Electrical conductances of 1-butyl-3-propylimidazolium bromide and 1-butyl-3-propylbenzimidazolium bromide in water, methanol, and acetonitrile at (308, 313, and 318) K at 0.1 MPa

Article abstract of DOI:10.1021/je300339q

Two ionic liquids (ILs), 1-butyl-3-propyl imidazolium bromide ([BPim][Br]) and 1-butyl-3-propyl benzimidazoliumbromide ([BPbim][Br]), have been synthesized from their appropriate imidazole and benzimidazole precursors and were characterized by NMR spectroscopic technique. Their electrical conductances have been measured as a function of their concentrations in water, methanol, and acetonitrile at three different temperatures. The limiting molar conductances (Λ⁰), the association constants (K_A), and the values of the association diameters (R) of these ILs have been obtained by analysis of the conductance data using the Fuoss conductance equation. These ILs have been found to remain unassociated in water and methanol, whereas these exhibit slight ionic association in acetonitrile within the investigated temperature range. The temperature elevation causes an increase in their limiting molar conductances in the three solvents under investigation. The IL cations [BPim]⁺ and [BPbim]⁺ exist as unsolvated species in aqueous solutions, whereas substantial solvation was noticed for the [BPim]⁺ ion in methanol and acetonitrile solutions.

Full text of DOI:10.1021/je300339q

Article
pubs.acs.org/jced
Electrical Conductances of 1‑Butyl-3-propylimidazolium Bromide
and 1‑Butyl-3-propylbenzimidazolium Bromide in Water, Methanol,
and Acetonitrile at (308, 313, and 318) K at 0.1 MPa
Sumanta Gupta, Amritendu Chatterjee, Sajal Das, and Basudeb Basu
Department of Chemistry, North Bengal University, Darjeeling 734013, India
,
†
Bijan Das*
Department of Chemistry & Biochemistry, Presidency University, 86/1 College Street, Kolkata 700 073, India
ABSTRACT: Two ionic liquids (ILs), 1-butyl-3-propyl imidazo-
lium bromide ([BPim][Br]) and 1-butyl-3-propyl benzimidazolium
bromide ([BPbim][Br]), have been synthesized from their appro-
priate imidazole and benzimidazole precursors and were char-
acterized by NMR spectroscopic technique. Their electrical
conductances have been measured as a function of their concentra-
tions in water, methanol, and acetonitrile at three different tem-
0
peratures. The limiting molar conductances (Λ ), the association
constants (K ), and the values of the association diameters (R)
A
of these ILs have been obtained by analysis of the conductance
data using the Fuoss conductance equation. These ILs have been
found to remain unassociated in water and methanol, whereas
these exhibit slight ionic association in acetonitrile within the
investigated temperature range. The temperature elevation causes an increase in their limiting molar conductances in the three
+
+
solvents under investigation. The IL cations [BPim] and [BPbim] exist as unsolvated species in aqueous solutions, whereas
+
substantial solvation was noticed for the [BPim] ion in methanol and acetonitrile solutions.
INTRODUCTION
EXPERIMENTAL SECTION
■
■
Ionic liquids (ILs) constitute a group of molten organic electro-
lytes whose chemical and physical properties could be conveniently
engineered by judicious choice of their anions, cations, and the
substituents. They possess specific properties, for example, negligible
vapor pressures, noninflammability, good thermal stability under
ordinary conditions, and outstanding catalytic properties, and
above all they offer excellent solubility for inorganic and organic
Chemicals. Acetonitrile (purity: 99 %) was purchased from
E. Merck, India; it was purified by distillation with P O followed
2
5
by redistillation over CaH . Methanol (purity: 99.9 %) was
2
procured from Acros Organics and was distilled three times.
