Indian Journal of Chemistry
Vol. 52A, November 2013, pp. 1377-1382
Unusual salting effects in ionic liquid solutions
Raju Nanda & Anil Kumar*
Physical & Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India
Email: [email protected]
Received 2 September 2013; revised and accepted 3 October 2013
Unusual salting effects of salting-out agents like LiCl and NaCl and salting-in agents like LiClO4 and NaClO4 in ionic
liquid solutions are reported. It is observed that the salting behavior in water can be altered in the presence of ionic liquids. It
is demonstrated that the salting-in agents in the presence of the ionic liquids with higher alkyl chain can display unusual fall
and rise in the viscosity of the systems. On the other hand, no change in the behavior of the salting-out agents is noted in
these ionic liquids. A tentative explanation is proposed for the observed viscosity data in these systems.
Keywords: Ionic liquids, Salting effects, Viscosity, Cationic ring
It has long been established that the ions of strong
electrolytes enable the water molecules to orient
around themselves. The concept of “water structurebreakers” and “water structure-makers” was first
introduced by Gurney1 and Frank and Wen,2 who
postulated that each ion is surrounded by three
distinct regions of water structure. Several years ago,
the notion of the water structure with regard to the
presence of ions was proposed.3 In the first layer, the
water molecules are tightly bound to the ion. The
second region is extended farther away from the ion
and is known as the region of structure breaking. Only
at larger distances, where the ionic field is weak, the
water molecules constitute the “normal” ice-like
structure. Small (in terms of crystallographic radii)
and strongly hydrated ions reinforce the “normal”
structure of water and the region of the structurebreaking is not observed. In contrast, large
less-hydrated ions disturb the ice-like structure and
generate an extensive region of structure breaking.
The quantitative scale to indicate the structuremaking and breaking abilities of ions is indicated by
the viscosity B-coefficients obtained from the analysis
of viscosity data using Jones-Dole equation.4 The
positive and negative values of the B-coefficients
indicate the structure-making and-breaking abilities,
respectively of the ions. The B-coefficient values of
the constituent ions are additive and yield the
B-coefficient value for the salt.5
In fact, the structure-breaking ions are also referred
to as chaotropes, whereas structure makers are known
as kosmotropes2 and have been widely used to
describe denaturation of biological molecules like
proteins and nucleic acids.6 Similarly, the effect of salts
was observed on the solubility of organic compounds
and was termed as salting-out and in behavior.7
For the past decade, there has been an upheaval in
the research activity with respect to the synthesis,
characterization and applications of special materials
called ionic liquids.8-14 Ionic liquids have wide
applications in a variety of processes because of
insignificant vapor pressure, recyclability, wide
liquidus range, wide electrochemical window, high
thermal stability and non-flammability, to name a
few.15,16 Ionic liquids are molten organic salts
compresing an asymmetric organic cation and a
symmetric/asymmetric organic/inorganic anion.
Some years ago, we reported that the highly
viscous nature of ionic liquids can be detrimental for
carrying out both inter-and intra-molecular
Diels-Alder
reactions.17,18
Subsequently,
we
demonstrated that it was possible to achieve lowering in
the viscosity of ionic liquids by adding a small amount
of solvent called co-solvent.19 A co-solvent may be
understood as a very small amount of a different solvent
added to the main bulk solvent medium. Isothermal
titration calorimetry was carried out on some ionic
liquids in water to unravel ion-ion and ion-water
interactions that may be responsible for a sudden fall in
high viscosity of many ionic liquids.20 In order to
understand how a solvent system comprising ionic liquid
and a co-solvent can impact the rate constants of
Diels-Alder reactions, we later investigated the role of
viscosity of the solvent system on the rate constants.21,22
1378
INDIAN J CHEM, SEC A, NOVEMBER 2013
During our extensive research program on ionic
liquids, we discovered unusual results on how
viscosity of the ionic liquid system can be altered in
the presence of salts. In this paper, we report these
unusual findings with the help of accurate viscosity
measurements. In other words, we demonstrate that
the addition of a specific type of salt when added to
the ionic liquid solutions can exhibit both decrease
and increase in the viscosity. The ionic liquids
selected for this purpose were 1-butyl-3methylimidazolium bromide [BMIM][Br], 1-hexyl-3methylimidazolium bromide [HMIM][Br] and
1-octyl-3-methylimidazolium bromide [OMIM][Br].
The structures of the cations of these ionic liquids are
shown in Fig. 1. Water was used as the solvent.
