Comparative Investigation of the Ionicity of Aprotic and Protic Ionic Liquids in Molecular Solvents by using Conductometry and NMR Spectroscopy

Article abstract of DOI:10.1002/cphc.201501156

Electrical conductivity (σ), viscosity (η), and self-diffusion coefficient (D) measurements of binary mixtures of aprotic and protic imidazolium-based ionic liquids with water, dimethyl sulfoxide, and ethylene glycol were measured from 293.15 to 323.15 K. The temperature dependence study reveals typical Arrhenius behavior. The ionicities of aprotic ionic liquids were observed to be higher than those of protic ionic liquids in these solvents. The aprotic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, [bmIm][BF₄], displays 100 % ionicity in both water and ethylene glycol. The protic ionic liquids in both water and ethylene glycol are classed as good ionic candidates, whereas in DMSO they are classed as having a poor ionic nature. The solvation dynamics of the ionic species of the ionic liquids are illustrated on the basis of the ¹H NMR chemical shifts of the ionic liquids. The self-diffusion coefficients D of the cation and anion of [HmIm][CH₃COO] in D₂O and in [D₆]DMSO are determined by using ¹H nuclei with pulsed field gradient spin-echo NMR spectroscopy.

Full text of DOI:10.1002/cphc.201501156

DOI: 10.1002/cphc.201501156
Articles
Comparative Investigation of the Ionicity of Aprotic and
Protic Ionic Liquids in Molecular Solvents by using
Conductometry and NMR Spectroscopy
Sachin Thawarkar, Nageshwar D. Khupse, and Anil Kumar*[a]
Electrical conductivity (s), viscosity (h), and self-diffusion coeffi-
cient (D) measurements of binary mixtures of aprotic and
protic imidazolium-based ionic liquids with water, dimethyl
sulfoxide, and ethylene glycol were measured from 293.15 to
323.15 K. The temperature dependence study reveals typical
Arrhenius behavior. The ionicities of aprotic ionic liquids were
observed to be higher than those of protic ionic liquids in
these solvents. The aprotic ionic liquid, 1-butyl-3-methylimida-
zolium tetrafluoroborate, [bmIm][BF4], displays 100% ionicity
in both water and ethylene glycol. The protic ionic liquids in
both water and ethylene glycol are classed as good ionic can-
didates, whereas in DMSO they are classed as having a poor
ionic nature. The solvation dynamics of the ionic species of the
1
ionic liquids are illustrated on the basis of the H NMR chemi-
cal shifts of the ionic liquids. The self-diffusion coefficients D of
the cation and anion of [HmIm][CH3COO] in D2O and in
1
[D6]DMSO are determined by using H nuclei with pulsed field
gradient spin-echo NMR spectroscopy.
1. Introduction
Ionic liquids are composed of organic cations as well as either
inorganic or organic anions, and they possess interesting phys-
ical properties such as low vapor pressure, high thermal stabili-
ty, wide electrochemical window, and large liquid range. These
properties render ionic liquids useful to employ in many appli-
cations in several fields[1,2] such as a solvent in organic synthe-
sis, catalysis, separation and extraction processes, electrochem-
ical cells, and solar energy cells.[3,4] In view of great need to de-
velop clean and renewable energy sources, ionic liquids have
also been explored as electrolytes for energy conversion devi-
ces such as lithium-ion batteries and fuel cells.[5] However, the
high viscosity of ionic liquids reduces the optimum per-
formance in many electrochemical devices that employ such
compounds as electrolytes. The effect is further complicated
by the high sensitivity of viscosity not only to the choice of
constituent ions but also to the presence of impurities and co-
solvents.[6] In particular, the addition of co-solvents such as
water or methanol can strongly affect the physical and chemi-
cal properties of ionic liquids such as viscosity, electrical con-
ductivity (s), and reactivity as well as solvation and solubility
properties.[7] Water and ethylene glycol are protic solvents that
are useful for enhancing electrical conductivity. Previous inves-
tigations by our group have revealed a drastic reduction in the
viscosity of ionic liquids upon the addition of small amounts of
co-solvent.[8] Recently, several binary ionic liquids/co-solvent
systems have been shown to perform better than the pure
ionic liquids, and such systems have been used in many appli-
cations.[9,10]
To use such binary mixtures in applications, it is essential to
study fundamental properties such as the thermal behavior of
the mixture, and the conductivity, viscosity, and self-diffusion
coefficients, etc. of ionic liquid solutions. Knowledge of such
fundamental properties and of ion–ion and ion–solvent sys-
tems in a solution will help to achieve the maximum efficiency
of a system by tuning the nature and amount of cations,
anions, and solvent. It is essential to elucidate the properties
of mixtures that are very different from those of pure ionic liq-
uids and molecular solvents. In binary mixtures, the structural
organization of ions differs from those of pure ionic liquids.
The effective use of binary mixtures in electrolytic systems re-
mains limited because of the lack of experimental data on
physical properties such as conductivity, viscosity, and diffu-
sion, and because of a lack of understanding on the molecular
level of interactions present in the system. Ionic liquids possess
unique characteristics because of their ionic nature; however,
obtaining an estimate of the ionicity of such liquids through
conductivity measurements is very important for their use as
an electrolyte. Previous reports have shown that many pure
ionic liquids lie below the reference line indicating incomplete
ionization of ionic liquids, which was explained on the basis of
the Walden plot.[11–13] An adjusted Walden plot that allows for
differences in ionic size was shown to offer an improvement to
this approach for a series of ionic liquids. Some ionic liquids
studied by the authors showed ionicity values that were close
to ideal, which was described in terms of a model in which the
ion correlations had similar influence on both the diffusive and
conductive motions. It was established by the authors that the
ionicity was not essentially a measure of ion availability in the
chemical sense.[12] The group of MacFarlane has demonstrated
[a] S. Thawarkar, N. D. Khupse, Dr. A. Kumar
Physical and Materials Chemistry Division,
CSIR-National Chemical Laboratory,
Pune 411 008 (India)
E-mail: a.kumar@ncl.res.in
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201501156.
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that there is a clear distinction between the behavior of simple
primary versus tertiary amines. Proton transfer is more com-
plete in primary amines than in tertiary amines. This effect was
attributed to the hydrogen-bonding ability of the ammonium
ions to provide a good solvating environment for the ions pro-
duced by the proton transfer.[13] Ionicity provides information
on molecular interactions between ions and ion–solvent inter-
actions.[14] A literature survey suggests that the data on ionici-
ties are available for only pure ionic liquids. Krossing et al. de-
scribed the ionicities of imidazolium and ammonium based
ionic liquids.[15] Rebelo et al. investigated ways to generate
high ionicity by adding inorganic salts to pure ionic liquids.[16]
Mu et al. studied the ionicity of acetate-based protic ionic liq-
uids.[17] Unfortunately, a survey of the literature reveals that
available data on the ionicities of ionic liquids in molecular sol-
vents are very limited. Buchner et al. compared the ionic asso-
ciation behavior of different classes of ionic liquids in acetoni-
trile.[18] Sagara et al. measured the temperature dependent
transport properties of ionic liquids.[19] Watanabe et al. illustrat-
ed the ionicity and proton transfer mechanism of binary protic
ionic liquid mixtures for fuel cell reactions.[20] However, there
remains a lack of data on the role of molecular level interac-
tions on iconicity in different solvent systems for seeking their
better applications.
ity indicates less ionic aggregation whereas poor ionicity indi-
cates high ionic aggregation.
