Densities, excess molar and partial molar volumes for water + 1-butyl- or, 1-hexyl- or, 1-octyl-3-methylimidazolium halide room temperature ionic liquids at T = (298.15 and 308.15) K

Article abstract of DOI:10.1016/j.molliq.2012.12.018

Experimental densities for seven mixtures of water + 1-butyl-3- methylimidazolium iodide, [C₄mim][I], + 1-hexyl-3-methylimidazolium chloride, [C₆mim][Cl], + 1-hexyl-3-methylimidazolium bromide, [C ₆mim][Br], + 1-hexyl-3-me

Full text of DOI:10.1016/j.molliq.2012.12.018

Journal of Molecular Liquids 180 (2013) 12–18
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Densities, excess molar and partial molar volumes for water+1-butyl- or,
1
-hexyl- or, 1-octyl-3-methylimidazolium halide room temperature ionic
liquids at T=(298.15 and 308.15) K
Nandhibatla V. Sastry ⁎, Nilesh M. Vaghela, Pradip M. Macwan
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar - 388120, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2012
Received in revised form 15 December 2012
Accepted 17 December 2012
Available online 2 January 2013
Experimental densities for seven mixtures of water+1-butyl-3-methylimidazolium iodide, [C mim][I],
4
+
1-hexyl-3-methylimidazolium chloride, [C
Br], +1-hexyl-3-methylimidazolium iodide, +[C
Cl], +1-octyl-3-methylimidazolium bromide, [C
mim][I]) were measured across the composition at T=(298.15 and 308.15) K and atmospheric pressure.
6
mim][Cl], +1-hexyl-3-methylimidazolium bromide, [C
6
mim]
[
[
[
6
mim][I], 1-octyl-3-methylimidazolium chloride, [C
8
mim]
8
mim][Br] and +1-octyl-3-methylimidazolium iodide,
C
8
E
m
The excess molar volumes, V , were calculated and their compositional variation was mathematically repre-
Keywords:
sented by Redlich–Kister type equation. The partial excess molar volumes of water as well as the correspond-
ing RTIL were calculated across the composition. The partial molar volumes at infinite dilution for the
respective components and the corresponding standard transfer volumes for different combinations of
water+RTIL mixtures were also calculated.
1
-Alkyl-3-methylimidazolium halides
Aqueous mixtures
Densities
Volumetric properties
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
RTILs are usually highly viscous and their viscosities can drastically be
decreased by the addition of small amount of water. Most of the industri-
al requirements also desire simple means of altering the volumetric and
transport properties of RTILs. Moreover, the volumetric properties are
excellent tools to clearly understand the nature and predominance of a
given type of interactions present among the ions of RTILs, RTIL and
water and water……water in the aqueous solutions. Therefore,
water+RTIL systems as binary liquid mixtures are interesting both
from fundamental as well as applied aspects. The knowledge of the
thermophysical properties of the binary mixtures of water+hydrophilic
RTILs is not only very useful for designing the chemical processes involv-
ing ILs in aqueous media but also aids in inventing their novel and still
unexplored new practical applications. For example, the processes in-
volving homogenous mixtures of water and volatile organic solvents
that are currently in use needs to be improved by substituting the
given organic solvent with a RTIL. The accurate data on various
thermophysical properties such as heat capacities, phase equilibria, ex-
cess molar enthalpies and excess molar volumes etc. are highly desired
for the simulation and the design of industrial processes.
Room temperature ionic liquids (RTILs) are salts that are in liquid
form at the room temperature. RTILs consist of bulky and asymmetric
organic cations like 1-alkyl-3-methylimidazolium, or 1-alkylpyridinium,
or N-methyl-N-alkylpyrrolidinium, or alkyl-ammonium or -sulfonium
or -phopshonium etc. and any one of the negative ions from a wide
range of species namely simple halides to inorganic anions such as
tetrafluoroborate or hexfluorophosphate, or large organic anions like
bistriflimide, triflate, tosylate, formate, alkylsulfate, or alkylphosphate
etc. RTILs have unique and extraordinary properties such as non-
volatility, non-inflammability, reasonable thermal stability, wide liquid
range up to 573 K, high viscosity and also conductivity and wide electro-
chemical window etc. [1–3]. These novel properties distinguish RTILs
from the traditional organic molecular solvents. RTILs have high solvation
power and can dissolve a wide variety of solutes that include inorganic,
organic and polymeric materials. RTILs have also been successfully
employed either as solvents or as solvents cum catalysts for carrying
out several organic reactions [4–6]. The unique combination of physical
properties namely low volatility (or near zero vapor pressure) and
wide liquidus range project RTILs as best suitable green solvents i.e. as
suitable alternates to the damaging and volatile organic molecular sol-
vents [7]. RTILs are also known as designer solvents because their physi-
cal properties can be drastically altered just by replacing either of the ring
cation or counter anion [8].
In view of the above, attempt was made to analyze the limiting ap-
parent molar volumetric and compressibility properties (as derived
from the experimental densities and speeds of sound) of either dilute
solutions of 1-butyl-3-methylimidazolium bromide (bmimBr) in
aqueous or alcoholic (methanol, ethanol) media [9], or for 1-hexyl-
3-methylimidazolium bromide and water mixtures [10], or of
aqueous or alcoholic mixtures of 1-propyl-, 1-hexyl-, 1-heptyl,
1
-octyl-3-methylimidazolium bromides [11] in terms of RTIL–water
⁎
Corresponding author. Fax: +91 2692 236475.
