Binary Mixtures of Aprotic and Protic Ionic Liquids Demonstrate Synergistic Polarity Effect: An Unusual Observation

Article abstract of DOI:10.1007/s10953-020-00952-w

In this communication, we demonstrate the solute–solvent and solvent–solvent interactions in the binary mixtures of two aprotic ionic liquids, namely 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-butyl-1-methylpyrrolidinium bis(trifl

Full text of DOI:10.1007/s10953-020-00952-w

Journal of Solution Chemistry
https://doi.org/10.1007/s10953-020-00952-w
Binary Mixtures of Aprotic and Protic Ionic Liquids
Demonstrate Synergistic Polarity Efect: An Unusual
Observation
Sachin Thawarkar1 · Nageshwar D. Khupse2 · Anil Kumar1
Received: 20 June 2019 / Accepted: 11 December 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
In this communication, we demonstrate the solute–solvent and solvent–solvent inter-
actions in the binary mixtures of two aprotic ionic liquids, namely 1-butyl-3-meth-
ylimidazolium bis(trifuoromethylsulfonyl)imide and 1-butyl-1-methylpyrrolidinium
bis(trifuoromethylsulfonyl)imide, with the protic ionic liquid 1-methylimidazolium ace-
tate. The synergistic efects as expressed by the solvatochromic parameter are noted. This
observation is in contrast to the mixing of protic ionic liquids 1-methylpyrrolidium acetate
and 4-methylmorpholine acetate with 1-methylimidazolium acetate, respectively. It appears
that the synergistic efects in the binary mixtures of aprotic and protic ionic liquids are
caused by the formation of hydrogen bonds, since cations are dominant H-bond donors
while anions are dominant H-bond acceptors. Preferential solvation models are used to
describe the solute–solvent interactions in the binary ionic liquid mixtures.
Keywords Ionic liquid · Binary mixture · Polarity · Synergetic efect · Preferential
solvation
1 Introduction
An accurate knowledge of solute–solvent interactions is essential in order to explore efec-
tive application of ionic liquids [1–3]. Two types of mixtures involving ionic liquids are
possible: (1) an ionic liquid with solvent and (2) one ionic liquid with another. The prob-
lem of solute–solvent interactions in ionic liquid solvent systems has recently been dis-
cussed by several workers including us [4–10]. For example, positive and negative devia-
tions from the linear mixing of polarity parameters of the ionic liquid–solvent systems have
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1095
3-020-00952-w) contains supplementary material, which is available to authorized users.
* Anil Kumar
dranilkumar1956@gmail.com; a.kumar@ncl.res.in
1
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008,
India
2
Centre for Materials for Electronic Technology, Panchwati, Pashan Road, Pune 411008, India
1 3
Vol.:(0123456789)

Journal of Solution Chemistry
been analyzed in terms of ionic structures and nature of the solvent [8–10]. It was of great
interest to note the presence of hyper polarity in the solutions of ionic liquids with alcohols
like methanol and ethanol. We were intrigued whether a case of hyper polarity or the syn-
ergistic efect existed in the binary mixtures of ionic liquids alone. Our prime interest was
initiated with exploring the possibilities of using the mixtures of the ionic liquids in more
efcient applications to a variety of chemical processes.
In this communication, we present our experiments on two systems: (1) the polarity
efects on mixing of an aprotic ionic liquid with protic ionic liquid and (2) the polarity on
mixing with one protic ionic liquid with another protic ionic liquid. For this purpose, we
selected 1-butyl-3-methylimidazolium bis(trifuoromethylsulfonyl)imide [bmIm][NTf2],
1-butyl-1-methylpyrrolidinium bis(trifuoromethylsulfonyl)imide [bmPyrr][NTf2], 1-meth-
ylpyrrolidium acetate [HmPyrr][CH3COO], 4-methylmorpholine acetate [Hmmorph]
[CH3COO] and 1-methylimidazolium acetate [HmIm][CH3COO] ionic liquids. The abbre-
viations, structure and name of all the ionic liquids are given in Table 1. We have chosen
the [NTf2] anion based ionic liquids which is more stable and possesses a large electro-
chemical window and low viscosity. We have also preferred [CH3COO] based protic ionic
liquids because of low viscosity and precursors of these ionic liquids are commercially
available.
It is necessary to point out the polarity profle for the binary mixture of ionic liquids.
The microscopic properties of the solvents are always refected through their bulk or mac-
roscopic properties like dielectric permittivity, dipole moment, refractive index, and polar-
ity. The microscopic properties are correlated with the structure of ions and dynamics of
the solvation sphere of the dye. The microscopic parameters were determined from the
solvatochromic phenomenon of the dye. The pure solvents do not give rise to preferential
solvation, since the compositions of the solvation sphere and bulk solvent are identical.
In the case of ionic liquids, such preferential interactions may be manipulated to give the
desired outcome of any chemical processes. The polarity parameters for pure ionic liquids
at 25 °C are shown in supplementary table Table S1. As expected, the polarity of ionic
Table 1 The names, structure of cations and anions, acronym of the used ionic liquids
Name of ionic liquids
Structure of cations
Structure of anions
Acronym
1-Butyl-3-methylimidazolium
bis(trifuoromethylsulfonyl)
imide
[bmIm][NTf2]
1-Butyl-1-methylpyrrolidinium
bis(trifuoromethylsulfonyl)
imide
[bmPyrr][NTf2]
1-Methylpyrrolidium acetate
[CH3COO]–
[CH3COO]–
[HmPyrr][CH3COO]
[Hmmorph][CH3COO]
4-Methylmorpholine acetate
1-Methylimidazolium acetate
[CH3COO]–
[HmIm][CH3COO]
1 3

