Association of ionic liquids in solution: Conductivity studies of [BMIM][Cl] and [BMIM][PF6] in binary mixtures of acetonitrile + methanol: Ion association in ionic liquid solutions

Article abstract of DOI:10.1007/s10953-013-9999-4

The concentration dependence of molar conductance for 1-butyl-3- methylimidazolium chloride and 1-butyl-3-methylimidazolium hexafluorophosphate in binary mixtures of acetonitrile + methanol was investigated to explore the ion association behavior of imidazolium based ionic liquids. The limiting molar conductance (Λ_m⁰), association constants (K _a ⁰) and the maximal distance between the oppositely charged ions in ion pairs (R _ij ) in the mixed solvent mixtures were evaluated following the framework of Barthel's low-concentration chemical model. The investigated ILs display opposing trends in ion association behavior with change in solvent composition of acetonitrile + methanol binary mixtures. The results are discussed in light of ionic liquid and solvent specific ion-solvent and ion-ion interactions in the mixed solvent systems.

Full text of DOI:10.1007/s10953-013-9999-4

J Solution Chem (2013) 42:738–745
DOI 10.1007/s10953-013-9999-4
Association of Ionic Liquids in Solution: Conductivity
Studies of [BMIM][Cl] and [BMIM][PF6] in Binary
Mixtures of Acetonitrile + Methanol
Ion Association in Ionic Liquid Solutions
•
Roohi Jan Ghulam Mohammad Rather Mohsin Ahmad Bhat
•
Received: 22 May 2012 / Accepted: 12 September 2012 / Published online: 30 April 2013
Ó Springer Science+Business Media New York 2013
Abstract The concentration dependence of molar conductance for 1-butyl-3-methyl-
imidazolium chloride and 1-butyl-3-methylimidazolium hexafluorophosphate in binary
mixtures of acetonitrile ? methanol was investigated to explore the ion association
behavior of imidazolium based ionic liquids. The limiting molar conductance ðK0mÞ,
association constants (K0a) and the maximal distance between the oppositely charged ions
in ion pairs (Rij) in the mixed solvent mixtures were evaluated following the framework of
Barthel’s low-concentration chemical model. The investigated ILs display opposing trends
in ion association behavior with change in solvent composition of acetonitrile ? methanol
binary mixtures. The results are discussed in light of ionic liquid and solvent specific ion-
solvent and ion-ion interactions in the mixed solvent systems.
Keywords Ionic liquids ꢀ Ion association ꢀ Barthel’s low-concentration chemical model ꢀ
Association constants
1 Introduction
Ionic liquids (ILs) in recent years have been attracting a considerable attention from the
scientific and industrial community in general and electrochemists in particular [1–4]. On
account of their unique and tunable inherent physicochemical characteristics, ILs are now
increasingly being used as reaction media, novel solvents for separation and analysis and as
electrolytes in electrically, photochemically or chemically driven electrochemical setups
[1–3, 5]. A comprehensive understanding of ion-ion and ion-solvent interactions in ILs
and IL? co-solvent mixtures is a must to achieve the as desired characteristics of ILs in
such systems. Among the large variety of ILs known until now, those based on imi-
dazolium cations are probably the most intensively investigated and used, perhaps on
account of their high air, moisture, thermal, electrochemical and chemical stability [3, 6].
The imidazolium based ILs are very complex innovative systems which are capable of
R. Jan ꢀ G. M. Rather ꢀ M. A. Bhat (&)
Department of Chemistry, University of Kashmir, Srinagar 190006, India
e-mail: mohsinch@kashmiruniversity.ac.in
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739
interacting simultaneously with other molecules via dispersive, ionic, p-p, dipolar and
hydrogen bonding interactions. These interactions when understood completely can be
intelligently made use of in changing the course of chemical or electrochemical reactions
through a proper use of ILs as solvents or electrolytes [4, 7–11].
