Chemical Tuning of Zwitterionic Ionic Liquids for Variable Thermophysical Behaviours, Nanostructured Aggregates and Dual-Stimuli Responsiveness

Article abstract of DOI:10.1002/chem.201802367

The design and synthesis of a series of zwitterionic ionic liquids (ZILs) to understand the structure–property relationship towards an increase of the thermal stability, a variation of the glass transition temperature, the shape-tuning of nanostructured aggregates and the tuning of the stimuli responsiveness are demonstrated. The substitution reaction of imidazole with various aliphatic and aromatic bromides followed by the reaction of the corresponding substituted imidazoles with bromoalkyl carboxylic acids of varying spacer length produces the ZILs. In aqueous solution, a ZIL molecule either exist in its ionic liquid (substituted imidazolium bromide) form or its zwitterionic (substituted imidazolium alkyl carboxylate) form with an isoelectric point (pI) depending on the pH value of the solution. Upon changing the pH to near or above the pI, the aqueous ZIL solution undergoes transition from a transparent to a turbid phase due to the formation of insoluble hierarchical nanostructured aggregates of various morphologies, such as spheres, tripods, tetrapods, fern-like, flower-like, dendrites etc. depending on the pH of the solution and the nature of the alkyl/vinyl/aryl substituents. Upon heating the solution a phase transition occurs from turbid to transparent, exhibiting a distinct reversible upper critical solution temperature (UCST)-type cloud point (T_cp). It is observed that the cloud point varies with the nature of the substituent, an increase of the concentration of the ZIL as well as with changes of the pH of the solution.

Full text of DOI:10.1002/chem.201802367

A Journal of
Accepted Article
Title: Chemical Tuning of Zwitterionic Ionic Liquids for Variable
Thermophysical Behaviours, Nanostructured Aggregates and
Dual-Stimuli Responsiveness
Authors: Yajnaseni Biswas, Pratyush Ghosh, and Tarun K Mandal
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To be cited as: Chem. Eur. J. 10.1002/chem.201802367
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Chemistry - A European Journal
10.1002/chem.201802367
Chemical Tuning of Zwitterionic Ionic Liquids for Variable
Thermophysical Behaviours, Nanostructured Aggregates and Dual-Stimuli
Responsiveness
Yajnaseni Biswas, Pratyush Ghosh and Tarun K. Mandal*
Abstract: In this article, we demonstrate design and synthesis of a series of zwitterionic
ionic liquids (ZILs) to understand the structure-property relationship towards the increase of
thermal stability, variation of glass transition temperature, shape-tuning of nanostructured
aggregates and tuning of stimuli-responsiveness. The substitution reaction of imidazole with
various aliphatic and aromatic bromides followed by the reaction of the corresponding
substituted imidazoles with bromoalkyl carboxylic acids of varying spacer length produces
these ZILs. In aqueous solution, ZIL molecule exist either in its ionic liquid (substituted
imidazolium bromide) form or zwitterionic (substituted imidazolium alkyl carboxylate) form
with an isoelectric point (pI) depending upon the solution pH. Upon changing the pH to near
or above pI, the aqueous ZIL solution undergoes transition from transparent to turbid phase
due to the formation of insoluble hierarchical nanostructured aggregates of various
morphologies such as spheres, tripods, tetrapods, fern-like, flower-like, dendrites etc.
depending on solution pH and nature of alkyl/vinyl/aryl substituents. Upon heating the
solution phase transition occurs from turbid to transparent, exhibiting a distinct reversible
upper critical solution temperature (UCST)-type cloud point (T ). It is observed that the
cp
cloud point varies with the nature of substituents, increase in ZILs’ concentration as well as
with change of solution pH.
……………………………………………………………………………………………………………………
Y. Biswas, P. Ghosh and Prof. T. K. Mandal
Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Fax: 91-33-2473 2805, E-mail: psutkm@iacs.res.in
1
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Chemistry - A European Journal
10.1002/chem.201802367
Introduction
Ionic liquids (ILs) are organic salts with many unique properties such as negligibly small
[
1]
vapour pressure, high ionic conductivity, and high chemical and electrochemical stability.
[
2]
They have been extensively studied as solvents in chemical industry, for performing
[
1,3-4]
[5-7]
[8]
chemical reactions including catalysis,
for extraction
and for electrolysis because of
their above mentioned properties including attractive thermophysical properties such as high
thermal stability, glass transition and low melting temperature, especially for room
[
9-11]
temperature ILs (RTILs).
temperature range as solvents. These thermophysical properties can be changed by modifying
The later properties are the key to know their liquid operating
[
12-15]
the structure of cation or anion of IL,
for example, by attaching alkyl chain of varying
[
9,14-15]
[13]
length
or bulky aromatic moiety to the imidazolium cation. Thus, it is important to
synthesize new ILs having distinct glass transition and/or melting, which are very less
reported.
Because of many interesting properties of ILs, there is a fundamental interest on this
martial to use them as alternative to traditional solvents on the one side, but on the other side
there exist a lot of practical applications. This requires in-depth study of their
physicochemical properties, those can be fine tuned by the design of new ILs with interesting
chemical structures. However, the easiest way to fine-tune these properties is to mix the
existing ILs with conventional solvents, particularly with water. The IL-based mixed systems
showing thermoreversible phase behaviours is also important in proceeding from the start of
the reaction to the separation of products. Moreover, less attention has been paid to study the
various solution properties of mixed systems comprising of ILs. In this direction, some
groups have already been reported to study the phase behaviour of mixed systems such as IL-
[16]
[6]
[17]
[18]
[19]
super critical CO2, IL-water, IL-alcohol, IL-mixed solvent and IL-K PO -water
2
4
etc. The detail study of composition-dependent phase behaviours of such IL-based mixed
systems at different temperatures are necessary for their use in optimized extraction process.
Therefore, the synthesis of new ILs and the systematic study of their phase behaviour are still
important when they are mixed with conventional organic solvents including water. As an
alternative system, there is also some recent research on deep eutectic solvents (DESs) having
[
20-21]
similar characteristics and properties with the conventional ILs.
In particular, a major focus of many researchers was on IL-water mixed system as
[
22]
they are expected to be potential media for many biological and chemical processes. It is
known that the physicochemical properties of ILs can be tuned towards specific tasks by
2
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Chemistry - A European Journal
10.1002/chem.201802367
[8,23-24]
exploiting the diversities of component organic ions.
The hydrophilic/hydrophobic
balance of component ions of IL in IL-water mixture is also an important physicochemical
parameter that governs their thermoresponsive properties to either upper critical solution
[
25-27]
[28-30]
temperature (UCST)-type
There are several reports on LCST-type thermomorphic behaviours of IL-water mixtures.
Ohno and coworkers investigated LCST-type phase behaviours of many different ILs in
and/or lower critical solution temperature (LCST)-type.
[
1]
[
22,29,31-32]
water,
with different cloud points (T s) depending on the counter anions, such as 4-
cp
[
33]
[31]
styrenesulfonate,
trifluoroacetate
etc. In addition to these studies, Ohno et al. also
[
22]
describes a detailed study of a list of existing LCST-type ILs in recent reviews. In regard
to thermoresponsive IL-water systems, UCST-type systems are more popular as they can be
utilized as extraction and separation solvents which further broadens their applications in
[
34-36]
[34-37]
catalysis.
However, only few UCST-type IL-water systems are so far reported.
The
important one is the binary mixture of imidazolium-based ILs ([C mim]BF (n = 4, 6, 8)) and
n
4
water which shows tuanble UCST-type transitions depending on the length of alkyl chain and
[
37]
also on the hydrophobicity of the anions.
The reports of both UCST- and LCST-type
transitions of [P4,4,4,4]-type ILs with fumarate and maleate anions respectively further
[
25]
enriched this area. The pH-responsiveness is rather a rare occurrence in IL-water systems.
Only a very few such systems are reported where pH-sensitivity is originated from the
[
14]
associated
large
anions
such
as
8-hydroxypyrene-1,3,6-trisulfonate,
4-
[
38]
[39]
sulfonatocalix[6]arene,
and
carboxylbenzenesulfonate . Further, Binnemans and
coworkers developed a pH-responsive betaine-based IL system with an insoluble-to-soluble
phase transition upon heating for selective dissolution of metal oxides under aqueous acidic
[36]
condition.
