Exploring physicochemical aspects of N-alkylimidazolium based ionic liquids

Article abstract of DOI:10.1016/j.molliq.2013.02.021

Physicochemical aspects (structural, thermodynamic and transport) of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([BDMIM][BF₄]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) room temperature ionic liquids (RTILs) are reported. The results based on temperature dependence of surface tension, conductivity, photoluminescence and spectroscopic measurements on RTILs and their mixtures with acetonitrile (ACN) as cosolvent are interpreted in-terms of structure-composition-property relations. The presented observations clearly indicate that a highly structured and organized arrangement of constituents prevails in the investigated RTILs, which is sensitive to temperature variations and cosolvent addition. Thermodynamic and structural parameters estimated from temperature dependency of interfacial tension demonstrate that surface characteristics are dominated by covalent interactions in imidazolium based RTILs. From composition dependence of measured parameters in RTIL-cosolvent mixtures it is shown that ACN mixes nonideally with RTIL, and interestingly the investigated RTILs retain their inherent structural order up to high dilution limits (0.3 volume fraction of RTIL), beyond which these behave as associated electrolytes. The presented findings seem useful for arriving at molecular basis of physicochemical aspects and future applications of RTILs and their binary mixtures especially with cosolvents for biphasic catalysis and heterogeneous electron transfer reactions, wherein temperature elevation and cosolvent addition are currently advocated as operationally simple means to speed up the mass transport.

Full text of DOI:10.1016/j.molliq.2013.02.021

Journal of Molecular Liquids 181 (2013) 142–151
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Exploring physicochemical aspects of N-alkylimidazolium based ionic liquids
b
b
Mohsin Ahmad Bhat a, , Chanmeet K. Dutta , Ghulam Mohammad Rather
⁎
a
Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India
b
Department of Chemistry, University of Kashmir, Srinagar 190006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Physicochemical aspects (structural, thermodynamic and transport) of 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIM][BF4]), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([BDMIM][BF4]) and
1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) room temperature ionic liquids (RTILs) are
reported. The results based on temperature dependence of surface tension, conductivity, photoluminescence
and spectroscopic measurements on RTILs and their mixtures with acetonitrile (ACN) as cosolvent are
interpreted in-terms of structure–composition–property relations. The presented observations clearly indicate
that a highly structured and organized arrangement of constituents prevails in the investigated RTILs, which is
sensitive to temperature variations and cosolvent addition. Thermodynamic and structural parameters estimated
from temperature dependency of interfacial tension demonstrate that surface characteristics are dominated by
covalent interactions in imidazolium based RTILs. From composition dependence of measured parameters in
RTIL–cosolvent mixtures it is shown that ACN mixes nonideally with RTIL, and interestingly the investigated
RTILs retain their inherent structural order up to high dilution limits (0.3 volume fraction of RTIL), beyond
which these behave as associated electrolytes. The presented findings seem useful for arriving at molecular
basis of physicochemical aspects and future applications of RTILs and their binary mixtures especially with
cosolvents for biphasic catalysis and heterogeneous electron transfer reactions, wherein temperature elevation
and cosolvent addition are currently advocated as operationally simple means to speed up the mass transport.
© 2013 Elsevier B.V. All rights reserved.
Received 10 February 2012
Received in revised form 16 January 2013
Accepted 21 February 2013
Available online 14 March 2013
Keywords:
Room temperature ionic liquid (RTIL)
Surface tension
Excess surface energy
Specific conductance
Cosolvent
Photoluminescence
1. Introduction
where interface plays a significant role. Hence the use of RTILs as sol-
vents for such applications requires a comprehensive understanding
Room temperature ionic liquids (RTILs) are attracting significant
attention as novel ‘green’ alternatives to conventional organic solvents
for biphasic catalysis, solvent extraction, synthesis and electrochemical
investigations [1–8]. Experimental and simulation studies have clearly
established that a combination of dispersive and non-dispersive forces
prevailing among the constituents lead to a characteristic structural
organization and attractive physicochemical aspects in RTILs [9]. On
account of this unique structural organization solution chemists refer
to RTILs as nanofluids wherein three dimensional polar networks are
permeated by non-polar domains. These uniformly distributed domains
of opposite polarities make RTILs a low dimensional analog of micellar
media. Supramolecular structure and nanoscale ordering in the RTILs
have emerged as key to molecular level understanding of their physico-
chemical characteristics and their impact on various processes [10]. The
structural organization of RTILs markedly affects the stability and trans-
port of analytes and the intermediates involved in biphasic catalysis,
heterogeneous electron transfer and associated chemical reactions
thereby affecting the thermodynamics and kinetics of these phenomena
about their transport and interfacial properties vis a vis their character-
istic structural organization. The use of RTILs together with organic and
inorganic solvents as binary or ternary mixtures has been attempted for
various technological processes. It is worth mentioning that RTIL
mixtures with cosolvent possess altered and, in some cases, improved
physicochemical properties [11,12].