Materials and Methods
The ionic liquids employed in the current
investigation were synthesized by a single step atom
economic reaction with equimolar mixture of
1-methyl imidazole and 1-alkyl bromide at 70 oC for
12 h in an oil bath as reported in the literature.20-26 The
excess base was removed by adding ethyl acetate and
the putting the reaction mixture to a rotavapor at
80 °C for 10 h in order to remove any ethyl acetate.
The final product was dried under high vacuum prior
to use for another 24 h at <0.01 Torr pressure. The
1
H NMR spectra of these ionic liquids agreed with the
reported values and did not show any traces of the
base used during the synthesis.21-26 The water content
of the pure and dried APILs as measured by
Karl-Fischer coulometer did not exceed 50 ppm.27,28
LiCl, LiClO4, NaClO4, guanidinium chloride
(GnCl), guanidinium sulphate (Gn)2SO4 and urea
(purchased from Sigma Aldrich and Fluka with
99.5% w/w purity) were heated in an oven at 150 °C
for 5 h before their use. Deionized water having
specific conductance <0.055×10-6 S cm-1 was used
throughout the investigation. The salt solutions were
prepared on the basis of molality with an accuracy of
±0.0002 mol kg-1.
The viscosity, η, of the solution was measured
using a Brookfield ultra-rheometer with cone plate
arrangement. The amount of sample required for the
measurement of viscosity was 0.5 mL. The η values
were obtained by using Eq. (1),
η = (100/RPM) (TK) (Torque) (SMC)
…(1)
where RPM, TK (0.09373) and SMC (0.327) are the
speed, viscometer torque constant and spindle
multiplier constant, respectively.
The instrument was calibrated against the η data of
aqueous CaCl2, MgCl2 and [BMIM][BF4] solutions of
different concentrations with an accuracy of ±1%.29,30
The precision in the measurement of the η data as
obtained from the average of the triplicate
measurements is 0.25%. All the measurements were
carried out at 298.15±0.01 K as monitored by a Julabo
constant temperature thermostat bath.
Results and Discussion
Effect of structure makers
The viscosities of aqueous ionic liquid solutions
were first measured in the presence of LiCl, which is
a strong structure-maker as evident from its
B-coefficient value of +0.141 dm3 mol-1 and from
other properties.5 It was observed that the η values of
aqueous [BMIM][Br], [HMIM][Br] and [OMIM][Br]
solutions with xIL = 0.06 increased monotonously on
addition of LiCl. The increase in the η value was
sharp in the case of aqueous [OMIM][Br], as
compared to that in [HMIM][Br] and [BMIM][Br].
These data together with those of pure aqueous LiCl
are shown in Fig. 2, indicating the extent of
enhancement of viscosity of aqueous [OMIM][Br] for
the purpose of illustration. The rise in the viscosity of
aqueous [BMIM][Br] and [HMIM][Br] systems upon
adding LiCl is not appreciable. However, the viscosity
of aqueous [OMIM][Br] increase twice on adding a
solution of 2.5 mol kg-1 LiCl. Since the rise in the
viscosity in the other two ionic liquids did not differ
from that in pure LiCl solution, the effect in
[OMIM][Br] system may be attributed to the long
alkyl chain length attached to imidazolium ring. This
suggests that additives like LiCl, which is a structuremaker, can be used to enhance the viscosity of the
ionic liquid solutions.
Fig. 1 – Structure of the cations of the ionic liquids employed in the study.
NANDA & KUMAR: UNUSUAL SALTING EFFECTS IN IONIC LIQUID SOLUTIONS
1379
cations. Due to this repulsion caused by
hydrophobic forces, the water molecules would
remain away from [OMIM]+ cations and would
form iceberg-like structures around the [OMIM]+
species in terms of Frank’s model.30 On adding
LiCl to this solution, the possibility of forming
these icebergs in the solution will be greatly
enhanced due to the structure-making effect of Li+
cations and hence, due to the formation of bigger
and more rigid ice-like structure of water molecules
and also the increased number of components in the
form of Li+ and Cl-, the viscosity tends to rise
sharply in the case of [OMIM][Br]-water system.
Fig. 2 – The variation of η values with varying concentrations of
LiCl in the ionic liquid solutions of xIL = 0.06. {[BMIM][Br]
(■); [HMIM][Br] (); [OMIM][Br] () in comparison with those
of aqueous solutions of LiCl (∇)}.