E-mail address: nvsastry17@gmail.com (N.V. Sastry).
interactions. It has been suggested that in the dilute solution region,
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.12.018

N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
13
RTIL–water interactions are stronger than IL–methanol or IL–ethanol
interactions because of poorer hydrogen bonding capacity of the lat-
ter [9,11]. However at high concentrations of RTILs, the hydration
around the IL molecules increases and this leads to so called hydro-
phobic or cage association in which the alkyl chains occupy the
intermolecular cavities of an ice-like water structures [10,11].
≥98%), bromooctane (Spectrochem, India, 98%) were dried over
fused calcium chloride and freshly distilled prior to use. Butyl iodide
(analytical reagent grade) was purchased locally and was twice
recrystallized before use.
2.1. Synthesis of ionic liquids
Gomez et al. [12] have reported the excess molar volumes, VE
and
m
viscosity deviations for water+1-hexyl-3-methylimidazolium chlo-
ride or +1-octyl-3-methylimidazolium chloride binary mixtures
The general procedure of the synthesis involved a direct reaction
between 1-methylimidazole and excess amount of respective halo-
alkane (iodobutane, chlorohexane, bromohexane, chlorooctane or
bromooctane) in acetonitrile at 473.15 K for 1.5 to 3 h. The reactions
were conducted under nitrogen blanket in a round bottom flask equipped
with reflux condenser and magnetic stirrer. The resulting products were
cooled to room temperature by adding 100 cm3 of analytical reagent
grade ethyl acetate under thorough mixing. The excess ethyl acetate
was then decanted and the procedure was repeated four times.
across the composition and at different temperatures. The VE
values
were negative with a minimum around a water mole fraction of
0.55 and become positive in water rich mole fractions. The authors
m
~
had attributed these trends in terms of strong hydrogen bonding be-
tween the RTIL and water and dissociation of the ions of the ionic liquids
at higher concentrations. Even though the strong hydrogen bonding in-
teractions between the ionic liquid molecules and water quite obviously
and predominately contribute to the interaction between the two com-
ponents, the nature of the ring cation as well as hydration characteris-
tics of counter anion are also expected to play a very important role in
the overall RTIL–water interactions. Systematic thermophysical and
thermodynamic studies on water+RTIL systems (in which the cation
of RTILs is fixed and while the hydrophilic character of anion is varied)
are very scarce in the literature. Only one such study on excess molar
volumes and excess molar enthalpies for water+RTILs based on
6 8
2.2. [C mim][I] and [C mim][I]
These ionic liquids were synthesized from their respective chlo-
ride counter parts by an ion exchange reaction. Equimolar solutions
of chloride based IL and sodium iodide solutions were mixed in ace-
tone by adding the sodium iodide solution drop wise under stirring
at room temperature over a period of three hours. The sodium chlo-
ride formed was then slowly got precipitated out during the reaction
and the same was removed by filtration. The supernatant liquid was
dried in an oven at 343.15 K to evaporate most of the acetone. The
product thus obtained was further purified by two cycles of dissolving
it first in dichloromethane followed by an extraction of the final prod-
uct into triple distilled water. Through this process, the organic impu-
rities present (if any) were removed. The ionic liquid samples were
characterized by 1H NMR. The chemical shifts and assigned protons
for individual RTILs are:
1
-ethyl- or 1-butyl–3-methylmidazolium cation with different anions
namely methylsulfate or ethylsulfate, trifluoromethanesulfonate, or
tetrafluoroborate has been reported [13]. It was suggested that the rel-
ative affinity of different anions towards water is crucial in deciding the
type and magnitude of RTIL–water interactions in the bulk state.
Therefore it is thought that the systematic measurements on the
thermophysical properties of water+ionic liquid systems consisting
of RTILs prepared from the same cation but different anion of varying
hydrophilic character would be very useful and help understand the
nature and type of interactions in such systems. The availability of dif-
ferent RTILs with variation in the chemical structure of the cations for
binary liquid mixture studies always poses a limitation because the
[C
7.0 Hz, N-(CH
1.865 (2H, m, N-CH
(2H, t, J=7 Hz, N-CH
d, H4), 8.769 (H, s, H2).
[C mim][Cl]. 0.790 (3H,t, J=7.2 Hz, N-(CH
N -CH -CH -(CH -CH ), 1.807 (2H, m, N-CH
3.840 (3H, S, N-CH ), 4.137 (2H, t, J=7 Hz, N-CH
(CH -CH ), 7.382 (H, d, H5), 7.427 (H, d, H4), 8.664 (H, s, H2).
[C mim][Br]. 0.808 (3H,t, J=7.0 Hz, N-(CH -CH ), 1.256 (6H, m,
N -CH -CH -(CH -CH ), 1.843 (2H, m, N-CH -CH -(CH -CH ),
3.899 (3H, S, N-CH ), 4.192 (2H, t, J=7 Hz, N-CH -CH
(CH -CH ), 7.462 (H, d, H5), 7.501 (H, d, H4), 8.767 (H, s, H2).
[C mim][I]. 0.891 (3H,t, J=7.0 Hz, N-(CH -CH ), 1.349 (6H, m, N
-CH -CH -(CH -CH ), 1.928 (2H, m, N-CH -CH -(CH -CH ),
3.966 (3H, S, N-CH ), 4.264 (2H, t, J=6.8 Hz, N-CH -CH
(CH -CH ), 7.517 (H, d, H5), 7.568 (H, d, H4), 8.818 (H, s, H2).