Therefore, the development of new IL-solvent systems and studies of their
physical properties are necessary considering their different applications including metal
extraction.
Recently, zwitterionic (ZI) compounds are also attracted considerable attention
because of their interesting solution properties that makes them potential for applications in
[
40]
[41]
[42]
[43]
biomedical engineering, solar cells, ion transport, oil recovery and as antifouling
materials. In context of stimuli-responsive property, most of studies has been conducted on
polymeric zwitterionic systems responsive towards different external stimuli including pH,
[
44]
temperature and ionic strength. Whereas, there are only very few reports on responsive ZI
small molecules. The recent important one is the thermomorphic phase transitions of the
[
45]
aqueous solution of ammonium-based sulphobetaine without any pH-responsiveness.
3
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10.1002/chem.201802367
Recently, Stoffelen et al. reported azo-carboxybetaine-based self-assembled zwitterionic
[
46]
supramolecular nanoparticles with pH- and photo-responsiveness in water. An another far
less investigated ZI molecule, known as zwitterionic surfactant was shown to be aggregated
[
47]
to reverse micelles in different solvents. Xie et al. developed a small molecular hydrogel
based on the zwitterionic dendritic surfactant showing temperature-induced gel-to-gel phase
[48]
transition. Therefore, it would be interesting to synthesize new functional small molecules
that combine the properties of ionic liquid as well as zwitterionic moieties so that they exhibit
responses to many different stimuli.
With this aim, we design and synthesize a series of molecules, namely zwitterionic
ionic liquids (ZILs) bearing an IL moiety, alkyl/vinyl/aryl-imidazolium bromide and a
zwitterionic moiety of alkyl/vinyl/aryl-imidazolium alkyl carboxylate of varying spacer
length by a two-step substitution reaction (Scheme 1). The studies of thermophysical
properties of these ZILs allow us to examine the effect of different substituent R-groups
particularly on their glass transition temperatures (T s) and their thermal stabilities. ILs with
g
such properties and their variations are very less reported considering a huge library of
different ILs. It is shown that the variation in pH could induce reversible solubility phase
transition in water for all ZILs, except those having butyl substituents (Scheme 1). It is
observed for the first time that the ZIL molecules can easily aggregate into hierarchical
nanostructures of huge variation in their morphologies ranging from small spheres, large
spheres, tripods, tetrapods, flower-like, dendrites, fern-like etc. depending upon their
chemical structures and pH of the solutions. There are only few systems where spherical
[46]
particles are reported due to aggregation of either IL or ZI molecules.
The aqueous
solutions of these ZILs also exhibit reversible and tuanble UCST-type solubility phase
transitions because of the presence of ZI moiety.
Results and discussion
Synthesis of alkyl/vinyl/aryl imidazolium-based ZILs
A series of ZILs bearing an IL moiety of either alkyl or vinyl or aryl imidazolium bromide
and a ZI moiety consisting of imadazolium cation and either acetate or isobutyratate or
valerate anions were prepared in two-step nucleophilic substitution reactions (Scheme 1).
First step yielded six different substituted imidazoles (precursor units) with moderate yields
(Table S1). ESI-MS spectra (Figures S1-S6) of all alkyl/vinyl/aryl imidazoles showed signals
corresponding to their molecular ions whose mass exactly matched with their theoretical
4
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Chemistry - A European Journal
10.1002/chem.201802367
mass, indicating their successful syntheses. The appearance of characteristic signals of
imidazole ring protons at about δ 7.5, 7.0 and 6.9 ppm along with those of phenyl ring
protons (δ 7.3-7.0 ppm) for EtBzIm and VBzIm (Figure S7 and Figure S8) or alkyl protons (δ
3
7
.8-0.7 ppm) for BuIm and OctIm (Figure S9 and Figure S10) or aromatic ring protons (δ
.8-7.3 ppm) for MeNapIm and MeBpyIm (Figure S11 and Figure S10) confirmed their
1
3
successful syntheses in their purest form. The C-NMR spectra (Figures S7-S12) and FTIR
-1
spectra (Figure S13) with characteristic bands at 1435 cm (N-CH - deformation) and 3110
2
-1
cm (C=N deformation of imidazole ring) further provided the evidence of formation of all
these alkyl/vinyl/aryl imidazoles.
R'
C
R
N
R
N
HO C
Br
2
n
50o 20h
C,
R'=H
N
n=4,
n=1, R'=CH
R'=H
R'
Br
R'
R
X
3
H O/ Acetone
60o 24h
N
C
N
H
2
N
C,
n=1,
(1:2, v/v)
CH CN
R'
3
n
Alkyl/vinyl
halides
aryl
CO H
2
Imidazole
Alkyl/vinyl
imidazoles
aryl
/
Zwitterionic ionic liquids
/
Br
Br
Br
Br
Br
N
Br
N
N
N
N
N
N
N
N
N
N
N
HO C
HO C
2
HO C
2
2
HO C
2
CO H
CO H
2
2
[EtBzImVa][Br]
[EtBzImIBa][Br] [EtBzImAa][Br]
[VBzImVa][Br]
[VBzImIBa][Br] [VBzImAa][Br]
R=
R=
Ethyl benzyl
Vinyl benzyl
CH3
CH
H C
3
H C
H C
3
3
(
2
CH )
7
(
2
)
3
(CH2)3
N
2
(CH )
3
N
Br
Br
N
Br
Br
N
Br
C
Br
N
N
N
N
N
N
N
N
HO C
HO
2
2
CO H
2
CO H
CO
H
CO
H
2
2
2
[BuImVa][Br]
[BuImIBa][Br] [BuImAa][Br]
R= Butyl
[
OctImVa][Br]
[MeNapImVa][Br] [MeBpyImVa][Br]
R=
Methyl
naphthalene
R=
Methyl
biphenyl
R= Octyl
Scheme 1.
Syntheses of different alkyl/vinyl/aryl imidazoles and their corresponding
ZILs.
5
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In the second step, six different ZILs, designated as [EtBzImVa][Br],
VBzImVa][Br], [BuImVa][Br], [OctImVa][Br], [MeNapImVa][Br] and [MeBpyImVa][Br]
[
were then finally obtained with good yields (Table S1) by separately reacting BVa with
above mentioned substituted imidazoles (Scheme 1). Note that the introduction of different
R-groups (e.g. aliphatic or aromatic) of different hydrophobicities is to control the degree of
zwitterionic characters in the obtained ZILs, which in turn can tune their pH- and thermo-
responsive behaviours in water and will be discussed in detail later in this section. In addition
to this, the separate reactions of EtBzIm, VBzIm and BuIm with BAa and BIBa instead of
BVa were also carried out to obtain two new sets of six ZILs (for yields see Table S1) of
different pH- and thermo-responsiveness due to any change that may occur in the degree of
zwitterionic characters of the acidic counterparts (Scheme 1). The characterization via ESI-
1
13
MS (Figures S14-S25), H-/ C-NMR (Figures S26-S37) as well as FTIR (Figure S38)
spectroscopy unambiguously revealed the successful syntheses of all these twelve ZILs. In
these ZILs, the attachment of alkyl carboxylic acid with the nitrogen atom of imidazole group
1
13
was ascertained from H-NMR, C-NMR and FTIR spectra showing shifting of
characteristic signals (at δ 7.5, 7.0 and 6.9 ppm, Figure S7-S12) of substituted imidazoles to
signals (at δ 8.7, 7.4 and 7.3 ppm, Figure S26-S37) for imidazolium cation, appearance of
carbonyl signals (at δ 178 to 171 ppm, Figure S26-S37) and appearance of a new band at
-1
1
718 cm (Figure S38) due to >C=O stretching of –COOH respectively. All the ZILs were
obtained as light yellowish tacky and waxy solids, whereas [MeNapImVa][Br] and
MeBpyImVa][Br] were obtained as white powder at room temperature. Note that the
[
tackiness remained even after prolong drying. This result indicated that the tackiness is the
inherent property of these ZILs and not caused due to the presence of water.