The water molecules are already engaged with the
cationic species of the ionic liquids through nonbonding specific interactions particularly in the case
of smaller cations like [BMIM]+ and [HMIM]+. Also,
the coordination numbers of these cations are larger
as compared to those of strong electrolytes indicating
the involvement of more water molecules during the
cation-water interactions. When aqueous solution of
LiCl is added to this system, (in which the network of
water-ionic liquid is already very pronounced due to
the presence of ionic liquid species), Li+ ions have to
compete with the cations of the ionic liquids in order
to manifest their usual structure-making effect, which
appears to be difficult due to a variety of interactions
of the cations and anions of ionic liquids. Secondly,
the size of the Li+ cations being very small, these ions
can fit into the intermolecular spaces in between the
two hydrated cations of ionic liquids without
disturbing the pre-existing ionic liquid-water
complexes. Thus, it is only the number of components
–
that would be increased in the form of Li+ and Cl ions
in the system. Due to the increased number of
components in the system, the viscosity tends to
increase. This increase in viscosity is milder for both
[BMIM][Br] and [HMIM][Br] and many folds
higher for [OMIM][Br]. The reason for this
abnormally
high
viscosity
of
aqueous
[OMIM][Br]-LiCl system can be attributed to
hydrophobicity. [OMIM]+ cations render maximum
hydrophobicity as compared to the two other
Rather than examining the effect of another
conventional structure-making salt like NaCl, MgCl2,
etc, an uncommon salt, guanidinium sulphate
(Gn2SO4) was selected for the study. Guanidinium
cation, Gn+ is a mild structure-breaker, while SO42- is
a good structure-maker.31 As seen in our earlier
studies32, on quantifying the role of salting-out and
salting-in agents on altering the reaction rates of
Diels-Alder reactions, the salting-in effect of Gn+
species is over-compensated by the salting-out
ability of SO42-. This salt has also been noted to
enhance the transition temperature of the proteins,
thus acting as a stabilizer, unlike other guanidinium
salts.33 The contrasting effect of Gn2SO4 was also
observed during the investigation on efficacy of various
protein denaturants (several guanidinium salts)33 as
hydrophobic bond breakers34 (the molecular basis of
the relationship between the nature of ionic solutes
and surface tension is described). It was observed
that all the guanidinium salts (the salting-in agents)
showed a strong disruptive effect and increased the
critical micelle concentration by several orders of
magnitude, whereas Gn2SO4 stabilized the micelles.
The structure-making ability of Gn2SO4 is also
confirmed by other solution properties.35 The
addition of Gn2SO4 was also noted to enhance the
viscosities of aqueous ionic liquids as shown in Fig.
3. However, the effect of Gn2SO4 on the viscosity of
the ionic liquid solutions is milder as compared to
that of LiCl.
Guanidinium chloride (GnCl) is a denaturant often
used to investigate the denaturation of proteins and
nucleic acids.33 An earlier study has shown it to be a
mild salting-in agent.34 Addition of GnCl in
[BMIM][Br] did not bring any great change in the
viscosities, whereas the viscosities in [OMIM][Br]
system decreased with an increase in the salt
1380
INDIAN J CHEM, SEC A, NOVEMBER 2013
Fig. 3 – The variation in the η versus with varying concentrations
of Gn2SO4 in the ionic liquid solutions of xIL = 0.06.
{[HMIM][Br] (∇), and, [OMIM][Br] (∆); a similar plot for GnCl
in [HMIM][Br] (●), [OMIM][Br] (■)}.
Fig. 4 – The variation in η with respect to varying concentrations
of LiClO4 at xIL = 0.06. {[OMIM][Br] () and [HMIM][Br]
()}.
concentration (Fig. 3). In the [OMIM]Br system, the
increase in the viscosity appears to be due to the alkyl
groups attached to the imidazolium ring of the cation.
The effect of GnCl on the viscosities of the ionic
liquids is very small, and cannot be quantified due to
lack of information on the microviscosity of the
environment. Urea, a conventionally known
denaturant33 and a controversial salting-agent did not
show any clear and confirmed trend of the fall and
rise in the viscosity of the ionic liquid solutions.