[C mim][Cl]. 0.783 (3H,t, J=7.0 Hz, N-(CH -CH ), 1.205 (10H, m,
N CH -CH -(CH -CH ), 1.802 (2H, m, N-CH -CH -(CH -CH ),
.816 (3H, S, N-CH ), 4.113 (2H, t, J=7 Hz, N-CH -CH -(CH -CH ),
.351 (H, d, H5), 7.395 (H, d, H4), 8.627 (H, s, H2)
mim][Br]. 0.824 (3H,t, J=7.0 Hz, N-(CH -CH
N CH -CH -(CH -CH ), 1.906 (2H, m, N-CH -CH
.965 (3H, S, N-CH ), 4.277 (2H, t, J=7.2 Hz, N-CH
), 7.562 (H, d, H5), 7.586 (H, d, H4), 8.995 (H, s, H2)
mim][I]. 0.828 (3H,t, J=7.0 Hz, N-(CH -CH ), 1.287 (10H, m,
N CH -CH -(CH -CH ), 1.901 (2H, m, N-CH -CH -(CH -CH ),
3.984 (3H, S, N-CH ), 4.299 (2H, t, J=7.4 Hz, N-CH -CH
CH -CH ), 7.589 (H, d, H5), 7.607 (H, d, H4), 8.995 (H, s, H2).
4
mim][I]. 1H NMR (400 MHz, D
-CH ), 1.333 (6H, m, N -CH
-CH -CH -CH ), 3.916 (3H, S, N-CH
-CH -CH -CH ), 7.456 (H, d, H5), 7.519 (H,
2
O,δ ppm), 0.936 (3H,t, J=
-CH -CH -CH ),
), 4.218
2
)
3
3
2
2
2
3
methylimidazolium halides with alkyl chains (higher than
C
8
2
2
2
3
3
atoms) and most of the pyridine based ILs are solids at room temper-
ature. In the present study, we report the volumetric properties (de-
rived from experimental densities) for seven binary mixtures of
2
2
2
3
6
2
)
7
-CH
3
), 1.225 (6H, m,
-(CH -CH ),
-CH
water+1-butyl-3-methylimidazolium iodide, [C
-methylimidazolium iodide, [C mim][I], +1-octyl-3-methylimid-
azolium iodide, [C mim][I], +1-hexyl-3-methylimidazolium bromide,
mim][Br], +1-octyl-3-methylimidazolium bromide, [C mim][Br],
1-hexyl-3-methylimidazolium chloride, [C mim][Cl], +1-octyl-3-
methylimidazolium chloride, [C mim][Cl], across the composition and
at T=(298.15 and 308.15) K. The excess molar volumes, (V
4
mim][I], +1-hexyl-
2
2
)
2 3
3
2
-CH
2
2
)
3
3
3
6
3
2
2
-
8
2
)
3
3
[C
6
8
6
2
)
7
3
+
6
2
2
)
2 3
3
2
2
2
)
3
3
8
3
2
2
-
E
m
), partial
2
)
3
3
molar volumes of water (V1), partial excess molar volumes of water
6
)
2 7
3
ꢀ
ꢁ
ꢀ
ꢁ
E
∞
V 1 , partial molar volumes of water at infinite dilution
V
, partial
2
2
)
2 3
3
2
2
2
)
3
3
1
molar volumes of ionic liquids (V 2), partial excess molar volumes of
3
2
2
-
ꢀ
ꢁ
E
ionic liquids V2 and partial molar volumes of ionic liquids at infinite
2
)
3
3
ꢀ
ꢁ
∞
2
8
2
)
7
3
dilution,
V
were calculated. The standard transfer volumes were
2
2
)
2 5
3
2
2
2
)
5
3
also calculated for different combinations of water+RTILs. The analysis
of the volumetric functions was done to adjudge the effect of anion en-
vironment on the overall volumetric changes at infinite dilution. The
chemical structures of the RTILs are particularly chosen so that effects
for example, of the chain length of the alkyl branch and hydrophilic
3
7
3
2
2
)
2 5
3
[
C
8
)
2 7
3
), 1.245 (10H, m,
-(CH -CH ),
-CH -(CH
2
2
)
2 5
3
2
2
2
)
5
3
3
3
2
2
2 5
) -
−
−
−
character of simple non-hydrolyzable anions namely Cl or Br or I
on the bulk interactions can be assessed in aqueous RTIL systems.
CH
3
[C
8
2
)
7
3
2
2
)
2 5
3
2
2
2
)
5
3
2
. Experimental
3
2
2
-
(
2
)
5
3
1
-Methylimidazole (Alfa Aesar, 99%) was freshly distilled over
potassium hydroxide. Chlorohexane (Merck, ≥98%), bromohexane
analytical reagent grade of local make, 98%), chlorooctane (Merck,
The water content of the RTILs was determined by using Karl
Fischer titrator and water mass fractions were found to range from
(

14
N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
4
5
.5×10− to 7.8×10−6 depending upon the length of the alkyl chain.
where V is the volume of a mixture containing 1 mole of IL+water, x
1
⁎
The purity of ionic liquids was estimated by potentiometric titration
and x
2
are the mole fractions of water and IL, respectively. V1.m and
by argentometric method in which 10 cm3 of aliquot solution was ti-
V
2.m are the molar volumes of the pure components.
⁎
−
3
trated with AgNO
of NaCl using AgCrO
3
solution standardized against a 0.015 mol dm
as indicator. The estimated purity of the ionic
Eq. (1) can be re-casted as:
4
liquids is ≥97% (on mole basis). Ionic liquids were stored in glass bot-
tles under inert atmosphere and were dried for 24 h at T=343.15 K
under vacuum before use.