2. Results and Discussion
2.1. Electrical Conductivity of Aprotic Ionic Liquids
Conductivity values s have been measured for all the systems
at atmospheric pressure and in the temperature range of
293.15 to 313.15 K. The s values mainly depend on tempera-
ture, concentration of ions in solution, and the mobility of the
ion. As expected, the s values of all the aprotic ionic liquids in
molecular solvents increase with increased temperature from
293.15 to 313.15 K. The plot of lns against 1/T is shown in Fig-
ure S1. Now, we consider the specific concentration of ionic
liquids throughout all the system; however, the production of
the ions depends on solvent properties and on the structure
of the ionic liquids. Finally, the mobility of ions plays a major
role in electrical conductivity, which depends on the viscosity
of the medium and on solute–solvent interactions present in
the system. Here, we focus on a comparative study of the
s values of ionic liquids, [bmIm]Br, [hmIm]Br, [omIm]Br, [bmIm]
[BF4], and [omIm][BF4] in water, DMSO, and ethylene glycol at
298.15 K (Table 1). The s value is higher in water than in either
The present work is focused on temperature-dependent
transport properties: electrical conductivity (s), viscosity (h),
and self-diffusion coefficient (D) of nine protic and aprotic
ionic liquids. The aprotic ionic liquids investigated were 1-
butyl-3-methylimidazolium bromide ([bmIm]Br), 1-hexyl-3-
methylimidazolium bromide ([hmIm]Br), 1-octyl-3-methylimida-
zolium bromide ([omIm]Br), 1-butyl-3-methylimidazolium tetra-
fluoroborate ([bmIm][BF4]), and 1-octyl-3-methylimidazolium
tetrafluoroborate ([omIm][BF4]). The protic ionic liquids em-
ployed in the investigation were 1-methylimidazolium formate
([HmIm][HCOO]), 1-methylimidazolium acetate ([HmIm]
Table 1. Electrical conductivity (s) of aprotic ionic liquids in molecular
solvents at 298.15 K.
Solvent
Ionic liquid s [mScmÀ1
]
[bmIm]Br [hmIm]Br [omIm]Br [bmIm]
[BF4]
[omIm]
[BF4]
water
8.05
2.65
0.50
7.69
2.36
0.47
7.43
2.28
0.44
9.93
3.44
0.77
7.51
2.50
0.46
DMSO
ethylene
glycol
[CH3COO]),
1-methylimidazolium
propionate
([HmIm]
[CH3CH2COO]), and 1-butylimidazolium acetate ([HbIm]
[CH3COO]), where H indicates a proton. Water, dimethyl sulfox-
ide (DMSO), and ethylene glycol were used as molecular sol-
vents. These ionic liquids were selected for study based on the
nature and structural arrangement of cations and anions. The
molecular solvents were selected on the basis of their proper-
ties: polarity, relative permittivity (e), hydrogen-bond donating
(a) and hydrogen-bond accepting ability (b), and viscosity
(h).[21]
DMSO or ethylene glycol for all aprotic ionic liquids and fol-
lows the order water> DMSO> ethylene glycol. Furthermore,
for each solvent, the s values are observed to be [bmIm]Br>
[hmIm]Br>[omIm]Br, which is imparted due to an increase in
alkyl chain length. The [BF4]À based ionic liquids show higher
s values than those of [Br]À based ionic liquids. In the series of
[RmIm]Br (R is alkyl chain length) aprotic ionic liquids, the
s value of [bmIm]Br is higher than that of [hmIm]Br and
[omIm]Br in each of the solvents, and [bmIm][BF4] has a higher
s value than [omIm][BF4].
As part of our ongoing investigation on ionic liquids sys-
tems,[22–27] we now measured three transport properties: elec-
trical conductivity, viscosity, and self-diffusion coefficient of
binary mixture of ionic liquids with solvents. According to
Angell and co-workers,[11,28] ionic systems can be classified as
super ionic, good ionic, poor ionic, and nonionic on the basis
of Walden plots. Deviation from the ideal line represents the
ionicity in a binary mixture of ionic liquids and solvents, which
can be used to classify the systems as super ionic, good ionic,
or poor ionic.[11] A departure from the reference KCl line indi-
cates the existence of ion pairs or ionic aggregates. High ionic-
The change in the s values of ionic liquids in different mo-
lecular solvents depends on the solvent properties and struc-
ture of the ionic liquids. In previous work, we have also de-
scribed the effect of solvent properties and the structure of
ionic liquids on the limiting molar conductivity.[25] Herein, sol-
vent properties such as relative permittivity (e), solvatochromic
N
parameters [i.e. polarity (ET )], hydrogen-bond donating ability
(a), hydrogen-bond accepting ability (b), and viscosity (h) of
solvents are shown to play a major role in determining the
s values of systems.
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2.2. Effect of Relative Permittivity (e)
and nonspecific interacting components, including columbic
interactions, the various dipole–dipole interactions, hydrogen-
bonding and electron-pair donor–acceptor interactions.[29–31]
The s values of all the aprotic ionic liquids were higher in
water than in either DMSO or ethylene glycol because of the
very high relative permittivity of water (78.4) followed by those
of DMSO (46) and ethylene glycol (37.7). The high relative per-
mittivity means that the columbic interactions between cations
and anions of ionic liquids are reduced, leading to higher dis-
sociation of cations and anions. Higher numbers of free ions
available to carry the electric current in water–ionic liquid sys-
tems thus become available. Figure 1 shows the decrease in
N
The solvatochromic parameters ET(30) or ET , a and b explain
the solvent dependent phenomena at the molecular level and
give information about the solvation ability of the medium.[21]
N
The values of ET , and the a and b parameters for molecular
solvents water, DMSO, and ethylene glycol are given in Table 2.
Table 2. Solvent properties of molecular solvents: ETN, a, b, h, and e at
298.15 K.[21]
No.
