Influence of tetra ethyl ammonium bromide (C2H5)4NBr on the aggregation behavior of surface active ionic liquid 1-tetradecyl-3-methylimidazolium bromide [C14mim][Br]

Article abstract of DOI:10.1016/j.molliq.2020.113431

In this work, we tend to explore the effect of organic electrolyte i.e. tetra ethyl ammonium bromide (C₂H₅)₄NBr on the aggregation behavior of 1-tetradecyl-3-methylimidazolium bromide [C₁₄mim][Br] by utilizing electrical conductivity measurement, refractive index measurement, UV–Visible spectroscopy as well as FT-IR spectroscopic study. First of all, the synthesis of [C₁₄mim][Br] is carried out in the laboratory, then the critical micelle concentration (CMC) of [C₁₄mim][Br] is determined using conductivity measurements at temperatures 298.15 K, 308.15 K and 318.15 K in the absence and presence of (C₂H₅)₄NBr at different concentrations 1 mM, 2 mM and 5 mM. The reduction in value of CMC is observed with respect to increasing concentration of (C₂H₅)₄NBr and with the increase in temperature, the value of CMC also increases. These results are observed due to the establishment of reinforced hydrophobic interactions between the hydrophobic parts of surface active ionic liquid and the electrolytes. Then we reflected different thermodynamic parameters such as, standard free energy of micellization (ΔG_m⁰), standard enthalpy of micellization (ΔH_m⁰), and standard entropy of micellization (ΔS_m⁰) in the temperature range 298.15 K–318.15 K, utilizing the value of CMC obtained from conductivity measurements. A graphical contrast of these thermodynamic parameters has been made. Further refractive index measurement and UV–visible spectroscopic study have been employed to verify the CMC obtained using conductivity measurement, which is found to be very close to that obtained via conductivity study. The alterations produced in the respective system have been confirmed via FT-IR spectroscopic study, which signify the variations produced by electrolyte on the aggregation of surface active ionic liquid in aqueous media.

Full text of DOI:10.1016/j.molliq.2020.113431

Journal of Molecular Liquids 313 (2020) 113431
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Influence of tetra ethyl ammonium bromide (C2H5)4NBr on the
aggregation behavior of surface active ionic liquid
1-tetradecyl-3-methylimidazolium bromide [C14mim][Br]
⁎
Harsh Kumar , Gagandeep Kaur, Priya
Department of Chemistry, Dr B R Ambedkar National Institute of Technology, Jalandhar 144 011, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2020
Received in revised form 19 May 2020
Accepted 22 May 2020
Available online 25 May 2020
In this work, we tend to explore the effect of organic electrolyte i.e. tetra ethyl ammonium bromide (C2H5)4NBr
on the aggregation behavior of 1-tetradecyl-3-methylimidazolium bromide [C14mim][Br] by utilizing electrical
conductivity measurement, refractive index measurement, UV–Visible spectroscopy as well as FT-IR spectro-
scopic study. First of all, the synthesis of [C14mim][Br] is carried out in the laboratory, then the critical micelle con-
centration (CMC) of [C14mim][Br] is determined using conductivity measurements at temperatures 298.15 K,
308.15 K and 318.15 K in the absence and presence of (C2H5)4NBr at different concentrations 1 mM, 2 mM and
5 mM. The reduction in value of CMC is observed with respect to increasing concentration of (C2H5)4NBr and
with the increase in temperature, the value of CMC also increases. These results are observed due to the establish-
ment of reinforced hydrophobic interactions between the hydrophobic parts of surface active ionic liquid and the
electrolytes. Then we reflected different thermodynamic parameters such as, standard free energy of micelliza-
tion (ΔG0m), standard enthalpy of micellization (ΔH0m), and standard entropy of micellization (ΔS0m) in the temper-
ature range 298.15 K–318.15 K, utilizing the value of CMC obtained from conductivity measurements. A graphical
contrast of these thermodynamic parameters has been made. Further refractive index measurement and UV–
visible spectroscopic study have been employed to verify the CMC obtained using conductivity measurement,
which is found to be very close to that obtained via conductivity study. The alterations produced in the respective
system have been confirmed via FT-IR spectroscopic study, which signify the variations produced by electrolyte
on the aggregation of surface active ionic liquid in aqueous media.
Keywords:
1-Tetradecyl-3-methylimidazolium bromide
[C14mim][Br]
Tetra ethyl ammonium bromide (C2H5)4NBr
Critical micelle concentration
Refractive index
UV–visible
FT-IR
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
liquids helps us to know how these components participate in various
applications because ILs based on the imidazolium ring are also biolog-
Ionic liquids are a class of molten electrolytes having specific proper-
ties such as excellent dissolution capacity, no vapor pressures, high ion
conductivity, outstanding catalytic properties, no flammability etc.
which make them potential candidates in many fields including prepa-
ration of novel nanomaterials, organic synthesis, homogeneous cataly-
sis, and electrochemistry [1–4]. In addition, the ionic liquids
containing a long hydrophobic alkyl chain are termed as surface active
ionic liquids (SAILs) that possess surface active properties similar to
conventional surfactants. Due to their structure, inherent charge and
surface active nature, these SAILs can self-assemble in aqueous solution
to form different types of aggregates such as micelles, vesicles and
lyotropic liquid crystals etc. [5–11]. The length of alkyl chain, type of
head group as well as nature of counterions effect the self-aggregation
of surface active ionic liquids [12–14]. Understanding the influence of
specific salt on the micellization behavior of imidazolium based ionic
ically active due to the imidazolium ring [15]. The organic or inorganic
salts interact with amphiphiles by salting in and salting out mechanisms
depending upon the conditions [16]. CMC as well as other parameters
get modified due to the various interactions occurring between SAIL
and the electrolyte [15,17–20].Generally CMC of surfactants get reduced
upon addition of inorganic electrolytes [21–23].
