Do anions influence the polarity of protic ionic liquids?

Article abstract of DOI:10.1039/c2cp23256a

Polarity studies in two classes of imidazolium-based protic ionic liquids (PILs) possessing [HSO₄]^-, [HCOO]^-, [CH ₃COO]^- and [CH₃CH₂COO]^- anions were carried out using a solvatochromic method from 298.15 to 353.15 K. For 1-methylimidazolium class of PILs, E_T(30) was found to be independent over the entire range of temperature, while E_T(30) was noted to decrease with a rise in temperature in the case of 1-butylimidazolium class of PILs containing [CH₃COO]^- and [CH ₃CH₂COO]^- anions. The E_T(30) value decreases in both the classes upon varying the anions ([HSO₄] ^-, [HCOO]^-, [CH₃COO]^- and [CH ₃CH₂COO]^-). The E_T(30) value is controlled by hydrogen bond acceptor basicity, β, and dipolarity/ polarizability, π*. The E_T(30) value for PILs varies inversely to the strength of the coulombic interaction between ions in PILs. Strong interactions between ions lead to lower E_T(30) values. Unlike the poor thermal effect on E_T(30), the Kamlet-Taft parameters i.e. α, β and π* have pronounced thermal effect in the imidazolium-based PILs. Variation in the Kamlet-Taft parameters is controlled by the stabilization of ions and the degree of proton transfer from Bronsted acid to Bronsted base. the Owner Societies 2012.

Full text of DOI:10.1039/c2cp23256a

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PAPER
Do anions influence the polarity of protic ionic liquids?w
Shashi Kant Shukla, Nageshwar D. Khupse and Anil Kumar*
Received 16th October 2011, Accepted 23rd December 2011
DOI: 10.1039/c2cp23256a
Polarity studies in two classes of imidazolium-based protic ionic liquids (PILs) possessing
[HSO4]À, [HCOO]À, [CH3COO]À and [CH3CH2COO]À anions were carried out using a
solvatochromic method from 298.15 to 353.15 K. For 1-methylimidazolium class of PILs,
ET(30) was found to be independent over the entire range of temperature, while ET(30) was noted
to decrease with a rise in temperature in the case of 1-butylimidazolium class of PILs containing
[CH3COO]À and [CH3CH2COO]À anions. The ET(30) value decreases in both the classes upon
varying the anions ([HSO4]À, [HCOO]À, [CH3COO]À and [CH3CH2COO]À). The ET(30) value is
controlled by hydrogen bond acceptor basicity, b, and dipolarity/polarizability, p*. The ET(30)
value for PILs varies inversely to the strength of the coulombic interaction between ions in PILs.
Strong interactions between ions lead to lower ET(30) values. Unlike the poor thermal effect on
ET(30), the Kamlet–Taft parameters i.e. a, b and p* have pronounced thermal effect in the
imidazolium-based PILs. Variation in the Kamlet–Taft parameters is controlled by the
stabilization of ions and the degree of proton transfer from Brønsted acid to Brønsted base.
Introduction
The era of ILs began with Protic Ionic Liquids (PILs) in
1914 with Walden, who discovered ethylammonium nitrate,
(mp 12.5 1C).9 PILs contain dissociable protons in their
continuum medium. Currently, the PIL materials have been
receiving a lot of interest towards their use as catalytic media
for biological applications, explosive formulations and as
lubricants.10 PILs containing imidazolium cations have
been used as catalytic media for many carbon–carbon bond,
carbon–oxygen bond and carbon–nitrogen bond forming
reactions.11 High ‘‘turn over number’’ of PILs makes them
useful catalysts for a variety of reactions.12 They have been
widely used in all kinds of chromatographic studies due to low
cost, high polarity, elevated miscibility with water, air/moisture
insensitivity and high selectivity to bind with polar solutes.13
Their polarity can be easily monitored by changing either
cations or anions, which makes them more useful in chromato-
graphic studies.13 PILs were known for their high dielectric
conductivities, caused by extended hydrogen bonding, which
makes them more polar.14 While we are much aware about the
thermal stability, density, molar volume, surface tension,
viscosity and ionic conductivity of the PILs, insufficient data
on the polarity of these materials make their use as a medium
somewhat difficult.