The as-synthesized ZILs became completely soluble in water as well as in polar
organic solvents, such as methanol, acetonitrile, DMSO, DMF etc indicating similar
behaviours that of ionic liquids (ILs). Thus, in solid state, ZIL exists as a IL type molecule. It
was observed that the aqueous solution of ZILs exhibited a pH of ~2.5, where their molecules
existed in their ionic liquid from. Above this pH, the ZIL molecules started to convert into
their zwitterionic form, the details of which will discussed later in this section. In aqueous
solution of pH 2.5, the degree of ionization of these ZILs (present as IL form) depends on
their chemical structures. Thus, as suggested by one of the reviewer, it is important to
determine the ionicity of the presented ZILs. The ionicity value may useful for understanding
their bulk behaviors. However, there is no such direct method to measure ionicity. It has been
6
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10.1002/chem.201802367
reported that the measurement of ionicity for protic IL was done by FTIR and NMR
[
49]
spectroscopy. However, these ZILs are not protic IL. Therefore, we determined ionicity by
measuring the solution conductivities at a concentration of 0.01 M and compared with that of
aqueous KBr solution (0.01M) (Table S2). According to the literature report, many research
groups have explained ionicity of ILs based on Walden rule (Λη=k, with Λ, η and k are the
[
50-51]
molar conductivity, viscosity and a constant dependent on temperature).
In this case, as
the concentration of ZIL solution is very low, the viscosity should also be low and almost the
same for all ZILs and KBr solution. Therefore, at a fixed temperature, the ionicity can be
considered as the fraction of ions that are available for conduction and contribute to the total
solution conductivity reading. It was observed that the measured solution conductivities were
different for different ZILs depending on their chemical structures and were lower than that
of aqueous KBr solution. We also measured the conductivities of few representative ZILs in
different organic solvents, the values were lower than that obtained for ZILs in water. This
results further summarized that the presented ZILs exist as IL type molecules in water at
pH~2.5 as well in organic solvents and their ions are participated in conduction in solution.
Thermal properties of bulk ZILs
The thermal stability of ZILs were examined by thermogravimetric analysis (TGA) and the
obtained thermograms (Figure S39) clearly showed a slight weight loss around 100 °C due to
the evaporation of residual water even after drying in vacuum. The main weight loss between
3
00 to 450 °C was attributed to the degradation of imidazolium groups containing bulky
substituents with a residual weight of 10-25 % depending upon the presence of R-groups in
ZILs. Grossly, all the ZILs were thermally stable up to ∼300 °C (Table 1), which is quite high
in comparison to the thermal degradation temperatures (T ) of other conventional
d
[
9,52]
imidazolium-based ILs.
Such high thermal stability can be ascribed to the presence of
bulky groups such as MeNap, MeBpy, EtBz, VBz and Oct etc. in these ZILs.
The appearance of T is very common for polymers because of their transformation
g
from glassy to amorphous state upon heating, rather it is rare phenomena for small organic
[
9,53-55]
molecules. However, recently, some research groups including us,
observed that some
ILs, especially RTILs often exhibited a T i.e., upon cooling, they can easily be transformed
g
into a glassy state from their liquid state without crystallization. With this expectation, we
also examined these ZILs by differential scanning calorimetry (DSC) to check whether they
exhibit a T or not. DSC thermograms (Figure S40) indeed exhibited a sharp T with low
g
g
7
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Table 1.
Thermal degradation temperature (T ), glass transition temperature (T ) and
d
g
transition breadth (TB) of all the ZILs
Sample name
T (°C) T (°C) TB (°C)
d
g
[
[
[
[
[
[
[
[
[
[
[
[
EtBzImVa][Br]
VBzImVa][Br]
BuImVa][Br]
OctImVa][Br]
267
345
256
247
-21.2
3.3
13
9
-51.9
-58.0
9.3
42
12
60
70
14
15
12
16
12
11
MeNapImVa][Br] 300
MeBPyImVa][Br] 295
11.5
EtBzImIBa][Br]
VBzImIBa][Br]
BuImIBa][Br]
EtBzImAa][Br]
VBzImAa][Br]
BuImAa][Br]
245
278
253
270
294
247
-19.0
9.8
-51.0
-18.7
10.5
-47.0
transition breadth (TB ~15 °C) for most ZILs (Table 1). Whereas the observed T s (9.3 and
g
1
1.5°C) for [MeNapImVa][Br] and [MeBpyImVa][Br] respectively were not so sharp and
have broad TBs ~ 60-70 °C (Table 1). Yuan et al. mentioned the appearance of similar broad
[56]
Tg ranges for polymerized ionic liquids.
These two ZILs are supposed to have less free
volume because of the presence of ᴨ-ᴨ interaction among the neighboring molecules which
probably the reason for broadening of their T s and observed at relatively higher
g
[
57]
temperature. It was further noticed from Table 1 that T s were mostly low and were largely
g
dependent on the nature of the substituents (R-groups). For example, [OctImVa][Br] bearing
octyl chain exhibited the lowest T = -58 °C. Whereas T s of ZILs bearing butyl groups,
g
g
[
BuImVa][Br], [BuImIBa][Br] and [BuImAa][Br] were -53, -51 and -47 °C respectively. The
sharp T signal for [BuImVa][Br] was clear from the enlarge view of the thermogram given in
g
Figure S40. It should be noted that the T increased with the decrease of the length of the
g
substituted alkyl chain considering Va-ZILs, [BuImVa][Br] and [OctImVa][Br]. This
8
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observation is similar to our earlier report for 3-alkyl substituted 1-methylimidazolium
[
9]
ascorbate ILs.
Furthermore, the ZILs with bulky rigid substituents such as
methylnaphthalene, methylbiphenyl, ethyl benzyl, vinyl benzyl showed higher T values
g
compared to those of octyl and butyl substituted ZILs (Table1). The explanation for such
complex T behaviours in these ZILs in not straightforward as the origin of glass transition
g
phenomenon in IL is not clearly known. However, some research groups mentioned that the
Tg behaviour in ILs is a consequence of presence of some kind of stability among IL
molecules due some short- and/or inter-mediate-range order in the glassy or metastable
[53,55]
supercooled liquid state.
Heating provide sufficient energy to increases the mobility of
the molecules and eventually breaks such stability of ILs, exhibiting a distinct T behaviour.
g
[
13]
However, some other research groups directly correlated the T with the free volume of IL
g
[
57-59]
or polymer
system. In general it is shown that more the free volume, the lower is the Tg.
Actually, the explanation of T behaviour by short- and/or inter-mediate-range ordering or by
g
free volume is essentially the same. The increase of free volume means decrease of short-
and/or inter-mediate-range ordering resulting in a lower T of the material. In this case, we
g
also propose similar explanation for the different T behaviours of ZILs. For example, as
g
mentioned above, the lower T of [OctImVa][Br] can be ascribed to it higher free volume
g
(because of less close packing of ZIL molecules) compared to that of [BuImVa][Br]. It has
also been reported for poly(n-alkyl methacrylates) that the T decreases from poly(methyl
g
methacrylate) to poly(n-dodecyl methacrylate) due to the increase of number of carbon atoms
[
59]
in the side chain, which prevents the polymer chains from close packing.
However, the
ZILs containing bulky groups (e.g., [MeNapImVa][Br], [MeBpyImVa][Br], [EtBzImVa][Br]
and [VBzImVa][Br]) are supposed to have less free volume because of the presence of ᴨ-ᴨ
interaction among the molecules. Hence, in a similar logic, these ZILs should have higher Tg
values, which was indeed the case as mentioned above (Table1). The higher T s of VBz-ZILs
g
can again be explained on the basis of higher ᴨ-ᴨ interaction due to extended conjugation
present in VBz moiety compared to that in EtBz of EtBz-ZILs of lower T s. In fact, it has
g
been reported earlier that the presence of a rigid ring system such as adamantane in a polymer
[
58]
increases its T and the thermal stability over the linear, unsubstituted polymer. It should
g
be noted that DSC thermograms (Figure S40) did not exhibit any peak/hump near 0 °C due to
the presence of trace amount of water as detected by TGA (Figure S39) may because of very
small amount. Furthermore, we did not observe any crystalline melting points in these ZILs
while heating up to 100 °C. The presence of the bulky pendent groups in imidazolium cations
9
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Chemistry - A European Journal
10.1002/chem.201802367
might hinders the ZIL molecules from crystallization due to the disturbance of the local
[
9,12]
ordering among the neighbouring molecules.