Solvent
Properties
N
ET
a
b
e
h [cP]
1
2
3
water
DMSO
ethylene glycol
1.00
0.44
0.79
1.12
0.00
0.90
0.50
0.76
0.52
78
46
37.7
0.89
1.99
16.9
N
As shown in Figure 2, the effect of the ET values of molecular
solvents on s of aprotic ionic liquids [bmIm][BF4] and [omIm]
[BF4] does not show any correlation. Here, water and ethylene
glycol are polar protic solvents whereas DMSO is a polar aprot-
N
ic solvent. The ET values for water, ethylene glycol, and DMSO
solvents are 1, 0.79, and 0.44, respectively. In the case of polar
protic solvents water and ethylene glycol, the s values for all
N
aprotic ionic liquids decrease in accordance with the ET value
(Table 2). However, in the case of DMSO, the s values are ob-
served to be higher than in ethylene glycol for all aprotic ionic
N
liquids, even though the ET is smaller for DMSO than for ethyl-
ene glycol. This contradicts the generalized correlation on the
N
N
basis of ET . Similar correlations like those of the ET values are
observed for the a values of water and ethylene glycol with
those of the s values of aprotic ionic liquids (Table 2). Hence,
the ability of water and ethylene glycol to form hydrogen
bonds plays a dominant role in the solvation of the ions of
aprotic ionic liquids independently. A smaller effect is observed
in the case of the b parameter of water and ethylene glycol
N
with s in the case of aprotic ionic liquids. Thus, ET , hydrogen-
bond donor ability of water and ethylene glycol helps the dis-
sociation of cation and anions in aprotic ionic liquids, which
dominates the columbic attraction force between cations and
anions in ionic liquids. This effect is limited to polar protic sol-
vents. However, in the case of polar aprotic solvent DMSO, the
Figure 1. Plots of the electrical conductivity s of ionic liquids vs. relative per-
mittivity (e) of solvents A) [bmIm]Br (solid square), [hmIm]Br (hollow circle),
[omIm]Br (hollow triangle); B) [bmIm][BF4] (solid square) and [omIm][BF4]
(hollow triangle) in water (red), DMSO (green) and ethylene glycol (blue) at
298.15 K.
N
effect of solvatochromic parameters ET , a, and b on the s of
all aprotic ionic liquids is observed to be weak. The effect of
solvatochromic parameters on s of [bmIm]Br, [hmIm]Br, and
[omIm]Br in molecular solvents are given in Figure S3.
s values for all ionic liquids from water to ethylene glycol be-
cause of the decrease in relative permittivity from water to
ethylene glycol. This indicates that higher ionic association is
present in DMSO and ethylene glycol systems than in water.
Importantly, the s values for all aprotic ionic liquids in ethyl-
N
ene glycol are smaller than in DMSO, although the ET and
a values are higher for ethylene glycol. These values arise be-
cause of the higher viscosity of ethylene glycol (16.90 cP) com-
pared with DMSO (1.99 cP). The high viscous drag opposes the
mobility of ions in a binary mixture of ionic liquids with ethyl-
ene glycol; this aspect is discussed further in Section 2.4.
2.3. Effect of Solvatochromic Parameters
Polarity is a general term that refers to all the interaction
forces between molecules, and is composed of several specific
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Figure 2. A) Plots of s vs. ETN; B) Plots of s vs. a for [bmIm][BF4] (solid
square) and [omIm][BF4] (hollow triangle) in water (red), DMSO (green), and
ethylene glycol (blue) at 298.15 K.
Figure 3. Plots of viscosity (h) vs. electrical conductivity (s) of A) [bmIm][BF4]
(solid square) and [omIm][BF4] (hollow triangle), and B) [bmIm]Br (solid
square), [hmIm]Br (hollow circle), and [omIm]Br (hallow triangle) in water
(red), DMSO (green), and ethylene glycol (blue) at 298.15 K.
2.4. Effect of Viscosity on s
2.5. Structure of Cations and Anions in Aprotic Ionic Liquids
Figure 3 shows a plot of the h values of solvents versus the
s values for the aprotic ionic liquids in solvent. The results
show that the s values of aprotic ionic liquids decreases with
increasing h values of the solvents. The bulk h values of water,
ethylene glycol, and DMSO are 0.89, 1.99, and 16.9 cP, respec-
tively. Thus, the higher the viscosity of the medium, the lower
will be the s value of the system. The s values of aprotic ionic
liquids decrease with molecular solvents in the order water>
DMSO> ethylene glycol. The s values of aprotic ionic liquids
decrease drastically from water to DMSO although the change
in viscosity is observed to be very small; however, a small de-
crease in the s values of aprotic ionic liquids from DMSO to
ethylene glycol is observed but the corresponding viscosity
change is significant. Thus, in addition to the viscosity of
medium, other factors are important for the solvation dynam-
ics of the ions of ionic liquids. This effect is similar to the effect
of relative permittivity of the medium on the s values of aprot-
ic ILs. Thus, in the case of aprotic ILs, only bulk properties of
viscosity h and dielectric constant e play dominant roles in de-
termining the s values of aprotic ILs.
Comparing the s values of [bmIm]Br, [hmIm]Br, [omIm]Br,
[bmIm][BF4], and [omIm][BF4] in water, DMSO, and ethylene
glycol reveals an inverse linear relationship between the
s values with an increase in the number of carbon atoms in
the alkyl chain of the cations of the aprotic ionic liquids. An in-
crease in alkyl chain length from butyl to octyl and the associ-
ated increase in the size of cation, decreases the mobility of
ions in all studied systems. The effect of the nature of the cat-
ions on s values is shown in Figure 4. Similarly, the experimen-
tal results shows that the s values of [BF4]À based ionic liquids
are higher than those of the respective [Br]À based ionic liquids
in each of the molecular solvents examined. The s values of
[bmIm]Br and [bmIm][BF4], with the same cation, are 8.05 and
9.93 mScmÀ1 in water, respectively, and for [omIm]Br and
[omIm][BF4] the values are 2.28 and 2.50 mScmÀ1, respectively.
Given that the size of the [BF4]À anion is larger than that of
the [Br]À anion,[21] the electrostatic interactions and hydrogen-
bonding ability of [Br]À anion with [bmIm]+ cation is stronger
than that of the [BF4]À anion with the [bmIm]+ cation, which
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for [HmIm][CH3COO] is higher in water (6.461) than in DMSO
(0.163) and ethylene glycol (0.42), which is due to the high
e (78) and h (0.89 cP) values of water, whereas DMSO and eth-
ylene glycol have low values of both e (46 and 37.7, respective-
ly) and h (1.99 and 16.9 cP, respectively).
