2] has a much lower e
value close to that of 1,2-dichloroethane. The predicted
dielectric constants suggest that water is miscible with
[C2MIM][EtSO4] over the whole concentration range,
whereas it shows a large miscibility gap in [C2MIM][NTf2].[22]
We are confident that our method is a more reliable
measure of the dielectric constant and polarity of an IL than
the use of solvatochromatic shifts and fluorescence probe
dyes.[25a] The unusually large negative solvatochromism of a
standard betaine dye has been used to introduce an empirical
scale of solvent polarity, referred to as the ET(30) or ENT scale.
For [C2MIM][NTf2], ET(30) values of about 52.6, 53.1, 47.6[25a]
as well as estimated values of about 46.5, 47.7 and 48.5 have
been reported.[25g] If one assumes a linear relationship
between ET(30) and e for simple organic solvents, dielectric
constants between 27 and 44 are obtained for this IL (see
Figure S1 in the Supporting Information). ILs having such
high dielectric constants would be definitely miscible with
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Table 2: Wavenumbers [cm 1] of the vibrational modes n3 and n1 of H2O
and D2O in the gas phase, dissolved in organic solvents,[17c,f, 19, 20, 24] and in
the ionic liquids [C2MIM][NTf2] and [C2MIM][EtSO4].[a]
H2O in solvent
n3
n1
e
gas phase, no anion
CCl4
benzene
1,2-dichloroethane
benzaldehyde
acetophenone
acetone
DMSO
3756
3708
3687
3671
3639
3612
3605
3504
3657
3616
3598
3592
3544
3535
3533
3429
2.2
2.3
10.65
17.0
18.3
20.7
46.7
y0
m
R
3707.97
4.43865
0.987174
3619.32
4.09055
0.988589
[C2MIM][NTf2]
[C2MIM][EtSO4]
3635
3540
3562
3469
D2O in solvent
n3
n1
e
gas phase, no anion
CCl4
benzene
acetone
DMSO
2788
2752
2734
2681
2608
2671
2643
2630
2580
2525
2.2
2.3
20.7
46.7
y0
m
R
2749.19
3.05007
0.993054
2640.4
2.52958
0.992204
[C2MIM][NTf2]
[C2MIM][EtSO4]
3635
3540
3562
3469
e(n3)[b]
e(n1)[b]
15.76
37.84
14.01
36.75
e(n3)[b]
e(n1)[b]
15.47
39.40
14.0
37.72
[a] In addition, the dielectric constants e for organic solvents from
experiments and the predicted e(n3) and e(n1) values for both ILs are
given. [b] The predicted values are obtained by using the linear
dependence of the vibrational modes of water and the dielectric constant
of the organic solvents. Values of the intercepts y0, slopes m, and
correlation coefficients R are given in the table.
water, in contradiction to experimental results. Additionally,
the obtained e values span a wide range which is in the order
of the dielectric constant itself, indicating that predictions
using this method are not really reliable.
If we apply our procedure to the IL [C4MIM][BF4] for
which n1 and n3 of dissolved water[20] are known, we predict a
dielectric constant of 14.9, which is close the e value of 11.7 0.7 determined by microwave dielectric spectroscopy.[28]
Again, ET(30) values[25a,g] for this IL give much too large
dielectric constants (e = 28–48). Moreover, depending on the
method and the probe molecule used, a wide range of values
results (see the Supporting Information).
Our proposed method may be limited by the assumption
that ILs are homogeneous fluids. In fact there is evidence that
imidazolium-based ILs have polar and non-polar regions, in
particular upon addition of controlled amounts of water.[30]
ILs may be regarded as nanostructured materials.[4, 31] This
must be checked by measuring more ionic liquids with
different kinds of cations and anions.
With a combination of FTIR and ab initio calculations we
have studied the structure of water molecules strongly
confined by hydrogen bonding in two ILs. We find that two
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Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
Angewandte
Chemie
used for the variable-temperature experiments. For each spectrum
128 scans were recorded at a spectral resolution of 1 cm 1. Solvent
subtraction (ILs) was carried out by using reference spectra obtained
at exactly the same temperatures as the sample spectra.
Density functional calculations were carried out using the
Gaussian 98 program.[33] All clusters were calculated with the
B3LYP nonlocal exchange correlation functional using a 6-31 + G*
basis set. The calculated harmonic vibrational frequencies were scaled
(0.96). Only the OH/OD-stretching vibrations were discussed.
Received: December 16, 2005
Revised: February 28, 2006
Published online: April 25, 2006
.
Keywords: density functional calculations · ionic liquids ·
IR spectroscopy · polarity · water
Figure 5. Predicted dielectric constants e for [C2MIM][NTf2] (^) and
[C2MIM][EtSO4] (&). The symmetric and asymmetric stretch frequencies of a) H2O and b) D2O dissolved in simple organic liquids are
plotted versus dielectric constants e of the solvents.[17a,d,f, 23, 24] The
linear relationships between both properties allows a prediction of e by
using the measured n3 and n1 values. The predicted e value can be
read off from the x-axis (a).
water configurations exist. In the DD structure both OH/OD
groups are bonded to the IL anion and in an SD structure only
one OH/OD group is strongly H-bonded while the other one
is “quasi free”. We could show that the vibrational modes of
single water molecules dissolved in ILs are sensitive probes
for their polarity. This is particularly important because
conventional methods to measure dielectric constants in ILs
fail and alternative polarity probes based on solvatochromatic
shifts and fluorescence probe dyes are not accurate. Our
method based on standard FTIR spectroscopy is generally
accessible and allows a straightforward determination of IL
polarities.
Experimental Section
Mixtures of 0.4 wt % and 1.0 wt % of H2O and D2O (Sigma Aldrich,
with a stated purity of 100 atom % D) and [C2MIM][NTf2] and
[C2MIM][EtSO4]) were prepared. [C2MIM][NTf2] was synthesized by
the group of P. Wasserscheid at the University of Erlangen. The
halogen-free method of preparation for [C2MIM][NTf2] has been
described in detail.[32] [C2MIM][EtSO4] (ECOENGTM212), provided
by Solvent Innovation, was used without further purification. The
purity of the sample was specified as 98 %. For all ionic liquids the
H2O content was determined by Karl Fischer titration.
Infrared measurements were performed with a Bruker Vector 22
FTIR spectrometer. An L.O.T.-Oriel variable-temperature cell
equipped with CaF2 windows having a path length of 0.025 mm was
Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
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Angew. Chem. Int. Ed. 2006, 45, 3697 –3702
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