In the past decade or so, ionic liquids (ILs) have been the
subject of intense study in the areas of organic synthesis,
catalysis, electrochemical applications, solvent extraction, synthesis
and stabilization of nanostructure materials to example a few.1–3
The growing awareness of ILs is due to their insignificant vapour
pressure, high thermal stability, low melting points, good solvating
capabilities, wide electrochemical window, inflammability, intrinsic
ionic conductivities, recyclability etc.4,5 Since the use of volatile
organic compounds (VOCs) leads to environmental pollution, ILs
because of their beneficial properties may be a useful replacement
for these VOCs.6
An IL consists of a poorly coordinating organic cation and
an organic or inorganic anion. The physicochemical properties
of ILs can easily be tailored by varying cations and anions,
and thus are called as ‘designers solvent’.7 The variations in
physicochemical properties of ILs, such as viscosity, conduc-
tivity, density and so on, were found to be more sensitive to
the change in cation due to the various possible specific and
non-specific interactions. Unfortunately, very little is known
about the variation in properties upon varying anions.4 The
efficiency of an IL in diverse applications, viz., chemical,
biochemical, and electrochemical, lies in the accessibility
of various parameters which makes them suitable as non-
aqueous media.8
Polarity of ILs is sensitive towards the nature of cations and
anions. Strength of the coulombic and van der Waals inter-
action between ions decides the magnitude of polarity. Recent
theoretical observations based on the principal component
analysis for a number of aprotic ionic liquids (APILs) demon-
strate the influence of structural variations on polarity.15
Polarity of ILs can be measured by a multiparameter approach,
Physical Chemistry Division, National Chemical Laboratory,
Pune 411 008, India. E-mail: a.kumar@ncl.res.in; Fax: +912025902636
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cp23256a
c
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electronic transition energy, ET(30), and Kamlet–Taft parameters,
a hydrogen bond donor acidity, b hydrogen bond acceptor
basicity and p* dipolarity/polarizability.8a,16 ET(30) is measured
by using Reichardt’s dye, while N,N-diethyl-4-nitroaniline
and 4-nitroaniline have been used for the measurement of
Kamlet–Taft parameters. The low volatility of solvatochromic
dyes makes them useful over a wide range of temperatures.
Polarity of ILs has been also measured by using microwave
dielectric spectroscopy as studied by Weingartner et al.17
However, for low viscous fluids, the fast elementary steps of
solvation dynamics and solvent-controlled chemical reactions
have been probed by using FTIR spectroscopy because
dielectric spectroscopy is valid for slow diffusive processes.18
The hyperpolarity of PILs as expected due to the extended
hydrogen-bonding is also supported by the microwave
dielectric spectroscopy, where strong orientational correlation
between dipoles can enhance dielectric polarization.18 The
behavior of ILs and their solvation capability also depend
upon the nature of the dissolved solute.19 The polarity of ILs
obtained using a polar solute differs significantly from that
obtained by using a non-polar solute. The spectroscopically
active salt [Py]I (1-ethyl-4-(methoxycarbonyl)pyridinium iodide)
also indicates the higher polarity of ILs due to the lower
concentration of ion-pairs while higher concentration was
noted in the molecular solvents of higher polarity.19
Table 1 The acronyms of ILs employed in the present work
Acronyms
Names of ionic liquids
[BMIM][BF4]
1-Butyl-3-methylimidazolium
tetrafluoroborate
[BMIM][PF6]
1-Butyl-3-methylimidazolium
hexafluorophosphate
[HmIm][HSO4]
[HmIm][HCOO]
[HmIm][CH3COO]
[HmIm][CH3CH2COO]
[HbIm][HSO4]
[HbIm][HCOO]
[HbIm][CH3COO]
[HbIm][CH3CH2COO]
1-Methylimidazolium bisulfate
1-Methylimidazolium formate
1-Methylimidazolium acetate
1-Methylimidazolium propionate
1-Butylimidazolium bisulfate
1-Butylimidazolium formate
1-Butylimidazolium acetate
1-Butylimidazolium propionate
Fig. 1 Structures of the PILs used for thermosolvatochromic studies.
A literature search reveals that a few studies exist on
the thermosolvatochromism of ILs and their solutions.20,21
The variation of alkyl chain length and the introduction of
functional groups on cations alter the polarity.20a,b An earlier
report from our group has shown the effect of cosolvents on
the polarity of APILs using a preferential solvation model.22
The strong synergism between an IL and methanol results into
the positive deviation from ideal mixing.
In continuation, a kinetic investigation of Diels–Alder
reaction in highly aqueous medium with IL as a cosolvent
was reported in promoting the kinetics of the organic reaction.23
The results indicated a pairwise interaction between the IL
cosolvent and the initial states of reactants than that of the
transition state, thereby lowering the rate constant (k2) value.