The presence of trace of water might also be
responsible for not showing any melting by these ZILs. It should be noted that for the
optimization of heating rate used in DSC measurements, a representative sample
([OctImVa][Br]) was heated from -89 to 100°C at different heating rates of 5, 10 and
2
0°C/min and the data were provided in Figure S41. The thermograms clearly revealed that
Tg was very prominent at a heating rate of 20°C/min compared to that observed using other
heating rates. obtained othe r. Also, there is no endothermic peak (within 100°C) even when
the heating rate was 5°C/min, indicating indeed the absence of any melting for this ZIL as
observed for all other samples.
pH-Responsive phase behaviours of ZILs in aqueous solution
[
46,60]
Among other pH-sensitive molecules,
the carboxylate groups of these ZILs can be
reversibly protonated/deprotonated upon reducing and/or increasing the solution pH. Thus, it
is expected that these ZILs would also exhibit pH-responsive soluble-insoluble phase
transition in aqueous solution. However, as can be seen from Table 2, the pH-responsive
Table 2.
pH-dependent aqueous solution properties of all the ZILs (1 wt%) at 25 °C.
Sample name
Isoelectric point (pI)
Phase transition
Transparent pH range
Turbid pH range
[
EtBzImVa][Br]
6.59
2-5.97
5.97-12
[
[
[
[
[
[
[
[
[
[
VBzImVa][Br]
BuImVa][Br]
5.7
4.76
5.00
5.19
3.84
6.48
5.7
2-5.7
2-12
2-5.5
2-5.3
2-3.9
2-6.5
2-6.5
2-12
2-12
2-12
5.7-12
-
OctImVa][Br]
NapImVa][Br]
BPyImVa][Br]
EtBzImIBa][Br]
VBzImIBa][Br]
BuImIBa][Br]
EtBzImAa][Br]
VBzImAa][Br]
5.5-12
5.3-12
3.9-12
6.5-12
6.5-12
-
4.68
6.38
5.7
-
-
[
BuImAa][Br]
3.98
2-12
-
1
0
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behaviours were clearly dependent on the nature of R-groups present in ZILs. For example,
all Va-ZILs (Table 2), except [BuImVa][Br] exhibited a transformation from transparent
(%T=100) to turbid ((%T=0) upon changing the pH in aqueous solution at 25 °C (see
photographs in Scheme S1A and Figure 1A). Interestingly, [BuImVa][Br] remained soluble
in the whole pH region from 2-11 at 25 °C (see photographs in Scheme S1C and Figure 1A).
IBa-ZILs (e.g., [EtBzImIBa][Br] and [VBzImIBa][Br]) (Table 2) also showed similar pH-
induced transitions from transparent (%T=100) to turbid phase (%T= 0) at 25 °C (see
photographs in Scheme S1A and Figure 1B). But, there was no change in transparency of
[
BuImIBa][Br] solution (1 wt%) upon changing the solution pH (see photographs in Scheme
S1C and Figure 1B). On the other hand, the solution of Aa-ZILs (Table 2) remained
transparent in the pH ranges from 2-11 at 25 °C (see photographs in Schemes S1B-S1C and
Figure 1C). Among the three Aa-ZILs, transparent solutions of [EtBzImAa][Br] and
[
VBzImAa][Br] (1 wt%) became turbid
[
EtBzImVa][Br]
[VBzImIBa][Br]
[EtBzImIBa][Br]
[BuImIBa][Br]
8
0
0
[VBzImVa][Br]
80
8
0
0
0
[
[
[
BuImVa][Br]
[EtBzImAa][Br]
[VBzImAa][Br]
[BuImAa][Br]
OctImVa][Br]
MeBpyImVa][Br]
4
[MeNapImVa][Br]
40
4
0
0
0
0
0
2
1
20
10
0
2
0
0
0
1
-
10
20
-10
-10
A
B
C
-
2
4
6
8
10
12
2
4
6
8
10
12
2
4
6
8
10
12
pH
pH
pH
Figure 1. The variation of ξ and %T (at λ = 600 nm) against pH of 1 wt% aqueous solutions
of (A) all ZILs bearing valeric acid, (B) all ZILs bearing isobutyric acid and (C) all ZILs
bearing acetic acid.
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suspensions upon cooling down from 25 to 6 °C (see photographs in Scheme S1B). The
reversible nature of pH-dependent phase transitions of these ZIL solutions at 25 °C was
clearly visualized from photographs in Scheme S1A.
We assume that this pH-induced phase transition from transparent to turbid is due to
the aggregation of ZIL molecules to aggregates, formed through the electrostatic interaction
between the oppositely charged groups of neighbouring ZIL molecules in solution (Scheme
2
), the details of which will be discussed later in this section. Dynamic light scattering (DLS)
data (Figure S42) indeed revealed such aggregation of ZIL molecules by showing changes in
D s from ∼2-3 to 2700-2800 nm when the pH of the transparent solutions (1 wt%) were
h
increased to anything above the soluble pH region. Such type of aggregate formation has
already been reported for other zwitterionic macromolecular system due to similar intra-
[
60]
and/or inter-molecular electrostatic interactions.
Notably, D s of 1 wt% solutions of
h
[
BuImVa][Br], [BuImIBa][Br], [BuImAa][Br], [EtBzImAa][Br] and [VBzImAa][Br] were
remained ∼2-3 nm in the pH range of 2-11 (Figure S42), which supported the %T value of 0
in this pH range as mentioned above (Figure 1) at 25°C.
In order to explain such complex, yet interesting phase behaviours, we measured the
zeta potentials (ξs) of aqueous solutions of these ZILs at different pHs. It was observed that
the positive ξ values of ZILs (at pH 2.5) decreased sharply with increasing pH and passed
through ξ = 0 at certain pHs, those were considered to be the pIs with a net charge of zero
and finally became negative (at pH 11) (Figure 1). As summarized in Table 2, the positive ξ
at pH 2 revealed that the ZIL molecule are existed as IL consisting of an imidazolium cation
Scheme 2. Schematic representation of pH-responsive soluble-to-insoluble and UCST-Type
insoluble-to-soluble phase transitions of ZILs.
1
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and a bromide counter anion along with an attached protonated carboxylate group (Scheme
). Upon increasing the pH, the deprotonation of carboxylic acid of ZIL started and
2
completed at a pH where ξ = 0, i.e., its pI (Figure 1 and Table 2). At this point, ZIL molecule
existed in its complete zwitterionic form in solution with equal numbers of imidazolium and
carboxylate ions. Further, with the increase of pH, the ξ should be zero as there is complete
deprotonation of carboxylic acid attached to imidazolium ion. But, Figure 1 clearly showed
that the ξ became negative with increase of pH of ZIL solution. Such negative ξ can be
ascribed to the effective decrease of positive charge of imidazolium ion due its screening by
−
the excess OH ions at higher pH. Thus, below pI, ZIL molecule remain as IL species and
would not undergo aggregation due to the electrostatic repulsion between similar charges of
the neighbouring molecules that result in a transparent solution (%T = 100) (see Figure 1 and
Scheme 2). On the other hand, near and above pI, the aqueous solution became turbid (%T =
0
) due to formation of insoluble ZIL aggregates (Figure 1). Such aggregates are actually
formed by the self-aggregation through intermolecular ionic cross-linking between the
oppositely charged imidazolium cation and carboxylate anion of neighbouring ZIL molecules
(Scheme 2). In fact, these aggregates become hydrophobic as they are charge neutral.
Moreover, one should note that each of these ZILs were insoluble in a certain pH range, the
minimum of which is little below its pI and the maximum of which is far above its pI (Table
2
). Below the pI, where ξ≠ 0, rather slightly positive (Figure 1), the fraction of ZIL molecules
are still existed in their zwitterionic form and able to form aggregates through intermolecular
electrostatic interactions, resulting in turbid suspension. However, as suggested by one of the
reviewer that one cannot rule out the possibility of intramolecular interactions between
imidazolium cation and carboxylate anion that may also play a role in the aggregation of
ZILs into different structures. Although, it seems that there is almost no possibility to have
intramolecular interactions between the imidazolium and carboxylate ions in same molecule
for ZIL-Aas and ZIL-IBas bearing acetate or isobutyratate anions as the spacer groups
containing only one single carbon and two bulky methyl substituent respectively. But the
presence of intramolecular interactions in valereate containing ZILs is certainly a possibility
and their possible role in aggregation cannot be ruled out.
