Influence of different branched alkyl side chains on the properties of imidazolium-based ionic liquids

Article abstract of DOI:10.1039/b807119e

Several new branched ionic liquids were synthesized under microwave irradiation applying two different synthetic approaches. Different already known ionic liquids, both linear and branched, were added to this set of new ionic liquids to investigate the influence of the branching on the thermophysical properties to elucidate first structure-property relationships. Thermogravimetric analysis was utilized to investigate the decomposition behavior and differential scanning calorimetry was used to study the influence of the branching on the thermal behavior, e.g. the melting point, the glass transition temperature, the freezing point and the cold crystallization temperature. Moreover, the water uptake of selected ionic liquids was analyzed. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b807119e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Influence of different branched alkyl side chains on the properties
of imidazolium-based ionic liquids†
abc
Tina Erdmenger,ab Ju¨rgen Vitz,ab Frank Wiesbrock‡,ab and Ulrich S. Schubert
*
Received 28th April 2008, Accepted 7th August 2008
First published as an Advance Article on the web 3rd October 2008
DOI: 10.1039/b807119e
Several new branched ionic liquids were synthesized under microwave irradiation applying two
different synthetic approaches. Different already known ionic liquids, both linear and branched, were
added to this set of new ionic liquids to investigate the influence of the branching on the thermophysical
properties to elucidate first structure–property relationships. Thermogravimetric analysis was utilized
to investigate the decomposition behavior and differential scanning calorimetry was used to study the
influence of the branching on the thermal behavior, e.g. the melting point, the glass transition
temperature, the freezing point and the cold crystallization temperature. Moreover, the water uptake
of selected ionic liquids was analyzed.
viscosity, and miscibility with other substances can be adjusted
Introduction
by selecting specific anions and, or cations.1,22 The anion mainly
controls their water miscibility, while the cation only has a small
influence on this property.3 Furthermore, the length of the alkyl
chains in the cations also influences these properties.6
Ionic liquids have been proposed as ‘green’ solvents due to their
negligible volatility and flammability. Regarding ecological
concerns, ionic liquids seem to be an attractive alternative to
replace conventional volatile organic solvents.1,2 In addition,
ionic liquids have interesting solvation properties and possess
a high compatibility with various organic compounds and
other materials.3,4 They can be recycled and reused after
accomplishing specific tasks due to their immiscibility with
a range of solvents.1,2,5
In this work, we were interested in the effect of branched alkyl
side chains of the cations on the properties of ionic liquids. Better
understanding of the structure–property relationships is a crucial
aspect for designing ionic liquids with tailor-made properties for
a specific application. It is already known that the length and the
degree of branching have an influence on the chemical and
physical properties of alkylated compounds, such as heat of
combustion, boiling point and melting point.23 Moreover, it has
already been reported that compact, highly branched chains
decrease the viscosity when introduced into ionic liquids, which
might improve e.g. extraction processes.24
Due to the large number of possible combinations of cations
and anions, and their dramatic influence on the properties, ionic
liquids are called ‘designer solvents’. In this regard, it is expected
that about a trillion different ionic liquids could exist.6,7 In recent
years, the fields of applications for ionic liquids has been signif-
icantly extended.8–10 Ionic liquids are already utilized in several
chemical reactions as catalysts,11,12 as reagents13 or as
solvents,7,12,14,15 for example in Friedel–Crafts reactions,11,14
polymerizations16,17 and ligand exchange reactions.11 Further
applications of ionic liquids can be found in separation processes
such as liquid–liquid extractions and chromatographic separa-
tions.1,18 Ionic liquids can also be applied as an electrolyte
material in electrochemical and in heterogeneous catalytic
processes,19,20 and it has been reported that ionic liquids can
improve the selectivity in certain chemical reactions.5,21
In order to investigate the effect of branched alkyl chains
on the properties of ionic liquids, new ionic liquids based on
1-(1-ethylpropyl)-3-methylimidazolium and 1-(1-methylbenzyl)-
3-methylimidazolium cations were synthesized. For a systematic
investigation, a series of known ionic liquids, both branched and
linear, were also synthesized, in order to investigate the influence
of branching on the properties of the ionic liquids in more detail.