A comprehensive study of polarity of ILs and molecular
solvents has been reported by Reichardt.24
Fig. 2 Solvatochromic probe molecules.
of ILs and their implications on chemical and electrochemical
processes.21,22,25 As a part of this ongoing program, we now
investigate the thermal effect on solvatochromism of probe
molecules in 1-methylimidazolium and 1-butylimidazolium classes
of PILs. We also address the question whether ‘hyperpolarity’
in PILs is anion-dependent. The acronyms and names of PILs
used in this work have been collected in Table 1, while Fig. 1
depicts the structures of these PILs. In Fig. 2 are shown the
solvatochromic probe molecules used in this investigation.
PILs are considered to be ‘hyperpolar’ in nature due to the
extensive hydrogen bonding between ions. The polarity of ILs
decreases with the increase in distance between ions due to
decrease in effective charge density between them. In this work,
we present the polarity of some PILs with a view to ascertaining
the role of anions. Herein, we represent another explanation
based on the Kamlet–Taft parameters for the variation in
polarity upon varying anions towards a particular cation.
Polarity study using the solvatochromic technique depends on
the preferential stabilization of well formed polar/apolar ground
or excited states of the probe molecule. The probability of
interaction between ions and the probe molecule directly
depends on the extent of coulombic interactions between them.
Strong coulombic forces between ions lead to weaker interaction
with the probe molecule; thereby reducing the polarity on the
solvatochromic scale.
Experimental section
Materials
The spectroscopic indicator dye 2,6-dichloro-4-(2,4,6-triphenyl-
pyridinium-1-yl)-phenolate (1) was obtained from Fluka
Analytical (98.5% purity). Reichardt’s dye (2) and 4-nitroaniline
(3) were used as obtained from M/s Sigma Aldrich. N,N-Diethyl-
4-nitroaniline (4) was purchased from Frinton laboratories
In our laboratory, we have embarked upon an extensive
research program on investigating the physicochemical properties
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(99% purity) (shown in Fig. 2). 1-Methylimidazole, 1-butyl-
imidazole and n-butyl bromide were distilled prior to their use.
Formic acid, acetic acid, propionic acid, sulfuric acid solutions
all obtained from Merck were used as obtained. Sodium
tetrafluoroborate and potassium hexafluorophosphate were
used as purchased from Merck.
under a nitrogen atmosphere and sealed with a septum.
The lmax was measured at different temperatures using a
UV-visible spectrophotometer. The temperature of the cell
was controlled using the single cell accessory having an
accuracy of Æ0.1 K.
N
Determination of ET and Kamlet–Taft parameters
Synthesis of protic ionic liquids
The equations required to calculate various parameters for
characterizing the polarity of ILs are given in Table 2. PILs
have lower pKa values, leading to the protonation of Reichardt’s
dye 30 (pKa = 8.6) and therefore, the long-wavelength intra-
molecular charge transfer (ICT) absorption band disappears.
The ET(30) data for all PILs were derived from ET(33), a
polarity scale of the less basic probe 2,6-dichloro-4-(2,4,6-
triphenylpyridinium-1-yl) phenolate (pKa = 4.78). For this
purpose, a simple linear regression method was used, involving
eight molecular solvents and one IL (see Fig. S1, ESIw).
PILs were synthesized by an atom economic process by mixing
equimolar amounts of acid and base.26 Dropwise addition of
base to acid was carried out for complete neutralization in an
ice bath in order to avoid heat generation. The reaction
mixture was stirred for 6 h at room temperature. The water
formed in the neutralization was removed by using a rotavapor
at 80 1C for 6 h. The last traces of water and other volatile
impurities were removed from PILs under reduced pressure at
70 1C for 10 h. The characterization and their purities were
determined by NMR spectroscopy. The water content of the
pure and dried PILs as measured by Karl-Fischer coulometer
analysis did not exceed 50 ppm. The halide contents were
estimated using Volhard titration due to the lack of a sophisti-
cated ion-chromatographic technique with us.27 Note that
the standard Volhard titration has been validated using a
chloride-selective electrode.28 The residual halide contents
(bromides) were less, >30 ppm.
ET(30) = 0.9119(Æ0.02956) ET(33) À 3.80634(Æ1.68048)
(1)
The calculated ET(30) values using the above formula were
found to be in good agreement with the measured ET(30)
values as shown in Table 3.