Rafati et al. investigated the effect of electrolytes on the micellization
behavior of ILs where the surface tension and CMC decreases upon ad-
dition of salts in the system [24]. Wang et al. reported the modulation
in the aggregation behavior of 1-decyl-3-methylimidazolium bromide
([C10mim]Br) by addition of sodium electrolytes followed the
Hofmeister series which can be explained by considering the hydropho-
bicity of anions [25]. Dong and his co-workers studied the interfacial
and bulk behavior of SAILs such as 1-decyl-3-methylimidazolium bro-
mide, [C10mim] [Br], 1-dodecyl-3-methylimidazolium bromide,
[C12mim] [Br], and 1-dodecyl-3-methylimidazolium tetrafluoroborate,
[C12mim][BF4] with or without sodium halide at room temperature
(298 K) by employing tensiometry and conductometry. A significant
⁎
Corresponding author.
E-mail address: manchandah@nitj.ac.in (H. Kumar).
https://doi.org/10.1016/j.molliq.2020.113431
0167-7322/© 2018 Elsevier B.V. All rights reserved.

2
H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
decrease in value of CMC and πcmc is observed upon addition of salts
[26]. Jiao et al. investigated the effect of three inorganic electrolytes
(LiCl, NaCl, and MgCl2) and four organic electrolytes, viz.
tetraalkylammonium bromides ((CH3)4NBr, (C2H5)4NBr, (C3H7)4NBr,
and (C4H9)4NBr) on the aggregation behavior of the anionic halogen-
free surface active ionic liquid, 1-butyl-3-methylimidazolium
dodecylsulfate ([C4mim][C12SO4]), in aqueous solution and found that
for a given electrolyte (i.e., NaCl), critical micelle concentration de-
creases with increasing electrolyte concentration [27].
reaction. Once the reaction completed, acetonitrile which was taken as
solvent was evaporated by employing rota-evaporator. Then the prod-
uct was washed with hexane in separating funnel to remove unreacted
substrate. The resulting product was recrystallized to yield a white solid.
At last, product was dried in vacuum for 8 h before use. The structure of
obtained SAIL was confirmed by employing 1H NMR and FT-IR spectro-
scopic studies.
2.3. Methods
However there are very few reports investigating the effect of elec-
trolytes on the aggregation of SAILs. The present investigation has
been undertaken with an aim to get clear and detailed information on
the effect of temperature and concentration of organic electrolyte
(C2H5)4NBr on the aggregation behavior of [C14mim][Br]. With this pur-
pose, the CMC values as well as other thermodynamic parameters of mi-
cellization have been evaluated by employing conductivity
measurement. The refractive index and UV–visible spectroscopy have
been employed to confirm the obtained CMC values. FT-IR spectroscopy
has been utilized to understand the various interactions prevailing in
the system constituting SAIL and organic electrolyte.
Sartorius CPA 225 D electronic balance was used to accurately weigh
the compounds with a precision of 0.00001 g to prepare the stock so-
lutions. For sample preparation, doubly distilled deionized water ob-
tained from Millipore, Milli-Q Academic water purification system had
been used which was having a conductanceb5 μS cm−1. Three times
measurements were made for each experiment, average value of
which was considered as final measurement.
2.3.1. Electrical conductivity measurement
A digital conductivity meter Systronics 306 having a cell with cell
constant unity had been used to measure the electrical conductivity of
samples. Before the measurements, the instrument was calibrated
using aqueous KCl solutions having the concentration in the range
0.01–1.0 mol kg−1. To obtain the value of conductivity at constant tem-
peratures i.e. 298.15 K, 308.15 K and 318.15 K, the solution was taken in
the double jacketed vessel, whose temperature was maintained with
accuracy of 0.1 K using refrigerated circulated water thermostat pro-
vided by Macro Scientific Works Pvt. Ltd. Delhi. The solution was prop-
erly stirred after each addition of SAIL to the cell, only then the value of
conductivity is noted. From the plot of conductivity versus concentra-
tion of SAIL, the critical micelle concentration of [C14mim][Br] in aque-
ous solution of 0 mM, 1 mM, 2 mM and 5 mM (C2H5)4NBr at different
temperatures is obtained at the breakpoint of two linear regimes as
shown in Fig. 2.
2. Experimental section
2.1. Materials
1-Methylimidazole purchased from HIMEDIA Laboratories Pvt. Ltd.
was used to synthesize the surface active ionic liquid 1-tetradecyl-3-
methylimidazolium bromide [C14mim][Br]. 1-Bromotetradecane and
hexane used for the synthesis were purchased from TCI Pvt. Ltd. aceto-
nitrile and tetraethyl ammonium bromide used in this study was pro-
cured from LOBA Chemie Pvt. Ltd. Mumbai India and Spectrochem Pvt.
