Synthesis, Characterization, Surface Properties and Micellization Behaviour of Imidazolium-based Ionic Liquids

Article abstract of DOI:10.1007/s11743-017-2021-1

Abstract: Ionic liquids (IL) have emerged as a prospective new material for a wide range of industrial applications, owing to their unique structures and properties. Hence, understanding the structure–property relationships of IL is very important for both fundamental and industrial applications. In this study, a series of imidazolium-based IL with different chain lengths, namely C₈mimBF₄, C₁₀mimBF₄ and C₁₂mimBF₄, were synthesized. Their molecular structures were confirmed by proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared (FTIR) analysis. Thermal gravimetric analysis (TGA) revealed that the IL synthesized were thermally stable in the desired temperature range. Dynamic light scattering (DLS) results of IL was analyzed in order to understand the effect of alkyl length on micellar size. Conductivity and surface tension measurements were carried out to determine the adsorption and aggregation characteristics in aqueous solutions. The influences of temperature as well as alkyl chain length on viscosity were also investigated. Surface adsorption parameters such as surface excess, minimum area, effectiveness and efficiency were also determined from equilibrium surface tension data. A careful analysis of the thermodynamic aspects of air-ionic liquid aqueous systems reveals that free energies of adsorption are favoured over micellization for all systems.

Full text of DOI:10.1007/s11743-017-2021-1

J Surfact Deterg
DOI 10.1007/s11743-017-2021-1
ORIGINAL ARTICLE
Synthesis, Characterization, Surface Properties and Micellization
Behaviour of Imidazolium-based Ionic Liquids
Prathibha Pillai1 Nilanjan Pal1 Ajay Mandal1
•
•
Received: 3 June 2017 / Accepted: 16 September 2017
Ó AOCS 2017
Abstract Ionic liquids (IL) have emerged as a prospective
new material for a wide range of industrial applications,
owing to their unique structures and properties. Hence,
understanding the structure–property relationships of IL is
very important for both fundamental and industrial appli-
cations. In this study, a series of imidazolium-based IL
with different chain lengths, namely C8mimBF4, C10-
mimBF4 and C12mimBF4, were synthesized. Their molec-
ular structures were confirmed by proton nuclear magnetic
resonance (1H NMR) and Fourier transform infrared
(FTIR) analysis. Thermal gravimetric analysis (TGA)
revealed that the IL synthesized were thermally stable in
the desired temperature range. Dynamic light scattering
(DLS) results of IL was analyzed in order to understand the
effect of alkyl length on micellar size. Conductivity and
surface tension measurements were carried out to deter-
mine the adsorption and aggregation characteristics in
aqueous solutions. The influences of temperature as well as
alkyl chain length on viscosity were also investigated.
Surface adsorption parameters such as surface excess,
minimum area, effectiveness and efficiency were also
determined from equilibrium surface tension data. A
careful analysis of the thermodynamic aspects of air-ionic
liquid aqueous systems reveals that free energies of
adsorption are favoured over micellization for all systems.
Introduction
Ionic liquids (IL) encompass a broad range of research
areas, having numerous applications owing to their supe-
riority as task-specific fluids. Lately, they have attracted
considerable interest as ‘‘green and designer’’ media for
chemical reactions and have emerged as potential
replacement for organic solvents in laboratory and indus-
trial applications for catalytic processes [1]. IL have unique
physical and chemical properties [2] with no measurable
vapour pressure, making them ideal substitutions for con-
ventional and volatile organic solvents [3]. As a result, IL
have been employed in bio-catalytic transformations [4, 5],
chemical synthesis [6], electrochemistry [7, 8] and sepa-
ration and analytical science [9, 10]. They provide con-
siderable variations in physical properties by undergoing
simple structural changes in anions or cations and, hence,
offer ample opportunities for the design and formulation of
customized IL solvent systems [11]. The applicability and
functionality of IL are attributed not only to the nature of
anions and cations in IL, but also to the interaction of IL
with other components and their microstructure in specified
systems.
Investigations on the aggregation behaviour of IL play
an important role in understanding their participation as
components in complex systems, whereas the analysis of
physical and thermodynamic behaviour of IL in aqueous
media aids in comprehending their applicability in different
industries. Lately, IL composed of 1-alkyl-3-methylimi-
dazolium cation (Cnmim ?) have been widely contem-
plated in the fields of colloid and interface sciences owing
to the ability of imidazolium-based aqueous solutions to
form aggregates with increase in the proximity among
alkyl chains attached to imidazolium group in the centre
[12]. Various studies have indicated the presence of micelle
Keywords Ionic liquids Á Synthesis Á Viscosity Á
Micellization Á Adsorption
& Ajay Mandal
mandal_ajay@hotmail.com
1
Department of Petroleum Engineering, Indian Institute of
Technology (Indian School of Mines), Dhanbad 826004,
India
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J Surfact Deterg
formation in aqueous solutions of IL. The thermodynamic
properties and micellization behaviour of cationic 1-decyl-
3-methylimidazolium bromide in solution and their
dependence on temperature have been studied by Naderi
and Sadeghi [13]. Kumar and Singh [14] studied the effects
of the aromatic ring, counter-ions and alkyl chain length on
the critical micelle concentration (CMC) of different IL,
namely [C8mim][Cl], [C4mim][BF4], [C4mpy][Cl] and
[C4mim][Cl] in aqueous solutions. Investigations on the
behaviour of 1-decyl-3-methylimidazolium bromide by
Gaillon and Sirieix-Plenet [15] revealed that IL have
varying aggregation morphologies in aqueous solution and
form lyotropic mesophases at low IL concentrations. The
effect of different halide ions on aggregation behaviour of
1-dodecyl-3-methylimidazolium have been investigated by
Ao and Kim [12]. Inoue et al. [16] explained the aggre-
gation behaviour of IL having long-chain imidazolium ions
with C12–C16 alkyl chains in water using conductivity
studies. Dong et al. [17] studied the surface adsorption and
aggregation behaviour of different IL, showing the superior
properties of BF4 anions in comparison to Br. Imida-
zolium-based IL containing BF4 have been found to display
better properties than that of halide [18, 19]. Though ample
research has been published on CnmimX (X = Br, Cl, I),
there has been limited work reported on long-chain IL
containing BF4 (CnmimBF4). It is, hence, important from
the practical and industrial point of view to investigate the
surface activity of these long-chained IL in aqueous solu-
tion, which exhibit properties superior to conventional
surfactants.