As suggested by earlier workers, Reichardt dye 30 (2) is not
suitable for the polarity measurement of protic ILs.29 They used
Nile red, the least basic solvatochromic dye that has been exten-
sively used for the polarity measurement of weak acids and protic
molecular solvents. However in our study we used Reichardt
dye 33 (1) and observed the long wavelength ICT absorption
band, which is absent in the case of Reichardt dye 30 (2).
Polarity measurements
The stock solutions of the required dyes were prepared in
methanol prior to their use. The stock solution was added
dropwise to a PIL, and the solvent was evaporated under
vacuum. The PIL was then transferred to a quartz cuvette
N
Table 2 Equations to determine the ET and Kamlet–Taft parameters
Polarity parameters
Empirical equations
N
Normalized electronic transition energy (ET
)
ET(30) (kcal molÀ1) = hcnmax = 28 591/lmax(nm) = 2.8591nmax where lmax is the maximum
wavelength of the lowest energy band of Reichardt’s dye (2)
ETN(30) can be normalized between water and TMS
ET = [ET(solvent) À ET(TMS)]/[ET(solvent) À ET(water)] = [ET(solvent) À 30.7]/32.4
Hydrogen bond donor acidity (a)
Hydrogen bond acceptor basicity (b)
a = 0.0649 ET(30) À 0.72p* À 2.03
n(3)max = 1.035n(4)max À 2.8b + 2.64 where n(3)max and n(4)max are the maximum wave numbers
of 4-nitroaniline (probe 3) and N,N-diethyl-4-nitroaniline (probe 4), respectively
n(4)max = 27.52 – 3.182p*
Polarity index (p*)
Table 3 Polarity parameters of aprotic ionic liquids, protic ionic liquids and molecular solvents at 298.15 K
ET(33)/kcal molÀ1
ET(30)/kcal molÀ1
p*
a
b
N
ET
Ionic liquids/solvents
[HmIm][HSO4]
[HmIm][HCOO]
[HmIm][CH3COO]
[HmIm][CH3CH2COO]
[HbIm][HSO4]
[HbIm][HCOO]
[HbIm][CH3COO]
[HbIm][CH3CH2COO]
[BMIM][PF6]
[BMIM][BF4]
Methanol
Acetone
Ethylene glycol
Tetrahydrofuran
73.89
65.62
59.50
55.70
76.04
66.20
65.01
64.35
63.6
56.0
50.5
47.0
65.5
56.6
55.5
1.02
0.78
0.61
0.50
1.07
0.80
0.77
1.17
1.10
1.03
1.50
1.12
0.97
0.88
1.225
0.812
0.61
0.81
0.85
0.10
0.58
0.73
0.73
0.506
À0.06
1.415
0.945
0.938
54.9
0.75
1.01
0.806
0.63
53.64 (52.375)a
52.4 (53.6)b
55.6 (55.4)c
43.0 (42.04)d
56.1 (56.30)e
37.8 (38.11)f
0.71 (0.669)a
0.67 (0.707)b
0.77 (0.762)c
0.38 (0.35)d
0.78 (0.790)e
0.22 (0.206)f
1.04 (1.032)a
1.05 (1.047)b
0.735 (0.730)c
0.71 (0.70)d
1.024 (0.932)e
0.590 (0.60)f
0.702 (0.634)a
0.615 (0.627)b
1.049 (1.05)c
0.250 (0.20)d
0.874 (0.95)e
À0.002 (0.0)f
0.241 (0.207)a
0.716 (0.376)b
0.625 (0.610)c
0.54 (0.54)d
0.588 (0.51)e
0.609 (0.54)f
61.61
65.13
51.34
65.73
45.59
a
Reported values are given in the parentheses. Ref. 21b. Ref. 21b. Ref. 20a. Ref. 20a. Ref. 2c. Ref. 30.
b
c
d
e
f
c
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the influence of the anion on polarity. However, the thermo-
Results and discussion
solvatochromic trend mimics the thermal effect on a than on b.
An increase in temperature from 298.15 to 353.15 K causes
slight lowering in the a value leading to negligible or small
The polarity parameters ET(30), a, b, and p* for 1-methylimida-
zolium and 1-butylimidazolium classes of PILs were initially
determined at 298.15 K as shown in Table 3. The measured
values are in good agreement with the reported data for some
ILs and organic solvents. The trends in polarity for [HmIm]+
and [HbIm]+ classes of PILs were found to be:
N
effect on the ET value.
N
ET parameter
The ETN values for 1-methylimidazolium and 1-butylimidazolium
classes of PILs show a similar trend with the variation in anion,
but a slightly different trend with temperature. The ETN value for
1-methylimidazolium class of PILs remains unaffected with the
change in temperature from 298.15 to 353.15 K. A strong
stabilization of the dye molecule in the ground state cannot be
weakened by an increase in temperature. A very small decrease in
the a and b values in the temperature range of 298.15 to 353.15 K
supports strong association between PILs and the dye molecule.
[HmIm][HSO4] > [HmIm][HCOO] > [HmIm][CH3COO]
> [HmIm][CH3CH2COO]
[HbIm][HSO4] > [HbIm][HCOO] > [HbIm][CH3COO]
> [HbIm][CH3CH2COO]
A comparison of polarity in PILs and APILs
N
The temperature dependence of the ET parameter for several
Polarity studies in APILs and PILs show opposite behavior
with respect to Kamlet–Taft parameters. In APILs, hydrogen-
bond donating (HBD) ability, a, has a major effect on the
PILs is shown in Fig. 3(a) and (b). The polarity index (p*)
depends on the interaction of a dye molecule with its cybotactic
environment. The strong interaction of a dye with its environment
leads to a high value of p*. The high p* value stabilizes the charge
of the dye molecule in the excited state; thereby influencing the
polarity.
N
ET value, while hydrogen-bond accepting (HBA) ability, b,
N 22a,24
.
has a negligible effect on the ET
HBD ability is solely
controlled by the cation while HBA ability depends on the
nature of anion. Similar observations were obtained in the case
of hydroxylic solvents and water, where HBD ability promotes
high polarity (ETN). Introduction of an alkyl substituent greatly
affects the HBD ability. In imidazolium-based ionic liquids, the
polarity decreases as the length of alkyl chain increases from
The p* value for 1-methylimidazolium class of PILs was noted
to be either constant or to decrease slowly with temperature from
298.15 to 353.15 K. The observed trends in the ETN values for
1-butylimidazolium class of PILs are found to be slightly
N
different with increasing temperature. The ET value for
N
ethyl to octyl. From [EMIM][NTf2] to [OMIM][NTf2], ET
N
decreases from 0.658 to 0.630. This decrease in ET is in the
[HmIm][HSO4] was noted to be 1.02 at 298.15 K. An increase
in the basicity of anion from [HSO4]À to [HCOO]À as indicated
by the increased b value from 0.61 to 0.81, respectively, leads to
strong coulombic interactions between ions in PILs, which
same order of decreasing HBD ability (0.627 for [EMIM][NTf2]
and 0.595 for [OMIM][NTf2]).20b Similar observations were
noted for the pyridinium, pyrrolidinium and phosphonium-
based ionic liquids where ETN values vary as the HBD ability.20b,22a
Influence of substituents on a was observed similar to the
influence on polarity, indicating that a is a major contributor
to polarity.20a,21b Hydrogen bond accepting tendency b depends
on the anion. The presence of an alkyl substituent on the cation
has a little effect on b value.20b
N
lowers the ET value from 1.02 for [HmIm][HSO4] to 0.78 for
[HmIm][HCOO] at 298.15 K. A further increase in the b value
(0.85) for [CH3COO]À anion lowers the ET value to 0.61 at
N
298.15 K. The b value for [CH3CH2COO]À was observed to be
lower than those for [HCOO]À and [CH3COO]À suggesting that
polarity should be higher for [HmIm][CH3CH2COO] than for
[HmIm][HCOO] and [HmIm][CH3COO]. However, the polarity
for [HmIm][CH3CH2COO] was observed to be lower than for
[HmIm][HCOO] and [HmIm][CH3COO]. This anomalistic decrease
Contrary to APILs, for which the constituent cation
governs polarity, the anion has a major contribution towards
N
polarity (ETN) in PILs. In spite of very high a values, the ET
of PILs were observed to be low when compared to those of
N
in polarity (ET value) in spite of a lower b value (0.10) for
the molecular solvents, for which higher a values always
resulted in higher polarity (ET value). This indicated the
N
partial reduction in the cationic control on polarity by the
N
anion. The ET value fluctuates according to their hydrogen
bond accepting tendency (b). Variation in the polarity with
anion is in the same order as that of the b value. The a has no
direct effect on the polarity of PILs. However, [HmIm][HSO4]
and [HbIm][HSO4] show high polarity due to the presence
of oxyanions. HBA tendency for [HSO4]À, [HCOO]À and
[CH3COO]À anions was found as 0.61, 0.81 and 0.85, respec-
tively, which is in the order of their decreasing polarity. In the
case of the [CH3CH2COO]À anion the b value was noted as
N
Fig. 3 The ETN–T plots of (a) [HmIm][HSO4] (’), [HmIm][HCOO]
(J), [HmIm][CH3COO] (m), [HmIm][CH3CH2COO] (,); (b)
[HbIm][HSO4] (’), [HbIm][HCOO] (J), [HbIm][CH3COO] (m),
[HbIm][CH3CH2COO] (,).
0.10, which is against the order of ET value. Probably, high
p* (1.50) value controls this behavior. A substitution of –CH3
by –C4H9 on the imidazole ring favors high ET values. It
N
appears that the steric crowding around the cation plays down
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of cation. Probably, retrotransfer of a quaternized proton to a
more basic organic anion is responsible for this dramatic
behavior.31 Further, PILs form extensive hydrogen bonding
which stabilizes both cation and anion, therefore, the probability
of reverse proton transfer becomes low. Due to these opposing
phenomena, a very small proportion of neutral moiety exists
along with the PIL that leads to the lower a value, which is far
beyond the a value for pure CH3CH2COOH i.e. 1.12.31b
The order of HBD tendency for 1-methylimidazolium class
of ILs was found to be:
[HmIm][HSO4] > [HmIm][HCOO] > [HmIm][CH3COO]
> [HmIm][CH3CH2COO]
Fig. 4 The a–T plots for (a) [HmIm][HSO4] (’), [HmIm][HCOO]
(J), [HmIm][CH3COO] (m), [HmIm][CH3CH2COO] (,); (b)
[HbIm][HSO4] (’), [HbIm][HCOO] (J), [HbIm][CH3COO] (m),
[HbIm][CH3CH2COO] (,).
Further, the HBD tendency for 1-butylimidazolium class
of ILs was noted to be slightly higher than that for 1-methyl-
imidazolium class of ILs as shown in Table 3. The presence of
butyl chain in 1-butylimidazole surprisingly increases the
HBD acidity. The order of HBD value for 1-butylimidazole
class at 298.15 K was observed as:
[HmIm][CH3CH2COO] is quite surprising. However, a high p*
value (1.50) indicates higher polarity. We are currently carrying
out these measurements on a variety of PILs to ascertain this
behaviour.
Substitution of the –CH3 group by –C4H9 remarkably
N
increases the ET value. In the 1-butylimidazolium class of
[HbIm][HSO4] > [HbIm][HCOO] > [HbIm][CH3COO]
> [HbIm][CH3CH2COO]
PILs comparatively lower values of p* and b while higher
a value decide the extent of coulombic interaction between
ions and hence polarity in PILs. This lowering in Kamlet–Taft
parameters can only be attributed to the presence of the
butyl chain.
b Parameter
The hydrogen bond acceptor (HBA) basicity b was observed
to decrease with an increase in temperature from 298.15 to
353.15 K. The b value increased with the increase in temperature
from 298.15 to 353.15 K for [HmIm][HSO4] and [HbIm][HSO4].
Contrary to this, the b values for PILs containing [HCOO]À,
[CH3COO]À and [CH3CH2COO]À anions were found to be high
and decreased with a rise in temperature (Fig. 5). Exceptionally
low b values for [HmIm][CH3CH2COO] may be due to the
presence of a neutral moiety along with the ionic species.
However, b for [HmIm][CH3CH2COO] is very small than the
b for pure CH3CH2COOH (0.45). Let us consider the following
equilibria:
Discrepancies in the ETN value for 1-butylimidazolium class
of PILs were seen with an increase in the temperature from
298.15 to 353.15 K. There was no thermosolvatochromic
N
change found in the ET value for [HbIm][HSO4] and
N
[HbIm][HCOO], but the decreasing trend in the ET value was
observed for [HbIm][CH3COO] and [HbIm][CH3CH2COO].
Thermosolvatochromic response for [HbIm][CH3COO] and
[HbIm][CH3CH2COO] may be due to high thermal sensitivity
N
of the Kamlet–Taft parameter. The ET value was found to
decrease for [HbIm][CH3COO] from 0.77 to 0.71 and for
[HbIm][CH3CH2COO] from 0.75 to 0.70 in the temperature
range from 298.15 to 353.15 K. Thus, [HbIm][CH3COO] and
[HbIm][CH3CH2COO] show positive thermosolvatochromic
response.
HA + B = [BH+] + AÀ
The strong acid (low pKa) favors the equilibrium towards
right due to complete transfer of proton showing high b value.
a Parameter
In spite of very close similarity in the behavior of ETN and a in
the temperature range of 298.15 to 353.15 K, it was noted that
a had no direct hold on the polarity of PILs (Fig. 4). For
1-methylimidazolium class of PILs the a parameter was observed
to be constant over the temperature change from 298.15 to
353.15 K. For 1-butylimidazolium class of PILs, a small increase
in a was noted for [HbIm][HSO4] and [HbIm][HCOO] with rise
N
in temperature, while ET remained constant. However, for
[HbIm][CH3COO] and [HbIm][CH3CH2COO], a decreased from
0.938 to 0.907 and 0.806 to 0.760, respectively, within the
temperature range of 298.15 to 353.15 K. The exceptionally high
a value for [HmIm][HSO4] and [HbIm][HSO4] may be due to
the presence of oxyanions. The unusually higher a value makes
them more polar than water. Surprisingly a negative a value
for [HmIm][CH3CH2COO] was noted, which indicated the
dominance of HBA basicity of anion over the HBD acidity
Fig. 5 The b–T plots for (a) [HmIm][HSO4] (’), [HmIm][HCOO]
(J), [HmIm][CH3COO] (m), [HmIm][CH3CH2COO] (,); (b)
[HbIm][HSO4] (’), [HbIm][HCOO] (J), [HbIm][CH3COO] (m),
[HbIm][CH3CH2COO] (,).
c
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However, the stabilization of cations and anions by hydrogen
bonding opposes the formation of neutral species. Thus, it was
assumed that the presence of ionic species favors high b value
and a combination of neutral species i.e. solvents and ionic
species favors low b value.
imidazolium ring makes it more asymmetric, allowing a close
approach of the cation to create the nanoscale regions with
aliphatic character in which the probe molecule may partition.32
This leads to the lower dipolarity/polarizability ratio for
1-butylimidazolium class of PILs. With a rise in temperature,
p* values decrease linearly. The observed trend in p* values for
1-butylimidazolium class of PILs is as:
Further the observed trends for 1-methylimidazolium and
1-butylimidazolium classes of PILs were noted to be:
[HbIm][CH3CH2COO] > [HbIm][HSO4] > [HbIm][HCOO]
> [HbIm][CH3COO]
[HmIm][HSO4] > [HbIm][CH3CH2COO] > [HbIm][HCOO]
> [HbIm][CH3COO]
N
Thus, polarity measurement (ET value and Kamlet–Taft
[HbIm][HSO4] > [HbIm][CH3CH2COO] > [HbIm][HCOO]
> [HbIm][CH3COO]
parameters) in PILs depends on the coulombic interaction which
is governed by the basicity of anion and dipolarity/polarizability.
The cation has a very small effect on the polarity of PIL.
Thus, the anomalous trend in b values for PILs containing
[HCOO], [CH3COO] and [CH3CH2COO] anions is attributed
only to the higher basicity (pKa) and to the relative stability of
these anions by hydrogen bonding.
Fajans’ rule and polarity of ILs: reverse polarization
In 1923 Fajans developed his quanticule theory called as
‘‘Fajans’ rule’’ to explain the development of partial covalent
character in ionic bonds.33 According to this theory a small,
highly charged cation pulls the electron cloud of an anion,
thereby developing covalent character in ionic bonds. Inter-
action of a cation of high charge density (hard) with an anion
of high polarizability (soft) generates a covalent character
between the ions in which the cation penetrates the electron
cloud of the anion. Polarization of a large anion by a small
cation is a very frequent phenomenon. However, is it possible
to see the polarization of a large cation by a small anion? This
turnaround of interaction between ions leading to covalent
character is known as reverse polarization. However, practically
it seems difficult because a soft cation holds its electron cloud,
while a hard anion cannot repel its electron cloud to become
a nucleus. For reverse polarization both the requirements
are essential.
p* Parameter
The dipolarity/polarizability parameter, p*, is based on the
p–p* electronic transition of N,N-diethyl-4-nitroaniline. Its
magnitude depends upon the interactions of a chromophore
with its cybotactic environment. With a rise in temperature the
interaction weakens, giving rise to a decrease in p* values. The
high value of dipolarity/polarizability for 1-methylimidazolium
class of PILs reveals that strong interaction occurs between the
probe and PILs. With increasing temperature, non-linear
decrease in the p* values was observed as shown in Fig. 6.
As evident, the trend in p* value is:
[HmIm][CH3CH2COO] > [HmIm][HSO4] > [HmIm][HCOO]
> [HmIm][CH3COO]
ILs are composed entirely of bigger ions and there is always
a possibility to develop covalent character between them.
Polarity study in different classes of APILs suggests them to
be of low polarity, comparable to short chain alcohols. In all
these classes of ILs it is difficult to correlate the polarity with
cations and anions. In order to make an IL, it is desirable to
incorporate a bulky cation having diffuse positive charge with
various kinds of anions. A bulky cation with diffused charge
behaves as a polarizable cation that can be easily distorted by
an anion. In this condition there is a possibility of reverse
polarization in IL. Hardness of anions, therefore, acts as a
guiding criterion to develop the covalent character in ILs
changing polarity. A strong interaction between ions creates
more covalent character and therefore changes the polarity as
measured by the solvatochromic method. Among various classes
of ILs it was observed that soft anions such as [NTf2]À, [PF6]À
interact weakly with soft cations (such as alkylimidazolium)
while hard anions such as [BF4]À, [N(CN)2]À approach much
closer towards the soft cations. The magnitude of this inter-
action governs the development of covalent character and
An exceptionally high dipolarity/polarizability ratio for
[HmIm][CH3CH2COO] was recorded. A very low b value (0.10)
and a moderate a value (0.66) can be attributed to the strong
interactions between the dye and [HmIm][CH3CH2COO].
The p* values for 1-butylimidazolium class of PILs were
found to be slightly lower than those of the 1-methylimida-
zolium class of PILs. The introduction of a butyl chain on the
N
thereby polarity of ILs. The ET values for [BMIM][NTf2] and
[BMIM][PF6] were measured as 0.645 and 0.669, respectively,
while for [BMIM][N(CN)2] as 0.629.34 This small difference in
Fig. 6 The plots of p*–T for (a) [HmIm][HSO4] (’), [HmIm][HCOO]
(J), [HmIm][CH3COO] (m), [HmIm][CH3CH2COO] (,); (b)
[HbIm][HSO4] (’), [HbIm][HCOO] (J), [HbIm][CH3COO] (m),
[HbIm][CH3CH2COO] (,).
N
ET value arises due to the strong interactions of [N(CN)2]À
towards [BMIM]+ as compared to those of [NTf2]À and [PF6]À
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offering less interactions for [BMIM]+. Similarly, for
N
Acknowledgements
[EMIM][NTf2], [EMIM][PF6] and [EMIM][N(CN)2], the ET
values were observed as 0.658, 0.676 and 0.648, respectively.34a,35
SKS is grateful to UGC, New Delhi, for awarding him a
research fellowship. AK thanks DST, New Delhi, for a
J. C. Bose National Fellowship (SR/S2/JCB-26/2009).
In the pyridinium-based ILs i.e. [BP][BF4] and [BP][NTf2], the
N
ET values recorded were 0.639 and 0.648, respectively.22a This
again confirms the role of anion as a guiding factor to determine
the polarity. An increase in the alkyl chain length has a similar
effect on the ETN value for [OP][BF4] and [OP][NTf2], 0.606 and
0.617, respectively.22a
Notes and references
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PILs due to the extensive hydrogen bonding act as hyperpolar
media. A transfer of proton from Brønsted acid to Brønsted
base generates the conjugate counterpart of acid and base.
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development of covalent character. However, oxyanions such
as [HSO4]À, [NO3]À, [H2PO4]À etc. due to the extensive network
of hydrogen bonding generate highly polar PILs. The current
polarity study in 1-methylimidazolium and 1-butylimidazolium
classes of PILs with [HSO4]À, [HCOO]À, [CH3COO]À and
[CH3CH2COO]À as counter anions shows exceptionally low
ETN values except for [HSO4À]. The lower polarity arises due to
the increasing polarizability of anions. The stronger interaction
of 1-methylimidazolium with the increasing polarizability of
anions ([HCOO]À, [CH3COO]À and [CH3CH2COO]À) leads to
N
the greater degree of polarization and hence, the ET value
becomes exceptionally low. The ETN values for [HmIm][HCOO],
[HmIm][CH3COO] and [HmIm][CH3CH2COO] were noted as
0.78, 0.61 and 0.50, respectively. The lowest value for
[HmIm][CH3CH2COO] is close to that of dimethylsulfoxide
(0.444), acetonitrile (0.46) and 2-propanol (0.50) acting as
non-polar media. Similarly, for 1-butylimidazolium-based PILs
with [HCOO]À, [CH3COO]À and [CH3CH2COO]À anions, the
N
ET values were found to be 0.80, 0.77 and 0.75, respectively.
The presence of –C4H9 lowers the polarizing power of the
N
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