Ltd. Mumbai India respectively. Tetraethyl ammonium bromide was
dried under vacuum for 48 h by keeping in vacuum desiccators before
use. Table 1 contains the list of all the chemicals used in experimental
work along with their source, CAS number, purification method and
mass fraction purity.
2.3.2. Refractive index measurements
A compact and convenient Abbemat 300 refractometer by Anton
Paar was used to determine the refractive index of the samples,
which was having a resolution of 0.00001 nD. Very low volume of
sample is required for the measurement by using this instrument.
The measurement can be made in temperature range of 4 °C to
85 °C with temperature probe accuracy of 0.05 °C. Before measure-
ment the instrument was always checked by measuring the refrac-
tive index of water. The instrument allows us to take the readings
2.2. Synthesis of ionic liquid
In 500 ml round bottom flask, 1-methylimidazole was taken in ace-
tonitrile. Then 1-bromotetradecane was added in above reaction mix-
ture in the ratio 1:1 dropwise and with continuous stirring. After the
complete addition, the reaction mixture was refluxed for 24 h at 80 °C.
Thin layer chromatography was used to know about the completion of
Table 1
Specification of chemicals.
Chemicals
Sources
CAS no.
Purification
Mass
Structure
method
fraction
purity#
1-Methylimadazole
HiMedia Laboratories Pvt. Ltd.,
India
616-47-7
No further
purification
N0.99
1-Bromotetradecane
Hexane
TCI Chemicals (India) Pvt. Ltd.
TCI Chemicals (India) Pvt. Ltd.
LOBA Chemie Pvt. Ltd., India
Spectrochem Pvt. Ltd., India
112-71-0
110-54-3
75-05-8
71-91-0
No further
purification
No further
purification
No further
purification
Vacuum drying
N0.97
N0.96
N0.995
N0.98
Acetonitrile
Tetraethylammonium bromide
1-Tetradecyl-3-methylimidazolium bromide
[C14mim][Br]
Synthesized in the laboratory
471907-87-6 Vacuum drying
N0.99##
#
As declared by supplier.
As per NMR analysis.
##

H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
3
within seconds. The measurement was taken at least 3 times, the
3. Result and discussions
mean value of which was considered as the final reading. After
each measurement the measuring prism was cleaned.
3.1. Characterization of synthesized ionic liquid
2.3.3. UV–visible spectroscopy
To confirm whether the desired ionic liquid [C14mim][Br] has been
synthesized or not, the characterization of synthesized ionic liquid is
done by utilizing 1H NMR and FT-IR spectroscopic studies. The results
obtained via both the techniques are shown in Fig. 1. 1H NMR (Bruker
400 MHz) spectrum of [C14mim][Br] in CDCl3 used as a solvent is as
shown: 0.77 (3H, Triplet), 1.22 (22H, Multiplet), 1.81 (2H, Pentet),
4.02 (3H, singlet), 4.22 (2H, Triplet), 7.38 (1H, Singlet), 7.57 (1H, Sin-
glet), 9.98 (1H, Singlet). The characteristic absorption bands in FT-IR
spectra of [C14mim][Br] is as shown: +NR4 stretching: 3482 cm−1 and
3436 cm−1, aromatic C\\H stretch: 3068 cm−1, aliphatic C\\H stretch:
2919 cm−1 and 2854 cm−1, C_N stretch: 1632 cm−1; C_C stretch:
The critical concentration was determined from a plot of the absor-
bance at a fixed wavelength versus the sample concentration [28]. The
spectra was recorded after each addition of [C14mim][Br] to the cuvette.
The absorbance of [C14mim][Br] was measured in the absence and pres-
ence of organic electrolyte (C2H5)4NBr at different concentrations i.e.
1 mM, 2 mM and 5 mM at λmax = 264 nm. All the measurements
were made at room temperature 298.15 K by employing Shimadzu UV
3600 UV–Vis spectrophotometer with a quartz cuvette having path
length of 1 cm. The concentration corresponding to the point of inter-
section of two line segments in the plot of absorbance versus concentra-
tion of [C14mim][Br] corresponds to the point of CMC as shown in Fig. 6.
1577 cm−1, CH2 bending: 1472 cm−1, and C\\N stretch: 1176 cm−1
.
The data obtained for the synthesized ionic liquid is quite consistent
with the literature data [7,29], which shows that the desired ionic liquid
has been synthesized.
2.3.4. FT-IR spectroscopic study
FT-IR spectroscopic studies were employed to analyze various struc-
tural alterations taking place in the SAIL/electrolyte system. For this var-
ious FT-IR spectra were recorded using Agilent technologies Carry 630
FT-IR Spectrophotometer in the wave number region from 4000 cm−1
to 400 cm−1 for different samples of [C14mim][Br] in aqueous solution
of 0 mM, 1 mM, 2 mM and 5 mM (C2H5)4NBr with concentration of
[C14mim][Br] corresponding to concentration below CMC, at CMC and
above CMC to analyze the variations occurring in the system near the
point of critical micelle concentration.