Prediction of composition–structure–property relations in RTILs and
their binary mixtures is an interesting and challenging task for present
day physical chemists. Various theoretical and experimental approaches
have been made use of to work out such relations for RTILs [13–16]. In
the experimental approaches researchers have made use of the magni-
tudes and trends in variations in some measurable parameters in
response to intelligently planned perturbations in selectively chosen
experimental variables to arrive at a basic understanding of such rela-
tions in RTILs. Thus while surface tension measurements have proved
to be very useful in prediction of microscopic ordering and estimation
of thermodynamic variables [17–19], the conductance [20–22] and op-
tical responses [23–26] have been quite effectively used for understand-
ing the bulk characteristics and transport phenomena in fluids and fluid
mixtures. In view of these facts we decided to make use of temperature
variation and cosolvent addition as perturbations in experimental
⁎
Corresponding author. Tel.: +91 194 242 4900; fax: +91 194 242 0333.
E-mail address: mohsinch@kashmiruniversity.ac.in (M.A. Bhat).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.02.021

M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
143
variables and surface tension, conductance and optical responses as the
3. Results and discussion
measurable parameters to explore the molecular basis of physicochem-
ical aspects of imidazolium based RTILs: 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIM][BF4]), 1-butyl-2, 3-dimethylimidazolium
tetrafluoroborate ([BDMIM][BF4]) and 1-butyl-3-methylimidazolium
hexafluorophosphate ([BMIM][PF6]) and their binary mixtures with
acetonitrile (ANC) as cosolvent. Imidazolium based RTILs were chosen
because of their air and moisture stability while we selected ACN as
cosolvent on account of its electrochemical, solubility and polarity
features [27]. Present study is expected to be very useful for applications
wherein the temperature elevation [28–30] and cosolvent addition are
employed as means to overcome the viscosity limitations in imidazolium
based RTILs [31–33]. Since fluid structuring in RTILs may be very sensi-
tive to rise in temperature and/or addition of cosolvents, a cautionary
approach needs to be followed in using any one of these options to
overcome viscosity related challenges in RTILs. In light of these facts
the present work also serves our aim to explore and understand the
impact of temperature and cosolvent on structural, transport and interfa-
cial properties of alkylimidazolium based RTILs. Besides providing new
insight into the structural features and structure–composition–property
relations for imidazolium based RTILs and their binary mixtures, the
study is expected to be of considerable importance for selection of opti-
mal experimental conditions, structure and/or composition for RTIL or its
binary mixture with cosolvent for a specific application.
3.1. Temperature variations
3.1.1. Surface tension measurements
Surface tension (γ) is a useful parameter for characterization of
fluids and their solutions [18,19,39]. According to Langmuir [40], each
part of a molecule possesses a local surface energy, hence γ of a fluid
is an index of the type of molecular orientation at its surface. Thermody-
namic modeling of fluid surfaces suggests that while magnitude of γ is a
useful index of cohesive energy prevailing at the surface, its tempera-
ture dependence can serve as a reliable means for estimation of thermo-
dynamic characteristics of fluids and fluid mixtures. Many groups in
recent past have used inter-facial tension and its dependence on molec-
ular structure and temperature as a means to model the molecular
ordering and estimate the thermodynamic variables associated with
RTIL–air interface [41–43]. In view of all these reports, we measured
the γ of the chosen RTILs as a function of temperature in the range
293–323 K and the results are depicted as Fig. 1. Like most other liquids
γ varies linearly with temperature, obeying Eq. (1)
γ ¼ a−b:T
ð1Þ
with a regression coefficient ≈0.99. In the entire temperature range in-
vestigated γ retains the order, [BDMIM][BF4] > [BMIM][BF4] > [BMIM]
[PF6] in accordance with an earlier report by Freire et al. [43]. The mea-
sured values of γ seem slightly larger in magnitude than those reported
for [BMIM][BF4] [44] and [BMIM][PF6] [45]. While this manuscript was
being compiled, a report by Russo et al. [42] highlighting the impact of
number of conceivable impurities on γ of imidazolium based RTILs
appeared in the literature. In this report the authors suggest that the
addition of experimentally expected impurities to RTILs tends to either
decrease or have essentially no effect on the surface tension but never
causes an increase in surface tension, corroborated by the findings of
Freire et al. [43], that γ and thermodynamic variables estimated from
its temperature dependence are not significantly affected by the pres-
ence of water.
It is expected that both cations and anions of investigated RTILs will
contribute to the measured γ with the relative contribution depending
upon their number density and orientation at the surface. Electro-
neutrality requirement warrants that the surface of RTILs cannot be
preferentially enriched by any of the constituent ions; instead bulk stoi-
chiometry should be preserved up to the surface, a fact proved through
direct recoil spectrometry (DRS) experiments by Gannon et al. [46].