Furthermore, Table 2 showed that the different ZILs have different pIs, which are
difficult to explain at this point. However, it should be noted that for ZILs bearing same R-
groups but having different carboxylic acid moieties of varying alkyl spacers, there was
almost no change in their pIs (Table 2), as because the pK of these carboxylic acids were
a
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almost the same (range 4.5-4.84). However, the difference in pIs of different ZILs can
certainly be ascribed to the effect of presence of different proximal alkyl/vinyl/aryl
substituents of imidazolium cation. For example, the higher pI of [OctImVa][Br] compared to
that of [BuImVa][Br] can be correlated to the higher pK value of octyl imidazole than that of
a
[61]
butyl imidazole.
Interestingly, as mentioned above that the ZILs bearing butyl substituent
[BuImVa][Br], [BuImIBa][Br], [BuImAa][Br]) did not exhibit pH-induced phase transitions
Table 2), though these molecules in solution existed in their zwitterionic form at pI.
(
(
Therefore, one should expect aggregate formation due to electrostatic interaction among
opposite charges of neighbouring ZILs. But the formed aggregates are not probably stable
enough to give turbid suspension. This observation indicated that not only the electrostatic
interaction but also hydrophobic interaction among substituent R-groups of ZIL played an
important role in stabilizing its hydrophobic aggregates in solution (Scheme 2). The
combined electrostatic and the weak hydrophobic interactions among small butyl chain is not
enough to stabilize the formed aggregates of butyl-ZILs in water. However, in addition to
electrostatic interactions, the strong hydrophobic interactions (Scheme 2) among Oct, EtBz,
VBz, MeNap, MeBpy moieties of ZILs gave their corresponding aggregates enough
stabilities to remain stable in turbid suspensions (Table 2). Notably, in spite of having high
hydrophobic interaction, [VBzImAa][Br] and [EtBzImAa][Br] molecules did not undergo
aggregation and the solution remained transparent (%T ∼100) (Figure 1C) in the pH ranges of
2
-11 at 25 °C. The obvious reason is the weak ionic interaction among the zwitterionic ImAa
compared to that of ImVa and ImIBa moieties having high degrees of zwitterionic characters.
However, the stability of the hydrophobic aggregates of Aa-ZILs is so moderate that it would
not be thermally stable at 25 °C, but is enough to keep such aggregates stable at low
temperature (below 6 °C). This is the reason why we observed turbid suspensions for
[
VBzImAa][Br] and [EtBzImAa][Br] at higher pH (~7) and at low temperature (~6 °C).
Nanostructured ZIL aggregates of tunable morphologies: Effects of R-group and pH
The aggregates of various morphologies formed in turbid suspensions (at pH > pI, Table 2) of
different ZILs were systematically examined by Field emission scanning electron microscopy
(FESEM). As can be seen from Figure 2A, [EtBzImVa][Br] at pH 7 aggregated into
nanostructures of interesting tripod and tetrapod shapes. The formation of these types of pod-
[
62]
like morphologies is very common in bottom-up growth of metal nanostructures, but, the
formation of this type of self-assembled pod-shaped organic nanostructures is absolutely rare.
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The high magnification image of one such tripod clearly revealed that it is actually made up
with many spherical nanoaggregates (NAs) of average diameter (D) ∼20 nm (inset of Figure
2
A). It has been explained that the pod-like nanostructures were also grown from the initially
[
62]
formed metal nanoclusters.
On the other hand, at pH 7, [VBzImVa][Br] initially
aggregated into similar NAs of D~20 nm (inset of Figure 2B) that eventually underwent
fractal growth into fern-tree-like structures
4
0 nm
4
0 nm
20 nm
A
F
B
E
C
D
I
2
0 nm
1
00 nm
1 µm
2
0 nm
G
H
Figure 2. FESEM images of aggregates in suspensions of different ZILs bearing various R-
groups and alkyl spacer groups: (A) [EtBzImVa][Br] (pH 7) (Inset: enlarge view of a portion
image in panel A); (B) [VBzImVa][Br] (pH 7) (Inset: enlarge view of a portion image in
panel B); (C) [OctImVa][Br] (pH 7) (Inset: enlarge view of a portion image in panel C); (D)
[
MeNapImVa][Br] (pH 5.5); (E) [MeBpyImVa][Br] (pH 5.5) (Inset: enlarge view of a
portion image in panel E); (F) [EtBzImIBa][Br] (pH 7); (G) [VBzImIBa][Br] (pH 7); (H)
EtBzImAa][Br] (pH 7); (I) [VBzImAa][Br] (pH 7) (Inset: enlarge view of a portion image in
panel I).
[
1
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(Figure 2B). At pH 7, [OctImVa][Br] also aggregated into nanostructures of mainly flower-
like dendritic morphology (Figure 2C). The magnified image (inset of Figure 2C) clearly
showed some spherical particles (D∼20 nm) on the surface of this flower-like structures. As
mentioned above, these spherical nanoparticles are ZIL’s NAs and thus act as precursor to
form these flower-like nanostructures. We have also shown in our earlier report that growth
of flower-like zinc oxide (ZnO) nanostructure occurred by the association of spherical ZnO
[
63]
nuclei of D~20 nm.
Figure 2D) and [MeBpyImVa][Br] (Figure 2E) gave only large spheres of average diameter
100 nm. Both of these larger spheres are formed by the secondary association of the
initially formed spherical NAs (D∼20 nm) (see the magnified image in the inset of Figure
E), the formation of which will be discussed later in this section. Further, IBa-ZILs,
EtBzImIBa][Br] and [VBzImIBa][Br] also underwent similar pH-induced aggregation, but
However, upon aggregation at pH 5.5, both the [MeNapImVa][Br]
(
∼
2
[
gave different types of dendritic nanostructures depending on their R-groups as they attain
the pH 7 (Figure 2F and Figure 2G). However, both dendrites were also constituted of
spherical NAs (D~20 nm). Whereas, [EtBzImAa][Br] and [VBzImAa][Br] at same pH 7
associated into large spheres (D~100 nm) (Figure 2H) and dendrites (Figure 2I) respectively.
Overall, it can be summarized that one can tune the morphology of ZILs (with the same EtBz
group) aggregates from pod-shaped (Figure 2A) to dendrites (Figure 2F) to large spheres
(Figure 2H) through the structural variation of their zwitterionic counterpart from ImVa to
ImIBa to ImAa (Scheme 1) respectively. The aggregates’ morphologies are also dependent
on the nature of the R-groups attached with imidazolium cation of ZIL. From the above
mentioned discussion, it is very clear that the building units for all such structures are
spherical NAs (D∼20 nm). Therefore, we may propose that, initially at a certain pH above pI,
ZIL molecules associated through intermolecular ion-bridging and hydrophobic interaction
(Scheme 2) to NAs (D∼20 nm), which act as building unit, in each case, to produce higher
order nanostructures of various shapes such as sphere, pod-shaped structures, fern-like,
flower-like, dendrites etc (Scheme 3). The formation of such different nanostructures can be
ascribed to the secondary aggregation of the NAs from different ZILs of different extent of
hydrophobicities. Note that we are for the first time able to achieve self-assembled
nanostructures of such huge variation in their morphologies from small organic molecules by
introducing the zwitterionic and ionic liquid components.
Such self-assembled
nanostructures are rarely reported for small zwitterionic organic molecules, but there is one
[
46]
report of only spherical nanostructures from zwitterionic supramolecules. However, there
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are lots of reports of inorganic nanostructures of various morphologies prepared by several
[
9,63]
different synthetic and assembly techniques.
Further, to investigate the morphology changes upon pH variation, a representative
ZIL, [VBzImVa][Br] was subjected to two pHs (5.7 and 9). We observed tetrapod-like
structures at pH 5.7 (Figure 3A), which can be clearly seen from the high magnification
image in the inset. Such tetrapods are also formed through the secondary aggregation of the
Scheme 3. Schematic representation for the mechanism of formation of different shaped
nanostructured aggregates of ZILs.