In particular, the thermophysical properties, such as decompo-
sition and glass transition temperatures, and selected solution
properties, such as the water uptake, are discussed as an attempt
to elucidate structure–property relationships. Furthermore,
some of the described branched ionic liquids contain a chiral
center, and could be used as chiral ligands in asymmetric
reactions or as stationary phases for chromatography applica-
tions, as addressed elsewhere.25
Ionic liquids are composed of an organic cation and an inor-
ganic anion, and their properties, such as melting point, density,
aLaboratory of Macromolecular Chemistry and Nanoscience, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands. E-mail: u.s.schubert@tue.nl; Tel: +31 40 2474186
bDutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The
Netherlands
Results and discussion
cLaboratory of Organic and Macromolecular Chemistry, Friedrich-
Schiller-University Jena, Humboldtstr. 10, D-07743 Jena, Germany
† Electronic supplementary information (ESI) available: Synthesis, anion
exchange, IR, NMR and MALDI-TOF-MS data. See DOI:
10.1039/b807119e
Synthesis of branched ionic liquids
Ionic liquids are in general synthesized by quarternization of
amines, imidazoles, pyridines or phosphines through an alkyl-
ation reaction, as displayed in Scheme 1. The synthesis of ionic
‡ Current address: ICTM – Institute for Chemistry and Technology of
Materials, TU Graz, Stremayrgasse 16, AT-8010 Graz, Austria
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Scheme 1 Schematic representation of the general synthesis of 1-alkyl-3-
methylimidazolium-based ionic liquids.
Scheme 3 Schematic representation of an alternative synthesis route for
the synthesis of 1-(1-ethylpropyl)-3-methylimidazolium iodide.
liquids using ‘conventional’ thermal heating can be a time-
consuming process (typically 24 h under reflux conditions) but it
can be accelerated significantly by using microwave irradiation,
as reported previously.2,14,26 This acceleration is a consequence of
elevated reaction temperatures (above the boiling point of the
involved reagents), which can be easily and rapidly reached in
closed reaction vessels under microwave irradiation.
1-methylimidazole as reagents was unsuccessful; as a conse-
quence it was decided to investigate an alternative synthesis route
according to Scheme 3.27
In the first step of this new synthetic approach (Scheme 3),
imidazole was alkylated with 3-bromopentane using sodium
hydride as a deprotonating agent. Nearly full conveꢀrsion was
obtained after 5 days under conventional heating (60 C), while
only 30 minutes were required under microwave irradiation
(120 ꢀC). This acceleration in tꢀhe reaction is caused by the
With this approach, 1-(1-methylbenzyl)-3-methylimidazolium
chloride was synthesized (solvent-free conditions) under micro-
wave irradiation using similar procedures as described in litera-
ture for other ionic liquids.2,14,26 Nevertheless, the conversion for
the investigated ionic liquid was only 51%; this effect is thought
to be related to an elimination reaction of the branched alkyl
halide yielding side products according to Scheme 2. 3-Methyl-
1H-imidazolium chloride as one of the side products could be
identified by 1H NMR spectroscopy, whereas the corresponding
alkenes could not be detected due to their volatility.
ꢀ
applied higher temperature (120 C instead of 60 C), which is
above the boiling point of the utilized solvent (THF). Thereafter,
the obtained reaction mixture was purified by column chroma-
tography and a yield of 48% was achieved for the pure product.
In the second reaction step methylene iodide was added to the
alkylated imidazole at room temperature to yield 55% of the
desired ionic liquid.
It is known that linear ionic liquids can, in general, be purified
by extraction with ethyl acetate. However, this approach was not
successful for the case of branched ionic liquids, even though
different solvents (e.g. tetrahydrofuran, methyl ethyl ketone,
diethyl ether, and acetone) were investigated to separate the
3-methyl-1H-imidazolium chloride (determined by 1H NMR
spectroscopy) from the desired ionic liquid (Scheme 2). For this
reason, an alternative work-up procedure for the branched
ionic liquids was developed, which consisted of drying the
samples at 120 ꢀC under vacuum in an infrared light evaporator
(IR-Dancer). Under these conditions the 3-methyl-1H-imidazo-
lium chloride seems to decompose to 1-methylimidazole and
hydrogen chloride. However, the side products could be
successfully removed from the desired ionic liquid. Subsequently,
the remaining ionic liquid was finally purified by filtration
over silica gel to remove traces of decomposed material.