Effect of structure breakers
The most interesting finding of this investigation is
the effect of structure-breakers on the viscosity of the
studied aqueous ionic liquid solutions.5 Dramatic
results were observed for the viscosity of ionic liquids
upon the addition of a salting-in agent, for example,
LiClO4. The viscosities of both aqueous [BMIM][Br]
and [HMIM]Br solutions increased monotonously
upon the addition of LiClO4 (Fig. 4). However, the
addition of LiClO4 to [OMIM][Br] solution led
initially to a decrease in the viscosity of the mixtures,
and then later to a sharp increase. We also carried out
these investigations by adding NaClO4. We obtained a
similar trend in the viscosity values (Fig. 5). A list of
the inflection points in different systems is given in
Table 1.
The reason for viscosity increment given in the
previous section dealing with the structure-makers
holds good for these systems as well. The competition
between the ions of an ionic liquid and of the salt
Fig. 5 – The variation in η with respect to varying concentrations
of NaClO4 at xIL = 0.06. {[OMIM][Br] () and [HMIM][Br]
()}.
Table 1  Inflection points in different ionic liquid solutions in
the presence of salting-in agents
Ionic liquid
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
[OMIM] [Br]
Comp. of aq. ionic Salting-in Inflection point
liquid solution (xIL)
agent
(mol kg-1)
0.05
0.06
0.07
0.10
0.05
0.06
0.07
0.10
LiClO4
LiClO4
LiClO4
LiClO4
NaClO4
NaClO4
NaClO4
NaClO4
≈ 1.0
≈ 1.2
≈ 1.4
Not observed
≈ 0.95
≈ 1.2
≈ 1.4
Not observed
NANDA & KUMAR: UNUSUAL SALTING EFFECTS IN IONIC LIQUID SOLUTIONS
becomes the major factor to determine whether a
given salt would be able to show its conventional
behavior in aqueous mixtures containing the ionic
liquid as the third component. In the case of
[OMIM][Br]-H2O system, it should be recalled that
[OMIM]+ cations have the tendency to freeze in the
form of icebergs away from itself. When ClO4- ions
are added to the system, these ions due to their
structure-breaking tendency start separating the water
molecules from these iceberg structures leading to a
drop in the viscosity of the system. When the
concentration of the ClO4- ions is increased further,
there is a possibility of their complexation with the
[OMIM]+ cations due to the highly polarizable nature
of the anion. A possiblity of cross anion exchange at
higher concentration of the salt may arise to form
[OMIM]ClO4/[ClO4-[OMIM]+ClO4-]-/[ClO4-[OMIM]+
Br-]-/[OMIMBr] complexes. Hence, the viscosity of
the system may increase sharply beyond a specific
concentration.
Alternatively, the inflection point (where the
decreasing and increasing plots meet) is seen in the
case of [OMIM][Br] system. This inflection point is
shifted towards the higher molality of salt with an
increase in xIL from 0.05 to 0.07. The appearance of
inflection point and its shift with the increasing ionic
liquid mole fraction can be explained on the basis of
ion-induced dipole interactions. The ClO4- anion
because of its higher polarizability induces dipole
moment in alkyl side chain. This ion-induced dipole
interaction between alkyl chain and ClO4- decreases
the interaction between alkyl chains, and hence,
viscosity decreases up to the inflection point. At
inflection point, all the alkyl chains remain free and
hence a minimum in viscosity is observed. Beyond
this point the added NaClO4 and LiClO4 remain in the
solution and increase the viscosity of solution because
of increased friction like any aqueous solution of
electrolyte.
Some remarks on the recent work carried out by
Takada et al.35 should be considered in the context of
our investigation. They reported shear stress
measurements with a rheometer to conclude that
viscosity showed shear thinning behaviour indicating
some structural changes around the cations. Further,
Freire et al.36 in a recent paper have provided
evidence for unexpected interactions between an ionic
liquid and inorganic salt with the help of 1H and
molecular dynamics investigations, suggesting the
need of further probe into these results.
1381
Conclusions
In summary, we have emerged with a new
methodology, which can be used to attenuate the
viscosity of ionic liquid solutions. In addition, ionic
liquids can prove to be powerful in changing the role
of salts in their aqueous solutions. In the presence of
ionic liquids, the salting-in agents display unusual fall
and rise in viscosity, while the salting-out agents
show no such behaviour. This finding will be used in
our next work, in which we are studying denaturation
studies of some proteins in ionic liquids.
Acknowledgement
RN is grateful to Council of Scientific and
Industrial Research, New Delhi, India, for awarding
him a research fellowship. AK thanks Department of
Science and Technology, New Delhi, India, for
JC Bose National Fellowship (SR/S2/JCB-26/2009).
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