.ꢀ
ꢁ
E
m
3
−1
V
cm ⋅mol
¼ fðx M þ x M Þ=ρ g–fðx M =ρ Þ þ ðx M =ρ Þg
1
1
2
2
12
1
1
1
2
2
2
ð2Þ
2
.3. Methods
where M
1
, M2,
ρ
1
and ρ
2
are the molar mass and density of respective
pure components and ρ12 is the density of mixture of a specified mole
The binary solutions were prepared by mass in hermetically
fraction for water and IL, respectively.
sealed glass vials. The solutions of each composition were prepared
fresh and the densities were measured the same day. The mass mea-
surements, accurate to ± 0.01 mg, were made on a single pan analyt-
ical balance (Dhona 100 DS, (India)). The estimated uncertainty in the
mole fraction was ± 0.0001.
The VE
values were fitted through Redlich–Kister equation of type,
m
E
i
Y
¼ x ð1−x ÞΣa ð1−2x Þ
ð3Þ
1
1
i
1
i¼0
Densities, ρ of the pure liquids and their mixtures were measured
with a high precision vibrating tube digital density meter (Anton
Paar, DMA 5000). The instrument has a built-in thermostat for
maintaining the desired temperatures in the range of (273 to 363)
K. The repeatability of the temperature has been found to be
E
where Y =VE
m
and a
i
are the fitting coefficients. a
i
were estimated by
multiple regression analysis based on a least squares method. The
summary of a along with the standard deviation, σ between calculat-
ed and fitted values is given in Table 3. The graphical variation of
i
(
± 0.002 and ± 0.003) K within a given session and/or for two succes-
E
V
m
as a function of water mole fraction for water+RTIL mixtures at
sive sessions respectively. The uncertainty in the temperature during
the measurements, however is ± 0.01 K because Pt 100 measuring
sensors were used. The instrument was calibrated with air, four
times distilled and freshly degassed water. The repeatability in the
densities for the distilled water, pure ionic liquids and as prepared bi-
T (298.15 to 308.15) K is shown in parts a and b of Fig. 1. An exami-
nation of the trends in V curves gave an interesting insight into the
m
E
bulk interactions among different halide based RTILs and water mix-
tures. It can be seen that the volumetric behavior is highly dependent
−
−
−
on the nature of the halide ion. In the present study, Cl , Br and I
−
6
−3
nary mixture solutions was found to be better than 3×10
g·cm
.
ions are particularly selected because their relative hydration and
water binding capacity are not only inversely related but are also dif-
We have estimated the uncertainty in our measured densities of the
pure liquids by comparing our data with the literature values, as
listed in Table 1. This comparison gave a mean absolute deviation of
ferent in that order. The features of VE
m
curves for the binary mixtures
of water+ILs are as described below:
−
4
−3
9
.0×10
g·cm . It may be mentioned here that the availability
of densities of pure RTILs is still very limited in the literature. Even
among the available data, the values need to be taken with caution
as they are dependent highly on the purity levels. Our measured
densities for distilled water match excellently well with literature
−
Cl
based RTILs: The water+1-hexyl-, and +1-octyl-3-
methylimidazolium chlorides mixtures were characterized pre-
dominantly by negative lobes followed by near zero or slight pos-
itive values in the water rich region. The increase in the
temperature from (298.15 to 308.15) K increased the positive or
negative values (as the case may be). The V
−
5
but the experimental densities of the RTILs deviated by 1.0×10
−
3
−3
to 4.5×10
g·cm
with the literature values. Hence the precision
E
m
values for the
and accuracy for the densities reported in the present work are
−
6
−4
−3
6 8
water+[C mim][Cl] and+[C mim][Cl] mixtures at T=298.15 K
have also been reported by Gomez et al. [12]. The V
3
.0×10
and 9.0×10
g·cm
respectively.
E
m
values for
the same mixtures and at the same temperature but calculated
for the mole fractions of the present study, using the fitting coeffi-
cients reported by the authors are also shown in part a of Fig. 1. It
can be seen that our values match excellently with the literature
reported data.
3
. Results and discussion
The experimental data of ρ at T=(298.15 and 308.15) K for the
seven binary mixtures are listed in Table 2. Excess molar volumes,
V
E
m
are calculated using the equation:
−
E
Br based RTILs: The nature of the V
m
curves is similar to that in
−
water+Cl based RTIL mixtures i.e. sigmoidal with a negative
lobe covering most of the mole fractions of water followed by
slight positive values in the water rich region. The mole fraction
corresponding to the crossover from negative to positive region
E
m
⁎
⁎
V
¼ V–x V
þ x V
ð1Þ
1
1:m
2
2:m
was however higher in [C
8
mim][Br]+water mixtures.
Table 1
−
I
based RTILs: The binary mixtures of water+1-butyl-, or
Densities, ρ for pure water and RTILs at T (298.15 and 308.15) K along with the litera-
ture values.
1-hexyl-, or 1-octyl-3-methylimidazolium iodides are character-
ized by exclusive positive values across the composition. The
values increased with the increase in the carbon chain length of
alkyl branch from butyl to octyl chains.
ρ/(gm.cm−3)
T=298.15 K
Exp.
T=308.15 K
Exp.
Lit.
Lit.
[
[
[
[
[
[
[
C
C
C
C
C
C
C
4
6
6
6
8
8
8
mim][I]
1.484210
1.044202
1.229168
1.340202
1.008810
1.173207
1.233638
0.997045
1.480731
1.038401
1.222396
1.332801
1.003049
1.166677
1.225420
0.994029
E
The magnitude of equimolar (V
m
)x1=0.5 at T=298.15 K, for a
mim][Cl]
mim][Br]
mim][I]
mim][Cl]
mim][Br]
mim][I]
1.039670 [12]
1.033960 [12]
given alkylmethylimidazolium cation but with different halide anions
followed the order:
1.008882 [12]
0.997043 [11]
1.003060 [12]
0.994029 [11]
6
[C mim][Cl]b[C
6 6 8 8
mim][Br]b[C mim][I] and [C mim][Br]b[C mim]
Water
8
[Cl]b[C mim][I],

N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
15
Table 2
Experimental densities, ρ for water (1)+RTILs (2) at T=(298.15 and 308.15) K.