A
B
C
4
0 nm
20 nm
Figure 3. FESEM images of different ZILs’ aggregates in suspensions at different pHs: (A)
VBzImVa][Br] at pH 5.7; (B) [VBzImVa][Br] at pH 9 and (B) [EtBzImVa][Br] at pH 9.
[
1
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initially formed spherical NAs (D∼20 nm) as mentioned in Scheme 3. However, upon
increasing the solution’s pH from 7 to 9, these tetrapods underwent fractal aggregation into
dendritic fern-tree-like structures (Figure 3B). Moreover, the similar kind of dendritic
morphology was also observed for other ZIL, [EtBzImVa][Br] at pH 9 (Figure 3C). Such
morphology variation is probably due to the charged instability in the initially formed NAs
−
with adsorbed OH ion at high solution pH. Thus, in order to stabilize and to reduce surface
charge, the spherical NAs (D~20 nm) merged into various higher order fractal structures as
shown schematically in Scheme 3.
Thermoresponsiveness of aqueous ZIL Solutions at different pHs: tuning of cloud
points
As mentioned in Table 2, the aqueous solutions of ZILs were transformed from transparent to
colloidal turbid suspension upon raising the solution pH from anything below pI to anything
near or above their pI, due to the formation of insoluble ZIL aggregates. As can seen from the
photographs in the inset of Figure 4A, this turbid suspension became transparent upon
heating and turbidity reappeared upon cooling indicating a reversible UCST-type phase
transition. The turbidity curves in Figure 4A represents the variation of transmittance with
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
a
a
b
b
c
c
d
d
e
e
f
f
g
g
h
h
i
i
A
B
c
0
40
Temperature ( C)
80
0
20
40
60
80
0
10
Temperature ( C)
20
o
o
o
Temperature ( C)
Figure 4. Turbidity curves (measured at λ = 600 nm) in (---heating)/(cooling) cycles of
(A) 1 wt% solutions of ZILs bearing valeric acid moieties: (a) [EtBzImVa][Br] at pH 6.4; (b)
[VBzImVa][Br] at pH 6.0; (c) [OctImVa][Br] at pH 5.6; (d) [MeNapImVa][Br] at pH 5.0 and
(e) [MeBpyImVa][Br] at pH 4.0. Inset showed photograph of insoluble to soluble phase
transition upon heating. (B) 1wt% solutions of ZILs bearing isobutyric acid moieties: (f)
EtBzImIBa][Br] at pH 7.0 and (g) [VBzImIBa][Br] at pH 7.0. (C) 1wt% solutions of ZILs
bearing acetic acid moiety: (h) [EtBzImAa][Br] at pH 7.0 and (i) [VBzImAa][Br] at pH 7.0.
[
1
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temperature of different ZILs bearing valeric acid moiety (Va-ZILs) at different pHs. Note
that the turbidity of different ZILs in solutions were appeared at different pHs as they
contained different R-groups. Typically, the turbidity curve of 1 wt% [EtBzImVa][Br]
exhibited a gradually increase of the %T from 0 to 78 upon heating or visa-versa upon
cooling indicating T s at 51.3 (heating) and 57.7 °C (cooling) with a slight hysteresis (Table
cp
S3 and Figure 4A). Similarly, the 1 wt% solutions of [VBzImVa][Br], [OctImVa][Br],
[
(
MeNapImVa][Br], and [MeBpyImVa][Br] also exhibited a sharp UCST-type transitions
Figure 4A) with T s (cooling cycles) of 31.2, 75.0, 63.0 and 44.4 °C respectively (Table
cp
S3). These results substantiated that T s were different for different ZILs depending on the
cp
R-group present. Such variation is expected and will be discussed later in this section. It was
noticed that %T of these ZILs never touched to 100, though their solution looked very
transparent (Inset of Figure 4A), indicating the presence of some kind of weakly associated
ZIL aggregates. DLS data of 1 wt% solution of [EtBzImVa][Br] in the cooling run (Figure
5
A) further exhibited rapid increase of D s from 15 to 2806 nm with decrease of temperature,
h
signifying the formation of ZIL aggregates form their soluble molecules through the
electrostatic as well as hydrophobic interactions among neighbouring ZILs (Scheme 2). DLS
curves for other Va-ZILs also showed similar jump (13-15 nm to 2700-2842 nm) in their Dhs
upon cooling (Figure 5A). The obtained T s from these curves were summarized in Table S3
cp
3000
2000
1000
0
3000
2000
1000
0
3000
2000
1000
0
A
B
c
a
b
c
h
i
f
d
e
g
0
20
40
60
o
80
0
20
40
60
0
10
Temperature ( C)
20
o
o
Temperature ( C)
Temperature ( C)
Figure 5. Variation of D s of aggregates of different ZILs in water (1 wt%) with temperature
h
in the cooling cycles: (A) ZILs bearing valeric acid moieties: (a) [EtBzImVa][Br] at pH 6.4;
(
b) [VBzImVa][Br] at pH 6.0; (c) [OctImVa][Br] at pH 5.6; (d) [MeNapImVa][Br] at pH 5.0
and (e) [MeBpyImVa][Br] at pH 4.0. (B) ZILs bearing isobutyric acid moieties: (f)
EtBzImIBa][Br] at pH 7.0 and (g) [VBzImIBa][Br] at pH 7. (C) ZILs bearing acetic acid
moieties: (h) [EtBzImAa][Br] at pH 7.0 and (i) [VBzImAa][Br] at pH 7.0.
[
1
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and the values of those were very close to those measured by turbidimetry. These results
again substantiate that the difference in chemical structures of ZILs induce a drastic change in
their T s, which can be explained in the following way. At the experimental pHs (near pI), as
cp
the zwitterionic counterpart (ImVa) in these ZILs are the same. Therefore, one can expect to
have almost similar extent of zwitterionic character present in these ZILs (Table 2).
Therefore, such huge differences in their T s can only be ascribed to the requirement of
cp
excess energy to disrupt the additional hydrophobic interactions that caused by the bulky R-
groups present in the Va-ZILs in addition to ionic interactions between zwitterionic
counterparts (Scheme 2).
Further, T s of ZILs bearing isobutyric and acetic acid moieties obtained from the
cp
turbidity curves in Figure 4B and Figure 4C also showed that their values were dependent on
the nature of attached R-groups. For example, T s of [VBzImIBa][Br] and [EtBzImIBa][Br]
cp
at pH 7 were found to be 25.7 and 44.8 °C respectively in the cooling run. It is interesting to
note that the change of R-group from VBz to analogous EtBz resulted in an increase of Tcps
of IBa-ZILs by ~20 and 18 °C in the cooling and heating run respectively (Table S3). At this
point, we do not know the exact reason for such increase. But, a similar increase of T (~26
cp
°
C, in cooling run) on going from [VBzImVa][Br] (31.2 °C) to [EtBzImVa][Br] (57.7 °C)
was also noticed for Va-ZILs. Whereas, in cases of [VBzImAa][Br] and [EtBzImAa][Br],
there were almost no difference in their T s 7.4 and 7.8°C respectively. But, it should be
cp
noted that the phase transitions of these Aa-ZILs occurred at very low temperature compared
to that of Va-ZILs and IBa-ZILs. The probable reason is that the thermal stability of
aggregates of Aa-ZILs is less due to the presence of the weak ionic interaction among their
zwitterionic ImAa counterpart. On the other hand, the aggregates of Va-ZILs and IBa-ZILs
have high thermal stabilities as the ImVa and ImIBa moieties having high degree of
zwitterionic characters. Thus, higher T s are the consequence of higher energy required to
cp
break aggregates in the latter two cases. UCST-type phase behaviours were also reversible
under repeatable heating/cooling cycles for at least four cycles of all nine ZILs having
different R-groups as shown in Figure S43, indicating their high thermal stabilities during
such heating/cooling runs.