obtain the CMC. The influence of temperature as well as
chain length on viscosity is also investigated. Surface
adsorption parameters such as surface excess, minimum
area, effectiveness and efficiency are also determined from
equilibrium surface tension data. Thermodynamic param-
eters of different IL aqueous systems are further deter-
mined to understand the degree of spontaneity of the
micellization process in bulk aqueous phase and adsorption
at the air–liquid interface. In order to develop a compre-
hensive understanding of the physicochemical properties of
IL as a function of temperature and alkyl chain length, the
adsorption and micellization properties of these IL have
been elaborately analyzed in this study.
Experimental
Materials
1-Methylimidazole (99.5%), 1-bromooctane (98%), 1-bro-
modecane (99.5%), 1-bromododecane (99.5%), sodium
tetrafluoroborate (98.5%) and dichloromethane (99.5%)
were obtained from SRL Chemicals. Ethyl acetate and
acetone were purchased from TCI Chemicals. Distilled
water was used in all experimental studies.
Synthesis
The synthesis procedure of IL involves two steps. Initially
a halide intermediate is formed, followed by anion
exchange with tetrafluoroborate anion. The reaction
scheme for synthesis of IL is shown in Fig. 1. The reac-
tions between 1-methyimidazole and alkyl bromide [1-
bromooctane for (C8mimBF4), 1-bromodecane for (C10-
mimBF4) and 1-bromododecane for (C12mimBF4)] were
carried out at 343 K for 48 h under reflux conditions [21].
The reactants were used in a 1:1 molar ratio. The reaction
mixtures were washed with ethyl acetate to eliminate
unreacted materials and then heated to 343 K to remove the
remaining solvent. The resulting pale-yellow liquid was
distilled under low vacuum. The intermediate product was
then vacuum-dried at 353 K, followed by further reaction
with NaBF4 in the presence of acetone for 10 h at tem-
perature below 313 K (boiling point of acetone). Dichlor-
omethane was then added to the aqueous solution to
remove the residue. The liquid mass was subsequently
vacuum-dried to eliminate traces of dichloromethane to
obtain the final product [22, 23]. A photograph of the
synthesised IL is shown in Fig. 2. The two-step procedure
for synthesis of 1-methyl-3-alkylimidazolium tetrafluo-
roborate had a yield of approximately 78.65% for C8-
mimBF4, 75.54% for C10mimBF4 and 78.29% for
C12mimBF4. The chemical structures of the three
A comprehensive study is necessary to provide a com-
plete understanding of the surface interaction of IL mole-
cules as the self-organization and micelle formation ability
of IL in aqueous solution have significant effect on their
applications and the environment. IL have also been widely
explored as potential ionic surfactants in different areas and
applications such as extraction of organic compounds,
metal ions and radioactive isotopes, and as a template to
produce micro/mesoporous materials in microemulsions. In
petroleum industry, long-alkyl chain IL have been explored
as efficient chemicals for enhanced oil recovery (EOR) due
to their enhanced hydrophobic nature [20]. IL are able to
reduce the interfacial tension (IFT) to low values especially
under high-salinity and high-temperature conditions, thus
further aiding in oil recovery.
In this paper, the surface activity and micellization
characteristics of a series of imidazolium-based IL with
different chain lengths, namely C8mimBF4, C10mimBF4
and C12mimBF4 have been discussed. The synthesized IL
are characterized by Fourier transform infrared (FTIR),
proton nuclear magnetic resonance (1H NMR) and dynamic
light scattering (DLS) analyses. Aggregation and adsorp-
tion behaviours of IL in aqueous media are investigated to
123

J Surfact Deterg
Fig. 1 Reaction Scheme for synthesis of ionic liquid
with temperature. The TGA of synthesized IL was per-
formed in the temperature range between 298 and 573 K at
a heating rate of 5 Kelvin per minute (K/min) in an argon
environment [24].
Density Measurement
Density measurement of synthesized IL was carried out
using Anton Paar DMA 4500 M digital density meter. The
density determination was based on the measurement of the
oscillations of a vibrating U-shaped sample tube. This tube
was filled with the sample. The relationship between the
period of oscillation, s, and the density, q, of the mixture is
given by Eq. (1):
q ¼ A þ Bs2
ð1Þ
Fig. 2 Photographs of different ionic liquids synthesized in the
laboratory
The constants A and B represent instrument constants for
each individual oscillator and were determined by two-
point calibration measurements with samples of known
density. The densimeter incorporates an automatic correc-
tion for the viscosity of the sample. The apparatus is pre-
cise to within 1.0 9 10-5 g/cm3.
synthesised IL along with their molecular properties are
reported in Table 1.
Characterization of the Synthesised Ionic Liquids
A FTIR spectrophotometer (PerkinElmer, Spectrum Two)
was used to characterize the IL in the mid-IR region
ranging between 4000 and 400/cm. The wave length of the
peaks is characteristic of particular functional groups. The
purity of the synthesized IL was determined by 1H-NMR
analysis using a Bruker Avance 500 MHz NMR spec-
trometer. The thermal stability of the synthesized IL was
measured by thermal gravimetric analysis (TGA). A
thermo gravimeter analyzer (Netzsch-STA 449 Jupiter
bench model) was used to measure the weight loss of IL
Surface Tension Measurements
An Easy Dyne K20 tensiometer (KRUSS Germany) was
used for surface tension measurement of synthesized IL at
298, 308, 318 and 328 K. A platinum ring was slowly lifted
from the surface of IL solutions using the Du Nou¨y ring
method. The surface tension recorded was correlated to the
force required to raise the ring from the surface of the air–
liquid interface [25]. The platinum ring was washed with
123

J Surfact Deterg
Table 1 Chemical structure and properties of synthesised ionic liquids
Designation
Structure
Molecular weight (g/mol)
282.129
Density
(q/g/cm3)
1-Methyl-3-octylimidazolium tetrafluroborate (C8mimBF4)
1-Methyl-3-decylimidazolium tetrafluroborate (C10mimBF4)
1.09
310.18
338.38
1.05
1-Methyl-3-dodecylimidazolium tetrafluroborate (C12mimBF4)
0.99
acetone and flame-dried prior to each measurement. Mul-
tiple observations were taken for each sample and the range
of standard deviation did not exceed 0.1 mN/m. All
samples of IL were prepared in distilled water.
were prepared. For each system, at least three different
readings were taken. The uncertainty was estimated to
be 2% for measured data.
Conductivity Measurements
Dynamic Light Scattering
A Hanna HI-2003 conductivity meter was used for mea-
suring the conductivity values for different concentrations
of IL solutions at 298, 308, 318 and 328 K. A four-ring
potentiometric probe was dipped in IL solutions and the
conductivity values were recorded after allowing the
readings to stabilise. The temperature of IL systems under
analysis was controlled using a thermostatic bath. All IL
aqueous solutions were prepared in distilled water.
A Zetasizer (Nano-S90, Nanoseries Malvern) was used to
determine the particle size. The particle size distribution of
IL in aqueous solution was measured at 303 0.1 K. The
instrument was equipped with a He–Ne laser (633 nm,
max. 4 mV). The laser wavelength and the scattering angle
were kept at 633 and 90°nm, respectively. The Brownian
motion of particles or molecules in suspension causes laser
light to be scattered at different intensities. Analysis of
these intensity fluctuations yields the velocity of the
Brownian motion and, hence, the particle size using the
Stokes–Einstein relationship. Depending on the distribu-
tion of the particles in the sample solution, for spherical
particles, the hydrodynamic radius of the particle RH can
be calculated from its diffusion coefficient (Df) by the
Stokes–Einstein equation, Eq. 2:
Results and Discussion
Spectroscopic Characterization of Ionic Liquids
The different functional groups present in IL are charac-
terized by FTIR analysis. The FTIR spectra of synthesised
IL (C8mimBF4, C10mimBF4 and C12mimBF4) are shown in
Fig. 3. The strong bands at 3425–3626/cm confirm the
presence of primary and secondary amides due to the
stretching of N–H bonds. The significant peaks at
2926–3170/cm indicate –CH2 and –CH3 stretching vibra-
tions of alkyl groups at the N atoms of the imidazolium
ring. Characteristic peaks at 2852 and 2922/cm represent
stretching vibrations of C–H and C–CH3, respectively [27].
Although the absorption strengths are different for each IL,
the shifts in peak positions are very small. The band at
1570/cm corresponds to the ring stretching vibration of the
imidazole group [28]. For C12mimBF4, the peak at
1697/cm is primarily attributed to physically adsorbed
water, which is confirmed during TGA. The single peak in
Df ¼ kBT=6pgRH
ð2Þ
where kB is the Boltzmann constant (1.38 9 10-23 Nm/
K), T is the temperature (K) and g is the viscosity (Pa.s).
The particle size distribution of aqueous solution at the
CMC was analyzed at a 908 scattering angle with the aid of
21CFR part 11 compliant software [26].
Viscosity Measurements
Viscosity measurements were performed at atmospheric
pressure over the temperature range of 293–353 K with the
aid of a Bohlin Gemini 2 rheometer (Malvern Instruments
Limited, UK). Aqueous solutions of the IL at the CMC
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J Surfact Deterg
C
mimBF
4
(b, c, d). The spectral results are consistent with imida-
zolium groups at one end and terminal –CH3 group at the
other.
8
754
845
3425
1464
1465
629
1095
3170
2927
1570
(1) 1-Methyl-3-octylimidazolium tetra fluoroborate (C8-
mimBF4).
C
mimBF
10
4
1HNMR (500 MHz, CDCl3): d (ppm) 8.849 (1H,s),
7.45 (1H,s), 7.388 (1H,s), 4.183 (2H,t), 3.953 (3H,s),
1.868 (2H,t), 1.314 (10H,m), 0.862 (3H,t).
(2) 1-methyl-3-decyllimidazolium tetra fluoroborate
(C10mimBF4).
630
737
845
2930
2926
1570
1570
3430 3172
1077
C
mimBF
4
12
1HNMR (500 MHz, CDCl3): d (ppm) 8.843 (1H,s),
7.429 (1H,s), 7.356 (1H,s), 5.315 (2H,s), 4.176
(2H,t), 3.952 (3H,s), 1.865 (2H,t), 1.313 (12H,m),
0.871 (3H,t).
854
728
3626
3150
630
1060
1697
1455
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
(3) 1-Methyl-3-dodecyllimidazolium tetra fluoroborate
(C12mimBF4).
1HNMR (500 MHz, CDCl3): d (ppm) 8.884 (1H,s),
7.446 (1H,s), 7.371 (1H,s), 5.318 (2H,s), 4.182
(2H,t), 3.957 (3H,s), 1.866 (2H,t), 1.245 (16H,m),
0.874 (3H,t).
Fig. 3 FTIR spectrum of synthesised ionic liquids
Thermal Gravimetric Analysis (TGA)
Fig. 4 Chemical structure of 1-methyl-3-alkylimidazolium tetra
fluoroborate (numbered at different positions)
Thermal analyses show the thermal stability of IL with
increasing alkyl chain lengths. The thermal degradation
zone is identified in order to determine the reduction in
weight with increasing temperature due to thermal
decomposition. The TGA and differential thermogravi-
metric (DTG) curves of the IL are studied at a heating rate
of 5 K/min as shown in Fig. 6a, b. It is observed that both
C8mimBF4 and C12mimBF4 have two degradation stages,
whereas C10mimBF4 has three degradation stages. The first
degradation stage for C8mimBF4, C10mimBF4 and C12-
mimBF4 is observed with 7, 6 and 9% weight loss,
respectively. This is due to moisture vaporization and
degradation of light organic compounds. The second
degradation stage for C8mimBF4, C10mimBF4 and C12-
mimBF4 is mainly attributed to the decomposition of imi-
dazole rings with 78, 63 and 84% weight losses,
respectively [30]. A third degradation stage showing 20%
weight loss in C10mimBF4 must be due to a complex
degradation phenomenon, due to breaking of an imidazole
group from the hydrophobic alkyl chain by condensation of
cyclic rings in the sample. This phenomenon is character-
ized by the decomposition of imides to form nitriles and the
release of volatiles such as CO2 and H2O [31].
the region between 1455 and 1465/cm indicates asym-
metrical bending vibration of the methyl group (C–H
bond). The peak observed around 1095/cm is due to the
asymmetric structure of [BF4]- ion. This is indicative of
anion exchange of tetra-fluoroborate anion. Peaks in the
region between 840 and 850/cm are attributed to C–N
stretching vibration. It is, hence, concluded that the syn-
thesized IL is comprised of imidazolium-based ionic
compounds with a terminal methyl group at one end, and
tetrafluoroborate ions in their free state.
Synthesized IL are then characterised by using 1H NMR
spectrum in CDCl3. The general structure of 1-methyl-3-
alkylimidazolium tetra fluoroborate with position numbers
(a, b, c and so on) corresponding to different shifts is
shown in Fig. 4. Resonance signals corresponding to the
presence of functional groups at different positions are
shown in Fig. 5a–c. The presence of characteristic signal
(showing a triplet) at chemical shift (d) values (in ppm) of
0.862, 0.871 and 0.874 is due to the presence of the ter-
minal methyl (–CH3) protons at the ends (h). A strong
resonance signal at d near 1.264–1.245 ppm is obtained
due to the cumulative influence of (–CH2–)n groups
attached on the side to the terminal methyl groups (g). The
two new signals found at around d value 3.9 ppm (a) and d
value 4.1 ppm (e) are assigned to the methyl protons
attached to the nitrogen of imidazolium [29]. The signals
corresponding to the other methine protons of imidazolium
side groups are found to appear at d = 7.371–8.884 ppm
Surface Tension Measurements
IL exhibit low vapour pressure as a result of their ionic
nature. Hence, cohesive forces between IL molecules at the
interface are difficult to determine. Surface tensiometry
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Fig. 5 1H NMR spectra of
a C8mimBF4 , b C10mimBF4
and c C12mimBF4
(a)
N+
H
3C
N
CH3
10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
(b)
N+
H
3C
N
CH3
10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
(c)
N+
H
3C
N
CH3
9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
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100
95
90
85
80
75
Table 2 Properties of synthesised ionic liquids and conventional
surfactants
(a)
100
80
60
40
20
0
10%
Surfactant
CMC (mmol/L)
ccmc (mN/m)
Weight loss
SDS [33]
9.5
0.915
42.5
16.1
5.9
45
38
42
33
27
460 480 500 520 540 560 580 600
CTAB [34]
C8mimBF4
Temperature (K)
C
10mimBF4
12mimBF4
C
C12mimBF4
C10mimBF4
C8mimBF4
increasing IL concentration due to the adsorption of
molecules at the air–liquid interface. The reduction in
surface tension ceases when IL molecules begin to form
micelles, represented by an inflection point in surface
tension versus concentration plots [11]. Above the CMC,
the surface tension remains virtually constant with
increasing IL concentrations. This is due to the saturation
of the air–liquid interface with IL molecules. As it can be
seen from the graph, the CMC and surface tension at the
CMC (cCMC) decreases with increasing alkyl chain length
owing to the increasing hydrophobic character of IL [35].
This shows that denser packing and aggregation of IL
molecules at the interface is favoured and surface tension is
more effectively reduced with increase in the chain length
of the IL molecule. The CMC and surface tension values of
IL are compared with those of cetyltrimethyl ammonium
bromide (CTAB) and sodium dodecyl sulfate (SDS) in
Table 2. It is observed that CMC values of the synthesised
IL are higher in comparison to these conventional
surfactants.
300
400
500
600
700
800
900
Temperature (K)
(b)
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
C12mimBF4
C10mimBF4
C8mimBF4
300
400
500
600
700
800
900
Temperature (K)
Fig. 6 Thermal analysis of synthesized ionic liquids a TGA curve,
b DTG curve
The effect of temperature on surface tension values of
C8mimBF4 is examined in Fig. 8. The surface tension
values decrease with increasing temperature. Similar
behaviour is observed for C10mimBF4 and C12mimBF4.
For each IL, as the temperature increases, the CMC ini-
tially decreases and then increases, owing to the smaller
probability of hydrogen bond formation at higher temper-
atures [36]. The initial decrease in the CMC with temper-
ature is a consequence of the decreased hydrophilic nature
of the surfactant molecules. As the temperature increases,
the effect of hydrophobicity begins to exert an influence
and finally predominates as the CMC reaches a minimum
value. Surface tension values decreases with increase in
temperature due to the elevation of molecular thermal
activity and increased adhesive action of the IL molecules
with air, resulting in reduced surface tension values along
the curvature of the interface [37].
80
C
C
C
mimBF
4
8
mimBF
4
10
12
mimBF
70
60
50
40
30
20
4
10-4
10-3
10-2
10-1
100
Concentration (mol/l)
Fig. 7 Surface tension of synthesized ionic liquid at varying
concentrations
Dynamic Light Scattering Analysis of the Ionic
Liquid Solutions
provides a measure of the cohesive forces at the liquid–gas
boundary [32]. This data is crucial in identifying the CMC
and investigating the adsorption behaviour at the air–liquid
interface. The IL surface tension isotherms are shown in
Fig. 7. Surface tension values initially decrease with
DLS technique is utilized to determine the particle size
distribution of micelles in aqueous solution. The
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J Surfact Deterg
46
44
42
40
38
36
For C12mimBF4, two well-distinguished peaks in the size
ranges 171.0–477.7 and 1790–2780 nm are observed at the
CMC (0.59 9 10-2 mmol/L). This must be attributed to
the presence of large agglomerates [41, 42]. In C12mimBF4
aqueous solutions with greater hydrophobicity, the formed
micelles begin to aggregate into clusters, resulting in a
second peak of low intensity in the higher-size range [34].
The interesting behavior of C12mimBF4 is attributed to the
entanglement of surfactant micelles, which are less spher-
ical in comparison to other IL systems.
70
65
60
55
50
45
40
35
0.02
0.04
0.06
0.08
0.10
298 K
308 K
318 K
328 K
(
a
)
Viscosity of Aqueous Ionic Liquid Solution
10-4
10-3
10-2
Concentration (mol/l)
10-1
100
The rheological behavior of aqueous IL solutions at the
CMC is measured in the temperature range between 298
and 363 K as shown in Fig. 10a. With rise in temperature,
viscosity is found to decrease for solutions with different
IL concentrations. A possible explanation for this trend is
due to a subsequent decrease in intermolecular kinetic
energy and an apparent reduction in viscosity [43]. Ion–ion
interactions such as van der Waals interaction and
Fig. 8 Plots of surface tension vs. concentration of C8mimBF4 at
different temperatures
C12mimBF4
C10mimBF4
20
15
10
5
C8mimBF4
C
C
C
mimBF
4
8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
(a)
mimBF
10
12
4
4
mimBF
0
100
1000
Size(r.nm)
Fig. 9 Size distribution using DLS for the synthesised ionic liquids at
the CMC and 273 K
300
310
320
330
340
350
360
370
Temperature (K)
synthesized IL has a positively charged surface. At low
concentrations, IL molecules are unassociated monomers.
As the concentration of IL is initially increased, the self-
aggregation of IL monomers begins to occur, resulting in
the formation of micelles. It is generally found that the size
distribution in the bulk aqueous phase is primarily depen-
dent on the conformational alteration of the molecular
arrangement, irrespective of the quantitative nature of
volumetric profiles [38, 39]. The intensity-weighted size
distributions of C8mimBF4, C10mimBF4 and C12mimBF4
solutions at the CMC and 298 K are depicted in Fig. 9. The
aggregates exhibit either bi-modal or mono-modal func-
tions [40]. During analysis of C8mimBF4 and C10mimBF4
aqueous solutions, single peaks are observed with maxi-
mum intensities in the range 82–229 nm at the CMC
(4.25 9 10-2 mmol/L) and 128-308 nm at the CMC
(1.61 9 10-2 mmol/L), respectively. These peaks corre-
spond to the presence of large aggregates in aqueous phase.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
C
C
C
mimBF
8
4
(b)
mimBF
10
12
4
4
mimBF
300
310
320
330
340
350
360
370
Temperature (K)
Fig. 10 Plot of a apparent viscosity and b specific viscosity
dependence of C8mimBF4, C10mimBF4 and C12mimBF4 as a function
of temperature
123

J Surfact Deterg
bFig. 11 Changes in surface parameters a Cmax, b Amin, c pC20 and d
Pcmc of C8mimBF4, C10mimBF4 and C12mimBF4 as a function of
temperature
1.3x10-6
1.2x10-6
1.1x10-6
1.0x10-6
9.0x10-7
C
mimBF
4
8
(a)
C
mimBF
10
4
4
C
mimBF
12
hydrogen bonding will also influence the apparent viscos-
ity. The viscosity of aqueous IL solution is observed to
increase with increase in the size of the alkyl side chain
length of the cation due to stronger van der Waals inter-
actions [44]. Specific viscosity is also plotted against
temperature as shown in Fig. 10b. Initially, specific vis-
cosity decreases and then gradually increases with tem-
perature which must be attributed to the dependence of
micellar size on temperature [44].
298
308
318
328
Temperature (K)
C
C
mimBF
8
4
(b)
180
160
140
120
100
80
mimBF
10
4
4
C
mimBF
12
Adsorption Behavior At The Air–Liquid Interface
A comprehensive analysis of adsorption parameters at the
air–liquid interface is investigated from surface tension
data at different temperatures. The amount of IL adsorbed
at the interface per unit area can be calculated using the
Gibbs adsorption equation. As per the Gibbs’ law, IL
molecule adsorption at the air–liquid interface results in
reduced surface tension under equilibrium conditions.
Surface excess concentration, (Cmax), is an effective
parameter to calculate the degree of adsorption of IL in the
air–liquid interface [11], obtained by the Gibbs isotherm
Eq. (3):
60
40
20
0
298
308
318
328
Temperature (K)
(c)
328
ꢀ
ꢁ
1
oc
C8mimBF4
C10mimBF4
C12mimBF4
318
308
298
Cmax ¼ À
ð3Þ
RT o ln c
T
where Cmax corresponds to surface excess concentration of
IL measured in mol/m2,
is the gas constant
R
(R = 8.314 J/mol/K), T represents the absolute tempera-
ture measured in K, c represents interfacial or surface
tension measured in mN/m, c is the IL concentration
measured in mol/L and (oc=o ln c) corresponds to surface
tension isotherm slope below the CMC [12]. In addition to
the surface excess, the minimum area per molecule, surface
effectiveness and efficiency of the surfactant are important
parameters that give an idea about the adsorption ability of
the synthesized IL.
4.5
5.0
5.5
6.0
6.5
pC20
7.0
7.5
8.0
8.5
9.0
60
55
50
45
40
35
30
25
20
15
10
5
C8mimBF4
C10mimBF4
C12mimBF4
(d)
To determine surface activity of IL, it is important to
take into consideration the packing density of IL molecules
at the air–liquid interface. The orientation and degree of
packing of the adsorbed IL molecules can be deduced from
the area per molecule at the interface. From surface excess
concentration, minimum area, Amin, occupied per IL
2
0
˚
molecule (in A /molecule) at the air–liquid interface is
298
308
318
328
obtained using Eq. (4):
Temperature (K)
123

J Surfact Deterg
Table 3 Surface properties of
aqueous solutions of
C8mimBF4, C10mimBF4 and
C12mimBF4
Ionic liquid Temp. (K) CMC (mol/L) ccmc
Cmax
Amin
2
pC20 Pcmc
(mN/m)
cmc/C20
(mNÁm-1
)
(lmol/m2) (A )
˚
C8mimBF4 298
0.0425
0.0451
0.0461
0.0501
0.0161
0.0202
0.0219
0.0242
0.0059
0.0069
0.0080
0.0088
42.15
0.0908
1.0974
1.1871
1.2152
1.0428
1.1391
1.2106
1.2667
1.1703
1.2282
1.2727
1.2933
182.82 4.622 29.4
151.28 4.839 33.7
1.77
3.18
308
318
328
38.12
37.11
35.61
32.70
32.01
30.78
29.30
27.32
26.01
25.73
25.19
139.77 5.000 34.7
136.62 5.049 36.2
146.86 5.356 39.2
145.75 5.585 40.7
137.10 5.769 41.1
131.07 6.000 42.6
141.91 6.472 44.6
135.17 6.962 45.9
130.45 7.051 46.2
128.37 7.167 46.7
4.82
5.61
C
10mimBF4 298
3.66
308
7.78
318
13.12
24.24
17.53
64.03
87.64
132.35
328
C
12mimBF4 298
308
318
328
1020
forces between IL molecules and air at the interface and
higher pC20 efficiency values.
Amin
¼
ð4Þ
NACmax
The efficiency of the surfactant (Pcmc) is defined as a
minimum value of surface tension and, hence, is indicative
of the ability of aqueous system to lower the surface ten-
sion of solvent [11]. It can be calculated using Eq. (6):
where NA is Avogadro’s number and Umax corresponds to
the surface excess concentration measured in mol/m2. The
values of Umax and Amin obtained from Gibbs adsorption
isotherm reflect the molecular arrangement of IL molecules
at the air–liquid interface. Figure 11a, b shows an
increasing trend in Cmax and decreasing Amin values with
temperature. As the temperature increases, each molecule
occupies less area at the interface. This is the result of
enhanced forces of adhesion between IL molecules and air
that aids in increase of interfacial curvature [45]. Maxi-
mum surface excess increases with alkyl chain length in the
IL tail as longer alkyl chains have higher hydrophobicity
and, hence, leads to greater adsorption of more molecules
at the interface [46]. The greater the adsorption, the smaller
is the effective area (Amin) of the IL molecule at the surface
[47]. IL molecules with longer alkyl chain lengths form
more compact monolayers that occupy less area.
Pcmc ¼ c0 À ccmc
ð6Þ
where ccmc and co are the surface tensions at the air-
aqueous interface for solutions at CMC and pure solvent,
respectively. A greater value of Pcmc or smaller value of
ccmc indicates a better ability to reduce surface tension of a
solution. The Pcmc values plotted against temperature are
shown in Fig. 11d. It is evident that the ability to reduce
surface tension of aqueous solutions is significantly
enhanced with increasing temperature.
The relative effects of various microenvironments or
structural factors on adsorption and micellization can be
measured by determining its effect on the ratio, CMC/C20
[44]. Increase in CMC/C20 ratios indicate that micellization
is less spontaneous in comparison to adsorption and vice
versa. A larger CMC/C20 ratio signifies a greater tendency
to reduce the surface tension. Both chain length and tem-
perature increase the CMC/C20 ratio, showing that
adsorption is favored over micellization. Hence, analysis of
values of CMC/C20 ratio provides a useful insight into
adsorption and micellization processes. Table 3 shows an
integrated overview of different parameters illustrating the
physicochemical properties of IL systems with different
chain lengths at the air–liquid interface at different
temperatures.
The adsorption efficiency, pC20, and surface tension
reduction effectiveness, Pcmc , are additional parameters
that show the surface activity of the IL molecules in
solution [48]. The adsorption efficiency is calculated using
Eq. (5):
pC20 ¼ À log C20
ð5Þ
where pC20 is the negative log of the IL concentration
which reduces the surface tension of pure solvent by 20
mN/m. From Fig. 11c, it is found that the pC20 value
increases in the order of C8mim BF4 \ C10mimBF4 \
C12mimBF4. This result is due to the increased
hydrophobic nature of the IL. This indicates that C12-
mimBF4 is more surficially active compared to other IL.
Increasing temperature exhibits a synergistic effect on the
adsorption ability of IL systems, increasing the adhesive
Conductivity Measurements
Electrical conductivities of aqueous solutions of IL are
studied to evaluate their micellization behaviour. A plot of
123

J Surfact Deterg
4500
4000
3500
3000
2500
2000
1500
1000
500
Table 4 Comparison of CMC values from surface tension and con-
ductivity measurements
298 K
308 K
318 K
328 K
(a)
Synthesized IL
C8mimBF4
Conductivity
CMC (mol/L)
Surface tension
CMC (mol/L)
0.0425
0.0451
0.0461
0.0501
0.0161
0.0202
0.0219
0.0242
0.0059
0.0069
0.0080
0.0088
0.0424
0.0443
0.0451
0.0460
0.0177
0.0193
0.0209
0.0225
0.0066
0.0073
0.0080
0.0088
C10mimBF4
0
0
10
20
30
40
50
60
Concentration (10-3mol/l)
C12mimBF4
298 K
308 K
328 K
338 K
(b)
2500
2000
1500
1000
500
Increasing IL concentration in the pre-micellar region
increases the concentration of current carriers, thus
increasing the conductivity of the solution rapidly. The
lower mobility of micelles as compared to that of free ions
and reduction in the effective charge on micelles, due to
counter ion binding on the micellar surface, results in a
smaller slope in the post-micellar region of the conduc-
tivity isotherms [50].
0
0
5
10
15
20
25
30
35
Concentration (10-3mol/l)
A comprehensive understanding of micelles to counter-
ion binding is required for understanding aggregation and
micellization in aqueous solutions. Conductivity measure-
ments can be used to determine the degree of micelle to
counter-ion binding (b), by using Eq. (7):
298 K
308 K
318 K
328 K
(c)
1400
1200
1000
800
600
400
200
0
b ¼ ð1 À aÞ
ð7Þ
The degree of ionization, a, of the micelle is obtained
using the s2/s1 ratio, where s2 and s1 represent the slopes
above and below the CMC, respectively [51]. From
Table 4, it is found that the CMC values obtained from
both tensiometry and conductivity experiments are in close
agreement with each other.
0
2
4
6
8
10
12
14
16
Concentration (10-3mol/l)
Thermodynamics of Micellization
Fig. 12 Plots of electrical conductivity (j) vs. concentration of
C8mimBF4, C10mimBF4 and C12mimBF4 at different
temperatures
a
b
c
The phase separation model can be used to determine
thermodynamic parameters of micellization. Standard
Gibbs free energy micellization change [52] can be
obtained using the following Eq. (8):
conductivity versus concentration for all investigated IL
indicates an apparent change of slope at the CMC, as
shown in Fig. 12a–c. Two distinguishing linear regimes
representing the pre-and post-micellar regions are observed
in each specific conductivity isotherm. The CMC values of
the IL are obtained at the point of intersection of the linear
portions of conductivity versus concentration plots above
and below the breakpoint, as per Williams’ method [49].
DG0m ¼ RT ð1 þ bÞ ln Xcmc
where Xcmc is CMC in molar fraction units; Xcmc
ð8Þ
cmc
,
=
55:4
where CMC is in mol/L and the factor 55.4 accounts for the
fact that 1 L of water corresponds to 55.4 mol of water at
298 K. b is the degree of micelle counter-ion binding.
123

J Surfact Deterg
42.5
40.0
37.5
35.0
32.5
30.0
The derived equation showing DHm0 values, applicable
to the investigated set of air–liquid systems at different
temperatures, is depicted in Eq. (10):
C
C
C
mimBF
4
(a)
8
mimBF
10
12
4
4
mimBF
DHm0 ¼ ÀRT2ð0:5 þ bÞ ln Xcmc=dT
ð10Þ
Finally, the standard entropy change during the micelle
formation, DSom, is calculated [42] using the following
Eq. (11):
DS0m ¼ ðDHm0 À DGm0 Þ=T
ð11Þ
Figure 13a–c shows the thermodynamic parameters for
C8mimBF4, C10mimBF4 and C12mimBF4 at different
temperatures. DG0m is negative, indicating spontaneous
aggregation of IL molecules. It is also evident that the
value of standard Gibbs aggregation energy becomes more
negative with an increase in temperature and alkyl chain
length, indicating ease of aggregation with increasing alkyl
chain length of IL. A negative DHm0 value indicates the
exothermic nature of the micellization process, which can
be attributed to hydrophobic interaction between the sol-
vent and IL molecules [53].
A negative value of DG0m is mainly due to large positive
DS0m value. DHm0 is much smaller than TDS0m , indicating
that the entropy gain greatly affects the associated micel-
lization process. Hence, the micellization process is
entropy-driven. The driving force of micellization is the
tendency of a hydrophobic group of the IL to move from
the solvent environment to the interior of the micelle [54].
With increase in temperature, the binding ability of IL
molecules with water is reduced, thereby forming micelles.
This results in increase in the contribution of TDS0m to the
value of DG0m.
298
308
318
328
328
328
Temperature (K)
1.0
0.5
C
C
C
mimBF
4
8
(b)
mimBF
10
12
4
mimBF
4
0.0
-0.5
-1.0
-1.5
-2.0
298
308
318
Temperature (K)
45.0
42.5
40.0
37.5
35.0
32.5
30.0
27.5
C
C
C
mimBF
4
(c)
8
mimBF
10
12
4
4
mimBF
Thermodynamics of Adsorption
The adsorption data is used to calculate the Gibbs free
energy change due to adsorption, DG0ads [55]. DGaods is
obtained using the Eq. (12):
DG0ads ¼ DGm0 À Pcmc=Cmax
ð12Þ
298
308
318
From Table 5, it is observed that DG0ads has negative
values, indicating that IL studied can be spontaneously
adsorbed at the air–liquid interface. DG0ads and DGm0 values
are both negative, indicating the tendency for adsorption at
the air–liquid interface and to form micelles in solution. In
each system, a higher magnitude of DG0ads is observed as
compared to DG0m values, which depicts that adsorption at
the air–liquid interface is more spontaneous than micel-
lization in aqueous IL systems [56]. This result is also
supported by relatively high values of Pcmc and pC20, as
tabulated earlier in Table 3. It is observed that DG0ads is
Temperature (K)
Fig. 13 Changes in thermodynamic parameters a DG0m, b DHm0 and c
TDS0m of C8mimBF4, C10mimBF4 and C12mimBF4 as a function of
temperature
The standard enthalpy change required for micellization
in aqueous solution, DHm0 , is obtained by using Eq. (8) in
the Gibbs–Helmholtz Eq. (9):
ꢂ
ꢃ
oðDG0m=TÞ
oð1=TÞ
¼ DHm0
ð9Þ
123

J Surfact Deterg
Table 5 Micellization and adsorption parameters of different ionic liquids at different temperatures
DG0m (kJ/mol) DHm0 (kJ/mol) DSm0 (kJ/mol/K) T DS0m (kJ/mol) DG0ads
(kJ/mol)
Ionic liquid Temp (K) CMC (mol/L) Binding degree (b)
C8mimBF4 298
0.0424
0.0443
0.0451
0.0460
0.0177
0.0193
0.0209
0.0225
0.0066
0.0073
0.0080
0.0088
0.7604
0.8723
0.9073
0.9675
0.6557
0.7149
0.7851
0.8803
0.5080
0.6071
0.6877
0.7594
– 31.29
– 34.19
– 35.87
– 38.06
– 33.15
– 34.96
– 37.18
– 40.03
– 33.74
– 36.73
– 39.45
– 41.31
– 1.683
– 1.212
– 0.554
0.220
0.099
0.111
0.119
0.128
0.103
0.111
0.121
0.132
0.101
0.112
0.122
0.132
29.6
33.0
35.3
38.3
31.9
34.3
37.1
40.7
32.9
36.3
39.5
42.7
- 63.66
- 64.97
- 65.15
- 67.92
- 70.60
- 70.76
- 71.14
- 73.66
- 71.86
- 74.09
- 75.75
- 78.05
308
318
328
C10mimBF4 298
– 1.156
– 0.674
– 0.087
0.642
308
318
328
C12mimBF4 298
– 0.89
– 0.473
0.074
308
318
328
0.731
always more negative with increase in alkyl chain length.
This may be due to the increased hydrophobicity of the IL
molecule, which improves the propensity of IL molecules
to migrate to the interface [57]. Rise in temperature
improves the absorption tendency of IL molecules due to
increase in the curvature of the air–liquid interface [58].
that the adsorption tendencies were more favoured than
micellization in the air–liquid interface over the tempera-
ture range investigated.
Acknowledgements The authors would like to acknowledge the
Sophisticated Analytical Instrument Facility, IIT Bombay and Central
Research Facility, IIT (ISM), Dhanbad, for technical contributions in
this study.
Conclusions
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Prathibha Pillai is currently pursuing her Ph.D. degree from the
Department of Petroleum Engineering, Indian Institute of Technology
(Indian School of Mines), Dhanbad. She did her Bachelors and
Masters in Petroleum Engineering. Her area of research involves
synthesis and characterization of ionic liquids and its application in
enhanced oil recovery.
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surface activity, micellar formation and dye solubilization beha-
viour of cationic surfactants. Indian J Chem. 2002;37(A):118–24.
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of the sodium salts. J Chem Soc. 1956;117:579–86.
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mixed systems for imidazole based ionic liquid surfactant and
Triton X-100. J Mol Liq. 2017;232:55–61.
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micelle concentration and micellization thermodynamic of non-
ionic surfactants: polyoxyethylene sorbitan fatty acid esters.
J Chem. 2012;9(4):2268–74.
Nilanjan Pal is a Senior Research Fellow (SRF) in the Department of
Petroleum Engineering, Indian Institute of Technology (Indian School
of Mines), Dhanbad. He did his Bachelors in Mechanical Engineering
and Masters in Petroleum Engineering. He is currently pursuing his
Ph.D. degree. His present field of research involves the synthesis,
characterization, and optimization of different chemical agents
(surfactants, ionic liquids, and polymers) for effective application in
enhanced oil recovery processes.
54. Ao M, Huang P, Xu G, Yang X, Wang Y. Aggregation and
thermodynamic properties of ionic liquid-type gemini imida-
zolium surfactants with different spacer length. Colloid Polym
Sci. 2009;287(4):395–402.
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Wiley; 1978. p. 83–221.
Dr. Ajay Mandal is presently working as an Associate Professor in
the Department of Petroleum Engineering, Indian Institute of
Technology (ISM), Dhanbad. His research area includes synthesis
and characterization of surfactants, enhanced oil recovery, gas
hydrates, multi-phase flow system, and water flooding. He has
published more than 125 research papers in reputed peer reviewed
International and National Journals. He is a reviewer of many
International Journals. He has been awarded 2015 SPE South Asia
Regional Distinguished Achievement Award for Petroleum Engi-
neering Faculty. He is also a member in the editorial board of the
International Journal of Petroleum Engineering.
´
´
56. Manko DI, Zdziennicka AN, Janczuk BR. Thermodynamic
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