Interestingly, the s values for all the protic ionic liquids in
ethylene glycol are higher than those in DMSO, although
DMSO posses higher e and low h than ethylene glycol. This
contradicts the correlation of s of all protic ionic liquids with
e and h of the medium. Thus, it is important to consider the
microscopic properties rather than bulk or macroscopic prop-
erties of the medium to fully understand the factors affecting
the s values of protic ionic liquids. Thus, here we have consid-
ered microscopic parameters such as solvatochromic parame-
N
ters ET , a and b, which are found to be more specific com-
N
pared with e and h. The parameters ET , a, and b for water,
DMSO, and ethylene glycol are given in Table 2. Figure 5
Figure 4. Plots of s versus alkyl chain length, Cn of [CnmIm][BF4] (hollow) and
[CnmIm]Br (solid) in water (blue), DMSO (red), and ethylene glycol (black) at
298.5 K.
shows plots of the s values of protic ionic liquids as a function
N
of ET and a for water, ethylene glycol, and DMSO. Here, the
reduces the ionic strength of the ionic liquids. The separation
of cations and anions in [BF4]À based ionic liquids means that
larger numbers of free ions are available than for [Br]À based
ionic liquids.
2.6. Electrical Conductivity of Protic Ionic Liquids
The conductivity of [HmIm][HCOO], [HmIm][CH3COO], [HmIm]
[CH3CH2COO], and [HbIm][CH3COO] in water, DMSO, and ethyl-
ene glycol are shown in Table 3. For all the studied protic ionic
liquids, the s values decrease in the order [HmIm][HCOO]>
[HmIm][CH3COO]> [HmIm][CH3CH2COO]>[HbIm][CH3COO] in
Table 3. Electric conductivities (s) of protic ionic liquids in molecular sol-
vents at 298.15 K.
Solvent
Ionic liquid s [mScmÀ1
]
[HmIm]
[HCOO]
[HmIm]
[CH3COO]
[HmIm]
[CH3CH2COO]
[HbIm]
[CH3COO]
water
6.461
0.163
0.421
6.143
0.012
0.252
5.732
0.010
0.238
4.379
0.010
0.203
DMSO
ethylene
glycol
water, DMSO, and ethylene glycol. The s values of the protic
ionic liquids in DMSO and ethylene glycol are smaller than in
water. Thus, the order of s values for protic ionic liquids in mo-
lecular solvents is water> ethylene glycol> DMSO. However,
these results for all protic ionic liquids contracts to the role of
viscosity and relative permittivity. Here, the effect of the e and
h values of the medium show a relationship with the s values
of the protic ionic liquids in water and DMSO systems, and
water and ethylene glycol systems; however, the correlation of
s values of aprotic ionic liquids contradicts the correlation of
s values with the h and e values of the medium in the case of
DMSO and ethylene glycol systems. For example, the s value
Figure 5. A) Plots of s vs. ETN; B) plots of s vs. a for [HmIm][HCOO] (solid
square), [HmIm][CH3COO] (hollow circle), and [HmIm][CH3COO] (hollow trian-
gle) in water (red), DMSO (green), and ethylene glycol (blue) at 298.15 K.
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N
s values for all protic ionic liquids decrease with ET , and a de-
uids in water is nearly linear with an increase in alkyl chain
length; however, there is sharp of decrease in the s value of
protic ionic liquids in DMSO and ethylene glycol from [HmIm]
[HCOO] to [HmIm][CH3COO] and a gradual decrease for
[HmIm][CH3CH2COO]. Similarly, we have examined the role of
cations, which were changed from [HmIm]+ to [HbIm]+ with
common anion [CH3COO]À, in determining the s values in
water, DMSO, and ethylene glycol (Table 3).
The s values decrease from 6.143, 0.012, and 0.252 mScmÀ1
for [HmIm][CH3COO] to 4.379, 0.010, and 0.203 mScmÀ1 for
[HbIm][CH3COO] in water, DMSO, and ethylene glycol, respec-
tively. This is due to the increase in alkyl chain length from one
carbon atom to four carbon atoms on the imidazolium cation
ring.
creases in the order water> ethylene glycol> DMSO. Thus,
the role of the hydrogen-bond donor ability of the solvent
plays a crucial role in the solvation of cations and anions, irre-
spective of the nature of the protic ionic liquids. Similarly,
b values of the medium affect the conduction behavior in
protic ionic liquids to some extent. This is because the Grot-
thuss-type mechanism[32] occurs in binary mixtures of protic
ionic liquids with water and ethylene glycol. Here, the s value
of protic ionic liquids is higher in ethylene glycol than in
DMSO, as a result of the proton hopping mechanism in protic
ionic liquids. Given that protic ionic liquids are more polar
than aprotic ionic liquids,[33] the properties of protic ionic liq-
uids are more sensitive towards the surrounding medium or
solvents at the microscopic level. Thus, microscopic parameters
determine the conductivity of protic ionic liquids whereas mac-
roscopic properties determine the conductivity of aprotic ionic
liquids.
2.7. Interactions between Cations and Anions of Ionic
Liquids
Figure 6 shows the plot of s versus alkyl chain length of
anions of [Hmim][HCOO], [Hmim][CH3COO], and [Hmim]
[CH3CH2COO] in water, DMSO, and ethylene glycol. From the
results it has been concluded that the s value of protic ionic
liquids has an inverse relationship with alkyl chain length of
Cations and anions are held together by electrostatic and hy-
drogen-bond interactions. The electrostatic and hydrogen-
bond interactions primarily depend on charge density, the size
of the ions, and availability of protons on the ions. Here, we
have considered three ionic liquids [bmIm]Br, [bmIm][BF4], and
[HbIm][CH3COO] in which the size of the cations are almost
identical but ionic radii of the anions differ for [BF4]À, BrÀ, and
[CH3COO]À as 0.232, 0.195, and 0.159 nm, respectively. Thus, an
increase in the ionic radii of anions from [CH3COO]À to BrÀ to
[BF4]À, leads to an increase in the s values of the ionic liquids
in all the molecular solvents studied. Here, the e/r ratio (where
e is the charge on the molecule and r is the radius of the mole-
cule) increases from [BF4]À to [CH3COO]À and charge density
decreases from [CH3COO]À to [BF4]À.[21] The value of e/r for
[BF4]À, BrÀ, and [CH3COO]À is 4.31, 5.12, and 6.28, respectively.
Protic ionic liquids [HbIm][CH3COO] show stronger interac-
tions between cations and anions because of the large electro-
static interactions between cations and anions and also be-
cause of the strong hydrogen-bonding ability of the proton on
the cation with the anion. This leads to a decrease in the
number of free ions in solution. This effect is clear in Figure 7,
which shows that the s values decrease slowly from [BF4]À to
BrÀ but decrease sharply from BrÀ to [CH3COO]À. The order of
electrical conductivity is [bmIm][BF4]> [bmIm][Br]> [HbmIm]
[CH3COO] in water, DMSO, and ethylene glycol. Thus, aprotic
ionic liquids exhibit higher s values than protic ionic liquids
because of the stronger attraction of ions in protic ionic liq-
uids.
