Understanding the behavior of mixtures of protic-aprotic and protic-protic ionic liquids: Conductivity, viscosity, diffusion coefficient and ionicity

Article abstract of DOI:10.1016/j.molliq.2018.12.024

We have investigated the physicochemical properties such as electrical conductivity, viscosity and diffusion coefficient for the binary mixtures of protic ionic liquids with aprotic ionic liquids and of protic with protic ionic liquids at 298.15 K. A significant enhancement in the electrical conductivity is observed for the binary mixtures of ionic liquids, as compared to those of the constituent pure ionic liquids and varied with the composition of the mixtures. The viscosity of binary mixtures of protic with aprotic ionic liquids, 1?butyl?3?methylimidazoliumbis(trifluoromethylsulfonyl)imide [bmIm][NTf₂], 1?butyl?1?methylpyrrolidiumbis(trifluoromethylsulfonyl)imide [bmPyrr][NTf₂] and 1,3-dimethylimidazolium methyl sulfate, [mmIm][CH₃SO₄] decreases with an increase in the composition of the [HmIm][CH₃COO]. On the contrary, the viscosity for binary mixtures of protic with protic ionic liquid, 1?methylpyrrolidium acetate [HmPyrr][CH₃COO] and 4?methylmorpholine acetate [HmMorph][CH₃COO] increases upon the addition of 1?methylimidazolium acetate [HmIm][CH₃COO]. The self diffusion coefficients were determined for all the binary mixtures of ionic liquids by using Pulsed Gradient Spin Echo (PGSE) NMR method. Self diffusion coefficients of [bmIm][NTf₂]-[HmIm][CH₃COO], [bmPyrr][NTf₂]-[HmIm][CH₃COO], [mmIm][CH₃SO₄]-[HmIm][CH₃COO] are enhanced, while those of [HmPyrr][CH₃COO]-[HmIm][CH₃COO] and [HmMorph][CH₃COO]-[HmIm][CH₃COO] decreases on addition of [HmIm][CH₃COO]. This is converse in the case of viscosity. Furthermore, the above correlations were interpreted with the help of NMR spectroscopy on the basis of interactions of ions in the binary mixtures of ionic liquids. Finally, we have quantified the ionicity through the Nernst–Einstein equation and have confirmed the validity of the Walden rule for the binary mixture of ionic liquids.