3.2. Determination of critical micelle concentration
3.2.1. Electrical conductivity measurement
The self-aggregation of SAIL [C14mim][Br] has been studied by taking
the electrical conductivity measurements of [C14mim][Br] in the ab-
sence and presence of (C2H5)4NBr in aqueous medium at different tem-
peratures i.e. 298.15 K, 308.15 K and 318.15 K. Table 2 constitutes the
Fig. 1. 1H NMR spectra and FT-IR spectra of [C14mim][Br].

4
H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
Table 2
Conductivity values of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at different temperatures 298.15 K, 308.15 K and 318.15 K in the aqueous
media.
[C14mim][Br]/mM
κ (mS/cm)
0 mM (C2H5)4N+Br−
1 mM (C2H5)4N+Br−
2 mM (C2H5)4N+Br−
5 mM (C2H5)4N+Br−
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
0.000
0.263
0.519
0.769
1.013
1.250
1.481
1.707
1.928
2.143
2.353
2.558
2.759
2.955
3.146
3.333
3.516
3.696
3.871
4.043
4.211
4.375
4.536
4.694
4.848
0.0046
0.0249
0.0459
0.0650
0.0834
0.1010
0.1186
0.1357
0.1521
0.1670
0.1826
0.1921
0.1992
0.2042
0.2087
0.2134
0.2174
0.2214
0.2251
0.2290
0.2326
0.2361
0.2397
0.2429
0.2464
0.0115
0.0401
0.0671
0.0930
0.1104
0.1345
0.1543
0.1725
0.1991
0.2181
0.2375
0.2518
0.2652
0.2732
0.2815
0.2879
0.2938
0.3001
0.3054
0.3109
0.3163
0.3210
0.3256
0.3289
0.3331
0.0157
0.0409
0.0708
0.1010
0.1270
0.1541
0.1816
0.2062
0.2317
0.2528
0.2802
0.3026
0.3220
0.3348
0.3455
0.3563
0.3651
0.3719
0.3770
0.3830
0.3893
0.3959
0.4077
0.4140
0.4185
0.1151
0.1334
0.1521
0.1697
0.1845
0.2007
0.2156
0.2319
0.2482
0.2609
0.2652
0.2711
0.2744
0.2779
0.2830
0.2862
0.2890
0.2933
0.2958
0.2988
0.3023
0.3040
0.3066
0.3081
0.3125
0.1416
0.1646
0.1873
0.2095
0.2312
0.2506
0.2682
0.2861
0.3053
0.3241
0.3331
0.3405
0.3454
0.3502
0.3544
0.3591
0.3636
0.3676
0.3731
0.3751
0.3817
0.3886
0.3911
0.3951
0.3993
0.1767
0.1983
0.2235
0.2497
0.2714
0.2936
0.3175
0.3411
0.3625
0.3844
0.4036
0.4197
0.4317
0.4410
0.4486
0.4550
0.4608
0.4670
0.4727
0.4775
0.4822
0.4847
0.4920
0.4960
0.5018
0.1976
0.2129
0.2303
0.2492
0.2639
0.2813
0.2950
0.3066
0.3147
0.3183
0.3219
0.3264
0.3294
0.3327
0.3357
0.3381
0.3413
0.3439
0.3464
0.3494
0.3513
0.3535
0.3558
0.3580
0.3603
0.2401
0.2638
0.2828
0.3061
0.3253
0.3432
0.3636
0.3805
0.3994
0.4052
0.4091
0.4139
0.4190
0.4234
0.4275
0.4318
0.4352
0.4386
0.4418
0.4453
0.4529
0.4554
0.4578
0.4617
0.4641
0.2937
0.3183
0.3448
0.3654
0.3893
0.4159
0.4325
0.4514
0.4721
0.4922
0.5108
0.5182
0.5254
0.5308
0.5337
0.5406
0.5424
0.5483
0.5519
0.5554
0.5612
0.5685
0.5714
0.5728
0.5801
0.4172
0.4271
0.4436
0.4594
0.4719
0.4791
0.4901
0.4931
0.4954
0.4941
0.4970
0.4988
0.4996
0.5011
0.5028
0.5048
0.5072
0.5085
0.5108
0.5119
0.5162
0.5192
0.5217
0.5241
0.5262
0.5090
0.5297
0.5417
0.5601
0.5805
0.5936
0.6113
0.6178
0.6194
0.6212
0.6223
0.6234
0.6269
0.6298
0.6304
0.6332
0.6359
0.6372
0.6412
0.6459
0.6496
0.6542
0.6579
0.6598
0.6654
0.6128
0.6297
0.6484
0.6652
0.6814
0.7046
0.7234
0.7384
0.7485
0.7516
0.7530
0.7557
0.7583
0.7611
0.7632
0.7684
0.7733
0.7774
0.7814
0.7845
0.7862
0.7892
0.7916
0.7958
0.8015
Standard uncertainties s are s (T) = 0.1 K, s (κ) = 0.0001 mS/cm.
data obtained via electrical conductivity measurement in the presence
of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at different temper-
electrolyte concentration from 1 mM to 5 mM as observed from Fig. 4
(a). In general, in case of concentration of organic electrolyte (C2H5)
4NBr beyond 5 mM, it is expected that there will be more and more re-
duction of electrostatic repulsions between head groups, thereby lead-
ing to more decrease in value of CMC. In other way, it can also be
assumed that due to higher concentration reverse phenomenon will
happen and CMC may increase that has been observed and discussed
in some cases [37–41]. So in our opinion that makes it different aspect
of study at higher temperature and is beyond the scope of this paper
and will be studied separately.
atures. The specific conductivity increases with increase in concentra-
+
tion of free [C14mim]
and [Br]− ions in the solution. At the
concentration corresponding to the critical micelle concentration, a
break arises in the line. This happens because of the change in slope of
line near the CMC. At CMC, micelle formation takes place. The low mo-
bility of micelles as compared to free ions is responsible for the change
in slope of line [30]. The concentration corresponding to the point of in-
tersection of two line segments in the graph of specific conductivity ver-
sus concentration of [C14mim][Br] corresponds to the CMC value. The
value of CMC is firstly obtained in aqueous medium without the addi-
tion of organic electrolyte at all the studied temperatures as shown in
Fig. 2(a). The CMC values obtained by employing this method for
[C14mim][Br] in water has been compared with the previous literature
[31–35] as reported in Table 3. The values obtained are found to be
very close to each other. Then the same is repeated to find the CMC
value of [C14mim][Br] in presence of (C2H5)4NBr at different concentra-
tions as well as temperatures as shown in Fig. 2(b), (c), (d). The ob-
tained values of CMC in the presence of 0 mM, 1 mM, 2 mM and
5 mM of (C2H5)4N+Br at temperatures 298.15 K, 308.15 K and
318.15 K has been reported in Table 4.
It can be observed from Fig. 3(a) that the value of CMC decreases on
the addition of (C2H5)4NBr and increases with the increase in tempera-
ture. The decrease in CMC value upon addition of organic electrolyte can
be explained by considering the fact that the anion of electrolyte inter-
act with the cationic head group of SAIL leading to decrease in thickness
of electrical double layer at interface. This favors the micellization of
SAILs by reducing the electrostatic repulsions between the head groups
leading to decrease in CMC value upon addition of electrolyte [36]. On
the other hand, the increase in value of CMC with the increase in tem-
perature is due to disruption of water structure leading to delay in pro-
cess of micellization. The electrostatic repulsions between head groups
of SAIL monomers are reduced more with the addition of higher con-
centration of organic electrolyte (C2H5)4NBr in the system. Thus, more
reduction in CMC value has been achieved with the increase in
3.2.2. Refractive index measurement
Refractive index is very important property of any material, which
describes how fast light moves through a specific medium in relation
to how fast it moves through vacuum. The speed of light changes
when it pass through the interface of two different substances resulting
in change of direction of light, which is typically known as “refraction”.
The determination of CMC using refractive index measurement is based
on the principle that the surfactant monomers have lower refractive
index as compared to the micelle. The refractive index of solution in-
creases rapidly before CMC and slowly after CMC, leading to clear deter-
mination of CMC value at the breakpoint in the graph of refractive index
versus concentration of SAIL [42–44]. Table 5 shows the refractive index
values of different solutions of [C14mim][Br] in the absence and pres-
ence of (C2H5)4NBr at different temperatures in aqueous media. The
value of CMC is determined at the breakpoint obtained by plotting re-
fractive index versus concentration of [C14mim][Br] as shown in Fig. 4
and are reported in Table 6.
3.2.3. UV–visible spectroscopy
The interaction of [C14mim][Br] with organic electrolyte (C2H5)
4NBr has been studied using UV–visible spectroscopy. Absorption
measurements can be at a single wavelength or over an extended
spectral range. The critical micelle concentration was determined
from a plot of the absorbance at a fixed wavelength versus the sam-
ple concentration [28]. The determination of CMC of imidazolium

H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
5
Fig. 2. Specific conductance (κ) of [C14mim][Br] solutions in the presence of (a) 0 mM (b) 1 mM (c) 2 mM (d) 5 mM (C2H5)4N+Br− at different temperatures 298.15 K, 308.15 K and
318.15 K in the aqueous media.
Table 3
Comparison of experimentally obtained and literature values of CMC for [C14mim][Br] in aqueous solution by (a) conductivity, (b) refractive index, (c) UV–Visible spectroscopy, (d) surface
tension and (e) fluorescence (f) isothermal titration microcalorimetry at different temperatures.
Temperatures
CMC (mM) for [C14mim][Br]
Expt. CMC
Literature [31]
Literature [32]
Literature [33]
Literature [34]
Literature [35]
298.15 K
2.558a
2.516b
2.498c
2.756a
2.703b
2.975a
2.912b
2.50a
2.30d
–
2.53a
2.60a
2.80d
2.90e
–
2.69d
2.52f
2.53a
2.76d
2.39 d
308.15 K
318.15 K
–
–
–
2.74d
2.82d
–
–
based ionic liquid do not need the involvement of any probe for ab-
sorption of light because imidazolium ring present in these SAILs
can itself absorb in the entire UV-region [45–48]. The absorbance
values have been taken at 264 nm wavelength for various solutions
of [C14mim][Br] in the absence and presence of (C2H5)4NBr which is
presented in Table S1. All the measurements were made at room
temperature i.e. 298.15 K. The plot of absorbance versus concentra-
tion of [C14mim][Br] has been shown in Fig. 5. It is observed from
the Fig. 5(a) that the value of absorbance show increment upon
the addition of [C14mim][Br] in aqueous medium in the absence of
organic electrolyte. But the rate of increase in absorbance value
above CMC is less than that of the rate below CMC. This gives the
clear indication of point of CMC in the graph. [49]. On the other
hand, the absorbance value at λmax = 264 nm starts decreasing
upon the addition of organic electrolyte in aqueous media [50].
But the rate of decrease below CMC was different from the rate
above CMC, leading to recognize the CMC value as shown in Fig. 5
(b), (c), (d). The CMC value obtained using this method has been

6
H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
Table 4
3.3. Determination of degree of counterion dissociation (α)
Critical micelle concentration (CMC), degree of counterion dissociation (α), free energy of
micellization (ΔG0m), enthalpy of micellization (ΔHm0 ), and entropy of micellization (ΔSm0
)
The degree of counterion dissociation (α) is determined by eval-
uating the ratio between the slopes of post-micellar region to pre-
micellar region as obtained in the electrical conductivity measure-
ments. The obtained values of degree of counterion dissociation
(α) have been reported in Table 4. The variation of degree of coun-
terion dissociation (α) with the concentration of organic electrolyte
and temperature of system has been presented graphically in Fig. 3
(b). It can be observed from the Table that the value of degree of
counterion dissociation (α) decreases with the addition of organic
electrolyte, whereas it increases with the increase in temperature.
It means that the degree of counterion binding (β = 1 − α) in-
creases on the addition of electrolyte leading to formation of better
packed aggregates in the system. This leads to increase in charge
density of the stern layer of the micelles formed. Hence the number
of bromide ions bound to the micelles also increase [51]. But with
the increase in temperature of the system, the degree of counterion
dissociation (α) increases indicating high thermal agitation in the
system causes more dissociation of counterions at the surface of mi-
celle [52–55].
of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)
4N+Br− at different temperatures 298.15 K, 308.15 K and 318.15 K in the aqueous media.
(C2H5)
CMC
(mM)
α
ΔG0m
ΔH0m
ΔS0m
4N+Br−/mM
(kJ·mol−1
)
(kJ·mol−1
)
(J·mol−1·K−1
)
T = 298.15 K
0 mM
1 mM
2 mM
5 mM
2.6
2.2
1.8
1.4
0.300
0.269
0.236
0.1969
−42.08
−43.58
−45.12
−47.29
−9.49
−12.24
−15.07
−18.02
109.32
105.13
100.79
98.15
T = 308.15 K
0 mM
1 mM
2 mM
5 mM
2.8
2.2
1.9
1.6
0.340
0.308
0.269
0.218
−42.16
−43.87
−45.51
−47.66
−9.90
−12.78
−15.79
−19.03
104.71
100.88
96.42
92.92
T = 318.15 K
0 mM
1 mM
2 mM
5 mM
2.9
2.6
2.3
1.9
0.377
0.330
0.286
0.244
−42.23
−44.02
−45.74
−47.88
−10.32
−13.44
−16.67
−19.98
100.32
96.10
91.36
87.68
Standard uncertainties s are s (T) = 0.1 K, s (CMC) = 0.001 mM, s (ΔG0m) =
0.03 kJ·mol−1, s (ΔHm0 ) = 0.02 kJ·mol−1, s (ΔSm0 ) = 0.02 J·mol−1·K−1
.
3.4. Determination of thermodynamic parameters of micellization
compared with that obtained using electrical conductivity and re-
fractive index measurements, which are found to be very close to
each other as reported in Table 6.
The values of CMC obtained from electrical conductivity mea-
surements have been considered to obtain various thermodynamic
parameters of micellization, which are important to understand
Fig. 3. Plot of (a) critical micelle concentration (b) degree of counter ion dissociation α of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at
different temperatures 298.15 K, 308.15 K and 318.15 K in the aqueous media from conductivity measurements.

H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
7
Fig. 4. Plot of refractive index (n) of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at temperatures (a) 298.15 K (b) 308.15 K and (c) 318.15 K.
Table 5
Refractive index values of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at different temperatures 298.15 K, 308.15 K and 318.15 K in the
aqueous media.
[C14mim][Br]/mM
Refractive index (n)
0 mM (C2H5)4N+Br−
1 mM (C2H5)4N+Br−
2 mM (C2H5)4N+Br−
5 mM (C2H5)4N+Br−
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
0
1.33250
1.33252
1.33254
1.33256
1.33258
1.33260
1.33262
1.33264
1.33266
1.33268
1.33270
1.33271
1.33272
1.33273
1.33274
1.33275
1.33276
1.33278
1.33127
1.33129
1.33132
1.33133
1.33135
1.33137
1.33139
1.33140
1.33142
1.33144
1.33146
1.33148
1.33149
1.33150
1.33152
1.33153
1.33154
1.33155
1.32978
1.3298
1.33252
1.33256
1.33257
1.33260
1.33262
1.33265
1.33268
1.33269
1.33271
1.33272
1.33273
1.33274
1.33275
1.33276
1.33277
1.33278
1.33279
1.33281
1.33129
1.33131
1.33133
1.33135
1.33137
1.33140
1.33143
1.33144
1.33146
1.33148
1.33149
1.33150
1.33151
1.33152
1.33153
1.33154
1.33155
1.33156
1.32979
1.32981
1.32983
1.32985
1.32987
1.32990
1.32993
1.32994
1.32996
1.32998
1.33000
1.33001
1.33002
1.33003
1.33004
1.33005
1.33006
1.33007
1.33256
1.33258
1.33259
1.33262
1.33264
1.33267
1.33270
1.33272
1.33273
1.33274
1.33275
1.33276
1.33277
1.33278
1.33279
1.3328
1.33131
1.33133
1.33134
1.33137
1.33139
1.33142
1.33145
1.33147
1.33149
1.33150
1.33151
1.33152
1.33153
1.33154
1.33155
1.33156
1.33157
1.33158
1.32981
1.32983
1.32984
1.32987
1.32989
1.32992
1.32995
1.32997
1.32999
1.33001
1.33002
1.33003
1.33004
1.33005
1.33006
1.33007
1.33008
1.33009
1.33265
1.33268
1.33270
1.33272
1.33274
1.33276
1.33277
1.33278
1.33279
1.33280
1.33281
1.33282
1.33284
1.33285
1.33286
1.33287
1.33288
1.33289
1.33140
1.33143
1.33145
1.33147
1.33149
1.33152
1.33154
1.33155
1.33156
1.33157
1.33158
1.33159
1.33160
1.33161
1.33162
1.33163
1.33164
1.33165
1.32989
1.32992
1.32994
1.32996
1.32998
1.33000
1.33002
1.33004
1.33005
1.33006
1.33007
1.33008
1.33009
1.33010
1.33011
1.33012
1.33013
1.33014
0.26316
0.52632
0.78947
1.05263
1.31579
1.57895
1.84211
2.10526
2.36842
2.63158
2.89474
3.15789
3.42105
3.68421
3.94737
4.21053
4.47368
1.32982
1.32983
1.32985
1.32987
1.32989
1.32990
1.32992
1.32994
1.32996
1.32998
1.32999
1.33000
1.33001
1.33003
1.33005
1.33006
1.33281
1.33282
Standard uncertainties s are s (T) = 0.05 K, s (n) = 0.00001.

8
H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
Table 6
CMC values of [C14mim][Br] solutions in the presence of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at different temperatures in the aqueous media obtained using conductivity mea-
surement, refractive index measurement and UV–visible spectroscopy.
(C2H5)4N+Br/mM
CMC (mM)
Conductivity measurement
Refractive index measurement
UV–visible spectroscopy
298.15 K
308.15 K
318.15 K
298.15 K
308.15 K
318.15 K
298.15 K
0 mM
1 mM
2 mM
5 mM
2.558
2.158
1.834
1.416
2.756
2.245
1.939
1.628
2.975
2.613
2.311
1.856
2.516
2.116
1.821
1.363
2.703
2.244
1.911
1.536
2.912
2.591
2.281
1.845
2.498
2.136
1.791
1.411
Standard uncertainties s are s (T) = 0.1 K, s (CMC) = 0.001 mM.
Fig. 5. Variation of absorbance of [C14mim][Br] solutions in the presence of (a) 0 mM (b) 1 mM (c) 2 mM (d) 5 mM (C2H5)4N+Br− at λmax = 264 nm in the aqueous media.
the driving force of micellization [56]. These parameters are the
standard Gibbs' free energy change of micellization, ΔG0m, the stan-
dard enthalpy change of micellization, ΔH0m and the standard en-
tropy change of micellization, ΔS0m, which are reported in Table 4.
The standard Gibbs' free energy change of micellization, ΔG0m can
be evaluated by using Eq. (1).
The standard enthalpy change of micellization, ΔH0m can be evalu-
ated by using Eq. (2).
ΔH0m ¼ −RT2ð2−αÞ½dð lnXCMC Þ=dTꢀ
ð2Þ
where all the symbols have usual meaning described before. From
the Table 4, it can be observed that the standard Gibbs' free energy
change of micellization, ΔGm0 as well as the standard enthalpy
change of micellization, ΔH0m are having negative values for all the
studied systems indicating the process of micellization is spontane-
ous and exothermic in nature. Both these parameters become more
ΔG0m ¼ ð2−αÞRTð lnXCMC
Þ
ð1Þ
where R is gas constant, T is temperature and α is degree of counter ion
dissociation, XCMC is the value of CMC in the mole fraction unit.

H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
9
Fig. 6. Plot of (a) standard free energy of micellization ΔG0m (b) standard enthalpy of micellization ΔHm0 (c) standard entropy of micellization ΔS0m of [C14mim][Br] solutions in the presence
of 0 mM, 1 mM, 2 mM and 5 mM of (C2H5)4N+Br− at different temperatures 298.15 K, 308.15 K and 318.15 K in the aqueous media.
negative on the addition of organic electrolyte and with the in-
crease in temperature. The variation in these parameters toward
the more negative value is more significant in case of addition of or-
ganic electrolyte to the system as compared to that with the in-
crease in temperature which indicates that the addition of
electrolyte makes the micellization process more spontaneous and
exothermic. This is because the organic electrolyte reduces the re-
pulsions between the head groups of [C14mim][Br]. The repulsion
between the head groups is one of the major factors opposing
micellization.
process shifts from entropy to enthalpy driven. Similar behavior is
observed for conventional surfactants [57,58]. The variation of
these parameters with the change in concentration of (C2H5)4NBr
as well as with the change in temperature has been presented in
Fig. 6.
3.5. FT-IR spectroscopy
FT-IR spectral study of solutions in the presence of 0 mM, 1 mM,
2 mM and 5 mM (C2H5)4NBr at concentrations of [C14mim][Br]
below CMC, at CMC and above CMC have been carried out and pre-
sented in Figs. S1, S2, S3 and S4 shown in supporting information.
The peak observed in the wavenumber range 3000–3600 cm−1 is
due to N\\H stretching of imidazolium ring. The variation in the
peak for different concentrations of [C14mim][Br] in absence and
presence of organic electrolyte in aqueous media has been shown
in Fig. 7 and the wavenumbers are reported in Table 7. The spectro-
scopic study confirms the structural alterations prevailing in the
system constituting SAIL [C14mim][Br] and organic electrolyte
(C2H5)4NBr. The observed variations in the wavenumber indicate
that various molecular scale interactions are occurring between
SAIL and organic electrolyte [59].
The standard entropy change of micellization, ΔS0m can be evaluated
by using the values of the standard Gibbs' free energy change of micell-
ization, ΔG0m and the standard enthalpy change of micellization, ΔH0m by
using Eq. (3).
ꢀ
ꢁ
ΔS0m
¼
ΔH0m−ΔG0m =T
ð3Þ
The entropy change of micellization, ΔS0m is found to be large
positive value at all concentrations of (C2H5)4NBr and at all the
studied temperatures. The value of entropy change of micellization,
ΔS0m decreases with the addition of organic electrolyte and with the
increase in temperature which indicates that the micellization

10
H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
Fig. 7. FT-IR spectra of [C14mim][Br] solutions in the presence of (a) 0 mM (b) 1 mM (c) 2 mM (d) 5 mM (C2H5)4N+Br− in the wavenumber range 3000–3600 cm−1 at different
concentrations of [C14mim][Br].
Table 7
such as electrical conductivity measurement, refractive index mea-
surement, UV–visible spectroscopy and FT-IR spectroscopy. The
Wavenumber obtained from FT-IR spectra recorded in the presence of 0 mM, 1 mM, 2 mM
and 5 mM of (C2H5)4N+Br− at different concentrations of [C14mim][Br] in the aqueous
modulation in the aggregation behavior of SAIL [C14mim][Br] has
media.
been observed on addition of organic electrolyte and with the
[C14mim][Br] + concentrations of
[C14mim]
Wavenumber/cm−1
change in temperature as depicted in Fig. 8. The results obtained
by employing various techniques are quite comparable to one an-
other. The decrease in value of CMC is observed by using (C2H5)
4NBr as an additive indicating the addition of electrolyte favors
the process of micellization as observed for various conventional
ionic surfactants [60–62] and cationic SAILs [63,64]. The tempera-
ture dependence of CMC obtained by using electrical conductivity
measurement have been employed to calculate various thermody-
namic parameters of micellization and the trends obtained are ex-
plained in terms of change in concentration of organic electrolyte
as well as temperature of the system. The standard Gibbs' free en-
ergy of micellization, ΔG0m and standard enthalpy of micellization
ΔH0m have large negative value indicating the process of micelliza-
tion is feasible and exothermic in nature. The values of standard en-
tropy of micellization ΔS0m reveals that the aggregation of [C14mim]
[Br] shifts from entropy driven to enthalpy driven with high contri-
bution of organic electrolyte in the solution, indicating synergism in
micellization process. Thus our study is conductive to adjust the
CMC value of SAIL [C14mim][Br] aqueous solutions to the specific
value by addition of electrolyte as well as by adjusting temperature
for potential application of SAIL [C14mim][Br] in variety of fields.
(C2H5)
[Br]/(mM)
4N+Br−
[C14mim][Br] + 0 mM of (C2H5)
4N+Br−
1.262
2.524
3.786
1.068
2.137
3.205
0.908
1.815
2.723
0.682
1.363
2.045
3309
3275
3315
3274
3264
3256
3272
3274
3280
3275
3276
3270
[C14mim][Br] + 1 mM of (C2H5)
4N+Br−
[C14mim][Br] + 2 mM of (C2H5)
4N+Br−
[C14mim][Br] + 5 mM of (C2H5)
4N+Br−
4. Conclusion
In the current study, the effect of adding various concentrations
of organic electrolyte (C2H5)4NBr on the micellization behavior of
SAIL [C14mim][Br] have been analyzed by using various techniques

H. Kumar et al. / Journal of Molecular Liquids 313 (2020) 113431
11
Fig. 8. Scheme showing the modulation in the aggregation behavior of [C14mim][Br] with the addition of electrolyte as well as with the change in temperature.
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Declaration of competing interest
Authors declare no conflict of interest.
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Authors (H.K.) would like to thank Science and Engineering Re-
search Board (SERB), New Delhi for providing financial assistance to
carry out research work vide sanction order number EMR/2015/
002059. One of the authors, Gagandeep Kaur is thankful to Department
of Science and Technology (DST), New Delhi for providing DST INSPIRE
Fellowship (IF170753) with application reference number [DST/IN-
SPIRE/03/2017/001654]. The authors are also thankful to DST, New
Delhi for DST-FIST [CSI-228/2011] Program and The Director and Head,
Department of Chemistry for providing necessary facilities.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.113431.
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