Journal of Solution Chemistry
liquids is lower than that of water and methanol but is higher than that of solvents like
dichloromethane. The polarity values of the pure liquids conform to the values reported in
the literature [11–13]. After ascertaining the purity of our samples and observing the good
agreement between the literature values and our measured ETN values, we have focused our
attention on the binary mixtures of ionic liquids. It is important to note that the ETN values
of [bmIm][NTf2] and [bmPyrr][NTf2] measured by us are in agreement with literature val-
ues [5, 11, 14–16].
2 Experimental Section
2.1 Chemicals
1-methyl imidazole (≥99%), 1-methyl pyrrolidine (≥99%), 4-methyl morpholine (≥99%),
1-butylbromide (≥99%), bis(trifuoromethane)sulfonimide lithium salt (≥99%), dimethyl
sulfate (≥99%) and acetic acid (≥96%) were purchased from Sigma–Aldrich and used as
obtained. Solvents such as dichloromethane (99%) and ethyl acetate (99%) were obtained
from Merck and used after distillation.
2.2 Synthesis of Ionic Liquids
Aprotic ionic liquids such as [bmIm][NTf2] and [bmPyrr][NTf2] were synthesized accord-
ing to the literature reports [1, 6, 7]. In the frst step, the quarternization reaction of 1-meth-
ylimidazole and 1-methyl pyrrolidine with 1-bromobutane was carried out to form [bmIm]
Br or [bmPyrr]Br ionic liquids. In second step, [bmIm][NTf2] or [bmPyrr][NTf2] were
synthesized using metathesis reactions of lithium bis(trifuoromethane)sulfonimide salt
(LiNTf2) with [bmIm]Br or [bmPyrr]Br in water at room temperature. Extraction of aprotic
ionic liquids was carried out using the water–dichloromethane solvent system to give pure
ionic liquids. The halide contents were determined using the Volhard titration method [17].
The protic ionic liquids, [HmIm][HCOO], [HmPyrr][CH3COO] and [HmMorph]
[CH3COO] were synthesized and purifed according to the reported procedures [18]. The
protic ionic liquids were synthesized by neutralization reaction of acid such as acetic acid
with bases such as 1-methylimidazole, 1-methyl pyrrolidine and 4-methyl morpholine.
Drop wise addition of base to the acid was carried out in an ice bath to avoid excessive
heat generation due to the exothermic reaction. The reaction mixture of acid and base in
the molar ratio of 1: 1 was stirred for 6 h at room temperature. Water was removed by using
a rotavapor at 50 °C under reduced pressure. These protic ionic liquids were further dried
under vacuum at 50 °C for 6 h. Water contents of the ionic liquids were measured by a Karl
Fischer coulometer and are tabulated in Table 2 for all of the studied ionic liquids. The
1
purity of aprotic and protic ionic liquids were determined by H-NMR spectroscopy and
the ionic liquids are about 99% pure.
2.3 Polarity Measurements
The stock solutions of 2, 6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate dyes were
prepared in methanol prior to their use. The stock solution was added drop wise to the
binary mixtures of ionic liquids and methanol was evaporated under vacuum. The binary
1 3