Full text of DOI:10.1016/j.molliq.2018.12.024

Accepted Manuscript
Understanding the behavior of mixtures of protic-aprotic and
protic-protic ionic liquids: Conductivity, viscosity, diffusion
coefficient and ionicity
Sachin Thawarkar, Nageshwar D. Khupse, Dinesh R. Shinde, Anil
Kumar
PII:
S0167-7322(18)30857-2
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.12.024
MOLLIQ 10100
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
18 February 2018
1 December 2018
4 December 2018
Please cite this article as: Sachin Thawarkar, Nageshwar D. Khupse, Dinesh R. Shinde,
Anil Kumar , Understanding the behavior of mixtures of protic-aprotic and protic-protic
ionic liquids: Conductivity, viscosity, diffusion coefficient and ionicity. Molliq (2018),
https://doi.org/10.1016/j.molliq.2018.12.024
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ACCEPTED MANUSCRIPT
Understanding the Behavior of Mixtures of Protic-Aprotic and Protic-Protic Ionic Liquids:
Conductivity, Viscosity, Diffusion Coefficient and Ionicity
Sachin Thawarkar (a), Nageshwar D. Khupse,(b) Dinesh R. Shinde(a) and Anil Kumar(a)*
(a) Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune-
411008 (India). Email: a.kumar@ncl.res.in
(b) Centre for Materials for Electronic Technology, Panchwati, Pashan Road, Pune-411008
(India).
ABSTRACT: We have investigated the physicochemical properties such as electrical
conductivity, viscosity and diffusion coefficient for the binary mixtures of protic ionic liquids
with aprotic ionic liquids and of protic with protic ionic liquids at 298.15 K. A significant
enhancement in the electrical conductivity is observed for the binary mixtures of ionic liquids, as
compared to those of the constituent pure ionic liquids and varied with the composition of the
mixtures. The viscosity of binary mixtures of protic with aprotic ionic liquids, 1-butyl-3-
methylimidazoliumbis(trifluoromethylsulfonyl)imide
methylpyrrolidiumbis(trifluoromethylsulfonyl)imide
[bmIm][NTf2],
[bmPyrr][NTf2]
1-butyl-1-
1,3-
and
dimethylimidazolium methyl sulfate, [mmIm][CH3SO4] decreases with an increase in the
composition of the [HmIm][CH3COO]. On the contrary, the viscosity for binary mixtures of
protic with protic ionic liquid, 1-methylpyrrolidium acetate [HmPyrr][CH3COO] and 4-
methylmorpholine acetate [HmMorph][CH3COO] increases upon the addition of 1-
methylimidazolium acetate [HmIm][CH3COO]. The self diffusion coefficients were determined
for all the binary mixtures of ionic liquids by using Pulsed Gradient Spin Echo (PGSE) NMR
method. Self diffusion coefficients of [bmIm][NTf2]-[HmIm][CH3COO], [bmPyrr][NTf2]-
[HmIm][CH3COO], [mmIm][CH3SO4]-[HmIm][CH3COO] are enhanced, while those of
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[HmPyrr][CH3COO]-[HmIm][CH3COO]
and
[HmMorph][CH3COO]-[HmIm][CH3COO]
decreases on addition of [HmIm][CH3COO]. This is converse in the case of viscosity.
Furthermore, the above correlations were interpreted with the help of NMR spectroscopy on the
basis of interactions of ions in the binary mixtures of ionic liquids. Finally, we have quantified
the ionicity through the Nernst–Einstein equation and have confirmed the validity of the Walden
rule for the binary mixture of ionic liquids.
Keywords: Ionic liquid mixtures; electrical conducticity; viscosity; self diffusion coefficient;
ionicity.
1 INTRODUCTION: In last two decades, ionic liquids have attracted attention in
research and industries because of their interesting physicochemical properties such as
very negligible vapor pressure, wide electrochemical window, high thermal stability,
etc.[1]Because of these properties, ionic liquids are rendered as a class of material useful
in many applications as solvents, catalysts and electrolytes for electrochemical
devices.[2–5] Recently, there has been a significant use of ionic liquids as an electrolytic
materials in batteries, fuel cells, dye sensitized solar cells, etc.[6–10] However, the
conductivity of pure ionic liquids is low due to their higher viscosity, which prevents their
efficient use in commercial applied devices. Mainly, there are three ways to alter the
properties of ionic liquids; (1) by introducing a molecular solvent in the ionic liquids,[11–
14], (2) by selecting the proper cation or anions of ionic liquids[15,16] and (3) by mixing
the other ionic liquids particularly possessing low viscosities.[17] Several reports are
available on the physico-chemical properties such as conductivity, viscosity, polarity and
diffusivity of binary systems of ionic liquids with molecular solvents.[11,12] The
mixtures of an ionic liquid with a molecular solvent may offer high ionic conductivity and
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low viscosity. However, the performance of this mixture is superior compared to pure
ionic liquids especially in electrochemical devices. Unfortunately, the use of these
mixtures has some limitations due to the high vapor pressure of molecular
solvents.Interestingly, these properties, are tunable by selecting proper cations and anions
combinations.[18] To propose any systems for efficient application, it is necessary to
understand the basic physicochemical properties such as polarity, conductivity, viscosity
and diffusivity. The problem of loss of low vapor pressure is partially solved by mixing -
ionic liquids to each other.[19,20] Very few research groups have used the binary
mixtures of ionic liquids in electrochemical applications, which have shown improved
performance in the device. Furthermore, these investigations provide useful fundamental
information about the molecular interactions present in the binary systems. For example,
the binary mixtures of different ionic liquids have shown the enhancement in
conductivity, thermal stability and electrochemical stability, which result in the better
performance of the lithium electrochemistry using the mixture of binary electrolytes.[21]
Similarly, a mixture of iodide and tricynomethide-imidazoliums has been used as an
electrolyte for dye sensitized solar cells, which exhibits higher conductivity and improved
efficiency.[22] The binary mixtures of protic ionic liquids have been investigated by
measuring the properties such electrochemical window and ionicity.[23] It has been
noticed that a mixture of ionic liquids possesses a good prospect as electrolyte material in
devices as well as other chemical processes.[24,25] The structural arrangement and
physicochemical properties of ionic liquids depend on many interactions such as
electrostatic, hydrogen bonding, electron donor-acceptor, dipole-dipole interactions, etc
operating between the ions.[26,27] The insight of the physical properties of mixtures of
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ionic liquids is extremely important from experimental and theoretical points of view,
which is useful for designing systems on an industrial scale. In this work, we have
focused on the properties such as conductivity, viscosity and self diffusion coefficient of
mixtures
of
1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide
1-butyl-1-methylpyrrolidiumbis(trifluoromethylsulfonyl)imide
[bmIm][NTf2],
[bmPyrr][NTf2], 1,3-dimethylimidazolium methyl sulphate, [mmIm][CH3SO4], 1-
methylpyrrolidium acetate [HmPyrr][CH3COO] and 4-methylmorpholine
acetate[HmMorph][CH3COO] with 1-methylimidazolium acetate [HmIm][CH3COO],
respectively over the entire composition range at 298.15K. In this work, we have studied
the mixtures of protic ionic liquids with protic ionic liquids and with aprotic ionic liquids.
2 EXPERIMENTAL SECTION:
2.1 Chemicals: 1-methyl imidazole (
morpholine ( 99%), 1-butylbromide (
salt ( 99%), dimethyl sulphate ( 99%) and acetic acid (

99%), 1-methyl pyrrolidine (
99%), bis(trifluoromethane) sulfonimide lithium
96%) were purchased from
 99%), 4-methyl




Sigma-Aldrich and used as obtained. Solvents such as dichloromethane (99%) and ethyl
acetate (99%) 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 according to the literature reports.[1,11] In first step, the quarternization
reaction of 1-methylimidazole and 1-methyl pyrrolidine with 1-bromoalkane was carried out to
prepare [bmIm]Br or [bmPyrr]Br, respectively. In second step, [bmIm][NTf2] or [bmPyrr][NTf2]
was synthesized using metathesis reaction of bis(trifluoromethane) sulfonimide lithium salt
(LiNTf2) with [bmIm]Br or [bmPyrr]Br in water at room temperature.[28] The protic ionic
liquids, [HmIm][HCOO], [HmPyrr][CH3COO] and [HmMorph][CH3COO] were synthesized
4

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and purified according to the reported procedures.[29] The protic ionic liquids were synthesized
by neutralization reaction of an acid such acetic acid with a base such as 1-methylimidazolium,
1-methyl pyrrolidine or 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 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 60 C under reduced pressure. These protic ionic liquids
were further dried under vacuum at 60 C for 6 h. Water contents of the ionic liquids were
measured by a Karl Fischer coulometer (Model No. Metrohm KF-Coulometer 831). On a
thoughtful suggestion received from an anonymous reviewer, the water content of all the
synthesized ionic liquids was again checked. The water content in both [bmIm][NTf2] and
[bmPyrr][NTf2] was 52 ppm, while that in
[mmIm][CH3SO4], [HmIm][CH3COO],
[HmPyrr][CH3COO], [HmMorph][CH3COO], [HmIm][CH3COO] did not exceed 65 ppm.
1
Determination of purities and characterization was performed by H NMR spectroscopy, which
was in agreement with the reported values.[30]
2.3Conductivity measurements: The specific conductivities () were measured using a
Systronics Micro Controller Based Conductivity Meter 306 at 1 kHz supplied by Systronics
India Ltd from Ahmedabad, India. The conductivity meter was calibrated and cell constant was
determined with standard 0.1 M aqueous solutions of KCl.[31] The measured conductivity data
were accurate to be ± 1%, while concentration be 0.1%. The temperature of the jacket was
maintained using a JULABO thermostat F25-ME Refrigerated/Heating Circulator with an
accuracy of ±0.01 K.
2.4Viscosity measurements: Viscosity measurements were performed by using a
Brookfield-ultra rheometer-DV-III with cone plate. The calibration of the viscometer was
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carried out with the help of reported viscosity data of aqueous solutions of KCl having
different concentration. The measured viscosity data were accurate to ±1%. The
viscosities of the binary mixtures of ionic liquids were measured for entire range of
composition at 298.15 K. The compositions of binary mixtures were taken as
푥[
of ionic liquids. Temperature of the systems was maintained by a
]
HmIm [CH COO]
3
JULABO thermostat bath with an accuracy of ±0.01 K.
2.5PGSE-NMR Measurements: The pulsed-field gradient spin-echo (PGSE) NMR
technique was used to measure the self-diffusion coefficients of both the cation and anion
19
species by observing1Hand F nuclei. The PGSE−NMR measurements were performed
by using a Bruker Avance 400 ultra shield spectrometer with 9.4 T super conducting
magnet. The spectrometer was equipped with 2D sequence for diffusion measurement.
The self-diffusion coefficients were measured using stimulated echo sequence,
longitudinal-eddy-current delay (LED) and bipolar gradient pulses to improve the self-
diffusion coefficient measurements.[32]Self-diffusion coefficients, D, were calculated by
using the equation 1.[33]
I = Ioexp [- 2g22D (-/3 )]
(1)
where I is the measured intensity with field gradient g, I0 the equilibrium intensity, δ,
△
,
γ and D are the effective gradient pulse, diffusion time, gyromagnetic ratio and diffusion
coefficient, respectively. The samples were thermally equilibrated at each temperature for
30 min. before any measurements.
3 RESULTS AND DISCUSSION:
3.1 Viscosity of (ionic liquid-ionic liquid) binary systems: In order to demonstrate the
effect of [HmIm][CH3COO] compositions on the viscosity of binary mixtures of ionic
6

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liquids, we have chosen both protic-protic and protic-aprotic binary mixtures of ionic
liquids for this study. The viscosity (
[bmIm][NTf2]-[HmIm][CH3COO],

) for binary mixtures of ionic liquids;
[bmPyrr][NTf2]-[HmIm][CH3COO],
[mmIm][CH3SO4]-[HmIm][CH3COO], [HmPyrr][CH3COO]-[HmIm][CH3COO] and
[HmMorph][CH3COO]-[HmIm][CH3COO] were measured for entire composition range
at 298.15K and summarized in Table 1. The [HmIm][CH3COO] having viscosity 6.22 cP
was added to [bmIm][NTf2], [bmPyrr][NTf2] and [mmIm][CH3SO4] having viscosities
49.3, 74.9, 93.4 cP, respectively. There is a dramatic decrease in the
 values of the
binary mixture upon addition of small concentration of [HmIm][CH3COO] as shown in
Fig. 1 (a). This is due to the separation of cation and anion of aprotic ionic liquids on
addition of polar protic ionic liquids. As seen in Fig. 1, the viscosity of [bmIm][NTf2]
decreases by about 14% upon addition of 푥[
=
0.1. Similarly, a decrease of
20% and 4% in the viscosity of [bmPyrr][NTf2] and [mmIm][CH3SO4], respectively is
noticed, upon addition of 푥[ = 0.1 to pure ionic liquids at298.15K. At
]
HmIm [CH COO]
3
]
HmIm [CH COO]
3
푥[
= 0.5, the order of viscosities for the binary mixtures of aprotic ionic
]
HmIm [CH COO]
3
liquids is [mmIm][CH3SO4]-[HmIm][CH3COO] > [bmPyrr][NTf2]-[HmIm][CH3COO] >
[bmIm][NTf2]-[HmIm][CH3COO] and for the binary mixtures of protic ionic liquids
[HmMorph][CH3COO]-[HmIm][CH3COO] > [HmPyrr][CH3COO]-[HmIm][CH3COO].
Interestingly, in the case of binary mixtures of protic-protic ionic liquids, the addition of
[HmIm][CH3COO] in [HmPyrr][CH3COO] and [HmMorph][CH3COO], respectively, the

of the systems is exceeded than those of the pure component. The maxima of  for the
[HmPyrr][CH3COO]-[HmIm][CH3COO] and [HmMorph][CH3COO]-[HmIm][CH3COO]
7

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systems were obtained to be 6.78 and 7.78 cP at 0.5 and 0.8 푥[HmIm [CH COO] ],
]
3
respectively which is greater than viscous pure ionic liquids.
Table 1. The values of  of binary mixtures of [bmIm][NTf2], [bmPyrr][NTf2],
[mmIm][CH3SO4], [HmPyrr][CH3COO] and [HmMorph][CH3COO] with [HmIm][CH3COO] at
298.15K.

(cP)
푥[
[bmIm] [bmPyrr] [mmIm]
[HmPyrr] [ HmMorph ]
[CH3SO4] [CH3COO] [CH3COO]
]
HmIm [CH COO]
3
[NTf2]
49.3
42.8
38.4
33.5
28.8
24.5
20.2
15.5
12.2
6.22
[NTf2]
74.9
59.5
52.7
38.8
33.3
26.9
21.1
15.9
12.1
6.22
0.00
0.10
0.19
0.31
0.40
0.50
0.59
0.71
0.80
1.00
93.4
90.2
77.2
56.3
40.7
35.3
27.7
19.9
13.6
3.78
4.87
5.35
6.23
6.46
6.78
6.74
6.59
6.47
5.37
5.76
6.19
6.54
6.96
7.15
7.41
7.57
7.78
6.22
6.22
6.22
Standard uncertainties are as follows:
△
u(
) = 1 %,
△
u(x) = 0.1 %,
△
u(T) = ±0.01 K
(a)
(b)
7.8
6.5
90
60


5.2
30
3.9
0
0.00
0.33
0.66
0.99
0.00
0.33
0.66
0.99
x
x
[HmIm][CH COO]
[HmIm][CH COO]
3
3
Fig. 1The plots of

vs. 푥[HmIm [CH COO] with (a) [mmIm][CH3SO4] (
○
), [bmIm][NTf2]
]
3
(■) and [bmPyrr][NTf2] (△) and (b) [HmPyrr][CH3COO] (△) and [mmorp][CH3COO]
(■), respectively at 298.15K.
The viscosity of binary mixtures increases up to certain limit due to an increase in the
strength of hydrogen bonding interaction amongst the ions in binary mixtures with the
8

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composition of [HmIm][CH3COO] and hence form ordered structures (both are protic
ionic liquids therefore to enhance the hydrogen bonding). Furthermore, it is decreased due
to increase in composition of [HmIm][CH3COO] comparison to other protic ionic liquids.
3.2 Conductivity of (ionic liquid + ionic liquid) binary systems: In this study, we have
measured the conductivity by mixing (a) protic ionic liquids and aprotic ionic liquids (b)
protic ionic liquids and protic ionic liquids. The specific conductivity (
mixtures of ionic liquids systems, [bmIm][NTf2]-[HmIm][CH3COO], [bmPyrr][NTf2]-
[HmIm][CH3COO], [mmIm][CH3SO4]-[HmIm][CH3COO], [HmPyrr][CH3COO]-
[HmIm][CH3COO] and [HmMorph][CH3COO]-[HmIm][CH3COO] is measured for
whole composition range at 298.15K. The values for pure ionic liquids such as
[bmIm][NTf2], [bmPyrr][NTf2], [mmIm][CH3SO4], [HmPyrr][CH3COO],
) for the binary

[HmIm][CH3COO], [HmMorph][CH3COO] are 3.28, 2.01, 5.53, 3.03, 4.48 and 0.23
mScm-1, respectively which are lower than any composition of [HmIm][CH3COO] in the
binary mixtures of investigated ionic liquids. The
 values of the binary mixtures of ionic
liquids as function of mole fraction of [HmIm][CH3COO], 푥[
]are shown in
][
퐻푚퐼푚 퐶퐻 퐶푂푂
3
Fig. 2 and Table 2. The
 values of mixtures are noted to increase with composition of
[HmIm][CH3COO] up to 푥[퐻푚퐼푚 [퐶퐻 퐶푂푂] = 0.9 for aprotic ionic liquids. However, in the
]
3
case of binary mixtures of protic ionic liquid with protic ionic liquids, there is an increase
in the
further, decrease in the
[bmIm][NTf2], [bmPyrr][NTf2],
 values of the mixture up to 푥[HmIm [CH COO] = 0.59 in [HmPyrr][CH3COO],
]
3

value is noticed. The maxima of conductivity max values for
[mmIm][CH3SO4], [HmPyrr][CH3COO]and
[HmMorph][CH3COO] are 5.92, 5.11,7.17, 5.59 and 3.31mScm-1, respectively. Our study
of binary mixtures of ionic liquids, [mmIm][CH3SO4] shows higher conductivities to that
9

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of the other binary systems for entire composition range. Thus, it is observed that the

values of aprotic ionic liquids are higher than those of protic ionic liquids. The
of ionic liquids mixtures except [HmMorph][CH3COO] show positive deviation from
ideal linear mixing rule which is indicated by dashed line. Thus, the changes in values
for the binary mixtures of ionic liquids are observed to be non-ideal. Interestingly, the
values of mixtures of ionic liquids are higher than those pure ionic liquids
 values


at푥[
]in the range of 0.2 to 0.9 for [bmIm][NTf2], [bmPyrr][CH3SO4] and
][
HmIm CH COO
3
[mmIm][CH3SO4].
Table 2. The values of
[mmIm][CH3SO4], [HmPyrr][CH3COO] and [HmMorph][CH3COO] with
[HmIm][CH3COO], respectively at 298.15K.

of the binary mixtures of [bmIm][NTf2], [bmPyrr][NTf2],

(mScm-1)
[mmIm] [HmPyrr]
[CH3SO4] [CH3COO] [CH3COO]
푥[
[bmIm]
[NTf2]
3.276
3.515
3.836
4.172
4.450
4.856
5.230
5.655
5.928
4.479
[bmPyrr]
[NTf2]
2.010
2.300
2.620
3.010
3.391
3.810
4.237
4.712
5.110
4.479
[HmMorph ]
]
HmIm [CH COO]
3
0.00
0.10
0.19
0.31
0.40
0.50
0.59
0.71
0.80
1.00
5.528
5.480
5.729
6.090
6.370
6.579
6.764
6.993
7.178
4.479
) = 1 %,
3.029
3.691
4.247
4.887
5.270
5.537
5.593
5.193
5.025
0.229
0.447
0.662
0.959
1.578
2.016
2.416
2.736
3.310
4.479
4.479
Standard uncertainties are as follows:
△
u(

△
u(x) = 0.1 %, △u(T) = ±0.01 K
The enhancement or diminution of the

values of ionic liquid mixtures mainly depend
upon the mobility of ions and number of charge carriers which follows the equation 2.[34]
∑
 = 푛푖푞푖푖
(2)
where, ni = number of charge carriers of type i,q= charge and

= mobility of each ions.
10

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(a)
7.5
6.0
(b)
5.6
4.2

2.8
4.5

1.4
0.0
3.0
0.00
0.33
0.66
0.99
0.00
0.33
0.66
0.99
x
x
[HmIm][CH COO]
[HmIm][CH COO]
3
3
Fig. 2 The plots of  of ionic liquid mixture vs. 푥[HmIm [CH COO] with (a) [mmIm][CH3SO4] (○),
]
3
[bmIm][NTf2] (■) and [bmPyrr][NTf2] (△) and (b) [HmPyrr][CH3COO] (□) and
[HmMorph][CH3COO] (▲), respectively at 298.15K.(The solid line shows the experimental
data and dashed line indicates linear ideal mixing, error bars are shown).
A considerable deviation in conductivity was observed 1.44, 0.63, 1.64 and 1.11 mScm-1
for the binary mixtures of [bmIm][NTf2], [bmPyrr][NTf2], [mmIm][CH3SO4] and
[HmPyrr][CH3COO] with [HmIm][CH3COO], respectively. However, the
 values for
the binary mixtures of [HmMorph][CH3COO] with [HmIm][CH3COO] show some extent
of negative deviation from the linear mixing. We have also measured the diffusion of
ions in binary ionic liquid systems as described in next Section 3.3.The diffusion of ions
does not considerably deviate from the linear law of mixing. Consequently, it is observed
that ionic mobility is not responsible for the positive deviation in
 values of the ionic
liquids mixtures. Therefore, the enhancement in the values may be due to an increase

in number of charge carriers in binary systems and a decrease in viscosity of mixtures by
the addition of low viscous [HmIm][CH3COO] in high viscous [mmIm][CH3SO4],
[bmIm][NTf2] and [bmPyrr][NTf2]. The electrical conduction of ions depends on two
11

ACCEPTED MANUSCRIPT
different factors, i) the number of ions present in the transport charges and ii) the mobility
of ions in the binary mixtures of ionic liquids.[35] After the addition of
[HmIm][CH3COO] to [mmIm][CH3SO4], [bmIm][NTf2] and [bmPyrr][NTf2], it leads to
an increase in number of ions and lower in viscosity of the mixtures. But the mobility of
ions is observed to have decrease due to an increase in the concentration of ions.
However, the effect of increase in number of ions and decrease in viscosity are
dominating over the mobility factor. Therefore, the
 values of the binary mixtures of
ionic liquids are enhanced upon addition of [HmIm][CH3COO] in [mmIm][CH3SO4],
[bmIm][NTf2] and [bmPyrr][NTf2]. In another case, the binary mixtures of protic-protic
ionic liquids show that the
 values increase up to 푥[HmIm [CH COO] = 0.59 and afterwards
]
3
decrease with composition of [HmIm][CH3COO]. At concentration 푥[
=
]
HmIm [CH COO]
3
0.59, an increase in mobility of ions is the most favourable parameter and hence
conductivity is enhanced. Subsequently, there is an increase in viscosity, which decreases
the mobility of ions in the binary mixtures beyond 푥[HmIm [CH COO] = 0.6. However, in the
]
3
case of the binary mixtures of [HmMorph][CH3COO] and [HmIm][CH3COO], the

values of mixtures increase gradually with 푥[HmIm [CH COO] with a slight negative
]
3
deviation from linear mixing which is due to the continuous increase in the viscosity of
ionic liquid mixtures. It is a consequence of stronger the interaction between cation and
anion through hydrogen bonding formed from the electronegative oxygen atom present in
morpholine ring. This is also reflected from the conductivity of pure
[HmMorph][CH3COO] which is observed to be low compared to [HmIm][CH3COO]. The

values of the binary mixtures of [HmIm][CH3COO] with aprotic ionic liquids follow in
the order as [mmIm][CH3SO4]
+
[HmIm][CH3COO]
>
[bmIm][NTf2]
+
12

ACCEPTED MANUSCRIPT
[HmIm][CH3COO] > [bmPyrr][NTf2] + [HmIm][CH3COO]. Since, in all the above three
systems, the second component [HmIm][CH3COO] is common, the conductivity therefore
depends on the nature of first component of the systems. The
 values are higher for the
binary mixtures of [mmIm][CH3SO4]-[HmIm][CH3COO] due to the small size of both
the cation and anion of [mmIm][CH3SO4] compared to [bmIm][NTf2] and
[bmPyrr][NTf2]. Therefore, the mobility of [mmIm]+ and [CH3SO4]- ions is higher than
that of [bmIm]+ and [bmPyrr]+and [NTf2]- ions. Similarly, the
 values of the binary
mixtures of [bmIm][NTf2]-[HmIm][CH3COO] is higher than that of [bmPyrr][NTf2]-
[HmIm][CH3COO] binary systems though the size of the ions in both the system are
approximately the same. The interactions between cation and anion are stronger in
[bmPyrr][NTf2] than that of [bmIm][NTf2] which is due to the localisation of positive
charge on only one nitrogen atom of pyrrolidinium ring of [bmPyrr][NTf2], while it is
delocalized in the aromatic imidazolium ring of [bmIm][NTf2]. This leads to a decrease in
number of charge carrier ions in [bmPyrr][NTf2] systems than that of [bmIm][NTf2]. The

values of the binary mixtures of [HmPyrr][CH3COO]-[HmIm][CH3COO] are higher
than that of [HmMorph][CH3COO]-[HmIm][CH3COO] binary systems. In the binary
mixtures of [HmMorph][CH3COO] with [HmIm][CH3COO], the hydrogen bonding
ability of ion due to the morpholine ring is higher which reduces the mobility of ions and
increases the viscosity of the binary mixtures of ionic liquids. Thus, the
 values of
studied binary systems are higher than those of pure components excluding
[HmMorph][CH3COO]-[HmIm][CH3COO] system. This may be ascribed due to an
increase of the inter-ionic interactions upon addition of [HmIm][CH3COO].
13

ACCEPTED MANUSCRIPT
3.3 Diffusion coefficients of cations and anions in ionic liquid mixtures: The PGSE-
NMR techniques are used to measure the ions self-diffusion coefficient (D) without the
1
use of any additional probe molecules affecting the diffusion processes. In this study, H
19
and F NMR spectroscopy were used for the determination of the cationic and anionic
self-diffusion coefficients independently. In this work, we have explained the nature of
the ionic interactions as well as the site of the ion binding. For the binary mixtures of
[bmIm][NTf2]-[HmIm][CH3COO],
[bmPyrr][NTf2]-[HmIm][CH3COO],
[mmIm][CH3SO4]-[HmIm][CH3COO], the D of ions increases exponentially with
푥[HmIm [CH COO] as shown in Fig. 3 which is due to the lower viscosity of mixture. These
]
3
results are consistent with
 and  of the studied systems. Similarly, we have measured D
for the system of [mmIm][CH3SO4]-[HmIm][CH3COO] with 푥[HmIm [CH COO], the plot is
]
3
shown in Fig. S1. The values of D of ions for respective system are shown in Table
3.From the Table 3, it is observed that the relative order of the D of the ions remain as
[HmIm]+>[CH3COO]->[bmIm]+>[NTf2]-, [HmIm]+>[CH3COO]->[bmPyrr]+>[NTf2]-and
[HmIm]+>[CH3COO]-> [mmIm]+> [CH3SO4]-for respective binary systems. The D values
of cations and anions of ionic liquids in binary mixtures vary with size of ions and it is
inversely related. Importantly, it is also observed that the
 values of aprotic ionic liquids
direct the D of [HmIm]+ and [CH3COO]- and is shown in Fig. S 2. Similarly, for the
binary mixtures of [HmPyrr][CH3COO]-[HmIm][CH3COO] and [HmMorph][CH3COO]-
[HmIm][CH3COO], the D of ions has been observed to decrease with 푥[HmIm [CH COO] as
]
3
shown in Fig. 4. The D values are inversely related to the

values and the

values of the
14

ACCEPTED MANUSCRIPT
Table 3. Diffusion coefficients of cation and anions of [bmIm][NTf2]-[HmIm][CH3COO] and
[bmPyrr][NTf2]-[HmIm][CH3COO] ionic liquid systems.
[bmIm][NTf2] + [HmIm][CH3COO]
(10-10) m2s
[bmPyrr][NTf2] + [HmIm][CH3COO]
(10-10) m2s
[bmIm]
[NTf2]
[HmIm]
[CH3COO]
CH3-
[bmPyrr]
[NTf2]
[HmIm]
[CH3COO]
CH3-
C2-
Fluorine C2-
CH2 -
Fluorine C2-
proton of A-
proton proton
of C+ of A-
2.394 2.031
proton
of C+
-
of A-
proton proton
푥[
]
bmIm
[NTf ]
of C+
of C+
2.394
of A-
2.031
2
0.00
0.00
0.00
-
0.20
0.30
0.41
1.097
0.733
0.664
0.625
0.414
0.494
1.514 1.272
0.999 0.822
0.951 0.730
-
-
-
-
0.748
-
0.628
-
1.046
-
0.902
-
0.50
0.69
0.81
0.533
0.386
0.337
0.341
0.226
0.224
0.754 0.586
0.509 0.410
0.431 0.359
0.461
0.323
-
0.306
0.255
-
0.676
0.488
-
0.556
0.400
-
0.90
Pure
0.303
0.246
0.116
0.145
0.411 0.336
0.201
0.182
0.120
0.114
0.361
-
0.302
-
0.00
0.00
Standard uncertainties are as follows:
△
u(D) = 1 %,
△
u(x) = 0.1 %, △u(T) = ±0.01 K
(b)
(a)
2.4
2.4
1.8
1.2
0.6
0.0
1.6
0.8
0.0
0.00
0.33
0.66
0.99
0.0
0.3
0.6
0.9
x
x
[HmIm][CH COO]
[HmIm][CH COO]
3
3
Fig. 3 The plots of D of ions vs.푥[
(a) for [bmIm][NTf2]-[HmIm][CH3COO]
]
HmIm [CH COO]
3
system; [bmIm]+ (□), [NTf2]- (△), [HmIm]+ (□) and [CH3COO]- (△) (b) for [bmPyrr][NTf2] +
[HmIm][CH3COO] system; [bmPyrr]+ (□), [NTf2]- (△), [HmIm]+ (□) and [CH3COO]- (△).
15

ACCEPTED MANUSCRIPT
medium, since the
decrease in D of ions.
For the protic-protic ionic liquids mixtures, an increase in the

values of system increase with 푥[HmIm [CH COO]] which leading to a
]
3

values is not only because
of the increasing number of charge carriers but also for other additional factor such as
Grotthuss mechanism.[31]
3.2
(a)
(b)
4.0
2.8
3.2
2.4
2.4
2.0
1.6
1.6
0.0
0.3
0.6
0.9
0.00
0.25
0.50
0.75
x
x
[HmIm][CH COO]
3
[HmIm][CH COO]
3
Fig. 4 The plots of D of ions vs. 푥[
(a) for [[HmPyrr][CH3COO]-
]
HmIm [CH COO]
3
[HmIm][CH3COO] system; [HmPyrr]+ ( □ ),[HmIm]+ ( □ ) and [CH3COO]- (△) (b) for
[HmMorph][CH3COO] + [HmIm][CH3COO] system; [mmorph]+ ( □ ),[HmIm]+ ( □ ) and
[CH3COO]- (△).
Hence, the transport properties of ionic liquids are strongly governed by Columbic forces
such as van der Waals interactions and hydrogen bonding. In the protic ionic liquid
systems, more protons are available for strong hydrogen bonding, therefore D and

reduce with 푥[
. The relative order of the D of the ions at
]
HmIm [CH COO]
3
푥[
=
0.5 remains as [HmIm]+ > [HmPyrr]+ and [HmMorph]+ >[CH3COO]-
]
HmIm [CH COO]
3
for respective system.
16

ACCEPTED MANUSCRIPT
3.4 Interactions of cations and anions in the binary mixtures of ionic liquids:
Interactions between cations and anions of the binary mixtures of ionic liquids are
1
19
explained by using H and F NMR spectroscopy. In this study, we have considered the
chemical shifts ( ) of C2-proton of imidazolium ring and N-H proton for cationic

interactions whereas the -CH3 proton in acetate anion of [HmIm][CH3COO] and
 of
fluorine in [NTf2]- are considered for anionic interactions and the experimental values of
chemical shift are given in Table SI 5-9. The
 values are more sensitive for C2 proton
due to its more acidic nature which is directly interact with anionic species.[36–38] If the
concentration of ionic liquids in binary mixture changes, the
also change.
 of these protons should
Fig. 51H-NMR spectra for cations of the binary mixtures of [bmIm][NTf2]-[HmIm][CH3COO]
with pure [HmIm][CH3COO] (bottom) and pure [bmIm][NTf2] (top). C2-proton of
[bmIm]+(black arrow), [HmIm]+ (red arrow), C4, C5 proton of [bmIm]+ (blue arrow) and C4,
C5proton of [HmIm]+ (green arrow) with increasing the concentration of 푥[bmIm [NTf ] = 0.3, 0.5,
]
2
0.7, 0.9 from bottom to top.
For example, in the binary mixtures of [bmIm][NTf2] with [HmIm][CH3COO], the  values of
C2-proton of imidazolium ring for both the ionic liquids, N-H proton for [HmIm][CH3COO], -
17

ACCEPTED MANUSCRIPT
CH3 proton for [CH3COO]- and fluorine for [NTf2]- anion are shifted towards the downfield with
the addition of [HmIm][CH3COO].
Fig. 6(a) 1H-NMR spectra for anions of the binary mixtures of [bmIm][NTf2]-
[HmIm][CH3COO] with pure [HmIm][CH3COO] (bottom) and pure [bmIm][NTf2] (top).
Chemical shift of CH3-proton of [CH3COO]-with increasing the 푥[bmIm [NTf ] = 0.3, 0.5, 0.7, 0.9
]
2
from bottom to top and (b) 19F-NMR spectra for anions of the binary mixtures of[bmPyrr][NTf2]
-[HmIm][CH3COO], chemical shift of fluorine of [NTf2]- with increasing the 푥[
=
]
bmpyrr [NTf ]
2
0.3, 0.5, 0.7, 0.9 from bottom to top.
The  values for C2 proton of [bmIm][NTf2] downfield from 8.39 to 8.80, C2 proton of
[HmIm][CH3COO] from 7.57 to 7.80, for N-H proton from 13.30 to 15.22, for –CH3 proton of
[CH3COO]- from 1.83 to 1.95 and for fluorine from -80.39 to 79.85 ppm.
18

ACCEPTED MANUSCRIPT
From these values, it is clearly interpreted that the interactions between ions get stronger with an
increase with푥[HmIm [CH COO] . The downfield shift in  values is the direct consequence of
]
3
stronger interactions between cations and anions of ionic liquids.
Fig. 71H-NMR spectra of the binary mixtures of [HmPyrr][CH3COO]-[HmIm][CH3COO]
with pure [HmIm][CH3COO] (bottom) and pure [HmPyrr][CH3COO] (top)(a) C2, C4 and
C5-proton of [HmIm]+ and (b) –CH3 proton of [HmPyrr]+ and –CH3proton of [CH3COO]-
with increasing the 푥[HmPyrr [CH COO] = 0.3, 0.5, 0.7, 0.9 from bottom to top.
]
3
This indicates that the solvation of the cation or anion is increased by the surrounding ions in
ionic liquid binary mixtures. Hence, the numbers of ions are increased in the system. Similarly,
19
the 1H and F NMR spectra with systematic variation of of proton and fluorine of the ions for
the binary mixtures of [bmIm][NTf2]-[HmIm][CH3COO] and [bmPyrr][NTf2]
-
[HmIm][CH3COO], respectively with푥[
] are shown in Fig. 6. The  values of C2-
][
HmIm CH COO
3
proton and fluorine for neat [bmIm][NTf2] and [bmPyrr][NTf2]are 8.39 and -80.20 ppm, which
shifts to 8.80 and -79.68 ppm, respectively after the addition of 푥[ = 0.7. It is
]
HmIm [CH COO]
3
cleared that in neat [bmIm][NTf2], interaction between cation and anion is weaker than that of
19

ACCEPTED MANUSCRIPT
binary ionic liquid mixtures. The change in chemical shift (△) for C2 proton and fluorine are
0.41 and 0.55 ppm at 푥[
= 0.7, respectively.
]
HmIm [CH COO]
3
The
△
of N-H and –CH3 proton from 푥[
= 0.1 to 푥[HmIm [CH COO] 0.7 are
]
HmIm [CH COO]
3
]
3
1.43 and 0.05 ppm, respectively.The

values of the binary mixtures of [bmPyrr][NTf2]-
[HmIm][CH3COO] and [mmIm][CH3SO4]-[HmIm][CH3COO] are shown in Tables S6
and S7. The 1H and 19F NMR spectra with systematic variation in

values for proton and
fluorine of ions for the binary mixtures of [bmPyrr][NTf2]-[HmIm][CH3COO] as function
of 푥[HmIm [CH COO] of [HmIm][CH3COO], are shown in Figs. S6, S7. The for N-H
△

]
3
proton is changed as compared to that of –CH3 proton. It may be due to the more acidic
N-H proton and therefore it is more sensitive towards the change in environment rather
than –CH3 proton. Similar observations were found for [bmPyrr][NTf2]-
[HmIm][CH3COO] and [mmIm][CH3SO4]-[HmIm][CH3COO] systems. In the case of
mixtures of the protic ionic liquids with [HmIm][CH3COO], the behavior is observed to
be contradictory which is displaced by the aprotic ionic liquids. Since, there is up field
shift in the

values of –CH2 proton of pyrrolidinium ring, C2-proton of imidazolium ring
values of –
and –CH3 proton of acetate ion on the addition of [HmIm][CH3COO].The

CH2 proton, C2-proton and –CH3 proton are changed from 3.96 to 3.78, from 7.97 to
7.80, 2.18 to 1.95 ppm, respectively. This shows that interactions between cation and
anion in the binary mixtures become weak, which leads to an increase in the number of
cation and anions. Since acetate anion is common for both the ionic liquids and addition
of [HmIm][CH3COO] to [HmMorph][CH3COO] there will be no change in interaction in
ionic liquids. However, the up field shift in all
 values occurred due to the weakening of
interactions between cation and anion of both protic ionic liquids.
20

ACCEPTED MANUSCRIPT
3.5 Ionicity: Herein, the estimation and understanding of ionicity is essential part in the
view of electrolytes. A Walden plot has been used to estimate the ionicity roughly with
aqueous solution of KCl as reference line. The ideal line represents the complete
dissociation of ions and has a slope of unity. The deviation from this ideal line is a rough
estimation of ionicity. Angell et al have classified the ionic liquid or mixtures of ionic
liquids in the classes of super ionic, good ionic and sub ionic in nature.[39,40]Fig. 8
shows Walden plots for the binary mixtures of ionic liquids. The binary mixtures of protic
1.6
0.8
0.0

-0.8
-1.6
-1.6 -0.8 0.0
0.8
-1
1.6
log (1/ ) poise
Fig. 8 The Walden plots for binary mixtures of ionic liquids at 푥[HmIm [CH COO] = 0.5 with
]
3
[bmIm][NTf2] (■), [bmpyrr][NTf2] (△), [mmIm][CH3SO4] ( ○ ), [HmPyrr][CH3COO] and
[HmMorph][CH3COO].
and aprotic ionic liquid are located near the ideal line. According to Angell et al the
mixture of ionic liquids at 푥[
= 0.5 with [bmIm][NTf2], [bmPyrr][NTf2] and
]
HmIm [CH COO]
3
[mmIm][CH3SO4], are in the range of good ionic liquid and with [HmPyrr][CH3COO]
and [HmMorph][CH3COO], respectively, are in the range of sub ionic in nature. In order
21

ACCEPTED MANUSCRIPT
to explain the ionicity of binary mixtures of ionic liquids is more quantitatively on the
basis of self diffusion coefficient of cations and anions.
The molar conductivity of the binary mixtures has been determined from Nernst-Einstein
equation 3. [12,41]
2
∑
푁 푒 (푥 퐷 )
푎
푛
푖
푖
푚 =
(3)
퐾 푇
퐵
where, Na = Avogadro number (6.022x1023mol-1), e = electric charge on each ionic
carrier (1.602x10-19C), xi = molar ratio of ions, Di= self diffusion coefficient of ions, kB=
Boltzmann constant (1.38x10-23 JK-1), and T = absolute temperature (K).
Herein, for example, the calculation of NMR for the binary mixtures of ionic liquids
[bmIm][NTf2]-[HmIm][CH3COO], we have considered the D of C2 proton for both the
cations, fluorine for [NTf2]- and -CH3 proton for [CH3COO]- anions and which is
explained in the previous section. In Table 3, we have compared the ionicity of the binary
mixtures of ionic liquids calculated from Walden plots and D of ions. The values of
ionicities calculated from the both the methods are nearly equal. The order of ionicity of
binary mixtures at 푥[
] =0.5 is [mmIm][CH3SO4]-[HmIm][CH3COO] >
][
HmIm CH COO
3
[bmIm][NTf2]-[HmIm][CH3COO]
[HmPyrr][CH3COO]-[HmIm][CH3COO] > [HmMorph][CH3COO]-[HmIm][CH3COO]
which are consistent with the order of and D.
>
[bmPyrr][NTf2]-[HmIm][CH3COO]
>

22

ACCEPTED MANUSCRIPT
Table 4.Ionicity of the binary mixtures of ionic liquids at 푥[HmIm [CH COO]= 0.5 calculated from
]
3
Walden rule and diffusion coefficient.
Ionic liquid mixtures
% of Ionicity
Walden rule
imp/NMR
T
t+
t-
[bmIm][NTf2]+[HmIm][CH3COO]
27
26
0.58 0.42
[bmPyrr][NTf2]+[HmIm][CH3COO]
[mmIm][CH3SO4]+[HmIm][CH3COO]
[HmPyrr][CH3COO]+[HmIm][CH3COO]
24
35
6
23
27
7
0.57 0.43
0.58 0.42
0.56 0.44
[HmMorph][CH3COO]+[HmIm][CH3COO]
-
-
0.56 0.44
Standard uncertainties are as follows:
±0.01K.
△u(% Ionicity) = 1 %, △u(x) = 0.1 %, △u(T) =
Furthermore, we have also determined ionic transference numbers of cation and anions by
using self-diffusion coefficients of each ion; the ionic transference number ti is defined
as[42,43]
푥 퐷
푖
푖
푡푖= ∑
(4)
푥 퐷
푖
푖
where, xi=ith ion molar fraction and Di=ith ion diffusion coefficient of ions.
From Table 3, it can be concluded that the t+ values for all binary ionic liquid mixtures
are higher than the t- values. The cationic transport number t+ is higher than 0.5, while the
anionic transport number t- is less than 0.5. This indicates that cations carry more than
half of the current and less than half is carried by the anions, since cations are able to
move or diffuse faster than anions. These results are in agreement with our previous
work and other literature reports.[11,12] Interestingly, the t+ value decreases, while the t-
value increases with an increase in the concentration of [HmIm][CH3COO] in
[bmIm][NTf2], as shown in Fig. S 8.
CONCLUSIONS: In this work, we have investigated the transport properties of ions of
binary mixtures of ionic liquids. All the studied properties of ionic liquid mixtures depend
upon the composition of mixtures. The conductivity of all binary mixtures increases in
23

ACCEPTED MANUSCRIPT
comparison to their individual pure components, which is due to the increase in number
of ions in the binary mixture of ionic liquids. The viscosity of the mixtures of aprotic and
protic ionic liquids decreases with composition while it increases for the mixture of protic
ionic liquids. Self-diffusion coefficients of the ions in binary systems were determined by
1
19
observing H and F nuclei with the pulsed-field gradient spin-echo NMR technique.
Self-diffusion coefficient for each ion in the mixture of aprotic and protic ionic liquids
increases with푥[
while for the mixture of protic ionic liquids decreases.
]
][
HmIm CH COO
3
This contradict results are explained in terms of the strong H-bonding present in mixtures
of protic ionic liquids. These results were correlated to the deviation from the ideality of
systems as a function of푥[HmIm [CH COO].The results were also analyzed by investigating
]
3
the interactions between the ions of ionic liquids with the help of NMR spectroscopy. The
ionicity determined from Walden plot and NMR techniques mutually support each other.
ACKNOWLEDGEMENTS: SRT thanks CSIR, New Delhi, for awarding him a Senior
Research Fellowship, while A.K. thanks DST, New Delhi, for awarding him a JC Bose
National Fellowship (SR/S2/JCB-26/2009).
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24

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