Since the constituent cations of RTILs are comparatively of larger size,
the stoichiometric requirement of 1 cation: 1 anion at the surface war-
rants the dominance of cation on surface properties. As the actual
2. Experimental section
2.1. Chemicals
RTILs used were synthesized following a two step procedure [34] as
reported earlier [35]. Briefly, in the first step 1-methylimidazole or 1,2-
dimethylimidazole was refluxed with n-butyl chloride for 90 h under
argon atmosphere for the synthesis of 1-butyl-3-methylimidazolium
chloride and 1-butyl-2, 3-dimethylimidazolium chloride respectively. In
−
−
the next step the halide anion was exchanged with [BF]
or [PF]
4
6
using HBF4 or HPF6. The RTILs were vacuum dried and stored in desicca-
tors under inert atmosphere and were characterized through 1H NMR,
mass spectrometry and 13C NMR spectroscopy. The water content of
dried RTILs was less than 50 ppm, as analyzed by Karl Fischer titration.
The details of synthesis, purification, drying and characterization proce-
dures are described in supporting information.
2.2. Measurements
Surface tension measurements (within 0.1 mJ m−2) were made
with a K9 Tensiometer (Kruss, Germany) by the ring detachment
technique. Temperature was maintained at the desired value (within
0.1 °C) by circulating thermostated water from a HAAKE GH bath.
The measured values were corrected according to the procedure of
Harkins and Jordan built into the instrument software. Conductivity
was recorded by a digital microprocessor based conductivity meter
(CYBERSCAN CON 500) from Eutech instruments having sensitivity
of 0.1 μScm−1; details are reported elsewhere [36]. Steady state
photoluminescence (PL) spectra were recorded at room temperature
with a Shimadzu RF-5301PC spectrofluorometer. Fourier transform
infrared spectroscopy (FTIR) of liquid samples was performed with
a Thermo Scientific, NICOLET 6700 FT-IR spectrometer. Numerical
calculations and data fitting were performed through codes written
in Origin 6.0 (Microcal Software Inc.). Ab initio calculations were
performed to obtain gas phase optimized geometries and electronic
energies by using GAUSSIAN 03 set of codes [37], at the level of den-
sity functional theory using the exchange functional B3LYP [38] with
6-31G(d, p) basis set. Frequency analysis of the resultant geometries
was performed at the same level of theory and basis set, to check
whether the obtained structures are the stable minimum energy
structures and no imaginary frequencies were found.
Fig. 1. Temperature dependence of γ for [BMIM][BF4], [BDMIM][BF4] and [BMIM][PF6].
The straight lines show linear fits of the data (regression coefficient ≈0.99) in accor-
dance with Eq. (1).

144
M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
orientation of cations at interface will be determined by their interac-
tions with the counter anions and other cations, the extent of cationic
dominance on surface characteristics will be affected by the overall com-
position of RTIL and the prevailing experimental conditions. The
observed values of γ are close to 40.3 mJ m−2 as reported for imidazole
[47], an evidence for cationic dominance on surface properties in the
investigated RTILs. While cation–anion interactions in the presently in-
vestigated RTILs seem to be purely Coulombic, the cation–cation interac-
tions are expected to have considerable contribution from van der Waals
interactions also. Taking into consideration the conformers with the low-
est energy for constituent ions and the cation–anion combinations, ab
initio calculations revealed the ion-interaction energy to be −160.01,
−88.47 and −94.99 kcal·mol−1 for [BMIM][BF4], [BMIM][PF6] and
[BDMIM][BF4] respectively, indicating that the interaction energy is
stronger in presence of [BF4]− than [PF6]−. This is well expected as per
the Lewis basicity of anions and is evident from Fig. 2 that shows IR spec-
tra recorded for pure [BMIM][BF4] and [BMIM][PF6]. Lower frequencies
for C\H vibrations of imidazolium cation in [BMIM][BF4] in comparison
to [BMIM][PF6] are a direct evidence for weaker cation–anion interac-
tions in the latter [48]. Since stronger cation–anion interactions imply
lesser residual charge at the surface, a higher interaction energy among
RTIL constituents leads to higher γ values. Thus we may propose that
the stronger interaction energy among constituents in [BMIM][BF4] in
comparison to that in [BMIM][PF6] is responsible for higher γ of the
former. This is in accord with the hypothesis by Deetlefs et al. [49], that
an increase in ion size and hence diffuseness of charge on the RTIL ions
lead to a decrease in the overall interaction among the constituents
and, therefore, a decrease in surface tension. This feature of RTILs
makes them behave differently than conventional organic solvents
wherein increase in size of constituents leads to an increase in surface
tension.
Since introduction of methyl group at the C-2 position of imidazolium
cation has been proved to decrease the cation–anion interaction which
we also observed in our quantum mechanical calculations, higher value
of γ for [BDMIM][BF4] in comparison to that of [BMIM][BF4] seems
quite unexpected. Similar trend has earlier been reported by Hunt et al.
[50] for the melting point and viscosity of imidazolium based RTILs and
by Freire et al. [43] for γ value of [BDMIM][PF6] in comparison to
[BMIM][PF6]. Surface tension of fluids is actually a measure of the cohe-
sive forces among their constituents at the surface. In RTILs both disper-
sive (van der Waals) and non-dispersive (Coulombic) interactions
contribute to the overall interaction energy among constituent ions at
the interface. We attribute the higher surface tension of [BDMIM][BF4]
with respect to that of [BMIM][BF4] to increase in structural ordering of
constituents. Besides inhibiting butyl chain rotation due to steric repul-
sion, introduction of methyl group at C2 of imidazolium ring restricts
the number of otherwise thermodynamically allowed ion pair con-
formers for an imidazolium (cation) plus anion combination. Both
these factors are expected to enhance molecular order that ensures
greater van der Waals interactions and hence higher surface tension. In
other words, on substitution of C2–H of imidazole with C2–CH3, the loss
in Coulombic interactions due to steric (size) reasons is lesser than the
gain in dispersive (van der Waals) interactions that arises on account
of loss in entropy. Such ordering, as reported recently [41], increases
the number of overall units contributing to Coulombic attractions at
the surface and hence lead to an increase in the surface tension. Thus,
an increase in structural order leads to a net increase in dispersive as
well as non-dispersive interactions among the constituents of [BDMIM]
[BF4] [50] resulting in a higher surface tension in comparison to
[BMIM][BF4], an argument well supported by its higher viscosity [51].
Simulation studies, and neutron and X-ray reflectivity measurements
[52–55] have shown that at imidazolium RTIL/air interface, two types
of molecular arrangements prevail — one with side chain (butyl in pres-
ent study) normal to surface and other with side chain parallel to the sur-
face. The former with imidazolium rings parallel to the surface will have
lower surface tension than the later wherein their orientation is perpen-
dicular to the surface. Introduction of the methyl group at C-2 position in
imidazolium ring is expected to favor its orientation perpendicular to
surface and hence higher surface tension. Within this orientation, CH3
substituent, besides increasing the orientational order, will also enhance
van der Waals interactions among the cation rings at the surface [56],
that in turn will more than compensate the decreased Coulombic inter-
actions of imidazolium rings with their counter anions [57].
Linear fit parameters of γ vs. T plots were used to calculate ther-
modynamic variables for the RTIL/air interface. Thus, while the inter-
cept gives the enthalpy of surface formation per unit surface (Hs), the
slope is a measure of surface entropy (Ss) [58]. Assuming that surface
tension vanishes at critical temperature (Tc), the ratio of Hs to Ss gives
Tc. The resulting thermodynamic surface characteristics are listed in
Table 1. The estimated values are in good agreement with those
reported earlier [59–63]. According to Bloom, Davies and James
[64], the magnitude of Hs can be correlated to the bonding character-
istics of the fluid. The values observed for the investigated RTILs are
much lower than those reported for fused electrolytes but close to
those reported for organic liquids. This indicates that the surface ther-
modynamic characteristics are dominated by covalent interactions
[58,65]. Surface energy shows an increase with increase in cation
size in agreement with the report by Freire et al. [43], that increase
in cation size leads to charge dispersion and hence reduction of the
hydrogen bond strength. The surface entropies of the RTILs seem low
in comparison to those reported for conventional organic solvents, there-
by indicating a high surface organization of constituents at the RTIL sur-
face. This is in accord with predictions based on molecular dynamics
simulation calculations [66] which suggest that in imidazolium based
RTILs, the imidazolium cations are strongly aligned at the liquid/vapor
interface and thereby leading to negative or at least anomalously low
surface excess entropy. As evident from the slopes of the fit lines in
Fig. 1, which are a measure of sensitivity of surface interaction to temper-
ature, the former decrease with increase of latter in the order [BDMIM]
Table 1
Values of surface tension (γ) (standard error = 0.20), surface entropy (Ss), surface en-
thalpy (Hs), critical temperature (Tc), average interstice volume (ν), interstice volume
fraction (ϕ) and thermal expansion coefficient (α) for [BMIM][BF4], [BMIM][PF6] and
[BDMIM][BF4].
Properties
[BMIM][BF4]
[BMIM][PF6]
[BDMIM][BF4]
γ/mJ m−2 at 298 K 49.2 (46.6, Ref. [44]) 48.0 (47.92, Ref. [45])
50.8
0.0471
64.86
Ss/mJ m−2 K − 1
Hs/mJ m−2
0.0404
61.29
0.0343
58.25
Tc/K
1577.17
16.42
1698.34
17.04
1377.13
15.65
ν × 1024 cm3 at
298 K
ϕ × 102 at 298 K
α × 104 at 298 K
12.0
6.02
8.68
4.37
8.59
4.74
Fig. 2. FTIR absorption spectra in the range 2500–3500 cm−1 for pure [BMIM][BF4] and
[BMIM][PF6] ionic liquids. Frequency assignments were done as per reference [48].

M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
145
[BF4] > [BMIM][BF4] > [BMIM][PF6], an evidence for the higher γ value
of [BDMIM][BF4] in comparison to [BMIM][BF4] on account of stronger
dispersive interactions in the former. These results prove the predictions
of Law et al. [59] well, that the structure of cation and its orientation at
surface are mainly responsible for surface properties of RTILs. The calcu-
lated values of critical temperature show that thermodynamically [PF] 6−
combination with imidazolium cation is more stable than its [BF] 4− ana-
log and introduction of an additional alkyl chain leads to instability.
Making use of a classical statistical mechanics based theoretical
model originally proposed by Furth [67] and subsequently developed
by Bokris and Reddy [68], many groups [62,69–71] have reported the
use of experimentally measured values of γ for the estimation of bulk
characteristics in many RTILs. According to this model, now known as
interstice model, due to large size and asymmetric shape, the constit-
uent ions are not closely packed, instead they leave a lot of interstices
in between them. The model predicts that the average volume of an
interstice (ν) in RTIL is related to its γ through Eq. (2)
and Λm that we found are significantly greater in magnitude than the
values reported in the literature [28–30]. This perhaps is on account of
open to atmosphere condition of our setup which may allow RTILs to
absorb moisture because of their hygroscopic nature. Interestingly, we
found that instead of remaining constant as in conventional electrolyte
solutions, the Walden product decreased with increase of temperature.
This temperature dependence of Walden product seems unexpected in
light of earlier reports [33,74], which suggest that viscosity is the only
force impeding the motion of RTIL ions, while the dissociation of neat
RTILs is independent of temperature. Our observations, besides the ion
diffusion experiments [31], however indicate that the ionicity of RTILs
should be temperature dependent. From electrical conductance point of
view, RTILs can be modeled as an equilibrium mixture of dissociated
ions (ions free to conduct-solute) dissolved in the associated part (asso-
ciated ion clusters not able to conduct-solvent) [75,76], as per the equi-
librium
AþB−⇌Aþ þ B−
:
ð6Þ
ν ¼ 0:6791ðkT=γÞ3=2
ð2Þ
Sangoro et al. [77] on the basis of their pulsed field gradient nuclear
magnetic resonance and broad band dielectric spectroscopy measure-
ments have suggested that there is an Arrhenius type dependence for
the number density of charge carriers in imidazolium based RTILs.
Thus it may be argued that the equilibrium concentrations depend on
where k is the Boltzmann constant and T the temperature. The total
volume of interstices per mole formula unit (Vmi ) of RTIL (1:1 electro-
lyte) is given by Eq. (3)
Vim ¼ 2Nν
ð3Þ
N being the Avogadro's number, leading to an interstices volume frac-
tion (ϕ) in an RTIL of molar volume Vm equal to
Vmi
Vm
ϕ ¼
:
ð4Þ
The model envisages the interstices moving like ions, their expan-
sion with temperature being the cause for thermal expansion which
can be characterized in terms of the thermal expansion coefficient
(α) given by Eq. (5)
3Nν
α ¼
:
ð5Þ
V
mT
Using Eqs. (2) to (5), the characteristic values for ν, ϕ and α were es-
timated for the investigated RTILs and the same are presented in
Table 1. While the value of α estimated for [BMIM][BF4] in present
study is in good agreement with that reported by Harris et al. [29], the
value estimated for [BMIM][PF6] is a bit lesser in magnitude than that
reported in literature [72]. These values indicate that the packing of
constituents is more compact in [BDMIM][BF4] than in [BMIM][BF4],
supporting our contention that the entropic factors dominate the ener-
getic factors which lead to higher γ value of [BDMIM][BF4] in compari-
son to [BMIM][BF4]. In a recent study Larriba et al. [73] have proposed
that the interstice volume can be used to predict γ in imidazolium
based RTILs. Use of interstice volume as presented in Table 1, for estima-
tion of γ of presently investigated RTILs through this approach was
found to yield values in good agreement with the experimentally
observed values.
3.2. Conductivity measurements
Fig. 3A depicts the variation of specific conductance (κ) with temper-
ature for the investigated RTILs. Conductivity increases linearly with tem-
perature and is in the order [BDMIM][BF4] b [BMIM][PF6] b [BMIM][BF4],
in conformity with the trend expected on the basis of ion mobility and vis-
cosity. We used the reported values of density and coefficient of viscosity
[28–30] for calculation of molar conductance (Λm) and Walden product
(Λm. η) for [BMIM][BF4] and [BMIM][PF6]. The resulting values are plotted
as a function of temperature in Fig. 3B. It is pertinent to mention that the κ
Fig. 3. (A) Specific conductance (κ) of [BMIM][BF4], [BDMIM][BF4] and [BMIM][PF6]
RTILs as a function of temperature; (B) Walden product (Λm. η) as a function of tem-
perature. Density and η values were taken from references, [28–30].

146
M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
the dissociation constant of the above equilibrium (Kdis) which is related
to the free energy of dissociation (ΔGdois) of RTIL as
interactions among the ions that lead to higher values for ςanion and
cation will result in conductance values less than expected from vari-
ς
ations in η alone (Eq. (8)). It is in this context that deviations from
Walden rule observed in RTILs have recently led to the use of fraction-
al Walden rule [84], according to which
ꢀ
ꢁ
−ΔGodis
RT
Kdis ¼ exp
:
ð7Þ
Λm ꢀ ηβ ¼ constant:
ð12Þ
The greater the value of ΔGdois, the -smaller is the extent of dissoci-
ation. According to Bonhote et al. [78] the conductivity of an RTIL is
given by Eq. (8)
Thus a logarithmic plot of Λm vs. η should give a straight line with
slope β. The experimental data when plotted in the form of Eq. (12),
gave value of β equal to 0.76 for both [BMIM][BF4] and [BMIM][PF6].
Interestingly we found that, for both these RTILs, this value matches
the ratio of activation energy for Λm to activation energy for viscous
flow as obtained from the Arrhenius plots. Such a correlation has
also been reported recently by Schreiner et al. [83].
!
yF2ρ
6πNAMwη
h
i
κ ¼
ðςanionranionÞ−1 þ ðςcationrcationÞ−1
ð8Þ
where y is the degree of dissociation, F the Faraday's constant, ρ the
density, Mw the molecular weight and η the viscosity. ranion and rcation
are the anion and cation radii respectively and ςanion and ςcation, the
correction factors that take into account the specific interactions
among charge carriers in the RTIL. Increasing temperature is expected
to increase the extent of dissociation and hence ionicity of RTIL and at
the same time decrease the viscosity of the solvent. Thus the increase
in molar conductance with temperature can be attributed to both
these factors and hence the variation in Λm with temperature should
be more than that expected on the basis of variation in η alone, how-
ever, our observations are quite the opposite. On the basis of Eyring's
absolute rate theory [79] temperature dependence of Λm and η can be
expressed as [80]
4. Cosolvent addition
4.1. Surface tension measurements
Fig. 4 depicts the variations in γ with change in volume fraction of
RTIL in ACN. Surface tension increases with increase in mole fraction of
RTIL, but the variation is clearly not linear. The increase in γ is particular-
ly strong at higher concentrations, implying that the cosolvent is more
effective in breaking the cohesive interactions of RTILs in the low concen-
tration limit. Similar observations have been reported by Domanska et al.
[85] for addition of alcohols to imidazolium based RTILs. To make the
effect more clearly visible we calculated the γ-deviations for the mix-
tures using the equation
!
V2c=3F2
RT
−ΔG‡c
RT
Λm
¼
exp
ð9Þ
Δγ ¼ γ−ðXILγIL þ XACNγACNÞ:
ð13Þ
and
!
The results plotted in Fig. 4B demonstrate that mixing of RTIL with
ACN is nonideal and deviations from ideal behavior are more pro-
nounced for [BMIM][PF6] than for [BMIM][BF4]. This may be ascribed
to the more structured nature of the former than of the latter. To trace
the origin of interactions responsible for such nonideal mixing we
recorded the IR spectra at different dilutions (with ACN) of the two
RTILs, as are shown in Fig. 5. As evident the position of C2\H vibra-
tions (which is sensitive to hydrogen bonding) of the imidazolium
ring remains unchanged. A slight blue shift noticed in CH3− group
vibrations of the side chain on dilution is perhaps a result of increased
freedom of the said groups on account of dilution in RTIL. Since γ is a
measure of cohesive interactions among constituents at the interface,
the nonlinearity in γ vs. RTIL-mole fraction (a smaller slope in low
concentration range followed by a larger slope in the higher RTIL
concentration range) clearly implies some sort of transition in the na-
ture of forces responsible for nonideal variation of γ and Δγ. While
this manuscript was being prepared a report published by Ren et al.
[86] demonstrated that the excess volume vs. RTIL fraction for
1,2-dimethyl-3-hexylimidazolium bis(trifluoromethylsulfonyl)imide
passes through a minimum, while the curve as a whole is not sym-
metric. While the authors do not propose any justification for their
observations, it seems that such a variation carries interesting and
valuable information regarding RTIL/cosolvent mixtures. A compari-
son of the trends in γ, a measure of surface interactions, and excess
volume, a measure of bulk interactions, suggests that change in
mole fraction of RTIL changes the surface and bulk interactions in a
similar fashion. In the light of our observations and the data of the
above cited paper, we propose that introduction of RTIL into the
organic solvent leads to formation of some strong associates in the
lower mole fraction range, which are ultimately replaced by supra-
molecular aggregates of RTILs entrapping the small fraction of organic
solvent. To have a more clear picture before proposing a justification
for these observations we probed these mixtures for their conduc-
tance and optical responses.
hN
Vv
ΔG‡v
RT
η ¼
exp
:
ð10Þ
Vc and Vv are the volumes and ΔGc‡ and ΔGv‡ are the free energies of
activation of moving units for conductance and viscous flow respec-
tively. In case the two processes occur by the same mechanism, the
product, Λmη, can be presented as
!
F2hNV2c=3
RTVv
ΔG‡v−ΔG‡c
Λmη ¼
exp
:
ð11Þ
RT
Thus constancy of Walden product demands Vc = Vv and ΔGc‡
=
ΔGv‡. However in the present case the slopes of the plots of ln (Λm)
and ln(η) vs. 1/T (Fig. S1 and Fig. S2 supporting information) clearly
indicate that activation barrier for conductance is less than that for
viscous flow, well in accordance with the behavior for molten salts
[68]. The difference in these values can be attributed to mechanisms
responsible for the two processes. The mechanistic differences for
the two processes together with Eq. (11) suggest that the Walden
product will show a temperature dependence as depicted in Fig. 3.
These observations imply that increase of temperature does not
increase the Λm of RTILs to the extent expected from the extent of
decrease in their η. Similar behavior has been observed for polymer
electrolytes [81], where ion pairing has been suggested as the possi-
ble reason for the observed trend. Such variations as have been
reported earlier suggest that in RTILs, other than viscosity, there are
some additional forces that impede the motion of their constituent
ions in the conduction process.
In a recent report Kashyap et al. [82] suggested that increase in
temperature decreases the dielectric constant of the RTIL and hence
increases the magnitude of Coulombic interactions among the oppo-
sitely charged conducting ions. Thus while the lowering of η and the
increase in y should increase the molar conductance, the enhanced

M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
147
4.1.1. Conductivity measurements
Fig. 6A depicts the conductivity (κ) as a function of volume fraction
of RTIL in ACN and Fig. 6B depicts molar conductance (Λm) vs. mole frac-
tion of RTILs. For both [BMIM][BF4] and [BMIM][PF6], the traces pass
through maxima, indicating nonideality as observed in surface tension
plots. Such a conductance behavior of RTILs with addition of cosolvent
[86–89] has generated immense interest among researchers and
many explanations and advantages associated with it have been pro-
posed [89,90]. A correlation of the observed variations in γ, Δγ, k and
Λm with mole fraction of RTIL, in the light of earlier reports for similar
RTILs [89,90] indicates that a variety of factors, viz. formation of
cosolvent–RTIL and/or RTIL-supramolecular associates and entrapping
of the cosolvent within the RTIL-supramolecular aggregates, whose
fraction varies with mole fraction of the RTIL can be responsible for
the observed trends.
According to Dupont [91], pure imidazolium based RTILs form an
extended hydrogen bonded network due to self aggregation of mono-
meric cations under the influence of anions. Signatures of structural
organization in the investigated RTILs were clearly observed in their
UV–visible absorption and emission spectra as depicted by Figs. 7 and
8. The long tail of the absorption band in RTILs (Fig. 7) has been attrib-
uted to the presence of a large number of variedly-sized supramolecular
aggregates that are energetically different and characterized by specific
absorption maxima [24–26]. Direct evidence for presence of such supra-
molecular aggregates has been reported earlier [92,93].
More interesting features were observed in the emission spectra
(Fig. 8) which show two maxima, one in lower and other in higher
wavelength range. The position and relative intensity of both the
Fig. 4. (A) γ and (B) Δγ (calculated through Eq. (13)) as a function of composition in
acetonitrile for [BMIM][BF4] and [BMIM][PF6].
Fig. 5. FTIR absorption spectra in the range 2500–3500 cm−1 at changing dilutions
Fig. 6. (A) Specific conductance (κ) and (B) molar conductance (Λm) as a function of
with acetonitrile for (A) [BMIM][PF6] and (B) [BMIM][BF4].
composition in acetonitrile for [BMIM][BF4] and [BMIM][PF6].

148
M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
maxima were found to be excitation wavelength dependent. At lower
excitation wavelengths, the maxima in the low wavelength region of
emission spectrum are dominating. As the excitation wavelength
increases, the maximum at higher wavelength becomes more intense
while that in lower wavelength range decreases in intensity and finally
disappears. Another interesting observation is that above a particular
range, the upper wavelength maximum shows a significant red shift
with increase of excitation wavelength.
Paul et al. [25,94] have proved that the short wavelength emission in
imidazolium based RTILs is due to the monomeric form of the
imidazolium cation while excitation and relaxation of varied-sized
and energetically different associated forms of the constituent ions are
responsible for the tail in the absorption and long wavelength compo-
nents in their emission spectra. The aggregates are expected to have
their specific absorption and emission maxima. With changing excita-
tion wavelengths, different species get excited and hence different
emission behavior is observed. However, the theory of emission from
photo-excited assembly of variedly energetic species [95] demands
that emission will always be from the least energetic species, and
hence excitation wavelength dependence of maxima in emission spec-
trum of RTILs is unexpected. Short fluorescence lifetimes of excited spe-
cies and minimal interactions among energetically different aggregates
that reduce the chances of energy transfer among these species in RTILs
are perhaps the reason for the excitation wavelength dependence of
their emission spectra [24]. The shape and intensity of two component
excitation wave length dependent photoluminescence (PL) spectra, as
Fig. 8. PL emission spectra at changing excitation wave lengths recorded for pure
(A) [BMIM][PF6] (B) [BMIM][BF4].
shown in Fig. 8, thus also attest the presence of supramolecular aggre-
gates in the investigated RTILs.
Marked variations were observed in the absorption and emission
spectra of RTILs on dilution with ACN. Decrease in tail length of the
absorption spectra upon dilution with ACN (Fig. 9), indicates the break-
age of supramolecular aggregates in RTILs. However, persistence of
absorption tail up to high dilution limits implies the presence of these
structures, although probably of smaller size, in these solutions. Emis-
sion spectra recorded for varied RTIL/co-solvent compositions, as
depicted in Fig. 10 also lead to similar conclusions. While the high
wavelength emissions have been attributed to supramolecular aggre-
gates, the low wavelength emissions arise from monomeric forms of
RTILs [24,26]. As evident from the figure, the relative emission on
account of monomeric forms (low wavelength hump) increases on suc-
cessive dilutions with ACN in comparison to that from aggregated
forms, clearly indicating the transition of aggregates into monomers.
Normalizing the emission spectrum with respect to concentration,
(Fig. 10B) also indicates that addition of ACN breaks the network of
supramolecular aggregates in RTILs. The relative emission of monomer-
ic forms to aggregate forms changes from dominating aggregated forms
in pure RTIL to dominating monomeric forms in dilute states. However,
an interesting feature worth to be underscored is that the emission from
the aggregates does not vanish even at high dilutions, thereby indicat-
ing their existence even at these dilution limits. The positions of maxi-
ma in the traces in Fig. 6 and the emission spectra of Fig. 10 hence
imply that the structural order of RTILs is retained up to high dilutions
(RTIL volume fraction = 0.3), beyond which the RTIL exists as a con-
ventional electrolyte that is strongly associated.
Fig. 7. UV–visible absorption spectra for pure (A) [BMIM][PF6] and (B) [BMIM][BF4]
ionic liquids.

M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
149
Fig. 10. PL emission spectra of [BMIM][BF4] at excitation wave length of 330 nm at
changing dilutions with acetonitrile (A) as recorded spectra; (B) spectra normalized
with respect to concentration.
Fig. 9. UV–visible absorption spectra at changing dilutions with acetonitrile for
(A) [BMIM][PF6] and (B) [BMIM][BF4].
as cosolvent for RTIL should only be made after the expected extent of
structural distortions is known.
A holistic view of the observed trends in γ, Δγ, κ and Λm, emission
and absorption spectra makes us to propose that addition of RTIL to
the cosolvent (acetonitrile in present case) in low dilution limits
leads to formation of free conducting ions and their aggregates
besides ion + cosolvent complexes resulting in increase of γ, Δγ, κ
and Λm. Further addition of RTIL favors the formation of ion-pairs
and aggregates that, besides decreasing the fraction of conducting
species, increase the viscosity and γ as well. However, in low concen-
tration region the characteristics are dominated by the ion–solvent
complexes, responsible for increase in γ, Δγ, κ and Λm. At the stage
when RTIL aggregates begin to dominate the free ion–solvent com-
plexes, the solution starts behaving more like pure RTIL wherein the
cosolvent molecules are entrapped in RTIL aggregates. Higher cohe-
sive energy among RTIL constituents in the aggregates than that of
RTIL–cosolvent complexes, in turn, leads to increase in viscosity, γ,
and Δγ, and hence decrease in κ and Λm with further addition of
RTIL. Thus the maxima in Δγ, κ and Λm vs. mole fraction plots actually
represent the RTIL–cosolvent composition beyond which (more RTIL
fraction) the mixture behaves more RTIL like. In other words up to
the dilution limit by cosolvent, where the maxima are observed,
RTIL retains its chemical characteristics and in the present case this
limit is 0.3 volume fraction of the RTIL. Below this dilution limit the
investigated RTILs behave like strongly associated electrolytes. This
is in agreement with earlier reports based on absorption spectra
and kinetic measurements [96] and dielectric relaxation spectroscopy
[89] on N-alkylimidazolium based RTILs. Thus it may be inferred that
the addition of ACN leads to structural distortions of RTILs and its use
5. Conclusion
Imidazolium based RTILs and their mixtures of varying concentra-
tions in acetonitrile (ACN) were probed at different temperatures for
their surface tension, conductance and optical characteristics. The
recorded results of conductivity, surface tension and spectroscopic
(UV–visible, FTIR and photoluminescence) measurements indicate
that these RTILs are structured fluids wherein the structural ordering
is sensitive to cosolvent addition or temperature elevation. The
results warrant that use of these two options to decrease the viscosity
of RTILs for electrochemical investigations and biphasic catalysis, will
always be at the cost of loss in structural order at the interface and in
the bulk of RTIL. An interesting finding of the present work is that,
with cosolvent addition, RTILs preserve their characteristic structural
aspects up to a certain limit of dilution (ϕRTIL = 0.3, for acetonitrile).
Acknowledgments
M. A. Bhat would like to thank, University authorities-especially Vice
Chancellor, University of Kashmir, and Head, Department of Chemistry,
University of Kashmir, for sanction of study leave in his favor. C. K. Dutta
would like to thank Director School Education (J & K), for sanction of
study leave in her favor and Head, Department of Chemistry, University
of Kashmir, for allowing her to carry out some part of the presented
work in the Department. Authors would also like to thank Dr. S. K.

150
M.A. Bhat et al. / Journal of Molecular Liquids 181 (2013) 142–151
P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng,
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Haram (University of Pune) for his useful suggestions for the present
work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.molliq.2013.02.021.
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