Figure 6. Plots of s vs. alkyl chain length (Cn) of anions of [Hmim][HCOO],
~
&
[Hmim][CH3COO] and [Hmim][CH3CH2COO] in water ( ), ethylene glycol ( ),
*
and DMSO ( ) at 298.15 K.
anions in water, DMSO, and ethylene glycol. With an increase
in the number of carbon atoms in the structure of the anions
from [HmIm][HCOO] to [HmIm][CH3CH2COO], the value of s de-
creases from 6.461, 6.143 and 5.732, to 0.163, 0.012 and 0.010,
to 0.421, 0.252 and 0.238 mScmÀ1 in water, DMSO, and ethyl-
ene glycol, respectively. An increase in the alkyl chain length of
the anions is associated with an increase in the size of the
anions from [HmIm][HCOO] to [HmIm][CH3CH2COO] as 0.158,
0.159, and 0.162 nm, respectively.[21] The mobility of
[CH3CH2COO]À is slower because of its larger size and hence
the conductivity of [HmIm][CH3CH2COO] is lower in all the sol-
vents studied. The decrease in the s values of protic ionic liq-
2.8. Solvation Effects
1
Here, we illustrate the solvation of ions by using the H NMR
chemical shift (d) of the C-2 proton of imidazolium cations of
ionic liquids. The chemical shift of the C-2 proton is more sen-
sitive than the C-4 and C-5 protons because it is located be-
tween two nitrogen atoms of the imidazolium ring.[34–37] If the
concentration of the ionic liquids in solution is the same, the
chemical shift of the C-2 proton should be dependent on the
solvent. The experimental d value for the C-2 proton of neat
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Figure 7. Plots of s vs. ionic radii rx of anions for [bmIm][BF4] (red), [bmIm]Br
&
*
(green), and [HbIm][CH3COO] (blue) in water ( ), DMSO ( ), and ethylene
~
glycol ( ) at 298.15 K.
[bmIm][BF4] is 8.71 ppm and for neat [bmIm]Br is 9.67 ppm,
which indicates that the interaction of the C-2 proton with the
À
smaller and less polarizable Br anion is greater than that with
the [BF4]À anion, which larger and more polarizable.
The d shifts of the C-2 proton of the imidazolium ring of
[bmim]Br and [bmIm][BF4] in 0.1m D2O and [D6]DMSO are
given Table S7. The d values of [bmIm]Br in D2O and [D6]DMSO
are 8.61 and 9.19 ppm and for [bmIm][BF4] in D2O and
[D6]DMSO are 8.62 and 9.11 ppm, respectively. The lower d
values of [bmIm]Br and [bmIm][BF4] in D2O show the stronger
solvation of ions in water (D2O) than in DMSO ([D6]DMSO).
Thus, there is an increase in the s values for [bmIm]Br and
[bmIm][BF4] in water compared with those in DMSO. The up-
field shift in d is a direct consequence of weakening of the hy-
drogen bond between cations and anions and the interaction
with solvent molecules. Figure 8 shows that the d value of the
C-2 proton is strongly affected by the polarity of the solvent,
and it is reported that higher polarity leads to lower d values
of the C-2 proton.[38–40] In the case of [HmIm][CH3COO], we
have explained the solvation of ions on the basis of the d
value of CH3- proton of anions [CH3COO]. Given that the N-H
proton is more acidic than the C-2 proton of protic ionic liq-
uids and that it resonates at a more downfield position, we
consider the CH3- proton of the anion for analysis. The d value
of the CH3-proton in D2O (1.78 ppm) is lower than that in
[D6]DMSO (1.93 ppm). This indicates that there is a larger in-
crease in the solvation of the ions of protic ionic liquids in
water than in DMSO, which leads to higher s values for protic
ionic liquids in water than in DMSO.
*
Figure 8. A) Plots of e vs. A) chemical shift (d) for [bmIm]Br in DMSO ( ) and
&
&
*
D2O ( ) and for [bmIm][BF4] in [D6]DMSO ( ) and D2O ( ) and conductivity
~
~
(s) for [bmIm]Br in [D6]DMSO ( ) and D2O ( ) and for [bmIm][BF4] in
[D6]DMSO (!) and D2O (!). B) Enlarged plot between 7.5 to 10 ppm of d at
298.15 K.
with the Stejskal–Tanner equation.[41,22] The data show that cat-
ionic diffusion coefficients of [HmIm][CH3COO] (0.1m) in D2O
solution is higher than the anionic diffusion coefficients. The
coefficient values of the cation and anion of [HmIm][CH3COO]
increase with increased temperature. From these results, it is
observed that at higher temperature the rate of diffusion of
the cation is higher than that of the anion. The temperature
dependence of the diffusion coefficients of the binary system
of [HmIm][CH3COO] with D2O can be fitted by using the Arrhe-
nius equation [Eq. (1)].[42]
Ea
RT
D¼ DoexpÀ
2.9. Diffusion Coefficients (D) and Hydrodynamic Radius
or
ð1Þ
The temperature dependence of diffusion coefficients of cat-
ions and anions was obtained from PGSE-NMR measurements
(Figure S4). Both cationic and anionic diffusion coefficient, D,
were measured from the 1H nucleus. Fitting was performed
Ea
ln D ¼ ln Do À
RT
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by using the self-diffusion coefficient.[43,44]
NAe2
kT
ð3Þ
LNMR
¼
ðD þ D Þ
þ
where NA is Avogadro’s number, e is the electronic charge, k is
the Boltzmann constant, T is the absolute temperature, and D+
and D are the self-diffusion coefficient of the cation and
À
anion of [HmIm][CH3COO], respectively.
The values of Limp are less than LNMR, indicating that not all
of the diffusive species contribute to the conduction of electric
current because of the formation of neutral ion pair species in
molecular solvents. If the LNMR calculated from Equation 3 and
the molar conductivity, obtained from the electrical conductivi-
ty Limp, are equal then all the ions participate in ionic conduc-
tion.
In the case of ionic association, the molar conductivities,
Figure 9. The Arrhenius plots of cationic and anionic self-diffusion coeffi-
[45]
LNMR and Limp are different, and the ratio of Limp/LNMR
rep-
cients based on PGSE-NMR measurements for [HmIm][CH3COO] vs. 1/T for
À
[HmIm]+ ( ) and [CH3COO]
(
) in D2O at 298.15 K.
&
*
resents the ionicity of the binary mixtures. The values for the
ionicity of [HmIm][CH3COO] are lower than unity, confirming
the presence of ionic association in these binary systems
(Figure 10).
where, Ea is the activation energy for diffusion of ions and Do is
the maximum diffusion coefficient. The plot of lnD versus 1/T
give straight lines, as shown in Figure 9.