Journal of Solution Chemistry
Table 2 Molecular weight, % purity and water contents of the used ionic liquids [19]
Ionic liquids
Molecular weight
(g·mol–1)
% of purity
Water contain (ppm)
[bmIm][NTf2]
[bmPyrr][NTf2]
[HmPyrr][CH3COO]
[Hmmorph][CH3COO]
[HmIm][CH3COO]
419.36
422.21
119.00
147.10
142.21
99%
99%
99%
99%
98%
152 18
132 20
165 27
157 28
139 19
mixtures of ionic liquids of dye solution were kept to a quartz cuvette sealed with a sep-
tum under a nitrogen atmosphere. The λmax was measured at 25 °C using a UV–Visible
spectrophotometer. The temperature of the cell was controlled using the single cell Peltier
accessory having an accuracy of 0.1 °C. The EN parameter for ionic liquid mixtures was
calculated using equations 4, 5 which is a normaTlized form of the ET (30) parameter,
ET (solvent) − 30.7
ETN
=
(1)
32.4
where EN represents the polarity of binary mixtures of ionic liquids and ET (solvent) are
T
the ET(30) values of binary mixtures of ionic liquids derived from Eq. 3.
The polarity parameters are reproducible to 0.4%. In our study we used Reichardt dye
33 which has a long wavelength intra-molecular charge transfer (ICT) absorption band,
which is absent in the case of Reichardt dye 30:
28591
ET(33) =
× 4.184 (kJ ⋅ mol)
(2)
ꢀmax
The ET(30) data for all binary mixtures of ionic liquids were determined from ET(33),
a polarity scale of the less basic probe 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio) pheno-
late. For this purpose, we used Eq. 3:
ET (30) = 0.9119 × ET (33) − 3.80634
(3)
3 Results and Discussions
Polarity of the binary mixtures of ionic liquids: we have studied solvatochromic param-
eters to investigate the ion–ion interactions in the binary mixtures of ionic liquids. The
variations in the EN parameter with mole fraction of [HmIm][CH3COO] (x[HmIm] CH COO
)
]
[
T
3
for the binary mixtures of ionic liquids are shown in Figs. 1 and 2. From these fgures,
it is seen that the polarity of the binary mixtures of ionic liquids is afected by the con-
centration of [HmIm][CH3COO]. Protic ionic liquids have more ability to form stronger
hydrogen bonding with an anion compared to aprotic ionic liquids due to the presence of
a more acidic proton at the nitrogen atom. However, in the case of aprotic ionic liquids,
the hydrogen bond is formed by the hydrogen present at C-2 carbon of the imidazolium
ring cation with anion. The role of anion is also important in the formation of hydrogen
bonding in ionic liquids. The NTf−2 anion possesses low negative charge due to presence of
the electron defcient bis(trifouromethanesulfolnyl) functional group, hence it forms weak
1 3

Journal of Solution Chemistry
Fig. 1 Plot of EN
T
vs.x[HmIm] CH COO for the binary
0.850
0.825
0.800
0.775
[
]
3
mixtures of [bmIm][NTf2]–
[HmIm][CH3COO] (open trian-
gle); [bmPyrr][NTf2]–[HmIm]
[CH3COO] (flled square)
0.00
0.33
0.66
0.99
x
[HmIm][CH3COO]
Fig. 2 Plot EN vs.x[HmIm] CH COO
[
]
T
3
0.850
0.825
0.800
0.775
0.750
of [HmPyrr for the binary
mixtures][CH3COO]–[HmIm]
[CH3COO] (open square);
[HmMorph][CH3COO]–[HmIm]
[CH3COO] (open triangle)
0.00
0.33
0.66
0.99
x
[HmIm][CH3COO]
hydrogen bonds with C-2 proton of cation which causes NTf− to move out of plane of the
cation. The cations are dominant H-bond donors while anions2are dominant H-bond accep-
tor. In the case of protic ionic liquids, proton transfer from a Brønsted acid to a Brønsted
base is responsible for the hydrogen bonding within the case of aprotic ionic liquids; the
cations tend to have C2–H as the primary H-bond donor unit.
3.1 Protic Ionic Lquid–Aprotic Ionic Liquids Mixtures
In the binary mixtures of ionic liquids, the increase in local polarity experienced by the dye
molecule is due to ion–ion interactions.
For example, a positive deviation in polarity of the binary mixtures of [bmIm]
[NTf2]–[bmIm][PF6] and [emIm][NTf2]–[bmIm][PF6] have been observed [11]. Fig-
ure 1 shows the variation in the ETN parameter with addition of protic ionic liquid,
[HmIm][CH3COO], to aprotic ionic liquids [bmIm][NTf2] and [bmPyrr][NTf2]. A strong
1 3