Deionized triply distilled water was used for the preparation of
solutions; its specific conductivity was of the order of 0.1 mS·m−
1
at 308 K. The densities (ρ ) and the viscosity coefficients (η ) of
0
0
1−4
compounds. Recent years have, therefore, witnessed an upsurge
of interest in ILs in the field of scientific research opened up by the
possibility of their applications in novel eco-friendly and benign
the solvents thus purified are shown in Table 1, and these
2
0−24
properties agree well with those reported in the literature.
The relative permittivities (ε) of these solvents were obtained
5−11
20−23
industrial processes.
Plenty of physicochemical investigations
from the literature,
and these are also listed in Table 1.
have, so far, been performed on pure ILs as well as on their
The relative permittivity of methanol at 313 K is not available
in the literature, and this was obtained from an interpolation
of the available experimental relative permittivity values in the
12−15
mixtures with molecular solvents.
number of cases, the infinite dilution molar properties
However, only in a limited
1
6−19
were
21
obtained despite their importance in understanding the ion−ion
and ion−solvent interactions and the possibility of prediction of
ILs in specific applications such as IL-based chemical reactions.
This article reports the electrical conductances measured for
temperature range (278 to 328) K. 1-Bromopropane (purity:
8.5 %), 1-bromobutane (purity: 98 %), and benzimidazole
purity: 99.5 %) were purchased from Loba Chemie, India, and
9
(
imidazole (purity: 99 %) was purchased from SRL, India; these
were used without further purification.
1
3
-butyl-3-propyl imidazolium bromide ([BPim][Br]) and 1-butyl-
-propyl benzimidazolium bromide ([BPbim][Br]) dissolved in
water, methanol, and acetonitrile at (308, 313, and 318) K at
.1 MPa as a function of concentration of these ILs with a view
to unravel their association and solvation behavior.
Received: December 20, 2011
Accepted: November 29, 2012
0
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/je300339q | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Article
Table 1. Physical Properties of Water, Methanol, and
Acetonitrile at (308, 313, and 318) K at 0.1 MPa
method to that used for synthesizing IL 1, with imidazole (instead
a
of benzimiazole) as the starting material. This was characterized by
1
NMR spectroscopy [ H NMR (300 M Hz, CDCl ) δ (ppm):
ρ /g·cm−
3
3
η0/mPa·s
this work
Water
0
1
0.30 (s, 1H), 7.64 (s, 1H), 7.61 (s, 1H), 4.40−4.33 (m, 4H),
13
T/K
this work
lit.
lit.
ε
2
.02−1.87 (m, 4H), 1.42−1.32 (m, 2H), 1.01−0.92 (m, 6H); C
NMR (75 M Hz, CDCl ) δ (ppm): 137.27, 123.38, 123.26, 52.09,
3
2
2
2
2
2
2
22
22
22
22
22
22
308
313
318
0.99407
0.99221
0.99024
0.99406
0.99224
0.99024
0.7194
0.6533
0.5966
0.7194
0.6531
0.5963
74.82
73.15
71.51
50.47, 32.94, 24.46, 20.16, 14.23, 11.46]. The structure of this ionic
liquid is shown in Figure 1.
Electrical Conductance Measurements. Electrical con-
ductances were measured on a calibrated Orion 3-Star con-
ductivity meter at 2 kHz using a cell (dip-type) having a cell
Methanol
2
2
3
23
23
23
b
3
3
3
08
13
18
0.77718
0.77250
0.76770
0.77720
0.76770
0.76581
0.4747
0.4742
0.4174
30.74
29.81
28.92
−1
0.4440
0.4185
constant of 115 m . Uncertainties of the measurements were
3
23
always within 0.01 %. The conductivity cell was calibrated
2
7,28
Acetonitrile
following the methods described in the literature
by using
2
2
2
4
6
4
24
24
308
0.76564
0.3125
0.314
34.1
KCl (aq) solution. All measurements were carried out in a
water bath controlled to ± 0.05 K. The necessary correction for
the solvent specific conductance was done. Details of the experi-
2
5
25
0
0
.7662
.76560
34.54
25
29
3
3
13
18
0.75970
0.75501
0.3042
0.2903
33.85
33.12
mental method are available in the literature. The IL solutions
24
25
0.75271
0.293
0.289
were prepared on molality basis, and the molalities of the solutions
were converted into molarities by using their densities. The
reproducibility of the experimental data was checked by repeating
the measurements with different stock solutions.
2
5
25
0
.7559
a
The uncertainty u is u(T) = 0.05 K. Combined expanded
−3
uncertainties are U (ρ ) = 0.00003 g·cm and U (η ) = 0.005
mPa·s (level of confidence = 0.95). Obtained from interpolation of
c
0
c
0
b
Density Measurements. Density measurements were
carried out on an Anton Paar DMA-4500 M digital precision
densimeter. The precision of the density measurements was
the literature values from ref 21 (please see text).
−
5
−3
Synthesis of 1-Butyl-3-propylbenzimidazolium Bro-
3·10 g·cm . Calibration of the densimeter was done at each
temperature using dry air under ambient pressure and deionized
triply distilled water.
25
mide. To a round-bottomed flask containing benzimidazole
(
(
5.9 g, 50 mmol) in acetonitrile (30 mL) were added KOH
5.6 g, 100 mmol) and 1-bromobutane (7 g, 51 mmol). This
Viscosity Measurements. The kinematic viscosities (ν)
were determined with an Ubbelohde viscosimeter. The viscosimeter
was kept in a water bath maintained within ± 0.05 K of the
experimental temperatures. The absolute viscosities (η) have
then been calculated from the kinematic viscosity values by
using the density values. In each case, an average of triplicate
measurements was taken into account.
mixture, after refluxing for about 4 h, was cooled down to
ambient temperature. The solvent was then evaporated under
vacuum; 30 mL of dichloromethane was added to it, and
potassium bromide was removed by washing the material with
water using a separatory funnel. The nonaqueous layer was
dehydrated with Na SO , which on evaporation left behind the
2
4
crude product (1-butyl-benzimidazole). To the crude material
taken in a dry flask containing dry ethanol (20 mL) was added
RESULTS AND DISCUSSION
■
1
.1 equivalent amount of propyl bromide. The content of the
Table 2 lists the molar conductivity values obtained experi-
mentally as functions of the molality (m) of the IL solutions at
(308, 313, and 318) K at 0.1 MPa. These were analyzed by
flask was then refluxed for 24 h under nitrogen atmosphere, and
after cooling down to ambient temperature, evaporation of the
solvent was done under vacuum. The crude material, 1-butyl-3-
propyl benzimidazolium bromide, was finally recrystallized
3
0,31
using the Fuoss conductance equation.
A given set of con-
ductance data (c , Λ ; j = 1, ..., n) can provide three parameters,
j
j
0
from ethanol in the form of a white solid (1). This was char-
namely, the limiting molar conductivity (Λ ), the equilibrium
1
acterized by NMR spectroscopy [ H NMR (500 M Hz, CDCl )
constant for ionic association (K ), and the distance of the
3
A
δ (ppm): 11.32 (s, 1H), 7.66−7.19 (m, 4H), 4.56 (t, 2H, J =
closest approach of ions (R). For a given data set these three
parameters were obtained by an iterative solution of the following
set of equations:
1
2.4), 4.53 (t, 2H, J = 12.3), 2.06−1.91 (m, 4H), 1.41−1.34
13
(
m, 2H), 0.99−0.88 (m, 6H); C NMR (75 M Hz, CDCl )
3
δ (ppm): 143.74, 132.24, 132.20, 128.05, 114.08, 114.04, 49.90,
8.35, 32.26, 23.75, 20.68, 14.39, 11.90]. The structure of this
0
Λ = p[Λ(1 + RX + EL]
(1)
(2)
(3)
4
ionic liquid is depicted in Figure 1.
p = 1 − α(1 − γ)
2
2
K = (1 − γ)/cγ f
A
βk
(1 + kR)
−
ln f = 2
(4)
e2
εkBT
β =
Figure 1. Structures of the ionic liquids -1-butyl-3-propylbenzimida-
zolium bromide (1) and 1-butyl-3-propylimidazolium bromide (2).
(5)
(6)
K = K (1 + K )
A
R
S
2
6
Synthesis of 1-Butyl-3-propylimidazolium Bromide.
Synthesis of this IL (2) was accomplished by following a similar
where RX is the relaxation field effect, EL is the electrophoretic
countercurrent, γ is the fraction of unpaired ions, α is the
B
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Journal of Chemical & Engineering Data
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Table 2. Molalilty Dependence of the Molar Conductances of 1-Butyl-3-propylimidazolium Bromide and 1-Butyl-3-
a
propylbenzimidazolium Bromide in Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPa
T = 308 K
T = 313 K
T = 318 K
T = 308 K
T = 313 K
T = 318 K
04m/mol·kg−1
Λ/S·cm ·mol
2
−1
Λ/S·cm ·mol
2
−1
Λ/S·cm ·mol
2
−1
10 m/mol·kg
4
−1
Λ/S·cm ·mol
2
−1
Λ/S·cm ·mol
2
−1
Λ/S·cm ·mol
2
−1
1
1-Butyl-3-propylimidazolium Bromide in Water
1-Butyl-3-propylbenzimidazolium Bromide in Water
1
000.40
99.60
100.84
103.04
106.50
109.50
112.12
114.41
116.42
118.18
119.72
121.06
122.24
108.00
109.40
111.65
115.18
118.24
120.91
123.20
125.31
127.12
128.72
130.13
131.36
117.70
118.98
120.94
124.01
126.69
129.05
131.16
133.05
134.74
136.25
137.60
138.70
1030.11
924.81
830.46
788.15
588.31
439.66
328.87
246.17
184.36
138.11
103.49
77.57
89.21
90.76
92.06
92.62
94.90
96.77
98.07
99.14
100.06
100.90
101.65
102.35
99.31
106.06
8
7
5
4
3
2
1
1
1
98.74
62.10
69.63
26.16
19.02
38.91
78.98
34.14
00.54
101.90
104.02
105.66
107.06
108.29
109.39
110.38
111.26
108.92
111.25
113.27
114.95
116.56
117.88
119.08
120.20
7
5
5.36
6.51
1-Butyl-3-propylimidazolium Bromide in Methanol
1-Butyl-3-propylbenzimidazolium Bromide in Methanol
1
001.93
52.38
53.04
53.72
54.68
55.61
56.68
57.92
59.32
60.83
62.39
63.95
65.46
55.40
56.11
57.03
58.32
59.36
60.52
61.85
63.33
64.93
66.58
68.15
-
59.23
60.16
61.23
62.57
63.71
64.91
66.27
67.81
69.46
71.19
72.90
74.80
1012.50
910.23
772.52
578.14
432.94
324.32
243.02
182.15
136.53
102.37
76.75
51.41
53.07
54.88
56.82
58.04
59.06
60.09
61.20
62.41
63.80
64.98
66.27
55.43
57.28
59.29
61.37
62.63
63.67
64.72
65.87
67.12
68.46
69.83
71.32
61.11
62.91
64.81
66.76
67.94
68.96
70.02
71.23
72.57
74.00
75.49
77.17
9
7
5
4
3
2
1
1
1
01.74
66.46
74.86
31.14
23.38
42.51
81.88
36.42
02.31
7
5
6.73
7.55
57.55
1-Butyl-3-propylimidazolium Bromide in Acetonitrile
1-Butyl-3-propylbenzimidazolium Bromide in Acetonitrile
9
8
7
6
5
4
4
3
98.72
98.24
62.82
56.05
71.39
87.52
28.13
20.88
59.20
61.24
64.33
67.07
69.44
61.99
63.93
67.01
69.56
72.22
74.67
77.24
82.13
64.35
66.48
69.68
72.55
75.23
78.07
80.54
85.95
999.54
898.29
762.02
70.05
72.99
77.40
73.20
76.19
80.55
76.63
79.74
84.43
1-Butyl-3-propylbenzimidazolium Bromide in Acetonitrile
569.99
426.63
319.48
239.36
179.35
134.48
100.77
75.55
84.76
91.79
88.04
95.37
92.53
100.44
108.20
115.81
123.22
130.37
137.18
143.35
74.24
78.83
98.55
102.39
109.39
116.13
122.62
128.73
134.66
140.64
105.06
111.34
117.30
123.03
128.28
133.04
1
-Butyl-3-propylimidazolium Bromide in Acetonitrile
2
2
1
1
1
74.96
40.52
80.33
35.20
01.38
81.23
83.25
87.51
91.60
95.50
86.91
91.55
91.14
96.27
96.04
101.28
106.30
56.65
100.75
1-Butyl-3-propylbenzimidazolium Bromide in Water
1
1
330.01
200.00
95.87
97.37
102.58
103.93
a
−1
2
−1
Standard uncertainties u are u(T) = 0.05 K and u(m) = 0.00002 mol·kg , and the combined expanded uncertainty U is U (Λ) = 0.03 S·cm ·mol (level
c
c
of confidence 0.95).
3
0
fraction of contact pairs, K is the overall pairing constant
evaluated from the association constants of contact pairs (KS)
Following Fuoss, calculations were performed over a range
A
0
of R values to find the Λ and K values minimizing the
standard deviations, σ,
A
of solvent-separated pairs (K ), ε is the relative permittivity of
R
the solvent, e is the electronic charge, k is the Boltzmann
B
constant, k− is the radius of the ion atmosphere, c is the
molarity of the solution, f is the activity coefficient, T is the
temperature in absolute scale, and β is twice the Bjerrum
1
2
1/2
σ = [∑ [Λ (calcd) − Λ (obsd)] /(n − 2)]
j
j
(7)
of the fits; the value of R which fits the experimental data best
was, therefore, identified as the minimum of a σ vs R profile.
The Λ , K , and R values thus derived from the conductance
0
distance. The initial values of Λ for the iteration procedure
0
have been obtained by extrapolating the data according to
A
32
Shedlovsky and Fuoss.
data are recorded in Table 3.
C
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Table 3. Derived Conductivity Parameters for 1-Butyl-3-propylimidazolium Bromide and 1-Butyl-3-propylbenzimidazolium
Bromide in Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPa
0
2
−1
3
−1
IL
T/K
Λ /S·cm ·mol
K /dm ·mol
R/Å
σ%
A
Water
[
BPim][Br]
308
130.22 ± 0.03
139.89 ± 0.05
147.50 ± 0.02
108.83 ± 0.03
118.87 ± 0.02
129.03 ± 0.03
2.87 ± 0.01
2.57 ± 0.02
1.73 ± 0.01
0.74 ± 0.01
1.16 ± 0.01
1.50 ± 0.01
8.1
8.0
0.05
0.06
0.03
0.04
0.02
0.03
3
13
18
3
7.7
[
BPbim][Br]
308
12.6
11.4
10.1
3
3
13
18
Methanol
79.35 ± 0.01
[
BPim][Br]
308
2.31 ± 0.02
3.06 ± 0.05
3.09 ± 0.06
0.93 ± 0.05
0.58 ± 0.04
0.18 ± 0.06
4.6
4.8
4.9
5.8
5.8
5.6
0.02
0.04
0.05
0.04
0.04
0.06
3
3
13
18
84.75 ± 0.03
90.61 ± 0.04
79.21 ± 0.02
85.09 ± 0.02
92.03 ± 0.04
[
BPbim][Br]
308
3
3
13
18
Acetonitrile
[
BPim][Br]
308
128.10 ± 0.04
136.41 ± 0.08
145.89 ± 0.05
166.92 ± 0.05
177.04 ± 0.05
190.64 ± 0.01
32.83 ± 0.09
37.61 ± 0.12
42.97 ± 0.08
49.25 ± 0.09
54.76 ± 0.09
60.33 ± 0.01
7.7
7.2
0.02
0.02
0.02
0.02
0.02
0.01
3
3
13
18
7.2
[
BPbim][Br]
308
10.0
9.7
3
3
13
18
10.0
The representative plots (Figures 2 to 4) show the dependence
of the experimental Λ on the IL molality. Also included in these
figures are the calculated profiles in accordance with eqs 1 to 6.
Figure 3. Variation of molar conductivity of 1-butyl-3-propylimidazo-
lium bromide as a function of molality in methanol at different
temperatures at 0.1 MPa. The symbols represent the experimental
points, whereas the lines represent the calculations according to eqs 1
through 6. ▽, 318 K; □, 313 K; ○, 308 K.
Figure 2. Variation of molar conductivity of 1-butyl-3-propylimidazo-
lium bromide as a function of molality in water at different
temperatures at 0.1 MPa. The symbols represent the experimental
points, whereas the lines represent the calculations according to eqs 1
through 6. ▽, 318 K; □, 313 K; ○, 308 K.
also change in the same order as the limiting equivalent con-
33
ductivities of the ILs as a whole. It is now well-established
that some ions exist as bare species (unsolvated) in solution
with sizes being equal to their crystallographic sizes, whereas
others are solvated with a concomitant increase in their actual
sizes in solution. The order of variation of the limiting ionic
equivalent conductivities should, thus, point out the relative
ionic solvodynamic dimensions and hence provide information
on their solvation behavior in solutions.
0
The Λ values for the two ILs investigated are found to
increase with temperature, and these could be described by the
following linear relationship:
0
Λ = a + a (308.15 − T)
(8)
0
1
and the values of the coefficients of eq 8, together with the
correlation coefficients (r), have been listed in Table 4.
The ionic mobilities of the IL cations in water decrease in the
+
+
order: [BPim] > [BPbim] . This indicates that the larger IL
cation is less mobile than the smaller one in aqueous solution.
This is possible only if the ions remain unsolvated in water; had
An inspection of the variation of the limiting ionic equivalent
conductivities of the IL cations in the investigated solvents
could provide an important clue to their solvation in these
media. Since the anion is common to both the ILs, the limiting
ionic equivalent conductivities of the IL cations, according to
the Kohlrausch’s law of independent migration of ions, would
+
these IL cations been hydrated, the smaller ion [BPim] with a
higher charge density on its surface would have been more
+
hydrated compared to the bigger [BPbim] ion (with smaller
surface charge density), and hence their ionic mobilities should
D
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Journal of Chemical & Engineering Data
Article
other hand, the fractions of unpaired ions are found to be in the
range of 0.47−0.86 within the investigated concentration and
temperature ranges; this observation demonstrates that a sub-
stantial proportion of these ILs are associated and exist as ion
pairs in acetonitrile. Further, the extent of ion association in
acetonitrile increases as the temperature is elevated (cf. Table 3).
Now, an increase in temperature causes ion desolvation as well
as an attenuation of the product of the relative permittivity and
the absolute temperature (εT) for this medium. Desolvation
promotes ion association, and a decrease in the εT value also
enhances the value of the association constant according to eqs
3
to 5. Hence both these effects might have been contributed to
the increase in the K values with temperature in acetonitrile.
A
The existence of free ions for these two ILs in water might be
attributed to the high values of the relative permittivity of the
medium (ranging from 71.51 to 74.82 over the investigated
range of temperature), thus promoting the dissociation of these
electrolytes.
Since the relative permittivities of methanol and acetonitrile
are not very much different, one expects close correspondence
Figure 4. Variation of molar conductivity of 1-butyl-3-propylbenzimi-
dazolium bromide as a function of molality in acetonitrile at different
temperatures at 0.1 MPa. The symbols represent the experimental
points, whereas the lines represent the calculations according to eqs 1
through 6. ▽, 318 K; □, 313 K; ○, 308 K.
in the K values of these ILs in these solvent media if Coulombic
forces are only responsible for ionic association governed by the
relative permittivities of the media. The large differences between
Table 4. Coefficients of Equation 8
A
IL
a0
a1
r2
Water
the K values of the ILs in methanol and acetonitrile indicate that
A
[
[
BPim][Br]
130.56
108.81
1.7280
2.0200
0.9953
0.9996
the association/dissociation behavior of these ILs in methanol and
acetonitrile could not be accounted for only on the basis of the
relative permittivities of these two solvents. Non-Coulombic forces
might, therefore, play a decisive role in the ionic association
phenomena in acetonitrile. Further studies in this direction are,
therefore, necessary.
BPbim][Br]
Methanol
79.27
79.03
Acetonitrile
[
[
BPim][Br]
1.1260
1.2820
0.9994
0.9977
BPbim][Br]
[
[
BPim][Br]
127.91
166.34
1.7790
2.3720
0.9986
0.9929
BPbim][Br]
CONCLUSIONS
■
(
(
Two ionic liquids, 1-butyl-3-propyl imidazolium bromide
[BPim][Br]) and 1-butyl-3-propyl benzimidazolium bromide
[BPbim][Br]), have been synthesized respectively from their
have followed an opposite order. This observation thus suggests
+
+
that the IL ions [BPim] and [BPbim] exist as unsolvated
species in aqueous solutions.
appropriate imidazole and benzimidazole precursors and char-
acterized by NMR spectroscopic technique. The electrical con-
ductances of these two ionic liquids were measured as functions
of their concentrations in water, methanol, and acetonitrile at
different temperatures. Analyses of the conductance data on the
basis of the Fuoss conductance equation provided important
insight into the ion association and solvation behavior on the
investigated ionic liquid solutions.
+
+
The ionic mobilities of the IL cations [BPim] and [BPbim]
in methanol are approximately the same. This suggests that the
solvodynamic dimensions of these IL cations in methanol are
similar. Thus it can be inferred that at least the smaller cation
+
[
BPim] is solvated in methanol.
A comparison of the trend in the IL cationic mobilities in
acetonitrile with their bare sizes indicates that the bigger the
bare size of cation, the more mobile it would be. Therefore, the
solvodynamic dimensions of the cations in acetonitrile solution
AUTHOR INFORMATION
+
+
■
decrease in the order: [BPbim] > [BPim] . Thus there is apparent
+
Corresponding Author
E-mail: bijan_dasus@yahoo.com.
significant solvation of the [BPim] ion by the acetonitrile
*
molecules so that it turned into a bigger solvodynamic entity
+
than the [BPbim] ion.
Notes
†
The present study, however, does not rule out the possibility
Previous address: Department of Chemistry, North Bengal
+
of solvation of the [BPbim] ion in methanol and acetontrile
University, Darjeeling 734013, India.
solutions, although weaker solvation is expected for this ion
compared to [BPim] ion since the former with a bigger size
Funding
+
Financial assistance by the Department of Science and Technology,
New Delhi, Government of India (SR/S1/PC-67/2010 and SR/
FT/CS-017/2010) and the Department of Chemistry, North
Bengal University are gratefully acknowledged.
would possess a smaller surface charge density compared to the
latter.
The association constants of these two ILs in the three media
investigated are listed in Table 3. Now, the fractions of
Notes
unpaired ions (γ) can be obtained from the K values using eqs
A
The authors declare no competing financial interest.
3
to 5. The fractions of unpaired ions are found to be in the
range of 0.87−1.00 for the two ILs in water and methanol
within the investigated concentration and temperature ranges
indicating that a preponderant proportion of these ILs remain
in the form of free ions. For these ILs in acetonitrile, on the
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