Furthermore, we have determined the hydrodynamic radii
from the Stoke–Einstein equation [Eq. (2)].[15]
kT
CphRH
ð2Þ
D ¼
where, D is the diffusion coefficient, k is the Boltzmann con-
stant, T is the absolute temperature, and h the viscosity of the
solvent. RH is the hydrodynamic radius. The C factor is a con-
stant that is influenced by the strength of the interactions be-
tween the diffusing species and the solvent medium. It is gen-
erally taken as 6 but can be reduced to 4. To determine the hy-
drodynamic radius we gave C the value 6. The D values of the
cation and anion in D2O and [D6]DMSO are consistent with the
s values of [HmIm][CH3COO] in water and DMSO. The D values
of the cation and anion of [HmIm][CH3COO] in D2O is almost
1.5 times greater than in [D6]DMSO but the s value in D2O is
much greater than that noted in [D6]DMSO. Thus, it is clear
that some additional factor is also responsible for the electric
current in water. As mentioned in Section 2.6, water—being
a polar protic solvent—is responsible for the Grotthuss-type
mechanism.[32] The hydrodynamic radii (RH) of [HmIm]+ and
[CH3COO]À in D2O are 265 and 278 pm and for [D6]DMSO are
178 and 230 pm, respectively, calculated from the Stokes–Ein-
stein equation. The radii RH of [HmIm]+ and [CH3COO]À are
greater in D2O than in [D6]DMSO, which suggests that solva-
tion of ions occurs to a greater extent in water.
Figure 10. Plots of molar conductivity (Lm) vs. temperature (T) of [HmIm]
~
&
[CH3COO] in water for Limp
( ), LNMR ( ) at 298.15 K.
The Limp and LNMR values for [HmIm][CH3COO] are 61.43 and
67.62 Scm2 molÀ1 in water and D2O, respectively. On the other
hand, the Limp and LNMR values for [HmIm][CH3COO] are noted
as 0.12 and 40.82 Scm2 molÀ1 in DMSO ([D6]DMSO), respectively.
The ionicity values of [HmIm][CH3COO] calculated from the
Walden plot are 61.6 and 0.003% and ionicity values from
Limp/LNMR are 90 and 0.003% in water (D2O) and DMSO
([D6]DMSO), respectively, at 298.15 K. The effect of temperature
on the rate of increase of LNMR is greater than on Limp. Thus, it
is clear that temperature does not have a significant effect on
the dissociation of ions in [HmIm][CH3COO] with water.
To discuss the ionicity of binary mixtures of ionic liquids
with solvents in more qualitative and quantitative detail, we
measured the self-diffusion coefficients of all protons in the
cation and anion of [HmIm][CH3COO] individually, using the
PGSE-NMR method. The molar conductivity (LNMR) of binary
mixtures can be determined from the Nernst–Einstein [Eq. (3)]
Temperature dependent ionic transference numbers t have
been determined by using the self-diffusion coefficients of
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each ion; the ionic transference number ti is defined as by
Table 4. Linear fit parameters from the Walden Plot.
Equation (4):[46,47]
Ionic liquid
[bmIm]Br
Solvent[a]
f
logC [S·cm2·molÀ1
]
Ea,L/Ea,h
xiDi
P
water
DMSO
EG
0.7940
0.7955
0.8013
0.3301
0.0969
0.0912
0.8094
0.7926
0.8013
ti ¼
ð4Þ
xiDi
where xi is the ith ion molar fraction and Di is the ith ion diffu-
sion coefficient.
[hmIm]Br
water
DMSO
EG
0.8008
0.8384
0.8035
0.2919
0.0146
0.0867
0.8110
0.8387
0.8034
From Table S6, it is observed that the t+ values for all protic
ionic liquids are higher than the tÀvalues in D2O and
[D6]DMSO. The cationic transport number t+ for [HmIm]+ is
higher than 0.5, whereas the anionic transport number
tÀ[CH3COO]À is less than 0.5, suggesting [HmIm]+ carries more
than half of the current and less than half is carried by the
[CH3COO]À, because [HmIm]+ is able to move or diffuse faster
than anions. These results are analogous with our previous
work and with other reports.[25,43] At higher temperature, t+ in-
creases and tÀ decreases, which is consistent with the self-dif-
fusion coefficient of ions (Figure S4).
[omIm]Br
water
DMSO
EG
0.8088
0.2667
0.8242
0.8764
1.0240
0.8759 À0.0520
0.9662 À0.0868
[bmIm][BF4]
water
DMSO
EG
0.7259
0.7902
0.9195
0.5606
0.2565
0.0787
0.7306
0.7907
0.9205
[omIm][BF4]
water
DMSO
EG
0.7386
0.8191
0.8176
0.4162
0.0646
0.0420
0.7439
0.8192
0.8175
[HmIm][HCOO]
[HmIm][CH3COO]
[HmIm][CH3CH2OO]
water
DMSO
EG
0.8110
0.1879
0.8126
0.4340
0.7626
0.4341 À0.4855
2.10. Ionicity in Protic and Aprotic Ionic Liquids
0.7625
0.0072
Herein, we approximately quantify the ionicity of the binary
mixture on the basis of the Walden plot of logLm against
loghÀ1 with ideal 0.1m KCl as a reference line. The relation be-
tween molar conductivity and viscosity can be demonstrated
by Equation (5):[48,49]
water
DMSO
EG
0.7738
0.2431
0.7740
0.6520
0.6594
0.6519 À1.9843
0.6596 À0.0953
water
DMSO
EG
0.6858
0.3963
0.7046
0.7063
0.6488
0.7060 À2.1209
0.6487 À0.1310
log Lm ¼ log C þ ꢀ log hÀ1
ð5Þ
[HbIm][CH3COO]
water
DMSO
EG
0.8900 À0.1262
0.5982 À1.8568
0.6825 À0.2255
0.8905
0.5983
0.6864
The ø value is the slope of the line in the Walden plot,
which reflects the dissociation of the ions. The value of the ø
parameter reveals the difference of the activation energies of
the ionic conductivity and viscosity. In the present investiga-
tion, all ø values of binary mixtures are smaller than unity (ø
<1), indicating that the ionic conductivities of binary mixture
is, to some extent, reduced as a result of ion-pair formation.
The fitted ø values of binary mixtures are given in Table 4.
Another method that can be used to obtain similar values of ø
[a] EG: Ethylene glycol.
their relative permittivity. The fluidity of ethylene glycol is
lower than that of water and DMSO, which is the dominant
factor for the ionicity in the aprotic ionic liquids. According to
the classification by Angell et al.,[11,28] the plot for [bmIm][BF4]
with water and ethylene glycol are on the ideal KCl line and
other plots are located below the reference KCl line. The
[bmIm][BF4] with water and ethylene glycol shows ideal ionici-
ty or 100% ionic characters. Furthermore, the binary systems
of [omIm][BF4] with water, DMSO, and ethylene glycol are in
the region of a good ionic class but still lower than the ionic
region of [bmIm][BF4].
is the ratio of the temperature-dependent activation energies
[46]
for viscosity and molar conductivity, Ea,L/Ea,h
.