In view of the recent developments related to theory of conductance of electrolyte
solutions [12, 13], conductometry is increasingly being used as a reliable, affordable and
convenient bench level technique that provides valuable information about the ion-ion
and ion-solvent interactions in electrolyte solutions. In view of the fact that mixed sol-
vents enable the variation of properties such as dielectric constant and viscosity over a
wide range, conductometric investigations of ILs in mixed solvent systems seems very
interesting. Such studies are expected to shed new light on the mechanism and extent of
ion-ion and ion-solvent interactions and solvation of ILs in solutions, wherein non-
electrostatic forces contribute significantly to the equilibrium and dynamic characteristics
of constituent ions. In view of the above mentioned facts, we carried out detailed con-
ductometric investigations of 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) and
1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) in binary acetonitrile
(MeCN) ? methanol (MeOH) mixed solvent systems. The studies were aimed at exploring
the solvent and ion specific transport and ionic association characteristics of imidazolium
based IL electrolyte solutions. Such studies are of considerable interest for the optimal and
desired use of IL? mixed organic solvent systems in high energy batteries [14] and other
electrochemical systems, and for understanding the ion pair effect and organic reaction
mechanisms in such systems [7, 8, 15].
2 Experimental
MeCN (GR grade, 99.9 %) and MeOH (GR grade, 99.9 %) were purchased from Merck,
India and purified following reported standard methods [16]. Electrochemical purity grade
ILs used were synthesized following a two step procedure [17] as reported earlier [7, 8].
Briefly, in the first step 1-methylimidazole was refluxed with n-butyl chloride for 90 h
under argon atmosphere for the synthesis of 1-butyl-3-methylimidazolium chloride as a
white crystalline solid. In the next step the halide anion was exchanged with PF-6 by
treatment with HPF6. The ILs were vacuum dried and stored in desiccators under an inert
1
atmosphere and were characterized through H-NMR, mass spectrometry and 13C-NMR
spectroscopy. The water content of dried ILs was less than 50 ppm, as analyzed by Karl
Fischer titration. Binary solvent mixtures were prepared by mixing the required volumes of
MeCN and MeOH. The value of relative dielectric constant (er) and coefficient of viscosity
(g) of the solvent mixtures were obtained by interpolation of literature [18, 19] values for
MeCN ? MeOH mixtures at 298.15 K. The physical properties of the used mixture
composition are listed in Table 1.
Stock solutions of ILs were prepared by adding their required mass into the mixed
solvent system to be investigated. Conductivity was recorded by a digital microprocessor
based conductivity meter (CYBERSCAN CON 500) from Eutech instruments having a
sensitivity of 0.1 lSꢀcm-1; details are reported elsewhere [20]. During data processing the
conductivity of the solvent system was subtracted from the recorded values to obtain IL
only conductivity in the solution. Numerical calculations and data fitting was performed
through codes written in Origin 6.0 (Microcal Software Inc.)
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Table 1 Dielectric constantðꢀrÞ and viscosity (g) of methanol (MeOH) ? acetonitrile (MeCN) mixtures at
298.15 K; the values were calculated by interpolation of the literature reported [18, 19] values for these
parameters for MeCN ? MeOH mixed solvent systems
WMeOH% of solvent mixture
ꢀr
g/mPa.s
0
35.96
35.62
35.37
35.17
34.99
34.78
34.52
34.19
33.78
33.29
32.63
0.3413
0.3421
0.3486
0.3613
0.3791
0.4012
0.4270
0.4559
0.4874
0.5210
0.5438
10
20
30
40
50
60
70
80
90
100
3 Results and Discussions
The concentration dependent molar conductance values ðKmÞ for [BMIM][Cl] and
[BMIM][PF6] in MeCN ? MeOH mixtures with changing volume fractions of MeOH
(WMeOH%) are presented as Fig. 1. The conductance results were analyzed by following
the framework of Barthel’s low-concentration chemical model (lcCM) [13]. According to
this framework:
ꢀ
ꢁ
1=2
3=2
Km ¼ a K0m ꢁ SðaCÞ þ EaClnðacÞ þ J1aC ꢁ J2ðaCÞ
ð1Þ
where
1 ꢁ a
Ka0 ¼
ð2Þ
2
a2Cðy Þ
ꢂ
Fig. 1 Molar conductivities ðKmÞ of (a) [BMIM][Cl] and (b) [BMIM][PF6] in methanol ? acetonitrile
mixtures in the composition range 10–100 % methanol (by volume) in steps of 10 %. Lines show the results
of the lcCM calculations
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J Solution Chem (2013) 42:738–745
741
ꢂ
ꢃ
1=2
ꢁkDq
y
¼ exp
ð3Þ
ꢂ
1 þ kDR
kD ¼ ð16pNAqaCÞ
ð4Þ
ð5Þ
e2
q ¼
8pꢀ0ꢀrkBT
where K0m is the molar conductivity at infinite dilution, (1 - a) is the fraction of charged
ions bound as ion pairs, and K0a is the standard state association constant. The activity
coefficient of the free cation (y?) and free anion (y-) are defined as
2
ðy Þ ¼ y y
ð6Þ
ꢂ
þ ꢁ
where kD is the Debye parameter, e is the proton charge, ꢀ0 the dielectric constant of the
vacuum and ꢀr the dielectric constant of the solvent system. T is the Kelvin temperature
and NA and kB are the Avogadro’s and Boltzmann’s constants, respectively.
According to the lcCM model two oppositely charged ions are counted as ion pairs if
the inter-ionic distance, r, is within the limits of a \ r \ R, where a is the lower limit for
distance of closest approach and taken as the sum of their crystallographic radii
(a = a? ? a-) and R, the upper limit of distance of closest approach for ion pair formation
taken as R ¼ a þ n ꢀ s. Expressions for the coefficients S, E, J1 and J2 are given by Barthel
et al. [13]. The limiting slope S and the parameter E are fully defined by the known values
of ꢀr and g (Table 1) of the solvent system. The coefficients J1 and J2 are both functions of
the distance parameter R, representing the distance up to which oppositely charged ions
can move as freely moving particles in solution. In the present study, the data analysis was
carried out by a non-linear least square fitting procedure with the coefficients S, E and J1
preset to their calculated values and with K0m; Ka0 and J2 as adjustable parameters. For these
ꢁ
calculations a? = 0.330 nm [21], aCl = 0.181 nm [13] and aPF = 0.256 nm [22] were
ꢁ
6
used. In calculations for R, n was taken as 1 and in view of the literature reports [23],
s = 0.47 nm for MeOH and s = 0.58 nm for MeCN; s for the mixed solvent systems
employed in present study was calculated through the equation;
s ¼ XMeOH ꢀ 0:47 þ ð1 ꢁ XMeOHÞ ꢀ 0:58
where XMeOH is the mole fraction of MeOH in the mixed solvent system.
ð7Þ
Figure 1 compares the experimental Km values for [BMIM][Cl] (Fig. 1a) and for
[BMIM][PF6] (Fig. 1b) solutions in the mixed solvent systems of changing compositions
with the results of the lcCM calculations, Eqs. 1–5. The estimated values for K0m together
with the K0a and distance parameter Rij calculated from the last term of Eq. 1 are presented
in Table 2.
The value of Rij can be used as a compatibility control of the fit as it should be similar to
the input radius R ¼ a þ n ꢀ s chosen for calculation of J2 and y . As evident from the
Table 2, R and Rij are in good agreement for both the ILs. The vaules of K0m estimated for
the investigated ILs in the present work seem a bit higher than those reported in MeOH and
MeCN in earlier reports [24–26]. However the data presented in earlier reports related to
conductance of ILs in molecular solvents [24–27] indicates a significant variation in the
values of K0m, reported for the same ILs in the same solvents. Such discrepancies in the
reported values of K0m are being attributed to (a) estimation of Km0 through use of different
conductivity equations and (b) the use of concentration data in different ranges for the
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Table 2 Association constants(K0a), limiting molar conductivities (K0m), input radii (R) and parameter (Rij)
for [BMIM][Cl] and [BMIM][PF6] in MeCN?MeOH mixed solvent systems at 298.15 K
K0a dm3 ꢀ molꢁ1
K0m S cm2 ꢀ molꢁ1
WMeOH% of solvent mixture
R nm
Rij nm
[BMIM][Cl]
0
88.5
58.3
48.9
45.3
35.4
29.8
27.8
33.9
28.4
25.8
20.7
196.52
189.27
184.46
179.89
167.41
158.35
152.41
142.39
131.16
120.96
114.65
1.089
1.077
1.064
1.052
1.040
1.029
1.019
1.009
0.999
0.990
0.981
1.093
1.092
1.061
1.052
1.037
1.033
1.022
1.012
1.005
0.974
0.966
10
20
30
40
50
60
70
80
90
100
[BMIM][ PF6]
0
27.0
15.1
14.8
15.7
19.0
19.0
26.1
37.9
45.5
100.8
158.52
156.80
153.24
148.26
139.14
133.89
128.39
124.90
112.33
116.46
1.166
1.152
1.139
1.127
1.104
1.094
1.084
1.074
1.065
1.056
1.184
1.153
1.137
1.125
1.103
1.094
1.088
1.083
1.071
0.996
10
20
30
50
60
70
80
90
100
estimation of said values. As can be clearly seen from the entries in Table 2, the values for
K0m for [BMIM][PF6] and [BMIM][Cl] fall with the increase in MeOH fraction in the
mixed solvent system. This is in line with the decrease in the dielectric constant of the
solvent and increase in its viscosity with decrease of MeCN fraction in the solvent system.
Nevertheless, it was observed that the Walden product (K0m ꢀ g) does not remain constant as
expected in cases where ion-solvent interactions do not vary with changing composition
of solvent [12]. This implies that the changing mole fractions of MeCN and MeOH lead to
changes in ion-solvent interactions in the present case. Similar observations have been
made for RTILs in earlier studies [28].
Interestingly, for all solvent compositions (except at wMeCN = 0) investigated, the Km0
values for [BMIM][Cl] were found to be higher than the corresponding values for
[BMIM][PF6]. As depicted in Fig. 2a the relative difference between the K0m values for the
investigated ILs decreases with increase in WMeOH of the mixed solvent system. Since the
the investigated ILs have a common cation, the observed variations in K0m can be attributed
to differences in solvation and mobility characteristics of Cl- and PF6- in the mixed solvent
systems. As has been reported in the literature [29], while MeOH interacts strongly with
anions in such systems, MeCN on the contrary interacts preferentially with the imidazo-
lium cation suggesting that the observed variations in K0m are anion driven. In the light of
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743
Fig. 2 (a) Limiting ion conductance ðK0mÞ and (b) ion association constant (K0a) of [BMIM][Cl] and
(b) [BMIM][PF6] in methanol ? acetonitrile mixtures in the composition range 0–100 % methanol (by
volume) in steps of 10 %
our observations, it can be safely argued that while in MeCN the mobility of Cl- is more
than that for PF-6 , in MeOH solutions the reverse is true. This trend suggests that the
greater is the size of anion the smaller is its mobility in MeCN, while in MeOH the reverse
is true, which we attribute to differing ion solvating abilities of the two solvents. In view of
its smaller size, Cl- has a higher charge density than that of PF6-; thus the former species
interacts with MeOH more than the later which is more solvated by MeCN on account of
its higher polarizibility.
More interesting findings of the present investigations are the relative magnitudes of the
K0a and its variations in the mixed solvent systems for the two ILs with a common cation.
As seen from Table 2 and Fig. 2b, at lower fractions of MeOH in the mixed solvent system
the K0a of [BMIM][Cl] was higher than that observed for [BMIM][PF6], whereas in MeOH
rich solvent systems the reverse was observed. In addition while for [BMIM][Cl] the Ka0
value decreases with increase of MeOH fraction in the mixed solvent system, for
[BMIM][PF6] an increasing trend was observed. While the trend observed for variation of
K0a of [BMIM][Cl] with the change in content of MeOH fraction in the solvent system is
quite expected in light of the resulting variations in ꢀr; the trend observed in the case of
[BMIM][PF6] is quite unexpected. We observed a similar trend for the IL [BMIM][BF4].
From molecular dynamic studies [29] it has been established that hydrogen bonding solutes
like MeOH strongly solvate halide ions by forming hydrogen bonds, while non hydrogen
bonding solvents like MeCN interact more strongly with easily polarizable solutes through
ion-dipole interactions. In light of these reports, on account of its smaller size and higher
charge density, Cl- is expected to show weaker ion-solvent and stronger ion-ion
interactions with its imidazolium counterpart and hence larger K0a values in solvents like
MeCN. However, in solvents like MeOH where Cl- is expected to be strongly solvated due
to its ability to engage in strong hydrogen bonding interactions with the solvent molecules,
an opposite trend is expected. For the IL [BMIM][PF6], where the anion is more polar-
izible, the ion-solvent interactions are expected to be stronger and hence K0a values lower
than for the [BMIM][Cl] homologue in solvents like MeCN. However, in solvents like
MeOH the ion-ion interactions for [BMIM][PF6] are expected to be stronger and hence
expected to show K0a values larger than its chloride counterpart (Fig. 2).
In light of these facts we propose that for an IL with smaller anions, like [BMIM][Cl],
the stronger ion-ion plus weak ion-solvent interactions are replaced by weaker ion-ion
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and stronger ion-solvent interactions with increase of the MeOH fraction in the MeCN ?
MeOH mixed solvent systems, which is responsible for the resulting decrease in K0a. On the
contrary, in ILs with larger anions like [BMIM][PF6], the weaker ion-ion plus strong ion-
solvent interactions are replaced by stronger ion-ion and weaker ion-solvent interactions
with increase of MeOH fraction in the MeCN ? MeOH mixed solvent systems resulting in
the observed variations in K0a with changing fraction of MeOH. This is in agreement with
the report by Mohammed et al. [30] wherein it has been proved that, while the interaction
energy for the [BMIM][PF6]? MeCN system is negative, for [BMIM][PF6]? MeOH the
same quantity is positive. The present studies are an excellent example of tuning the
physicochemical characteristics of closely related IL electrolytes through alterations in
solvophobicity achieved through similar variations in the composition of the mixed solvent
systems.
4 Conclusions
Conductometric investigations of imidazolium based ILs, viz 1-butyl-3-methylimidazo-
lium chloride ([BMIM][Cl]) and 1-butyl-3-methylimidazolium hexafluorophosphate
([BMIM][PF6]) were performed in binary acetonitrile ? methanol. The data were analyzed
with Barthel’s low-concentration chemical model, establishing that the conducting
behavior and ion association is highly specific to the nature of IL and composition of the
mixed solvent system. Thus, while in MeOH ion-association and K0m were found to be
larger for [BMIM][PF6] than for [BMIM][Cl], in MeCN the opposite was observed. Also,
with an increase in MeOH fraction in MeCN ? MeOH mixed solvent systems, while Km0
was observed to decrease for both the ILs investigated, opposing trends were observed in
the variation of K0a. For [BMIM][PF6] there was an increase in Ka0 while for [BMIM][Cl] a
decrease in the corresponding value was observed.
Acknowledgments The authors would like to thank Prof. Josef Barthel (Universitat Regensburg, Ger-
many) and Prof. Marija Bester-Rogac (University of Ljubljana, Slovenia) for their guidance in our calcu-
lations for the presented studies. Authors are especially thankful to Prof. Barthel who provided us one of his
books and some classic papers related to conductivity of electrolytes, which helped us a lot in the calcu-
lations and the writting of the present MS.
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