Journal of Solution Chemistry
deviation in EN was found for solutions of dye in [bmIm][NTf2]–[HmIm][CH3COO] and
[bmPyrr][NTfT2]–[HmIm][CH3COO] of the ionic liquids. The [CH3COO]– anion has a
stronger coordinating ability than [NTf2]– because of the negative charge is more sta-
ble across the [NTf2]– moiety. Thus, [bmIm]+ and [bmPyrr]+ are loosely associated
with the [NTf2]– anion which is more stable because of the presence of fuorine atoms,
which is helpful to stabilize the negative charge on the nitrogen atom and larger size of
[NTf2]–. Hence, [NTf2]– is more stable and less interacting towards the cations; there-
fore the cations [bmIm]+ and [bmPyrr]+ are loosely bound to [NTf2]– and may interact
more strongly with the π-system of the dye which leads to higher polarity. Furthermore,
[CH3COO]– ions strongly interact with [bmIm]+ and [HmIm]+ which results in more
free [NTf2]– anions. These [NTf2]– interact more with dye molecules compared to pure
[bmIm][NTf2] or [bmPyrr][NTf2]. In the case of the binary mixtures of protic ionic liq-
uids in aprotic ionic liquids, the variations in the polarity values are diferent in nature,
as shown in Fig. 1. There are some interesting variations related to the addition of protic
ionic liquids [HmIm][CH3COO] in the aprotic ionic liquids. For the binary mixtures
of protic ionic liquids [HmIm][CH3COO] in aprotic ionic liquids [bmIm][NTf2] and
[bmPyrr][NTf2], respectively, EN shows a positive deviation suggesting a strong syner-
T
gistic efect. This shows that the polarity of the mixture is higher than that of the pure
ionic liquids. The observed polarity shows a strongly non-ideal behavior in the binary
mixtures of ionic liquids studied.
3.2 Preferential Solvation Model for Synergetic Efect
The polarity of the mixtures can be analyzed in terms of the preferential solvation
model. To explain the synergetic efect, the following model has been proposed and
it describes the hydrogen bond complex formation in binary mixtures of protic ionic
liquids–aprotic ionic liquids as shown in Fig. 1. We attempted to ft this model for
ionic liquids–ionic liquids binary mixtures and the estimated parameters are shown in
Table 3. The model is based on the two step solvent-exchange model frst proposed by
Skwierczynski and Connors [20]:
I(S1)2 + 2S2 ⇌ I(S2)2 + 2S1
I(S1)2 + S2 ⇌ I(S12)2 + S1
where S1 and S2 indicate the two pure solvents and S12 represents a solvent formed by the
interaction of solvents 1 and 2. I(S1)2, I(S2)2, and I (S12)2 indicate dye solvated by solvents
S1, S2, and S12, respectively. The two solvent exchange processes can be defned by two
preferential solvation parameters, f2/1 and f12/1, which measure the tendency of the indicator
to be solvated by solvents S2 and S12 with reference to solvent S1 solvation.
Table 3 Fitting parameters obtained from preferential solvation model (Eq. 8)
EN
EN
EN
Ionic liquids
Ionic liquids
f2∕1
f12∕1
T1
T2
T12
[HmIm][CH3COO]
[bmIm][NTf2]
[bmPyrr][NTf2]
0.785
0.773
0.804
0.804
0.795
0.788
8.04 0.41
14.23 0.72
27.43 1.35
17.93 0.91
1 3