ρ/(gm.cm−3)
Water (1)+[C
4
mim][I] (2)
Water (1)+[C
6
mim][Cl] (2)
298.15
Water (1)+[C
6
mim][Br] (2)
x
1
x
1
x
1
T in K
298.15
308.15
308.15
298.15
308.15
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0191
.0497
.0615
.1004
.1512
.2014
.2503
.3009
.3488
.3990
.4988
.5506
.5992
.6596
.7007
.7507
.8034
.8793
.9041
.9133
.9323
.9518
.9684
.9811
1.482978
1.481008
1.480236
1.477632
1.474021
1.470124
1.465916
1.461032
1.455831
1.449677
1.434728
1.425163
1.414678
1.398909
1.385806
1.366108
1.338642
1.276920
1.246647
1.233396
1.201613
1.160835
1.117125
1.075942
1.479342
1.477180
1.476362
1.473683
1.470107
1.466319
1.462244
1.457484
1.452352
1.446202
1.431058
1.421276
1.410555
1.394482
1.381191
1.361353
1.333861
1.272480
1.242425
1.229308
1.197701
1.157183
1.113690
1.072701
0.0300
0.0362
0.0609
0.1146
0.1842
0.3345
0.3545
0.4119
0.4617
0.4847
0.5568
0.5999
0.6597
0.7034
0.8031
0.9035
0.9098
0.9299
0.9512
0.9810
1.044459
1.044499
1.044648
1.044848
1.044976
1.045186
1.045218
1.045330
1.045394
1.045393
1.045236
1.044920
1.044036
1.042930
1.037997
1.026547
1.025445
1.021405
1.016126
1.006059
1.038356
1.038351
1.038327
1.038325
1.038435
1.038998
1.039089
1.039313
1.039434
1.039446
1.039220
1.038817
1.037749
1.036497
1.031313
1.020317
1.019305
1.015609
1.010841
1.001921
0.0447
0.0630
0.0631
0.1473
0.1892
0.2770
0.3453
0.3982
0.4844
0.4979
0.5595
0.6011
0.6629
0.7065
0.7993
0.8970
0.9109
0.9299
0.9367
0.9774
1.228661
1.228448
1.228446
1.227241
1.226420
1.224007
1.221287
1.218594
1.212867
1.211808
1.206294
1.201856
1.193922
1.187046
1.166674
1.126504
1.117412
1.102586
1.096388
1.044647
1.221820
1.221586
1.221583
1.220308
1.219481
1.217076
1.214400
1.211754
1.206143
1.205096
1.199696
1.195339
1.187554
1.180805
1.160804
1.121346
1.112456
1.097859
1.091791
1.040860
Water (1)+[C
6
mim][I] (2)
Water (1)+[C
8
mim][Cl] (2)
Water (1)+[C
8
mim][Br] (2)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0211
.0488
.0667
.0888
.1888
.2501
.3026
.3488
.3976
.4439
.4989
.5898
.6438
.6898
.8079
.9004
.9111
.9305
.9504
.9808
1.339252
1.337846
1.336852
1.335557
1.328933
1.324517
1.320630
1.317124
1.313306
1.309511
1.304616
1.294832
1.287275
1.279150
1.245132
1.187748
1.177317
1.154887
1.125577
1.060505
1.331384
1.329526
1.328331
1.326852
1.320219
1.316124
1.312542
1.309263
1.305595
1.301839
1.296837
1.286606
1.278683
1.270296
1.236262
1.180393
1.170282
1.148514
1.119948
1.056363
0.0278
0.0441
0.0769
0.0975
0.1389
0.2395
0.3045
0.3388
0.3812
0.4527
0.5098
0.5508
0.6015
0.6510
0.7212
0.7833
0.8989
0.9100
0.9322
0.9502
1.008917
1.008987
1.009147
1.009260
1.009516
1.010295
1.010883
1.011205
1.011598
1.012201
1.012567
1.012728
1.012761
1.012564
1.011774
1.010423
1.005598
1.004923
1.003426
1.002043
0.999326
1.003118
1.003166
1.003295
1.003395
1.003626
1.004399
1.004995
1.005323
1.005724
1.006324
1.006681
1.006812
1.006811
1.006550
1.005677
1.004249
0.999688
0.999105
0.997900
0.996869
0.995138
0.0250
0.0507
0.0692
0.1059
0.1986
0.2226
0.2975
0.3262
0.3886
0.4441
0.4840
0.6138
0.6499
0.7004
0.7928
0.8975
0.9115
0.9297
0.9494
0.9797
1.173259
1.173257
1.173224
1.173100
1.172466
1.172229
1.171304
1.170852
1.169619
1.168152
1.166785
1.159571
1.156456
1.150865
1.135183
1.100287
1.092844
1.081424
1.066006
1.032185
1.166702
1.166672
1.166621
1.166457
1.165702
1.165429
1.164360
1.163846
1.162474
1.160872
1.159417
1.151992
1.148854
1.143312
1.127973
1.094162
1.086984
1.075904
1.060995
1.028195
0
.9799
Water (1)+[C
8
mim][I] (2)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0190
.0510
.0904
.1558
.1930
.2071
.3000
.3570
.3858
.4450
.5290
.6197
.6530
.7373
.8540
.9047
.9390
.9500
.9724
.9790
.9810
1.232937
1.231702
1.230099
1.227281
1.225606
1.224960
1.220544
1.217654
1.216129
1.212787
1.207339
1.199793
1.196331
1.184790
1.155282
1.130561
1.103808
1.092227
1.060996
1.049060
1.045111
1.224690
1.223404
1.221735
1.218816
1.217086
1.216422
1.211919
1.209013
1.207491
1.204177
1.198822
1.191451
1.188076
1.176793
1.147960
1.123840
1.097796
1.086520
1.056211
1.044588
1.040760

16
N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
would result in positive VE
values [14–17]. Therefore, the observed
Table 3
m
Coefficients, a
i
(in cm3 mol−1) of Eq. (3) and root mean square deviation, σ (in cm3
for the mathematical representation of excess molar volumes for water
E
negative V
m
values for chloride or bromide based RTIL+water mix-
−
1
mol
)
tures are attributable to the predominance of structure making inter-
actions over RTIL–RTIL repulsive interactions in the bulk state. On
similar lines, it is not unreasonable to state that structure making in-
(1)+RTIL (2) mixtures at T=(298.15 and 308.15) K.
Water +
T/K
a
0
a
1
a
2
a
3
σ
−
[C
[C
[C
[C
[C
[C
[C
4
6
6
6
8
8
8
mim][I]
298.15
1.397
1.466
0.265
0.718
−1.052
−0.934
2.438
−0.984
−0.423
0.372
−1.695
−2.702
1.971
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
teractions in water+I based RTILs are probably responsible for the
3
08.15
298.15
08.15
298.15
08.15
298.15
08.15
298.15
08.15
298.15
08.15
298.15
08.15
observed positive excess molar volumes in these mixtures. Moreover
mim][Cl]
mim][Br]
mim][I]
−2.112
−1.972
−0.582
−0.484
3.493
−
I
ions being least hydrating and would occupy most of the outer
3
1.717
1.144
shell around the cation and therefore suppress the RTIL–water struc-
ture making interactions considerably.
−0.306
−0.201
−0.890
1.077
1.924
2.362
2.067
1.877
0.147
0.215
−1.780
−1.792
3.503
0.979
0.943
0.997
2.144
1.896
1.442
3
2.320
−3.200
−2.609
−0.233
−0.099
0.840
1.122
−1.334
−1.589
3
3.816
3.1. Partial molar volumes and transfer functions
mim][Cl]
mim][Br]
mim][I]
−2.226
−2.088
−2.413
−1.937
4.151
3
The partial molar volume for a given component in a mixture (Vi)
is generally defined by
3
3
4.547
1.693
V ¼ ð∂V=∂n Þ
ð4Þ
i
i
T;P;ni
⁎
The following equations can be obtained by differentiation of
Eq. (1) followed by its combination with Eq. (4):
and similarly for a fixed halide anion but with different alkylim-
idazolium cations followed:
ꢀ
ꢁ
E
m
⁎
E
V1 ¼ V þ V1:m −x2 ∂Vm =∂x2
ð5Þ
[C
4
mim][I]b[C
6
mim][I]b[C
mim][Cl].
8
mim][I];
8 6
[C mim][Br]b[C mim][Br]
P;T
and [C mim][Cl]b[C
8
6
and
Unlike non-electrolyte+water mixtures, the ionic liquid+water
systems contain ions and hence the interactions in such mixtures
are quite complex. The strong Columbic interactions between the
cation and anion on the one hand and the significant cation–cation
aggregation on the other hand lead to a kind of supra-molecular orga-
nization among IL molecules [14–17]. RTIL+water binary mixtures
are expected to be characterized mainly by two types of interactions
namely structure making and/or structure breaking. The net interac-
ꢀ
ꢁ
E
m
⁎
E
V2 ¼ V þ V2:m þ ð1−x2Þ ∂Vm =∂x2
ð6Þ
P;T
where V1 and V2 are the partial molar volumes of water and RTIL
respectively.
Differentiation of Redlich–Kister equation (Eq. (3)) with respect to
x
2
and substitution of the result into Eqs. (5) and (6) give the follow-
ing relations for the partial molar volumes of water V 1 and RTIL, V 2
18]:
tions get reflected in the sign and the magnitude of the excess
E
molar volumes. Negative V
m
values in general indicate that the mo-
[
lecular state in the bulk state is characterized by the presence of com-
pact structures. Specific interactions between the RTIL and water
would favor the formation of such compact structures. Free volume
effect, interstitial accommodation of one of the components into the
structure of the other and condensation of negative ions on the posi-
⁎
2
i
2
2
i–1
V1 ¼ V1:m þ x Σa ð1−2x Þ þ 2x ð1−x ÞΣia ð1−2x Þ
ð7Þ
ð8Þ
2
i
2
2
i¼0
i
2
i¼0
tive cationic surface would also contribute to the negative VE
values.
The predominance of structure breaking effects on the other hand
⁎
2
i
2
i–1
m
V2 ¼ V2:m þ x Σa ð1−2x Þ −2x ð1−x Þ Σia ð1−2x Þ
1
i
2
2
2
i
2
i¼0
i¼0
1
1
0
0
.5
.0
.5
.0
1.5
1.0
a
b
0.5
0.0
-
-
0.5
-0.5
1.0
0
-1.0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Xwater
Fig. 1. Excess molar volume, VE
(♦) [C mim][I], + (□) [C mim][Cl], + (○) [C
Redlich–Kister equation (Eq. (3)), —————— are the literature data from ref [12].
for water (1)+RTIL (2) binary mixtures at (a) T=298.15 K and (b) T=308.15 K: water+(■) [C
8
mim][Br] and+(Δ) [C mim][I]. The symbols are experimental points and the solid lines correspond to the correlations by the
mim][I], + (●) [C
mim][Cl], + (▲) [C
mim][Br],
m
4
6
6
+
6
8
8

N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
17
5
4
3
2
1
0
1
2
5
a
b
4
vii
3
vii
vi
2
1
vi
v
v
iii
0
iii
i
iv
ii
ii
iv
-
-
-1
-2
i
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Xwater
E
Fig. 2. Partial excess molar volume of water, V 1 plotted against the mole fraction of water in water+RTIL binary mixtures at (a) T=298.15 K and (b) T=308.15 K: Water+(i)
[C
6
mim][Cl], + (ii) [C
8
mim][Cl], + (iii) [C
6
mim][Br], + (iv) [C
8 4 6 8
mim][Br], + (v) [C mim][I], + (vi) [C mim][I], + (vii) ([C mim][I]. Lines are the guide to the eye.
E
1
The partial molar excess volumes of the ith component, V can
then be defined by
on the given RTIL. For example, water+[C
[Cl] mixtures were mostly characterized by negative values. The
curve for water+[C mim] [Br] was sigmoidal with initial negative
values followed by a positive lobe. While the same for water+[C8-
mim] [Br] was characterized by initial positive values which de-
creased and become negative with the increase in the water mole
6 8
mim] [Cl] or +[C mim]
6
E
⁎
V i ¼ V −V
ð9Þ
i
i:m
By inserting x
2
1
=1 (i.e. x =0) in Eq. (7), one gets
fraction. The curves for water+[C
characterized by positive values (only exception being the curve for
water+[C mim] [I] mixture, which displayed an initial negative re-
n
mim] [I] mixtures in general are
∞
⁎
i
V 1 ¼ V1:m þ Σa ð−1Þ
ð10Þ
i
i¼0
4
gion). The increase in temperature from (298.15 to 308.15) K in gen-
eral resulted in the increase of values in the respective region. These
described trends indicate that the liquid mixtures of water+halide
based alkylimidazolium RTILs are characterized by a complex mix of
interactions. To specify which type of interactions occurs at the mo-
2 1
and similarly, putting x =0 (i.e. x =1) in Eq. (8), one obtains:
∞
⁎
V 2 ¼ V2:m þΣai
ð11Þ
i¼0
∞
∞
where V1 and V 2 are the partial molar volume of water at infinite di-
lution in RTIL and the partial molar volume of RTIL at infinite dilution
in water. Eqs. (10) & (11) were used to calculate the partial molar
volumes of the respective components at infinite dilution across the
composition.
lecular level is out of scope of the present investigation. Otherwise,
E
1
the negative
V
values in general indicate the predominance of
E
water–water structure making interactions and while positive V 1
values as recorded for water+[C mim] [I] (where n=4, or 6, or 8)
n
mixtures suggest that the disruption of water–water interactions
(i.e. the hydrogen bonding net work within the water molecules
gets weakened in presence of the RTIL). The polarizability of the ha-
E
E
The profiles depicting the variation of V 1 and V as a function of
2
mole fractions of water or RTIL are shown in Figs. 2 and 3. It can be
E
−
-
−
seen from Fig. 2 that the V
1
values at high water mole fractions
lide ions follows the order: I >Br >Cl and in fact the detailed
Raman and IR spectral studies indeed confirmed that the hydrogen
bonding among the tetrahedrally coordinated water structures gets
(
≥0.8) are close to zero while the values in the rest of the mole
fraction range displayed positive or negative curvatures depending
5
5
a
b
4
4
vii
vii
vi
3
2
3
2
vi
1
0
v
1
0
v
i
ii
ii
-
1
2
3
4
-1
-2
-3
-4
iii
iv
iii
iv
i
-
-
-
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
XIL
E
Fig. 3. Partial excess molar volumes of IL, V 1 plotted against the mole fraction of ionic liquids in water+IL binary mixtures at (a) T=298.15 K and (b) T=308.15 K: Water+(i)
mim][Cl], + (ii) [C mim][Cl], + (iii) [C mim][Br], + (iv) [C mim][Br], + (v) [C mim][I], + (vi) [C mim][I], + (vii) ([C mim][I]. Lines are guide to the eye.
[C
6
8
6
8
4
6
8

18
N.V. Sastry et al. / Journal of Molecular Liquids 180 (2013) 12–18
Table 4
Partial molar volumes at infinite dilution, V 1 and V 2 and standard transfer volumes,V 1 tr andV 2 tr in water (1)+RTIL (2) mixtures at T=(298.15 and 308.15) K.
∞
∞
∞
∞
∞
(cm3 mol−1
1∞
tr (cm3 mol−1
T/K
V
1
)
V
)
Water+[C
17.248
18.078
Water+[C
18.477
19.295
6
mim][Cl]
Water+[C
17.839
17.966
Water+[C
20.707
21.081
6
mim][Br]
mim][Br]
Water+[C
20.975
21.386
6
mim][I]
mim][I]
[C
0.591
6
mim][Cl]→[C
6
mim][Br]
[C
3.136
3.420
[C
1.768
1.908
6
mim][Br]→[C
6
mim][I]
mim][I]
[C
3.727
3.308
[C mim][Cl]→[C
8
3.998
3.694
6
mim][Cl]→[C
6
mim][I]
mim][I]
2
3
98.15
08.15
−0.112
8
mim][Cl]
8
Water+[C
22.475
8
[C
8
mim][Cl]→[C
8
mim][Br]
8
mim][Br]→[C
6
8
2
3
98.15
08.15
2.230
1.786
22.989
∞
(cm3 mol−1
2∞
tr (cm3 mol−1
V
2
)
V
)
Water+[C
198.902
197.556
Water+[C
230.243
230.900
6
mim][Cl]
Water+[C
197.765
198.782
Water+[C
236.246
236.743
6
mim][Br]
mim][Br]
Water+[C
223.604
219.481
6
mim][I]
mim][I]
[C
−1.137
1.226
6
mim][Cl]→[C
6
mim][Br]
[C
25.839
20.699
[C
23.435
24.732
6
mim][Br]→[C
6
mim][I]
mim][I]
[C
24.702
21.925
[C mim][Cl]→[C
8
29.438
30.575
6
mim][Cl]→[C
6
mim][I]
mim][I]
2
3
98.15
08.15
8
mim][Cl]
8
Water+[C
259.681
8
[C
8
mim][Cl]→[C
8
mim][Br]
8
mim][Br]→[C
6
8
2
3
98.15
08.15
6.003
5.843
261.475
systematically weakened in the presence of large and highly polariz-
structure breaking and making interactions. The analysis of excess
molar volumes, partial molar volumes and related functions of these
able halide anions for aqueous sodium halide solutions [19].