Further, to check the effect of pH, the %Ts of cloudy solutions of two representative
ZILs, [VBzImVa][Br] and [EtBzImIBa][Br] (1 wt%) at different pHs were measured against
increasing temperature and T s were determined from their turbidity cooling curves (Figure
cp
S44). The plots describing T s against solution pHs (Figure 6A) clearly revealed almost a
cp
2
0
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Chemistry - A European Journal
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linear increase of T with increasing pH up to a certain value for both ZILs. However, in the
cp
case of [EtBzImVa][Br], the rate of increase of T against pH was comparatively lower than
cp
that of [VBzImVa][Br], which again support the effect of different extent of hydrophobic
interactions among various R-groups on the T . It is proposed that with the increase of
cp
solution pH, the degree of dissociation of carboxylic acid group of ZIL become higher which
in turn increases the charge difference between imidazolium cation and carboxylate anion.
Thus, the electrostatic interactions between the opposite charges of zwitterions increase and
80
70
60
50
40
30
20
74
70
66
62
A
B
[
VBzImVa][Br]
[EtBzImIBa][Br]
6
8
10
0.8
1.6
2.4
3.2
pH
[ZIL] (Wt%)
Figure 6. (A) Effect of pH on cloud point (T ) of different 1wt% ZIL solutions. (B) Effect
cp
of [MeNapImVa][Br]’s concentration on T s at pH 5.
cp
hence higher energy (higher T ) is required to disrupt the combined hydrophobic and high
cp
electrostatic interactions between the neighbouring zwitterions of ZILs at higher pH.
To study the effect of concentration, T s of a representative sample
cp
[
MeNapImVa][Br] at a fixed pH 5 were measured at its different concentrations from the
turbidity cooling curves (Figure S45). It was very clear from these curves that T increased
cp
almost linearly with ZIL concentration up to 3 wt% (Figure 6B). It should be noted that T of
cp
this ZIL solution above this concentration was not measurable because of its irreversible
coagulation and precipitation. The increase of concentration, increases the number of ZIL
molecules in solution and hence the intermolecular electrostatic interactions between the
oppositely charged zwitterionic groups and hydrophobic interaction between R-groups also
increase, which results in increase of Tcp.
2
1
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10.1002/chem.201802367
In this context, it is understood that there should not be any deprotonation of attached
carboxylic acid in the acidic pH region, i.e., below the pI, thus ZIL molecule can be
−
considered as a IL containing imidazolium cation and Br counter anion. It is, therefore,
expected that these ZIL would show some kind of responses toward foreign anions as many
cationic poly(ionic liquids) (PILs) also do due to intra- and/or inter-chain ionic cross-linking
[
64-65]
between the neighbouring PIL chains through the excess foreign anions.
Unfortunately,
we did not observe any turbidity formation upon addition of various inorganic anions such as
−
−
−
−
−
−
2−
NO , Cl , Br , I , SCN , ClO and SO up to a concentration of 5.0 M into the aqueous
3
4
4
many ZIL solutions (1 wt%). This observation may suggest the poor stability of the small
molecular ZIL aggregates through ionic-bridging in the presence of foreign anion. Whereas,
ionically-bridged PIL’s aggregates in presence of foreign ions are stable enough to give
turbid suspension in water. Further, it was noticed that upon addition of large hydrophobic
−
anion such as NTf , ZILs molecules get coagulated and phase separated out from their
2
aqueous solution and remained precipitated upon heating up to ∼90 °C.
Conclusions
In summary, the synthesis of a series of dual-responsive zwitterionic ionic liquids (ZILs)
containing both IL and ZI components of substituted imidazolium bromide and substituted
imidazolium carboxylate respectively were demonstrated, for the first time, by two-step
conventional substitution reactions. The as-synthesized ZILs have high thermal stabilities and
exhibiting interesting chemical structure-dependent glass transition behaviours, those are
normally absent in conventional ILs. Like conventional ILs, these ZILs were also soluble in
water but were showing responses to different pHs with a transition from transparent to turbid
phase depending on their chemical structures. It is proposed that the combined electrostatic
interactions between the oppositely charged neighbouring zwitterionic groups of ZILs along
with hydrophobic interactions between alkyl/vinyl/aryl substituents is responsible for
generation of ZIL aggregates in turbid suspension. ILs usually do not aggregate, but some IL
[
47]
surfactants do aggregate into micelles in aqueous solution. Whereas, these ZILs underwent
self-association into hierarchical aggregates of different morphologies from spheres to
tripods, tetrapods, fern-like, flower-like, dendrite etc. depending on solution pH and nature of
substituents. Further, the colloidal turbid suspension became transparent due to disruption of
ZIL aggregates upon heating, showing a distinct UCST-type reversible phase transition in
2
2
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Chemistry - A European Journal
10.1002/chem.201802367
aqueous solution. Importantly, the cloud point was easily tunable with respect to the change
of various parameters such as chemical structure of ZIL, its concentration and solution pH.
Experimental section
Materials
The chemicals such as [4-vinylbenzyl chloride (VBzCl; 90%), 1-bromobutane (BuBr; 99%),
1
9
-bromooctane (OctBr; 99%), α-bromoisobutyric acid (BIBa; 98%), bromoacetic acid (BAa;
7%), 5-bromovaleric acid (BVa; 97%), imidazole (Im; 99%)] and [4-ethylbenzyl chloride
(
EtBzCl; 98%), 2-(bromomethyl)naphthalene (MeNapBr; 98%) and 4-bromomethylbiphenyl
MeBpyBr; 98%)] were purchased from Aldrich and TCI, India respectively. Sodium
(
bicarbonate (NaHCO ) and sodium hydroxide (NaOH) were obtained from E. Merck, India.
3
Sodium nitrate (NaNO ), sodium chloride (NaCl), sodium bromide (NaBr), sodium iodide
3
(
NaI), sodium thiocyanate (NaSCN), sodium perchlorate (NaClO ), sodium sulfate (Na SO )
4 2 4
and bis(trifluoromethane)sulfonimide lithium salt (LiNTf ) were purchased from E. Merck,
2
India. All these chemicals were used as received without any further purifications.
Acetonitrile (ACN) and acetone were pot-to-pot distilled and collected in round bottom (RB)
flask prior to use. Milli-Q water was used in the preparation of various solutions.
Methods
Preparation of different alkyl/vinyl/aryl imidazoles: In the first step, six different
substituted imidazoles (Scheme 1) were synthesized by nucleophilic substitution reaction of
imidazole with various alkyl/vinyl/aryl halides using a slight modification of a reported
[
66]
protocol. The synthesis procedure were as follow: six two-neck RB flasks (250 mL) were
separately charged with imidazole (16.9 g, 248 mmol), NaHCO (6.52g, 77.6 mmol) and 124
3
mL of water/acetone (1:2, v/v) mixture. The mixtures were stirred at 25 °C for 1h under
continues purging of N2 for complete dissolution. The required amounts of any
alkyl/vinyl/aryl halide (62.06 mmol) (e.g., 9.23 mL EtBzCl; 8.75 mL VBzCl; 6.67 mL BuBr;
1
0.72 mL OctBr; 12.72 gm MeNapBr and 15.33 gm MeBpyBr) were then separately added
into the reaction mixtures and were refluxed under N atmosphere at 50 °C for 20 h with
2
(naphthalen-2-ylmethyl)-1H-imidazole (MeNapIm); 1-([1,1'-biphenyl]-4-ylmethyl)-1H-
imidazole (MeBpyIm)] were as follows. Typically, each of the mixture was filtered through
gauche funnel to remove unreacted salt after cooling. Acetone was then evaporated in a
rotary and the entire liquid mass was diluted with 350 mL diethyl ether followed by washing
2
3
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Chemistry - A European Journal
10.1002/chem.201802367
with 6 X 35 mL of water. The product and the unreacted imidazole were extracted into
aqueous phase by washing the organic phase with 100 mL of 2.0 M HCl for three times. The
partial neutralization with required volume of 4.0 M NaOH resulted a white suspension with
oily layer containing organic alkyl/vinyl/aryl imidazole and aqueous imidazolium chloride.
The final organic product was then extracted into diethyl ether (350 mL) by washing and was
collected over anhydrous Na SO using a separating funnel. Finally, ether was removed under
2
4
reduced pressure affording viscous yellow liquids (EtBzIm, VBzIm, BuIm and OctIm) or
white powders (MeNapIm and MeBpyIm) and were kept in refrigerator for further use. ESI-
1
13
MS (Figures S1-S6), H-/ C-NMR (Figures S7-S12), and FTIR (Figure S13) spectra and
their analyses were provided in the supporting information.