Journal of Solution Chemistry
f2∕1 = (x2s ∕x1s )∕(x20∕x10)2
f12∕1 = (x1s2∕x1s )∕(x20∕x10)2
(4)
(5)
Here, x1s, x2s and x1s2 represent the mole fractions of S1, S2 and S12 in the solvation
sphere of the dye molecules, respectively, whereas x10 and x0 are the bulk mole fractions of
S1 and S2. This entire mole fractions follow the equation as2
x10 + x20 = x1s + x2s + x1s2 = 1
(6)
The EN polarity of the mixed solvent is calculated as an average of the ETN values of sol-
vents S1,TS2, and S12 in the sphere of solvation of the indicator
ETN = x1s ETN1 + x2s ETN2 + x1s2ETN12
(7)
where ETN1, ETN2, ETN12 are the transition energies of the solvatochromic indicator com-
pletely solvated by the solvents S1, S2 and S12, respectively.
From Eqs. 4–8 a general equation that relates the ETN polarity of a binary mixture to the
ETN polarities of the two pure solvents, the preferential solvation parameters, and the solvent
composition can be derived according to
(
)
(
)
2
a x20 + c 1 − x20 x20
ETN = ETN1
+
(8)
(
)
(
)
(
)
1 − x20 2 + f2∕1 x02 2 + f12∕1 1 − x20 x01
where the terms a and c are defned by the following equations
(
)
a = f2∕1 ETN2 − ETN1
(9)
(
)
c = f12∕1 ETN12 − ETN1
(10)
where ETN12 is the polarity of the mixed solvent (S12) and is determined through Eq. 10.
The values of ETN12 are found to be higher than of both the pure components for the binary
mixtures of ionic liquids illustrating the synergistic efect.
This model is used for studying preferential solvation in order to explain synergism
in binary mixtures of protic ionic liquids with aprotic ionic liquids systems [21]. Herein,
we have studied two binary systems 1) [bmIm][NTf2]–[HmIm][CH3COO] and 2)
[bmPyrr][NTf2]–[HmIm][CH3COO] for which the three parameters f12/1, f2/1, and EN
T12
were determined. The synergism efect observed in the [bmIm][NTf2]–[HmIm]
[CH3COO] system and [bmPyrr][NTf2]–[HmIm][CH3COO] system are shown in Fig. 1.
Herein, the ETN values of both ionic liquids [bmIm][NTf2] and [HmIm][CH3COO] are
close to each other and hence synergism is observed. It can be observed that EN value
obtained from this model is greater than those for the pure ionic liquids which Tc1o2nfrms
the existence of synergism in mixture of protic–aprotic ionic liquids. The parameters
f12/1, f2/1, and as obtained from this model are shown in Table 3. The parameter f2/1 rep-
resents the tendency of protic ionic liquid 2 ([HmIm][CH3COO]) to solvate the dye with
reference to aprotic ionic liquid 1 ([bmIm][NTf2] and [bmPyrr][NTf2]) while parameter
f12/1 represents the tendency of the dye to be solvated by the mixture with reference to
aprotic ionic liquid 1. The values of f2/1 and f12/1 for the [bmIm][NTf2]–[HmIm]
[CH3COO] system are 8.04 and 27.43, respectively, and show that the preferential
1 3

Journal of Solution Chemistry
Fig. 3 Complex formation and solvation of dye in the binary mixtures of ionic liquids
Fig. 4 Plot of ΔEN vs for
T
the [bmIm][NTf2]–[HmIm]
[CH3COO] (open triangle)
and [bmPyrr][NTf2]–[HmIm]
[CH3COO] (flled square)
systems
0.006
0.003
0.000
-0.003
-0.006
N
T
E
0.00
0.25
0.50
0.75
1.00
x
[HmIm][CH3COO]
solvation by the mixture of [bmIm][NTf2]-[HmIm][CH3COO] is greater than for the
pure ionic liquids due to synergism. Similarly, the binary mixture of [bmPyrr][NTf2]
and [HmIm][CH3COO] shows synergism with parameters f2/1 and f12/1 as 14.23 and
17.93, respectively. Also, the ETN12 values estimated from the model are 0.795 and 0.788
for the [bmIm][NTf2]–[HmIm][CH3COO] and [bmPyrr][NTf2]–[HmIm][CH3COO] sys-
tems, respectively. A mixture of the [bmIm][NTf2]–[HmIm][CH3COO] system shows
stronger synergetic efect than for the [bmPyrr][NTf2]–[HmIm][CH3COO] system. The
synergism is observed for the HBA and HBD solvents by forming a hydrogen bond
complex, i.e. the binary mixture is more polar than either of the two pure solvents. The
complex formation and solvation of dye in the binary mixtures of ionic liquids are
1 3

Journal of Solution Chemistry
shown in Fig. 3. In Fig. 4 are shown the ΔETN values as a function of x[HmIm] CH COO in
[
]
3
the aprotic ionic liquids [bmIm][NTf2] and [bmPyrr][NTf2]. ΔENis defned as the difer-
T
ence between EN
and ETN,corr, where ETN,exp is the value of ETN obtained from our experi-
Tꢀ,exp
mental work while ETN,corr is the value of ΔETN obtained from this model.
3.3 Protic Ionic Liquid–Protic Ionic Liquids Mixtures
Figure 2 shows the plots of ETN of the binary mixtures vs of the protic ionic liquid [HmIm]
[CH3COO] in protic ionic liquids [HmPyrr][CH3COO] and [HmMorph][CH3COO],
respectively. For the binary mixtures of [HmPyrr][CH3COO]–[HmIm][CH3COO], it is
observed that the ETN values of the mixtures decrease with increase in concentration of
[HmIm] [CH3COO], while for the binary mixtures of [HmPyrr][CH3COO]–[HmIm]
[CH3COO], the EN values of the binary mixture increase with the addition of [HmIm]
T
[CH3COO].
Both protic ionic liquids [HmPyrr][CH3COO] and [HmMorph][CH3COO] show non-
ideal behavior upon the addition of [HmIm][CH3COO] indicating a strong solvent-specifc
interaction between the protic ionic liquids in mixtures. The shape of curves suggested
that cations are interacting more strongly with dye molecule due to stronger hydrogen
bonding. From the results, we conclude that the order of hydrogen bonding ability of the
studied ionic liquids towards dye molecule as [HmMorph]+ >[HmIm]+ >[HmPyrr]+.
Also, these behavior of mixed cation binary ionic liquids systems is due to more ef-
cient packing being possible between the [HmPyrr]+ or [HmMorph] and [HmIm]+ cations
which leads to greater charge density at the dye molecule. The following model is a more
generalized model and is defned by the solvent-exchange process. But, it cannot explain
the synergetic behavior observed in binary mixtures of protic ionic liquids with aprotic
ionic liquids.
In order to explain the preferential solvation in binary mixture of protic ionic liquid
with protic ionic liquid, the following model is used and estimated parameters are given in
Table 4:
(
)
(
)
ENT1 1 − x2o + ETN2f2∕1(x2o)2 + ETN12∕1f12∕1 1 − x2o x2o
ETN
=
(
)
(11)
(1 − x2o)2 + f2∕1(x2o)2 + f12∕1 1 − x2o x1o
ETN1 + ETN2
ETN12
=
2
Equation 11 correlates the polarities in the solution of ionic liquids binary mix-
tures. From this model, two parameters f2/1 and f12/1 were determined for the binary
Table 4 Fitting parameters obtained from preferential solvation model (Eq. 8)
EN
EN
EN
Ionic
Ionic
f2∕1
f12∕1
T1
T2
T12
liquids
liquids
[HmIm]
[HmPyrr]
0.746
0.843
0.804
0.804
0.775
0.824
0.088 0.004
0.185 0.009
12.27 0.84
1.16 0.081
[CH3COO]
[CH3COO]
[HmMorph]
[CH3COO]
1 3