E
−
−
The V vs XIL profiles (Fig. 3) have also presented interesting
mixtures revealed that in water+Cl or Br based RTIL mixtures,
structure making interactions are more prevalent and while
2
trends. Except at very high RTIL mole fractions, the profiles displayed
E
2
−
significant and notable changes in the sign and magnitude of V
water+I based RTIL mixtures are characterized predominantly by
E
values. The V vs XIL curves for water+(C
n
mim][I] (where n=4, or
structure breaking interactions. The relative polarizability of halide
anions plays a key role on the nature of interactions in such mixtures.
2
6
, or 8)) mixtures were characterized by positive values which de-
creased smoothly with the increase in the mole fraction of RTIL in
water+[C mim][I], or +[C mim][I] mixtures. The large and positive
values indicate that water addition significantly weakens the
4
8
E
2
V
Acknowledgment
RTIL cation–cation interactions that are otherwise predominantly
present in the supra molecular structures of pure RTILs [13–16]. The
The authors thank UGC-DAE Consortium for the Scientific Re-
search, Mumbai Center for funding a Collaborative Research Scheme
(CRS) under grant no. CSR/AO/MUM/CRS-M-127/08/448. Thanks are
also due to DAAD, Bonn, for the award of equipment grant for density
meter under the supra-regional program of the German Technische
Zussamen Arbeit, Bonn.
E
V
values for water+chloride or bromide based RTILs are negative
2
and become less negative with the increase in the IL mole fraction.
E
The negative V values may arise due to the contributions from either
2
RTIL–RTIL interactions and/or minimum structural disruptions in the
supra-molecular state of RTILs.
A comparison of the water or RTIL component behavior in
water+RTIL mixtures can be made by defining the standard transfer
∞
∞
volumes (partial molar volumes at infinite dilution, V 1 tr andV2 tr) and
the same is defined by the following relation:
References
[
[
1] M.J. Earle, K.R. Seddon, Pure and Applied Chemistry 72 (2000) 1391–1398.
2] P. Wasserscheid, R. Hal, A. Boesmann, Green Chemistry 234 (2002) 400–404.
m→n
¼ Y s∝ ðnÞ−Y
∝ðmÞ
Ys
ð12Þ
[3] C. Lagrost, D. Carrie, M. Vaultier, R. Hapoit, The Journal of Physical Chemistry. A
07 (2003) 745–752.
[4] T. Welton, Chemical Reviews 99 (1999) 2071–2083.
s
1
∞
(n)
s∞ (m)
where Y
s
and Y
are the partial molar volumes of water or RTIL
in water+RTIL mixtures, n and m correspond to the ionic liquids as
shown to the right and left hand part of the arrow marks shown in
the minor headings of column nos. 5–7 of Table 4. It can be seen
from the data given in columns 5–7 of Table 4 that V tr values for a
given RTIL are positive when Cl ion is replaced by Br and become
more positive upon the replacement of Cl by I as counter anion.
Similarly, successive replacement of Br with I also resulted into
positive transfer volumes. These trends clearly indicate that the larger
the counter anion and the higher its polarization, the stronger would
be its interaction with cation and such interactions induce water–
[
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∞
1
−
−
−
−
(
2005) 1029–1035.
[10] H. Shekaari, Y. Mansoori, R. Sadeghi, The Journal of Chemical Thermodynamics 40
2008) 852–859.
−
−
(
[11] R. Sadeghi, H. Shekaari, R. Hosseini, The Journal of Chemical Thermodynamics 41
(2009) 273–289.
[
[
[
[
[
[
12] E. Gomez, B. Gonzalez, A. Dominguez, E. Tojo, J. Tojo, Journal of Chemical and En-
gineering Data 51 (2006) 696–701.
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−
water structure breaking interactions. The replacement of Cl with
Br or I resulted into positive V 2∞ tr values for a given RTIL (an excep-
−
−
tion was found for [C
6
mim][Cl]→[C
6
mim][Br] at 298.15 K). Interest-
ingly, the replacement of Br with I ion also resulted in large and
positive transfer values. As mentioned earlier, I ions are closely
bound to the RTIL cation and can thus affectively neutralize the posi-
tive charge and therefore weaken the supra-molecular organization
−
−
−
of pure RTILs and contribute positively to V 2∞ tr
.
[18] Y. Maham, T.T. Teng, L.G. Hepler, A.E. Mather, Thermochimica Acta 386 (2002)
11–118.
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1
[
4
. Conclusions
(
The binary mixtures of water+(C
and X=Cl or Br and I are characterized by complex mix of
n
mim][X], where n=4, or 6, or
−
−
−
8