Synthesis of zwitterionic ionic liquids (ZILs):
A series of twelve different ZILs
(Scheme 1) were then synthesised by nucleophilic substitution reactions between as-
synthesized substituted imidazoles and bromoalkyl carboxylic acids of varying spacer length
in a molar ratio of 2:1 at 60°C. For ZILs bearing valeric acid chain (Va-ZILs), 8.14 mmol of
any of the substituted imidazoles [e.g., EtBzIm (1.52 gm), VBzIm (1.5 gm), BuIm (1 gm),
OctIm (1.46 gm,), MeNapIm (1.7 gm) and MeBpyIm (1.9 gm)] were separately added into
4
.07 mmol of BVa (736.93 mg) taken in six different RB flasks (100 mL) followed by
addition of 10 mL of dry ACN to prepare the corresponding ZILs, [EtBzImVa][Br],
VBzImVa][Br], [BuImVa][Br], [OctImVa][Br], [MeNapImVa][Br] and [MeBpyImVa][Br]
[
respectively. The reaction mixtures were then placed in a preheated oil bath at 60 °C and
continued stirring for 24 h. After completion of reaction, the products were isolated by
precipitation in diethyl ether. The resultant ZILs were then washed with diethyl ether for four
times for complete removal of unreacted components. ZILs were then subjected to prolong
drying at 50 °C in vacuum oven for three days to ensure complete removal of water.
Furthermore, ZILs bearing isobutyric acid chain (IBa-ZILs) ([EtBzImIBa][Br],
[
[
VBzImIBa][Br] and [BuImIBa][Br]), and acetic acid chain (Aa-ZILs) ([EtBzImAa][Br],
VBzImAa][Br] and [BuImAa][Br]) (Scheme 1) were also synthesised following the same
procedure as mentioned above. For example, 8.14 mmol of either EtBzIm (1.52 gm) or
VBzIm (1.5 gm) or BuIm (1 gm) were separately added into three RB flasks containing BIBa
(4.07 mmol; 679.8 mg) with 10 mL dry ACN and three RB flasks with BAa (4.07 mmol;
5
65.62 mg) and 10 mL dry ACN and were heated at 60 °C for 24 h. The purification and
characterization procedures were same as mentioned above. It should be mentioned that
TEMPO (45 mg, 0.28 mmol) was added into the reaction vessel as radical quencher for
2
4
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Chemistry - A European Journal
10.1002/chem.201802367
synthesis of vinylic ZILs ([VBzImVa][Br], [VBzImIBa][Br]and [VBzImAa][Br]). ESI-MS
1
13
(
Figures S14-S25), H-/ C-NMR (Figures S26-S37), and FTIR (Figure S38) spectra of these
ZILs and their analyses were provided in the supporting information.
pH-Responsive phase behaviours of aqueous ZIL solutions: To
study
pH-
responsiveness, the pH of aqueous solutions of different ZILs were systematically varied
from pH 2 to 11 using 1 M HCl and/or NaOH and were monitored by a pH meter (Oakton) at
2
5 °C. The isoelectric points (pIs) of ZILs were determined by monitoring zeta potentials (ξs)
at different pHs in a dynamic light scattering (DLS) instrument. Typically, the plot of pH
versus ξ gave the value of pI, means the pH at which the overall charge is zero, i.e., ξ =0. The
pH-induced soluble-insoluble phase transitions of these ZILs solutions were checked from
their % transmittance (% T) (at λ = 600 nm) at different pHs in an UV-Vis spectrophotometer
and also by measuring pH-dependent hydrodynamic diameters (D s) of ZILs solutions via
h
DLS.
Cloud point measurements: Cloud points (T s) of aqueous solutions of different ZILs (see
cp
chemical structures in Scheme 1) were determined by turbidity measurements in an UV-Vis
spectrophotometer equipped with a temperature controller. In a typical procedure, a
transparent aqueous solution of any ZIL (initial pH∼2.7) was first filtered through a
membrane filter (D∼ 0.45 µm) and the pH was adjusted to a desired value to obtain a turbid
suspension. The % T (at λ = 600 nm) of the resultant turbid suspension was monitored upon
-1
increasing/decreasing temperature at a rate of 1ºC min with an equilibration for 2 min in the
temperature window of -5 to 90 ºC in a UV-Vis spectrophotometer. The T was determined
cp
from the % T versus temperature curve considering the point at which the solution
transmittance reduced to half of its original value.
Characterizations
1
13
NMR spectroscopy: H-/ C-NMR spectra of substituted imidazoles and ZILs were
acquired from CDCl and D O/CD OD respectively in a Bruker DPX 500/400 MHz
3
2
3
spectrometer.
Electrospray ionization mass (ESI-MS) spectroscopy:
The molecular weights of
substituted imidazoles and ZILs were measured from their solution in CHCl and methanol
3
respectively in a quadrupole time-of-flight (Q-TOF) Micro YA263 mass spectrometer (ESI-
MS).
2
5
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Chemistry - A European Journal
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Fourier transform infrared (FTIR) spectroscopy: FTIR spectra of substituted imidazoles
and ZILs were recorded from their cast solutions (0.5 wt%) in DCM and methanol on KBr
pellets using PerkinElmer Spectrum 400 Spectrometer.
Thermogravimetric analysis (TGA): TGA thermograms of ZILs were recorded using a TA
-1
SDT Q600 instrument at a heating rate of 10 °C min under N atmosphere. All the samples
2
were dried over night at 60°C in vacuum before analysis.
Differential scanning calorimetry (DSC): The glass transition temperature (T ) of the ZIL
g
was measured using a Perkin-Elmer Diamond DSC equipped with an intercooler at a scan
-1
rate of 20 °C min . To remove previous thermal history, at least one cycle of heating and
cooling were done and T values were taken from the final heating scan. Note that ZILs were
g
dried at 50 °C in vacuum for three days before the DSC measurements.
Dynamic light scattering (DLS):
Zetasizer NANO ZS 90, Malvern (Model 3690),
equipped with a He-Ne laser (λ = 632.8 nm) was used to measure the D s and ξs of ZILs
h
aqueous solutions at different pHs.
UV-Vis spectroscopy: Hewlett Packard 8453 diode array UV-Vis spectrophotometer (Agilent
Technologies) equipped with a Peltier temperature controller was used to measure the %T of
ZIL solutions.
Field emission scanning electron microscopy (FESEM): FESEM (Model JSM-6700F) was
used to observe the morphologies of different ZIL aggregates at different pHs. Typically, for
analysis, 5 mg sample was dissolved into 500 μL Milli-Q water (1 wt%) and the pH of the
solution was adjusted to pH 7 to obtained turbid suspensions. However, for
[
MeNapImVa][Br] and [MeBpyImVa][Br], sample specimens were prepared at a pH 5.5,
since they were precipitated at pH 7. The turbid ZIL suspensions were then diluted 10 times
with THF for freezeing the formed nanostructures. The dispersed THF suspensions of
aggregates were then drop cast on small pieces of glass and allowed to dry overnight in air.
The piece of glass containing the sample was then mounted on a metal stub followed by
platinum coating to minimize charging and was imaged under the microscope operated at an
accelerating voltage of 5 kV.
Solution Conductivity. The conductivities of ZILs in water, methanol and acetonitrile were
measured using an Oakton conductivity instrument (Model no: pH/con 510 series) with the
-1
cell constants of 1 cm . The cell was calibrated with the aqueous KCl solution and each
sample solution was placed in a cell with a jacket for measurement. All measurements were
carried out at 25 °C and with 0.01(M) solutions.
2
6
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Supporting Information (SI)
ESI-MS, NMR, and FTIR spectroscopic data, solution conductivities, TGA and DSC
thermograms, DLS data, temperature-dependent transmittance data, cloud points,
photographs of ZIL’s solution exhibiting pH-responsive behaviours and tables containing
%
yields and UCST-type cloud points. This material is available free of charge via the
Internet.
Acknowledgements
Y.B. thanks IACS for providing fellowship. This research was supported by the grants from
SERB, DST, India.
Keywords: Ionic liquid • Zwitterion • Nanostructures • Aggregates • Stimuli-responsive •
Thermophysical property
Conflict of interest
The authors declare no conflict of interest
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