Journal of Solution Chemistry
mixtures of protic–protic ionic liquids. The parameter f2/1 represents the tendency of
protic ionic liquids 2 ([HmIm][CH3COO]) to solvate the dye with reference to protic
ionic liquid 1 ([HmPyrr][CH3COO] and [HmMorph][CH3COO]), while parameter
f12/1 represents the tendency of the dye to be solvated by the mixtures with refer-
ence to protic ionic liquid 1. The parameters f12/1 and f2/1 obtained from this model
are shown in Table 4. The f2/1 and f12/1 values for the protic ionic liquids [HmPyrr]
[CH3COO]–[HmIm][CH3COO] system are 0.088 and 12.27, respectively, and for
[HmMorph][CH3COO]–[HmIm][CH3COO] are 0.18 and 1.16, respectively. The
values of f2/1 < 1 indicate that the dye is more solvated by [HmPyrr][CH3COO]
and [HmMorph][CH3COO] with respect to [HmIm][CH3COO], though [HmIm]
[CH3COO] is more polar than [HmPyrr][CH3COO] but is less polar than [HmMorph]
[CH3COO]. Thus, the solvation sphere is enriched with [HmPyrr][CH3COO] and
[HmMorph][CH3COO] rather than with [HmIm][CH3COO]. This may be due to the
positive charge density of [HmIm][CH3COO] being delocalized over the imidazo-
lium ring while in the case of the [HmPyrr][CH3COO] and [HmMorph][CH3COO]
the positive charge density is more localized, therefore the solvation of dye is more
in [HmPyrr][CH3COO] and [HmMorph][CH3COO] compared to [HmIm][CH3COO].
Herein, for both systems, the f12/1 > f2/1 shows that mixtures form a complex which
has greater affinity toward the dye as compared to the pure [HmPyrr][CH3COO] and
[HmMorph][CH3COO]. Also, the value of f2/1 for [HmMorph][CH3COO]–[HmIm]
[CH3COO] binary system is greater than that of the [HmPyrr][CH3COO]–[HmIm]
[CH3COO] system.
This may be due to the cationic efect which dominates the solvation of the dye and
also the oxygen atom in the morpholine ring of [HmMorph][CH3COO] ionic liquid
decreases the positive charge at the nitrogen which directly afects the solute–solvent
interactions. This is apparently opposite to the trend observed in the protic–aprotic ionic
liquid mixtures. The fact that protic ionic liquids difer from aprotic ionic liquids by the
presence of only a butyl group makes such the diference more relevant. The plot of the
ΔEN values vs. composition of [HmIm][CH3COO] for the [HmPyrr][CH3COO]–[HmIm]
T
[CH3COO] and [HmMorph][CH3COO]–[HmIm][CH3COO] systems are shown in
Fig. 5.
Fig. 5 The plot of ΔEN vs.
T
x[HmIm] CH COO for the [HmPyrr]
[
]
0.008
0.004
0.000
-0.004
-0.008
3
[CH3C.x[HmIm] CH COO OO]–
[
]
3
[HmIm][CH3COO] (open
triangle) and [HmMorph]
[CH3COO]– [HmIm][CH3COO]
(open square) systems
N
T
E
0.00
0.25
0.50
0.75
1.00
x
[HmIm][CH3COO]
1 3

Journal of Solution Chemistry
4 Conclusion
In summary, we have presented the polarity values for the binary mixtures of protic ionic
liquids in aprotic ionic liquids and protic ionic liquids, respectively. Protic–protic ionic
liquid binary mixtures exhibited non-ideal behavior with composition while synergism is
observed in protic–aprotic ionic liquid binary mixtures. The behavior is explained using
preferential solvation models and demonstrated the presence of solute–solvent and sol-
vent–solvent interactions in binary systems. The parameters explained the strong synergism
which forms the strong hydrogen bond complexes between protic ionic liquids and aprotic
ionic liquids. Cations are dominantly H-bond donors, anions are dominantly H-bond accep-
tors. In protic cations the H-atom is often covalently bound to the heavy atom carrying the
formal charge. Aprotic cations tend to have C-H as the primary H-bond donor unit. Hence,
these models allow us to understand both synergetic as well as the non-synergetic efects in
binary mixtures of ionic liquids from the correlation parameters.
Acknowledgements SRT thanks CSIR, New Delhi, for awarding him a Senior Research Fellowship, while
A.K. thanks DST, New Delhi, for awarding him a J.C. Bose National Fellowship (SR/S2/JCB-26/2009).
References
1. Welton, T.: Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 99,
2071–2084 (1999)
2. Sheldon, R.: Catalytic reactions in ionic liquids. Chem. Commun. 23, 2399–2407 (2001)
3. Weingärtner, H.: Understanding ionic liquids at the molecular level: facts, problems, and controversies.
Angew. Chem. Int. Ed. 47, 654–670 (2008)
4. Reichardt, C.: Solvatochromic dyes as dolvent polarity indicators. Chem. Rev. 94, 2319–2358 (1994)
5. Reichardt, C.: Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium
N-phenolate betaine dyes. Green Chem. 7, 339–351 (2005)
6. Khupse, N.D., Kumar, A.: Contrasting thermosolvatochromic trends in pyridinium-, pyrrolidinium-,
and phosphonium based ionic liquids. J. Phys. Chem. B. 114, 376–381 (2010)
7. Khupse, N.D., Kumar, A.: Delineating solute–solvent interactions in binary mixtures of ionic liquids in
molecular solvents and preferential solvation approach. J. Phys. Chem. B. 115, 711–718 (2011)
8. Beniwal, V., Kumar, A.: Synergistic efects and correlating polarity parameters in binary mixtures of
ionic liquids. ChemPhysChem 16, 1026–1034 (2015)
9. Beniwal, V., Kumar, A.: Understanding positive and negative deviations in polarity of ionic liquid mix-
tures by pseudo-solvent approach. Phys. Chem. Chem. Phys. 18, 2385323863 (2016)
10. Beniwal, V., Shukla, S.K., Kumar, A.: Deviation of polarity from linearity in liquid mixtures contain-
ing an ionic liquid. Phys. Chem. Chem. Phys. 17, 31613–31617 (2015)
11. Fletcher, K.A., Baker, S.N., Baker, G.A., Pandey, S.: Probing solute and solvent interactions within
binary ionic liquid mixtures. New J. Chem. 27, 1706–1712 (2003)
12. Sarkar, A., Pandey, S.: Solvatochromic absorbance probe behavior and preferential solvation in aque-
ous 1-butyl-3-methylimidazolium tetrafuoroborate. J. Chem. Eng. Data 51, 2051–2055 (2006)
13. Sarkar, A., Trivedi, S., Baker, G.A., Pandey, S.: Multiprobe spectroscopic evidence for “Hyperpolar-
ity” within 1-butyl-3-methylimidazolium hexafuorophosphate mixtures with tetraethylene glycol. J.
Phys. Chem. B. 112, 14927–14936 (2008)
14. Crowhurst, L., Mawdsley, P.R., Perez-Arlandis, J.M., Salter, P.A., Welton, T.: Solvent–solute interac-
tions in ionic liquids. Phys. Chem. Chem. Phys. 5, 2790–2794 (2003)
15. Lee, J.M., Ruckes, S., Prausnitz, J.M.: Solvent polarities and Kamlet-Taft parameters for ionic liquids
containing a pyridinium cation. J. Phys. Chem. B 112, 1473–1476 (2008)
16. Muldoon, K.J., Gordon, C.M., Dunkin, I.R.: Investigations of solvent–solute interactions in room tem-
perature ionic liquids using solvatochromic dyes. J. Chem. Soc. Perkin Trans. 2, 433–435 (2001)
17. Seddon, K.R., Stark, A., Torres, M.J.: Infuence of chloride, water, and organic solvents on the physical
properties of ionic liquids. Pure Appl. Chem. 72, 2275–2287 (2000)
1 3

Journal of Solution Chemistry
18. Ohno, H., Yoshizawa, M.: Ion conductive characteristics of ionic liquids prepared by neutralization of
alkylimidazoles. Solid State Ion. 154, 303–309 (2002)
19. Thawarkar, S., Khupse, N.D., Shinde, D.R., Kumar, A.: Understanding the behavior of mixtures of
protic-aprotic and protic-protic ionic liquids: conductivity, viscosity, difusion coefcient and ionicity.
J. Mol. Liq. 276, 986–994 (2019)
20. Skwierczynski, R.D., Connors, K.A.: Solvent efects on chemical processes. Part 7. Quantitative
description of the composition dependence of the solvent polarity measure &(30) in binary aqueous–
organic solvent mixtures. J. Chem. Soc. Perkin Trans. 2, 467–472 (1994)
21. Bosech, E., Roses, M., Herodes, K., Koppel, I., Letto, I., Koppel, I., Taal, V.: Solute solvent and sol-
vent-solvent interactions in binary solvent mixtures. 2. Efect of temperature on the e,(30) polarity
parameter of dipolar hydrogen bond acceptor–hydrogen bond donor mixtures. J. Phys. Org. Chem. 9,
403–410 (1996)
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional afliations.
1 3