Table 4 compares the ø values obtained from the slopes of
the Walden plots in Figure 11 with those calculated from the
activation energies and are in very good agreement. The ionici-
ties for all protic and aprotic ionic liquids are calculated from
the Walden plot of logLm against loghÀ1 with KCl as reference
ideal line. The vertical distance measured from the KCl ideal
line to the point of the ionic liquid binary mixture is called DW
(deviation). The distance (DW) and ionicity calculated as %Ion-
icity=10(ÀDW) ”100, and data of ionicities for all systems are
given in Table 5.
In the case of protic ionic liquids, the binary systems of
[HmIm][HCOO], [HmIm][CH3COO], [HmIm][CH3CH2COO], and
[HbIm][CH3COO] with water and ethylene glycol fall in the
region of good a ionic class. The systems of [HmIm][HCOO],
[HmIm][CH3COO], [HmIm][CH3CH2COO], and [HbIm][CH3COO]
with DMSO are in the poor ionic region. These deviations from
the ideal line are observed to depend on the structures of the
ionic liquids and on the properties of the molecular solvents.
Ionicity in protic ionic liquids was observed to depend on
The binary system of aprotic ionic liquids [bmIm]Br,
[hmIm]Br, [omIm]Br with water, DMSO, and ethylene glycol
falls in the region of good ionic binary mixtures. The order of
ionicity of aprotic ionic liquids is ethylene glycol> water>
DMSO. However, the s values of aprotic ionic liquids in water
are higher than those in DMSO and ethylene glycol as per
N
microscopic properties such as solvatochromic parameters ET
and a, as noted above in the case of s.
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Table 5. Deviation (DW) from ideal KCl line and ionicity of binary mix-
tures of ionic liquids with molecular solvents at 298.15 K.
Ionic liquid
[bmIm]Br
Solvent
DW
Ionicity [%]
water
DMSO
EG
0.08
0.25
0.05
83.1
56.2
89.1
[hmIm]Br
water
DMSO
EG
0.11
0.24
0.08
77.6
57.4
83.0
[omIm]Br
water
DMSO
EG
0.11
0.25
0.10
77.6
56.2
79.0
[bmIm][BF4]
water
DMSO
EG
–
0.08
–
ca. 99.0
83.1
ca. 99.0
Figure 11. Walden plot of logLm against loghÀ1 The binary systems of ionic
[omIm][BF4]
water
DMSO
EG
0.10
0.24
0.10
79.0
57.5
79.0
&
*
liquids with solvents: ( ), water-[HmIm][HCOO]; ( ), DMSO-[HmIm][HCOO];
~
&
*
), EG-[HmIm][HCOO]; ( ), water-[HmIm][CH3COO]; ( ), DMSO-[HmIm]
(
[CH3COO]; (~), EG-[HmIm][CH3COO]; ( ), water-[HmIm][CH3CH2COO]; ( ),
&
*
DMSO-[HmIm][CH3CH2COO]; (~) EG-[HmIm][CH3CH2COO]; ( ), water-
&
[HmIm][HCOO]
[HmIm][CH3COO]
[HmIm][CH3CH2COO]
[HbIm][CH3COO]
water
DMSO
EG
0.18
1.39
0.19
66.0
4.07
64.5
~
&
*
*
[bmIm]Br; ( ), DMSO-[bmIm]Br; ( ), EG-[bmIm]Br; ( ), water-[hmIm]Br; ( ),
~
&
*
DMSO-[hmIm]Br; ( ), EG-[hmIm]Br; ( ), water-[omIm]Br; ( ), DMSO-
[omIm]Br; ( ),EG-[omIm]Br; ( ), water-[HbIm][CH3COO]; ( ),DMSO-[HbIm]
[CH3COO]; ( ), EG-[HbIm][CH3COO]; ( ), water-[bmIm][BF4]; ( ), DMSO-
~
&
*
~
&
*
water
DMSO
EG
0.21
2.52
0.35
61.6
0.003
44.6
~
&
*
[bmIm][BF4]; ( ), EG-[bmIm][BF4]; ( ), water-[omIm][BF4]; ( ), DMSO-[omIm]
~
[BF4]; ( ), EG-[omIm][BF4]from 293.15 to 313.15 K.
water
DMSO
EG
0.24
2.55
0.41
57.5
0.0028
38.9
The order of ionicity for protic ionic liquids in solvents was
found to be water> ethylene glycol> DMSO, and ETN and a pa-
rameters also decrease in the same way, which is due to the in-
dividual solvation of ions through hydrogen bonding.
water
DMSO
EG
0.34
2.50
0.48
45.7
0.0031
33.1
The lower ionicity is due to the stronger ionic interaction ex-
hibited by the protic ionic liquids, which possess higher polari-
ty compared with aprotic ionic liquids. Deviation (DW) from
the ideal KCl line and ionicities for all of the systems have
been calculated at 298.15 K. The calculated data are presented
in Table 5.
order water> ethylene glycol> DMSO. (3) Upon changing the
cations from [HmIm]+ to [HbIm]+ with the common anion
[CH3COO]À, the ionicity of [HbIm][CH3COO] decreases in each
of the molecular solvents. Thus, it is concluded from the data
of ionicities that 35–55% of ion pairs (or ion aggregation) are
present in protic ionic liquids in water. Similar results are ob-
tained in the case of ethylene glycol, but nearly all ions are in
paired or aggregated form for DMSO. The cations and anions
for protic ionic liquids in molecular solvents are strongly
bound together. The ionicity obtained according to the
Walden plot and from Limp/LNMR by measuring the s and ion
self-diffusion coefficient, show moderate interaction between
cations and anions for water; in DMSO, the cation and anions
are held strongly to form ion pair and clustered ions with non-
charge-neutral ion clusters.
The ionicity in aprotic ionic liquids can be classified by con-
sidering the following factors. (1) The increase in alkyl chain
length of the cation in [RmIm]Br (R=alkyl chain length); the
ionicity of binary mixture ionic liquids in water and ethylene
glycol decreases in the order [bmIm]Br> [hmIm]Br> [omIm]Br,
whereas in the case of DMSO, the ionicity is nearly equal for all
three ionic liquids [bmIm]Brꢁ[hmIm]Brꢁ[omIm]Br. (2) The ion-
icities of [bmIm]Br, [hmIm]Br, and [omIm]Br in each molecular
solvent is observed in the order ethylene glycol> water>
DMSO. (3) The ionicities of [Br]À based ionic liquids are ca. 80%
for water, ca. 56% for DMSO, and greater than 80% for ethyl-
ene glycol. This means that nearly 20% of the ions are not
available for charge transport in water and ethylene glycol,
and nearly 44% of the ions in DMSO, indicating the occurrence
of ion pairing or ion aggregation.