Nevertheless, this procedure was not successful for the case of
1-(1-ethylpropyl)-3-methylimidazolium bromide due to the fact
that 3-methyl-1H-imidazolium bromide could not be removed
effectively. A possible explanation for this finding is that the
corresponding side product is more stable than 3-methyl-1H-
imidazolium chloride, and a higher temperature would be
required to remove it, which might also lead to decomposition of
the desired ionic liquid. Thus, the synthesis of 1-(1-ethylpropyl)-
3-methylimidazolium bromide using 3-bromopentane and
Note that for this synthetic route iodide as counter-ion was
obtained, whereas all other ionic liquids, synthesized by the
alkylation of 1-methylimidazole, contain chloride as anion. For
this reason, an anion exchange step was selected for all
compounds in order to obtain the required branched ionic
liquids containing the same counter-ion. For this purpose, the
anion of the ionic liquids was exchanged for tetrafluoroborate,
which is known to be less hygroscopic than halide anions and, as
a result, allowing an easier handling. Thus, the exchange of the
anions was performed, using sodium tetrafluoroborate for all
ionic liquids containing chloride, and silver tetrafluoroborate for
the case of iodide.28 An overview of the cations of the different
ionic liquids synthesized in this work is depicted in Fig. 1.
Decomposition temperature
The heat of combustion is one of the chemical properties that is
influenced by branching. Branched alkanes have a slightly lower
heat of combustion and therefore they are more stable than their
linear analogues.23 The attractive forces increase in strength
relative to the repulsive forces when the structure becomes more
complex, leading to a higher stability of the branched alkanes.
Therefore it is expected that the decomposition temperatures,
which are related to the stability of the ionic liquids, should be
higher for branched ionic liquids than for their linear analogues.
On the other hand, Awad et al. proposed an SN1 reaction as the
decomposition mechanism for imidazolium-based ionic liquids
with an isobutyl group and tetrafluoroborate as counter-ion,
leading to lower decomposition temperatures than the linear
analogues.29
The results of the thermogravimetric analysis (TGA), depicted
in Fig. 2, revealed that the degree of branching has a small, but
not insignificant influence on the decomposition temperature of
Scheme 2 Schematic representation of the reaction scheme of 1-meth-
ylimidazole with branched alkyl chains yielding side products (3-methyl-
1H-imidazolium chloride and alkene).
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Fig. 3 Characteristic decomposition behavior (TGA) for imidazolium-
based ionic liquids, bearing chloride and tetrafluoroborate as counter-
ions.
Fig. 1 Overview of the chemical structures of the synthesized ionic
which would also explain the lower decomposition temperatures
in comparison to the alkyl chains. The tetrafluoroborate-
containing ionic liquids are more stable than the corresponding
halide-containing ionic liquids, and less hygroscopic as well. For
instance, the ionic liquid bearing chloride as counter-ion lost
around 20 wt% of water during heating, whereas a weight loss
of only around 3% was observed for the analogues with tetra-
fluoroborate as counter-ion, as displayed in Fig. 3. The water
absorbed by certain ionic liquids is due to their hygroscopic
character, and occurs during their handling under atmospheric
conditions.
liquids with different counter-ions.
The average decomposition temperature is mainly controlled
by the anions. For instance, the average decomposition temper-
ature for ionic liquids containing chloride and iodide was found
ꢀ
ꢀ
to be around 270 C, whereas an average value of 370 C was
observed for the cases of tetrafluoroborate-containing
ionic liquids. In all cases the linear ionic liquids bearing tetra-
fluoroborate as counter-ion were more stable than their branched
analogues, on average 30 ꢀC for alkyl groups and 100 ꢀC for
aromatic side groups, which is contrary to the behavior reported
for alkanes and for the observed behavior of ionic liquids with
chloride as counter-ion. On the other hand, these findings are
similar to those reported by Awad et al., where the ionic liquids
with isobutyl alkyl side chains showed a decomposition temper-
ature around 50 ꢀC lower than their linear analogues.29 The
differences between the chloride- and tetrafluoroborate-contain-
ing ionic liquids might be due to a different decomposition
mechanism. The degradation of tetrafluoroborate-containing
ionic liquids most probably leads to the formation of alkylimi-
dazole, alkyl fluoride and BF3, as described in the literature.
In addition, the formation of alcohol and HF is possible when
water is present.30 Nevertheless, the degradation mechanism is
less clear for tetrafluoroborate-containing ionic liquids, and
more understanding would be required. Currently, we have
no explanation for this different behavior.
Fig. 2 Decomposition temperatures of the different branched and linear
imidazolium-based ionic liquids, bearing chloride and tetrafluoroborate
as counter-ions.
the chloride-containing ionic liquids. The branched ionic liquids
were found to be on average 10 ꢀC more stable than their linear
analogues. In the literature, the decomposition of chloride-
containing ionic liquids has been ascribed to a dealkylation of the
N-alkyl substituents of the cation via an SN2 mechanism.30 The
findings described in this contribution support this decomposi-
tion pathway. The lower decomposition temperatures for linear
ionic liquids might be explained by the easier attack either of the
N-alkyl substituents of the cation via the chloride anion in
comparison to the branched analogues, where the dealkylation
most likely occurs at the methyl group. As a result, the ionic
liquids with linear alkyl chains degrade at lower temperatures.
An exception to this finding was the aromatic ionic liquids, which
displayed an inverse behavior; for instance the ionic liquid with
the non-branched aromatic group was around 30 ꢀC more stable
than that with the branched aromatic side group. The benzyl
analogues seem to degrade via a dissociative mechanism (SN1)
leading to an easier degradation of the branched analogues,
Melting point and glass transition temperature
The boiling point of alkanes is another physical property effected
by the degree of branching and increases with the length of the
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molecules. Branched alkanes with the same number of carbon
atoms as their linear analogues show in general lower boiling
points. The same trend is visible for the melting points of the
alkanes.23 The intermolecular interactions, in this case induced
dipole–induced dipole attractions, increase for longer chains and
decrease for branched alkanes due to a smaller surface area,
which causes less contact for intermolecular associations. Based
on this conclusion, a similar behavior is also expected for the
melting points of dialkylimidazolium-based ionic liquids.
Another property, related to the melting point, is the glass
transition temperature of a molecule. From the literature it is
known that the glass transition temperature is approximately
two-thirds of the value of the melting point.31 The range of values
for Tg/Tm has been found to vary from 0.58 to 0.78 for different
molecules and polymers.32 The same ratio is also expected for the
melting points and glass transition temperatures of the prepared
ionic liquids.
Fredlake et al.33 have described three different types of thermal
behavior for imidazolium-based ionic liquids, which have been
also observed in this contribution using differential scanning
calorimetry (DSC). As observed in Fig. 4, ionic liquids with well-
defined melting and freezing points (Fig. 4a) or with only a glass
transition temperature (Fig. 4b) could be found. However,
Fig. 4c displays a case of more complex thermal behavior; this
ionic liquid revealed a glassy transition at a subcooled liquid state
upon heating, followed by a cold crystallization and a melting
point. Nevertheless, for the most of the investigated cases only
a glass transition temperature (Tg) could be detected.
Fig. 5 Glass transition temperatures of the different branched and linear
imidazolium-based ionic liquids, bearing chloride and tetrafluoroborate
as counter-ions.
showed a melting point and a glass transition temperature.
The values were 0.59, 0.64 and 0.73 for [MC2MIM][BF4],
[EC3MIM][BF4] and [MBnMIM] [BF4] respectively, matching
well to the values found for other molecules (inorganic and
organic) and polymers in the literature.32 All results for ther-
mogravimetric analysis and differential scanning calorimetry
before and after the anion exchange are summarized in Table 1.
In general, the glass transition temperature was found to be
higher for ionic liquids with branched alkyl side chains, which is
an inverse behavior to the melting and boiling points of alkanes
(Fig. 5). The Tg for the tetrafluoroborate-containing ionic liquids
was on average 35 ꢀC lower than for the chloride-containing
analogues. For both counter-ions, the Tg of branched ionic
liquids was around 15 ꢀC higher than for the linear alkyl side
chain cases, and around 10 ꢀC higher for the aromatic side
groups. It was also found that the ionic liquids withꢀan aromatic
side group had higher Tg values (around 40 to 45 C) than the
investigated ionic liquids with an alkyl side chain. The Tg to Tm
ratio was determined for the three cases where the ionic liquids
Water uptake
Another important property of ionic liquids is their ability to
absorb water. Alkanes are known to be hydrophobic
compounds; therefore, an influence on the ability to absorb water
is expected for hydrophilic imidazolium-based ionic liquids
bearing different alkyl chains as side groups. Thus, in case of
water-soluble ionic liquids an increase in the alkyl chain length of
Table 1 Melting (Tm), freezing (Tf), cold crystallization (Tcc), glass
transition (Tg) and decomposition (Tdc) temperatures for imidazolium-
based ionic liquids with branched and non-branched alkyl (or aryl) side
groups
Entry Cation
Anion Tm/ꢀC Tf/ꢀC Tcc/ꢀC Tg/ꢀC Tdc/ꢀC
1
2
3
4
5
6
7
8
[C3MIM]+
ClÀ
ClÀ
ClÀ
ClÀ
—
—
—
—
—
—
—
124
—
—
—
—
—
67
—
—
68
73
—
—
—
—
—
—
—
45
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
À44
À41
À47
2
À34
À31
À19
—
269
268
262
278
277
278
273
284
248
393
380
408
384
370
362
366
363
288
[C4MIM]+
[C5MIM]+
[BnMIM]+
[MC2MIM]+ ClÀ
[MC3MIM]+ ClÀ
[MC4MIM]+ ClÀ
[EC3MIM]+ IÀ
[MBnMIM]+ ClÀ
[C3MIM]+
[C4MIM]+
[C5MIM]+
[BnMIM]+
[MC2MIM]+ BF4
[MC3MIM]+ BF4
[MC4MIM]+ BF4
[EC3MIM]+ BF4
[MBnMIM]+ BF4
9
10
À
À
À
À
À
À
À
À
À
10
11
12
13
14
15
16
17
18
BF4
BF4
BF4
BF4
À84
À80
À77
À33
À74
À68
À66
À56
À22
—
À30
—
—
À6
Fig. 4 Three different kinds of thermal behavior of imidazolium-based
35
ionic liquids as determined by DSC.
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their cations should lead to a lower water uptake.33 From alkanes
it is known that branching results in a more compact structure
and therefore a smaller surface area.23 Thus, we expect the
branched ionic liquids to absorb more water than their linear
analogues, because the branching allows the water molecules to
penetrate the ionic liquid more easily. Furthermore, the
hydrogen bonds of water molecules are less locally ordered
around the branched alkyl chains,23 and therefore it is thought
that alkyl-branched ionic liquids might be more hydrophilic than
their linear analogues. In addition, for ammonium-based ionic
liquids the water content for branched ionic liquids was reported
to be ꢁ0.4% higher than for their linear analogues, which is in
line with our expectation.34
a result of the decreased ability to self-assembly in branched ionic
liquids due to their lower surface tension, as reported for
ammonium-based ionic liquids.38 Thus, the water uptake for
branched alkyl side chains is more similar to the that for the
shorter linear alkyl chains than for the linear alkyl chains with
the same number of carbon atoms. This effect is more
pronounced in case of [MC3MIM][Cl], which has a water uptake
behavior more similar to [C3MIM][Cl] than to [C4MIM][Cl]. As
expected, ionic liquids with aromatic groups absorbed less water
than ionic liquids with alkyl side chains due to the higher
hydrophibicity of aromatic substituents. The introduction of
a methyl group to the benzyl group did not significantly change
the absorption of water, and only a marginal decrease was
measured.
As mentioned before, the cation of the ionic liquid only has
a slight influence on the hydrophobicity, while the anion mainly
dictates the water miscibility. Water uptake is expected to be
larger for ionic liquids containing a chloride counter-ion than for
tetrafluoroborate-containing ionic liquids, because the van der
The water uptake was plotted against the number of carbon
atoms in the alkyl and aryl side groups (Fig. 7) to investigate if
the difference in the water uptake between the alkyl and the aryl
group is due the different functional groups or not. A linear trend
was found, which shows that the water uptake mainly depends
on the number of carbon atoms in the side group and not on the
nature of the group (alkyl or aryl). In addition, the water uptake
of [C8MIM][Cl] was also measured, which were found to fit
perfectly to the observed linear trend, the values being below
those for the branched ionic liquid [MBnMIM][Cl]. From the
plot in Fig. 7, we can conclude that the water uptake within the
series of linear or branched ionic liquids is mainly controlled by
the hydrophobicity of the alkyl side chains, whereas the higher
water uptake of branched ionic liquids is a result of the decreased
ability to self-assembly.
3
˚
Waals volume for a chloride anion is 28 A , whereas for tetra-
3
fluoroborate it is 48 A as determined elsewhere.35 This results in
˚
a higher charge density for the chloride anion, and it is believed
that smaller anions will absorb more water.36 For these reasons,
the chloride-containing ionic liquids were chosen to investigate
the influence of the cation structure on the water uptake due to
their well-known hydrophilic nature. Thus, the water uptake of
the ionic liquids was investigated at three different humidities
ꢀ
(20, 50 and 80%) at 25 C. The measurement procedure used is
similar to that one described in literature.35,37 The results of
the water uptake measurements of the investigated chloride-
containing ionic liquids are depicted in Fig. 6.
Finally, the water uptake was measured for selected tetra-
fluoroborate-containing ionic liquids. From the literature it is
known that hydrophobic ionic liquids are able to absorb water to
a certain extent. Examples from the literature show that after
24 h exposure to air the water content of [BMIM][BF4] is 0.32 M
(ꢁ5 wt%).39 Ionic liquids that are even more hydrophobic,
namely [BMIM][PF6] and [BMIM][Tf2N], were reported to have
a water content of 0.08 M39 and 1.8% (v/v)40 for [BMIM][PF6],
and 0.10 M39 and 1.4 wt%41 for [BMIM][Tf2N]. These values
As observed for the investigated ionic liquids in Fig. 6, the
water absorption decreases as the length of the alkyl side chain
for both linear and branched side chains increases, which is in
agreement with previous observations.34,36 Fig. 6 shows that
[C2MIM][Cl] had the highest water uptake values of all the
investigated ionic liquids (up to 106 wt%). This water uptake is
quite reasonable for ionic liquids which are water-soluble. In
addition, it was found that the introduction of a branching point
increases the water uptake. According to Fig. 6, the branching
seems to improve the penetration of water, which might be
Fig. 7 Dependence of the number of C atoms in the side chain on the
water uptake of different linear imidazolium-based ionic liquids con-
taining chloride as counter-ion.
Fig. 6 Water uptake of different linear and branched imidazolium-
based ionic liquids containing chloride as counter-ion (at 25 ꢀC).
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obtained by mixing proper proportions (regulated by mass flow
controllers) of the dry and wet streams. The standard isotherm
measurement consisted of a number of subsequent steps. First,
the sample was dried at 60 ꢀC and 0% RH for a specific time until
the weight change was stabilized at less than 0.05% for a time
period of 60 minutes. In the second step, the temperature was
ꢀ
decreased to 25 C and the relative humidity increased to 20%.
The humidity was then increased step-wise (with steps of 30%
RH) to a maximum of 80% RH. The weight change of the sample
was stabilized after each step until it was smaller than 0.05% for
a time period of 60 minutes. In addition, the reverse isotherm was
also measured. In this case the humidity was decreased down
to 0% (in steps of 30% RH) and the samples were allowed to
stabilize after each step. In order to finalize the isotherm and to
compare the results with the initial weight of the sample, an
additional drying step was included (60 ꢀC at 0% RH). The
differences between absorption and desorption were less than
0.4% for 50% RH and less than 0.15% for 20% RH. The accuracy
of the measurement was checked by a second run. It was found
that the variation of the two runs was within the range of the
variation between absorption and desorption. Distilled water
was utilized for the humidity chamber of the water uptake
measurements.
Fig.
8 Water uptake of linear and branched imidazolium-based
ionic liquids containing chloride and tetrafluoroborate as counter-ions
(at 25 ꢀC).
depend on the impurities and the humidity in the air. Most values
reported in the literature are given without the present humidity,
and therefore differences in the saturated water content are
possible. As depicted in Fig. 8, the ionic liquids bearing a chloride
anion absorb approximately ten times more water than ionic
liquids with a tetrafluoroborate anion (the differences between
the linear and branched cases are more evident for the chloride-
containing ionic liquids, as expected due to the more hydrophilic
character of the chloride). It is worth noting that the amount of
water absorbed can be up to 10 wt% for a hydrophobic ionic
liquid under high humidity (80% RH).
Conclusions
A series of linear and branched ionic liquids were synthesized
under microwave irradiation to elucidate initial structure–
property relationships. In this regard, new ionic liquids were
synthesized, viz. 1-(1-ethylpropyl)-3-methylimidazolium iodide
and 1-(1-methylbenzyl)-3-methylimidazolium chloride and their
tetrafluoroborate-containing analogues. As well as these new
ionic liquids, a series of known linear and branched ionic liquids
were also synthesized to allow a systematic study of the ther-
mophysical properties of these substances.
Experimental
Materials
All ionic liquids used, besides 1-ethyl-3-methylimidazolium
chloride and 1-methyl-3-octylimidazolium chloride which were
donated by Merck, were synthesized. The synthesis of the ionic
liquids, the anion exchange and the characterization of the ionic
liquids (1H NMR, IR, MALDI-TOF-MS) can be found in the
ESI.†
It was found that branched ionic liquids containing chloride
were slightly more stable (10 ꢀC) than their linear analogues,
supporting a SN2 decomposition pathway. However, the tetra-
fluoroborate-containing ionic liquids revealed a 30 ꢀC higher
thermal stability for the cases with linear alkyl chains, which
might be due to a different decomposition mechanism initiated
by the anion. For both investigated anions the ionic liquid with
the non-branched aromatic group was more stable assuming
a more dissociative decomposition mechanism (SN1).
Melting points (onset of an endothermic peak on heating),
freezing points (onset of an exothermic peak on cooling), cold
crystallization temperatures (onset of an exothermic peak on
heating) and glass transition temperatures (midpoint of a small
endothermic heat capacity change) were determined on a DSC
204 F1 Phoenix instrument by Netzsch under a nitrogen atmo-
The tetrafluoroborate-containing ionic liquids revealed
a lower glass transition temperature than the chloride analogue
ionic liquids. For the branched ionic liquids the Tg values found
sphere from À100 to 150 C with a heating rate of 20 K minÀ1
ꢀ
(a first heating cycle to 150 ꢀC was not considered for the
calculations). Thermogravimetric analyses were performed on
a TG 209 F1 Iris by Netzsch under a nitrogen atmosphere in the
range from 25 to 600 ꢀC with a heating rate of 20 K minÀ1. The
water uptake measurements of the ionic liquids were performed
on a Q5000 SA thermo gravimetric analyzer from TA Instru-
ments containing a microbalance in which the sample and
reference pans were enclosed in a humidity- and temperature-
controlled chamber. The temperature in the Q5000 SA was
ꢀ
were 10 to 15 C higher than for the linear analogues for both
anions. The ionic liquids with an aromatic group showed the
highest Tg values of all the investigated ionic liquids.
Moreover, the water uptake of the ionic liquids was measured
and revealed a systematic dependency on the length of the alkyl
side chain and on the branching. It was found that the water
absorption decreases when the alkyl chain length for linear and
branched alkyl groups increases, due to the increasing hydro-
phobicity of the alkyl/aryl group. The ionic liquids with branched
alkyl chains showed a higher water uptake than their linear
analogues as a result of their decreased ability to self-assembly.
In general, the water uptake for branched ionic liquids was lower
controlled by Peltier elements. Dried N2 gas flow (200 mL minÀ1
)
was split into two parts, of which one part was wetted by passing
it through a water-saturated chamber. The desired relative
humidity (RH) for the measurements could subsequently be
5272 | J. Mater. Chem., 2008, 18, 5267–5273
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17 S. Maria, T. Biedro, R. Poli and P. Kubisa, J. Appl. Polym. Sci., 2007,
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than for ionic liquids with a shorter alkyl side chain (Cn ꢂ 2 À
Cn < CnÀ1).
The described results give a better insight into the structure–
property relationship of ionic liquids. The introduction of
branches to the side chains of ionic liquids will allow the
synthesis of tailor-made ionic liquids and the fine-tuning of the
chemical and physical properties, such as stability, water uptake,
and glass transition temperature.
Acknowledgements
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The authors would like to thank the Dutch Polymer Institute and
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as well as Merck and Solvent-Innovation for their collaboration.
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