3. Conclusions
Similarly, in the case of protic ionic liquids (1) as the alkyl
chain length of anions of protic ionic liquids increase, the
order of ionicity of the ionic liquids in each molecular solvent
is [HmIm][COO]> [HmIm][CH3COO]> [HmIm] [CH3CH2COO].
(2) The ionicity of protic ionic liquids in each solvent is in the
In this work, we have investigated the ionicities of five aprotic
ionic liquids and four protic ionic liquids in three different mo-
lecular solvents, water, ethylene glycol, and DMSO, by analyz-
ing conductivity, viscosity, and self-diffusion coefficients
through the application of PGSE-NMR techniques. Our study
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reveals that (1) the s values for protic and aprotic ionic liquids
are reflected through the systematic variation in the structure
of cation and anions and also through the size of the anions.
(2) The s values of aprotic ionic liquids is determined by mac-
roscopic or bulk properties such as relative permittivity and
viscosity the of molecular solvent, whereas the s values in
protic ionic liquids are determined by the microscopic proper-
Table 6. The w/w ratios of the ionic liquid to molecular solvent used in
the current study.
Ionic liquid
Water
DMSO
EG
[bmIm][BF4]
[omIm][BF4]
[bmIm]Br
[hmIm]Br
0.020
0.026
0.020
0.022
0.025
0.011
0.013
0.014
0.017
0.021
0.026
0.020
0.023
0.026
0.012
0.013
0.014
0.017
0.023
0.029
0.022
0.025
0.028
0.012
0.014
0.015
0.018
N
ties of solvents, solvatochromic parameters, ET , a. (3) The use
[omIm]Br
of PGSE-NMR spectral techniques clarifies the association of
cations and anions of ionic liquids in molecular solvents, which
explains the s values. For example, the degree of association
of ions is less in water than in DMSO. (4) Aprotic ionic liquids
are observed to be a better class of ionic salts (in solvents the
order is ethylene glycol> water> DMSO) than protic ionic liq-
uids (water> ethylene glycol> DMSO). (5) Ionicities obtained
from the Walden plots offer information on the ionicity of
binary mixtures readily, whereas the molar conductivity ratio
(Limp/LNMR) approach estimates the ionicity more precisely.
(6) The Stokes–Einstein radius [HmIm]+ and [CH3COO]À in
water and DMSO calculated from the self-diffusion coefficient
are consistent with the experimental values of ionic conductivi-
ty. The transference number of [HmIm]+ and [CH3COO]À is con-
sistent with the results obtained by using PGSE-NMR tech-
niques.
[HmIm][HCOO]
[HmIm][CH3COO]
[HmIm][CH3CH2COO]
[HbIm][CH3COO]
The w/w ratios of a given ionic liquid to water, DMSO, and ethyl-
ene glycol used in the above work are given in Table 6.
Conductivity Measurements
The specific conductivities s were measured with a Synchrotron
306 conductometer at 1 kHz. The conductivity meter was calibrat-
ed by using a 0.1m potassium chloride (KCl) solution.[52] The cell
constant was determined with standard aqueous solutions of KCl.
A 0.1m solution of ionic liquids was prepared and s was measured
at a temperature range from 293.15 to 348.15 K with constant stir-
ring. The molar conductivity was calculated by using the equation
Lm =(s/c)”1000, where s is the specific conductivity and c is the
concentration in mole per liter. The uncertainties were estimated
to be Æ0.1% for concentration and 1% for s, respectively. The
temperature of the jacket was maintained with a JULABO thermo-
stat with an accuracy of Æ0.01 K.
Experimental Section
Chemicals
1-Methylimidazole (99%), 1-butylimidazole (99%), 1-bromobutane,
1-bromohexane, 1-bromooctane, formic acid, acetic acid, propionic
acid were purchased from Sigma–Aldrich and used as obtained.
The molecular solvents, DMSO and ethylene glycol of high purity
were purchased from Merck, India. Milli Q water with specific con-
ductivity 5.5”10À6 mScmÀ1 was used throughout the experiment.
Viscosity Measurements
All the experiments were performed with a Brookfield-ultra rheom-
eter with a cone plate. The viscosity was measured with a 0.1m so-
lution of binary systems of ionic liquids with molecular solvents at
temperatures from 293.15 to 323.15 K. Temperatures of the sys-
tems were monitored with a JULABO thermostat bath with an ac-
curacy of Æ0.01 K. The calibration of the instrument was per-
formed by taking the reported viscosity data of aqueous solutions
of NaCl and KCl with different concentration. The accuracy of the
instrument was determined to be Æ1%.[52]
Synthesis of Ionic Liquids
Protic ionic liquids [HmIm][HCOO], [HmIm][CH3COO], [HmIm]
[CH3CH2COO], and [HbIm][CH3COO] were synthesized and purified
according to the reported procedure.[50] The protic ionic liquids
were synthesized by neutralization of acids such as formic acid,
acetic acid and propanoic acid with bases methylimidazolium and
butylimidazolium. Dropwise addition of base to acid was carried
out in an ice bath to avoid 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 RT. Water was removed by using a rota-
vapor at 808C under reduced pressure. These protic ionic liquids
were further dried under vacuum at 708C for 10 h. Aprotic ionic
liquids were synthesized according to the reported procedure,[51]
using the quarternization reaction between 1-methylimidazole and
PGSE-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 species by observing1H nuclei. The PGSE-NMR measure-
ments were performed with a Bruker Avance 400 ultra shield spec-
trometer with a 9.4 T superconducting magnet. The spectrometer
was equipped with a 2D sequence for diffusion measurement. The
self-diffusion coefficients were measured by using a stimulated
echo sequence, longitudinal-eddy-current delay (LED), and bipolar
gradient pulses to improve the self-diffusion coefficient measure-
ments.[53] The experiment was carried out at 0.1m solution and at
four temperatures ranging from 298.15 to 313.15 K with an incre-
ment of 5 K. Self-diffusion coefficients, D, were calculated by using
Equation (6):[54]
1-bromoalkane to form
a salt [RmIm]Br. In a second step,
[RmIm]BF4 was synthesized by using the metathesis reaction of
NaBF4 with [RmIm]Br in dichloromethane under inert atmosphere
at RT. The characterization and determination of purities were per-
formed by1H NMR spectroscopic analysis the data were in agree-
ment with reported values. The ionic liquids were dried under
vacuum to remove excess water; the water content of the ionic liq-
uids was measured before making the solution by a Karl–Fischer
coulometer and did not exceed 50 ppm for all ionic liquids.
I ¼ Io exp ½Àg2 g 2d 2D ðDÀd=3ފ
ð6Þ
ChemPhysChem 2016, 17, 1006 – 1017
www.chemphyschem.org
1016 ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Manuscript received: December 14, 2015
Accepted Article published: February 11, 2016
Final Article published: February 16, 2016
ChemPhysChem 2016, 17, 1006 – 1017
www.chemphyschem.org
1017
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim