Modulating the Aggregation Behavior of 1-Methyl-3-Octylimidazolium Chloride by Alcohols in Aqueous Media

Article abstract of DOI:10.1007/s11743-016-1859-y

The self-organization and aggregation behavior of a surface active ionic liquid (SAIL) 1-methyl-3-octylimidazolium chloride [C₈mim]Cl, was investigated in aqueous solutions of alcohols 1,2-propanediol and 1-propanol in different compositions using conductivity, surface tension and fluorescence measurements at room temperature. This surface active ionic liquid was synthesized by the reaction of 1-methylimidazole with 1-chlorooctane. Fluorescence spectroscopy was employed to get detailed insight into the local microenvironment of the aggregates, critical aggregation concentrations (CAC) and aggregation number (N_agg.). Degree of ionization (α), CAC and various thermodynamic parameters like the standard Gibbs free energy of aggregation (ΔG_agg. ⁰), standard enthalpy of aggregation (ΔH_agg. ⁰) and standard entropy of aggregation (ΔS_agg. ⁰) were calculated using conductivity measurements at different temperatures (288.15, 298.15 and 308.15?K). The surface activity of the IL in various mixed solvents was evaluated from surface tension measurements by calculating various surface parameters like the minimum surface area occupied by a single ionic liquid molecule (A_min), adsorption efficiency (pC₂₀), maximum excess concentration at the surface (Γ_max), effectiveness of surface tension reduction(Π_CAC), surface tension at CAC (γ_CAC), p (packing parameter) and CAC at different compositions. Increases in the CAC values were observed with the increase in the amount of alcohols which is attributed to the balancing between electrostatic and hydrophobic interactions. The results from different techniques show that the CAC increases with increase in the amount of the alcohol which is due to the solubilization of the IL molecules which delays the aggregation process. This shows that the spontaneity of the aggregation process of IL decreases with the increase in the concentration of alcohols in water.

Full text of DOI:10.1007/s11743-016-1859-y

J Surfact Deterg
DOI 10.1007/s11743-016-1859-y
ORIGINAL ARTICLE
Modulating the Aggregation Behavior of 1-Methyl-3-
Octylimidazolium Chloride by Alcohols in Aqueous Media
Amalendu Pal1 Alka Yadav1
•
Received: 10 October 2015 / Accepted: 18 July 2016
Ó AOCS 2016
Abstract The self-organization and aggregation behavior
of a surface active ionic liquid (SAIL) 1-methyl-3-
octylimidazolium chloride [C8mim]Cl, was investigated in
aqueous solutions of alcohols 1,2-propanediol and 1-pro-
panol in different compositions using conductivity, surface
tension and fluorescence measurements at room tempera-
ture. This surface active ionic liquid was synthesized by the
reaction of 1-methylimidazole with 1-chlorooctane. Fluo-
rescence spectroscopy was employed to get detailed insight
into the local microenvironment of the aggregates, critical
aggregation concentrations (CAC) and aggregation number
(Nagg.). Degree of ionization (a), CAC and various ther-
modynamic parameters like the standard Gibbs free energy
of aggregation (DG0agg.), standard enthalpy of aggregation
(DH0agg.) and standard entropy of aggregation (DS0agg.) were
calculated using conductivity measurements at different
temperatures (288.15, 298.15 and 308.15 K). The surface
activity of the IL in various mixed solvents was evaluated
from surface tension measurements by calculating various
surface parameters like the minimum surface area occupied
by a single ionic liquid molecule (Amin), adsorption effi-
ciency (pC20), maximum excess concentration at the sur-
face (Cmax), effectiveness of surface tension
reduction(PCAC), surface tension at CAC (cCAC), p (pack-
ing parameter) and CAC at different compositions.
Increases in the CAC values were observed with the
increase in the amount of alcohols which is attributed to the
balancing between electrostatic and hydrophobic interac-
tions. The results from different techniques show that the
CAC increases with increase in the amount of the alcohol
which is due to the solubilization of the IL molecules
which delays the aggregation process. This shows that the
spontaneity of the aggregation process of IL decreases with
the increase in the concentration of alcohols in water.
Keywords Ionic liquid Á Alcohols Á Conductance Á Surface
tension Á Fluorescence
Introduction
Ionic liquids (IL) occupy a special place among solvents
due to their unique physicochemical properties such as
high thermal stability, low toxicity, liquidity, low volatility,
high ionic conductivity, wide electrochemical window etc.,
which have attracted the attention of the scientific com-
munity [1–7]. The physicochemical properties of IL can be
tailored by the judicious selection of both the cation and
the anion as per the demands of different applications.
Hence IL are sometimes called designer solvents [8].
Recently, it is reported that IL possessing long aliphatic
chains undergo aggregation due to their inherent amphi-
philic nature and many research groups have investigated
the aggregation behavior of IL in aqueous medium [9–14].
IL form micelles or aggregates at low CAC values com-
pared to cationic surfactants having similar structures. This
allows for the growth of applications that use the surface
active ionic liquid in small quantity, which may also pos-
sess environmental advantages.
Nevertheless, limited investigations were carried out on
the self-organization behavior of IL with different organic
solvents and salts whose applications are not only limited
to the field of colloid and interface science but also have
tremendous applications in the field of micellar liquid
& Amalendu Pal
palchem21@gmail.com
1
Department of Chemistry, Kurukshetra University,
Kurukshetra 136119, India
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J Surfact Deterg
chromatography (MLC) [15]. Pino et al. [16] examined the
micellization and interfacial behavior of imidazolium
based IL [C16C4im]Br and [C12C12im]Br in various organic
solvent–water mixtures using tensiometry measurements.
An increase in the CAC values and in the minimum surface
area occupied by a single IL molecule was observed with
the increase in the concentration of organic additives [16].
Zheng et al. [17] have investigated the effect of sodium
halide salts on the micellization of [Cnmim]Br (n = 12, 14,
16) using surface tension and fluorescence methods.
Vaghela et al. [18] studied the effect of a non-electrolyte
2-butoxyethanol and two salts namely sodium chloride and
sodium sulfate on the aggregation behavior of [C8mim]Cl,
[C8mim]Br and [C8mim]I in aqueous media through small-
angle neutron-scattering and surface tension measure-
ments. Additionally, the self-assembly of IL molecules in a
solvent offer great promise in applications like nanomate-
rial synthesis [19], drug delivery [20], and dispersant
technologies [21]. Patel et al. [22] examined how
1-methyl-3-octylimidazolium chloride, [C8mim]Cl, affects
the micellization behavior of amitriptyline hydrochloride
(AMT), a drug which is amphiphilic in nature, using
electrical conductivity at different temperatures. It was
found that mixed micelles of AMT-[C8mim]Cl are more
stable than the micelles of [C8mim]Cl and AMT separately
[22].
fluorescence were not clear, so the weight percentages
chosen for the studied alcohols in binary aqueous solvents
were taken as 0, 10, 20, and 30 % (w/w). The temperature
dependence of conductivity enabled us to explore the ther-
modynamics of aggregation by evaluating the various ther-
modynamic parameters. Surface tension measurements were
used to address the interfacial behavior of [C8mim]Cl in the
presence of organic additives. A series of various surface
parameters like (Cmax), (Amin), (pC20), (PCAC), (cCAC),
p and CAC were obtained to evaluate the surface activity of
the IL in these organic solvents at different compositions.
The fluorescence spectroscopy was used for better under-
standing of the surrounding environment of IL aggregates
using pyrene as a polarity probe. The CAC values obtained
from our measurements agree with each other and with lit-
erature values obtained by Vaghela et al. [23] and Kumar
and Singh [24].
Experimental
Reagents
The IL, [C8mim]Cl, was synthesized in our lab.
1-Chlorooctane and 1-methylimidazole were procured
from Acros Organics with 99.0 % purity The different
compositions of alcohols i.e. (10, 20, 30 % w/w) were
prepared as mentioned in our earlier study [25]. The
alcohols studied 1,2-propanediol, 1-propanol and pyrene
were purchased from Sigma Aldrich with purity (C99 %).
Deuterium Oxide was purchased from SD Fine Chemicals,
India with purity of C99.9. Cetylpyridinium chloride was
acquired from Loba Chemie, India having purity of C99.0
and methanol from Rankem, India with C99.0 % purity.
The main motive of our work is to reveal the effect of
alcohols, viz 1,2-propanediol and 1-propanol, on the
aggregation behavior of 1-methyl-3-octylimidazolium
chloride [C8mim]Cl at different compositions using surface
tension, conductance, and fluorescence measurements. The
present work is of great importance in micellar liquid
chromatography (MLC). The hybrid micellar mobile pha-
ses were utilized in the applications of MLC. The afore-
mentioned paramount applications of SAIL have generated
our interest in investigating the effect of alcohols on their
aggregation and interfacial behavior on them.
Synthesis of 1-Methyl-3-Octylimidazolium Chloride
[C8mim]Cl
Molecular structure and molecular formulae of studied IL
and organic solvents are presented in Scheme 1. At higher
weight percentages of organic solvents, the breakpoints in
the curves obtained from surface tension, conductance, and
[C8mim]Cl was synthesized according to standard method
[8] by reacting n-chlorooctane (6.5 g, 43.77 mmol) with n-
methylimidazole (3.56 g, 43.35 mmol) in a round bottom
flask in acetonitrile media followed by refluxing the solu-
tion at 70 °C for 72 h under N2 atmosphere. The final
product was a light yellow colored viscous liquid, which
was then cooled to room temperature and washed several
times with ethyl acetate to remove unreacted starting
materials. Excess ethyl acetate was decanted and the
remaining solvent was removed by evaporation under
vacuum and further dried in vacuum oven for 72 h to get
the final product. Water content obtained in the IL was
found to be less than 290 ppm using Karl-Fischer titration
analysis. The product was flushed with N2 gas and kept in a
dry place before using.
Cl-
N
N+
1-octyl-3-methylimidazolium chloride
OH
HO
HO
1-propanol
1,2-propanediol
Scheme 1 Molecular structure and molecular formulae of IL and
organic solvents
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J Surfact Deterg
Instruments and Methods
Fluorescence Measurements
All solutions were prepared by weight using an electronic
balance (A&D Co. limited Japan, Model GR-202) with a
The fluorescence emission spectra samples were prepared
by dissolving pyrene in solutions of the long-chain imi-
dazolium IL. The fluorometer used blazed holographic
grating excitation and emission monochromators fitted
with a 150 W Xenon lamp, which was purchased from
Shimadzu (RF-5301PC). The cuvette used was made up of
quartz and purchased from Shimadzu. All the samples of
different compositions of alcohols were prepared accord-
ingly as mentioned in our earlier study [25]. The emission
spectra were taken at an excitation wavelength of 334 nm
in the range of 350–600 nm with the slit width of excitation
and emission, 3 and 1.5 nm, respectively. The ratio of the
first to the third vibronic peaks i.e. I1/I3 is very sensitive to
the polarity of the surroundings and can be used to obtain
the CAC of the IL.
1
precision of 0.01 mg. The IL was characterized by H
NMR spectra obtained using a Bru¨ker FT-NMR spec-
trometer operating at 300 MHz. D2O was used as an
external solvent. The chemical shifts are reported as d
units (ppm). The chemical shift values of peaks of interest
were obtained using peak pick facility. The d values in
ppm are 8.783 (s, H), 7.451 (d, H), 7.446 (d, H), 4.158 (t,
2H), 3.863 (s, 3H), 1.794 (m, 2H), 1.223 (m, 10H), 0.752
(t, 3H).
Surface Tension Measurements
All surface tension measurements were performed using
the ring detachment method with a Du Nou¨y tensiometer,
which was purchased from SD Hardsons Ltd. having a
precision of 0.01 mN m-1. The ring (platinum–iridium
with a mean circumference of 6.00 cm) was calibrated
before performing experiments. The experiments were
performed at room temperature 298.15 K using a Julabo
water thermostat with an uncertainty of 0.01 K in a
double walled water jacketed flow dilution cell. IL was
added into the water or water-alcohol solvent and stirred
for 4–5 min to allow for complete dissolution. The mea-
sured values were taken in triplicate, and the mean value
was considered. After each measurement, the ring was
washed with triply distilled de-ionized degassed water and
subjected to a high temperature flame to remove the resi-
dues of IL surfactant.
Results and Discussion
Surface Tension Measurements
The aggregation and interfacial behavior of the IL [C8-
mim]Cl in aqueous and aqueous solutions of alcohols were
investigated using surface tension at 298.15 K. Figure 1a,
b shows the variation in the surface tension of 1,2-
propanediol and 1-propanol with the concentration of IL.
From each plot, it can be noted that the values of the
surface tension decrease constantly as the concentration of
IL increases and after a particular point it becomes con-
stant. This is because in this region, aggregates start
forming in the bulk phase, and the concentration of IL at
the surface layer remains almost constant.
Conductance Measurements
The measured values of CAC obtained from different
experimental techniques are given in Table 1. Various
surface parameters like CAC, PCAC (surface pressure at
the CAC), cCAC (surface tension at the CAC), Cmax
(maximum surface excess concentration) and Amin (mini-
mum area occupied by a single molecule), pC20 and
p (packing parameter) were obtained from surface tension
measurements. The calculated surface-active parameters
are tabulated in Table 2. The breakpoint in the surface
tension versus concentration plot gives the CAC and the
value of surface tension at the CAC gives cCAC. PCAC
denotes the reduction in the surface tension to a minimum
value when the IL is added to the solvents. Therefore,
PCAC measures the effectiveness of IL in reducing the
surface tension of the solvent. PCAC can be calculated
using the following Eq. (1):
The conductivity measurements were obtained from
conductivity meter at
a
temperature range of
288.15–308.15 K with an uncertainty of 0.01 K in a
double walled water jacketed flow dilution cell. The
digital conductivity meter CM-183 microprocessor based
EC-TDS analyzer with an ATC probe and conductivity
cell with platinized platinum electrodes was purchased
from Elico Ltd., India. The calibration of the conductivity
cell was done before measurements with standard aqueous
potassium chloride solution by keeping the range of
concentration 0.01–1.0 mol/kg of known specific con-
ductance. The cell constant of the cell was 1.0014 cm-1
.
After each addition, the solution was stirred for 2–3 min
and the measured values were taken. The values were
measured in triplicate and the mean value was considered
for each addition. The measurements had an uncertainty
of less than 4 %.
PCAC ¼ c0 À cCAC
ð1Þ
123

J Surfact Deterg
concentration and the amount of IL adsorbed at the inter-
face can be calculated using the equation [26]:
(a)
80
70
60
50
40
30
0%
ꢀ
ꢁ
10%
20%
30%
1
oc
o ln C
Cmax ¼ À
lt
ð2Þ
½CŠ!0
nRT
T
where R is the gas constant, T is the absolute temperature,
C is the concentration of IL in bulk solution, n is 2 for IL
(as the IL upon dissociation in water gives two ions) and c
is the surface tension in the pre-CAC region. Cmax
decreases with the addition of organic solvent, which
indicates that the adsorption of IL molecules at the inter-
face decreases.
10
100
-1
Amin. is the minimum surface area occupied an IL
monomer at the surface of the solution in the region of the
CAC point which can be obtained by using the following
relation [26, 27]:
log C / (mmol kg )
(b)
80
70
60
50
40
30
0%
10%
20%
30%
1
Amin
¼
ð3Þ
NACmax
where NA is the Avogadro number. Therefore, Amin.tells us
about the packing density of the IL at the interface. The
increase in the concentration of additives results in
diminishing the packing of IL molecules at the interface
and consequently the minimum surface area covered by a
single IL monomer at the interface increases. It is well
established that the size of the hydrophilic head group is a
key factor in establishing the Amin. value, and hence the
Cmax value of IL.
10
100
log C / (mmol kg-1)
The adsorption efficiency, pC20 is defined using the
Fig. 1 Variation of surface tension as a function of [C8mim]Cl
concentration in aqueous or aqueous–alcoholic mixtures at 298.15 K:
a aqueous-1,2-propanediol; b aqueous-1-propanol for varying com-
position of mixed solvents: (black filled squares) 0 % (w/w); (red
filled circles) 10 % (w/w); (green filled up-pointing triangles) 20 %
(w/w); (blue filled down-pointing triangles) 30 % (w/w)
equation [26–28]:
pC20 ¼ À log C20
ð4Þ
where C20 is the concentration of IL required to reduce the
surface tension of pure solvent 20 mN m-1. The greater
value of pC20 depicts the larger adsorption efficiency and
hence better surface activity of the IL. The values of pC20
can only be calculated for pure water, 10 % 1,2-propane-
diol and 10 % 1-propanol as the surface tension could not
be reduced to C20 for the other remaining systems.
where co is the surface tension of the pure solvent.
According to the Gibbs adsorption isotherm, the adsorption
of IL at the air–water interface reduces the surface tension
of the solution. Cmax is the maximum surface excess
Table 1 Experimentally
Solvent
Critical aggregation concentration (mmol kg-1
)
I1/I3
Nagg.
determined CAC of IL in
presence of different additives
from various techniques, Nagg.
(aggregation number) and
relative intensity of vibronic
bands (I1/I3) of pyrene
Surface tension
Conductance
Fluorescence
Water
101.8a,b
116.1
121.1
131.0
119.8
125.1
135.0
101.6a,b
115.3
120.4
130.8
119.2
125.5
136.7
97.6b
115.0
123.0
130.9
119.1
126.0
135.8
1.25
1.31
1.32
1.34
1.29
1.30
1.32
26a,b
22
10 % 1,2-propanediol
20 % 1,2-propanediol
30 % 1,2-propanediol
10 % propan-1-ol
20 % propan-1-ol
30 % propan-1-ol
19
fluorescence at 298.15 K
17
20
16
14
The CAC and Nagg. values obtained are cited in references [23]a, [24]b
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Table 2 Various surface active
parameters of aggregation of IL
[C8mim]Cl at 298.15 K
Solvent
Surface active parameters
cCAC
PCAC
Cmax 9 1010
Amin
p
pC20
Water
29.0a
36.2
38.4
40.3
30.9
33.1
35.2
43.4a
27.3
19.2
15.4
20.0
13.6
9.9
1.40a
0.87
0.53
0.44
0.60
0.41
0.25
118.59a
190.84
313.26
377.34
276.72
406.94
664.12
0.351
0.218
0.133
0.110
0.151
0.102
0.063
1.82a
1.33
Nd
10 % 1,2-propanediol
20 % 1,2-propanediol
30 % 1,2-propanediol
10 % propan-1-ol
20 % propan-1-ol
30 % propan-1-ol
Nd
0.93
Nd
Nd
´
2
˚
c
respectively
CAC, PCAC, Cmax, and Amin are expressed in mN m-1, mN m-1, mol cm-2, and A molecule-1
,
Nd values not detected
a
The values of various surface active parameters obtained are cited in reference [23]
Table 3 Degree of counterion
dissociation, a and
thermodynamic parameters:
Solvent
Thermodynamic Parameters
DG0ad
Gsmin.
a
DG0agg.
DH0agg.
DS0agg.
standard free energy of
aggregation (DG0agg.), standard
enthalpy of aggregation
(DH0agg.), standard entropy of
aggregation (DS0agg.), standard
Gibbs free energy of adsorption
(DG0ad) and free energy of the
surface at equilibrium (Gsmin.) of
IL [C8mim]Cl at 298.15 K
Water
0.604a,b
0.684
0.743
0.784
0.717
0.774
0.796
-21.81a, b
-20.15
-19.12
-18.24
-19.54
-18.53
-17.94
-1.54a
-6.59
-8.07
-9.52
-9.41
-9.95
-11.30
67.99a
45.47
37.05
29.28
34.00
27.76
22.25
-52.83
-51.53
-55.40
-53.13
-52.93
-51.76
-57.42
20.71
41.61
72.50
91.59
51.45
81.23
140.92
10 % 1,2-propanediol
20 % 1,2-propanediol
30 % 1,2-propanediol
10 % propan-1-ol
20 % propan-1-ol
30 % propan-1-ol
Standard uncertainties are DGa0gg.
(JK-1 mol-1), DGa0d. 0.02 (kJ mol-1), Gms in.
The thermodynamic parameters obtained are cited in references [23]a, [24]b
=
0.02 (kJ mol-1), DHa0gg.
=
0.01 (kJ mol-1), DSa0gg.
=
0.02
=
=
0.02 (kJ mol-1), T = 1 9 10-2
K
The packing parameter (p), which predicts the shape of the
p decreases and the structure of the aggregates transforms
accordingly.
aggregates, was calculated using the following relation [27]:
By using surface active parameters, the standard Gibbs
free energy change required for adsorption (DG0ad.) and free
energy of a surface at equilibrium (Gsmin.) were also cal-
culated in order to investigate the effect of the organic
additive on the aggregation of [C8mim]Cl using following
equations [28, 30]:
p ¼ Vo=lcAmin:
ð5Þ
where Vo is the volume occupied by the long alkyl chain
groups in the center of the aggregate and lc is the length of
the long alkyl chain in the center of the aggregate were
determined by using Tanford’s formulae [29]:
ꢀ
ꢁ
˚
PCAC
Cmax
Vo ¼ ½27:4 þ 26:9 ðncÀ1ފ2 ðAÞ
ð6Þ
ð7Þ
DGoad: ¼ DGoagg:
À
ð8Þ
ð9Þ
˚
lc ¼ ½1:54 þ 1:26 ðncÀ 1ފ ðAÞ
Gsmin: ¼ Amin:
cCACNA
Commonly, the number of carbon atoms taken is one
less than the total number of carbon atoms in the hydro-
carbon chain (nc) because the first carbon atom attached to
the long alkyl chain is immensely solvated. The value of
p for pure [C8mim]Cl is slightly higher than 0.33 which
suggests that globule aggregates are formed. From Table 2,
the shape of the aggregates changes from globule to
spherical with increasing additive concentration [17]. This
may be because the molecules of organic solvent reside
primarily in the outer surface of the aggregates which
consequently increases the Amin.. Therefore, the value of
where DG0agg. has its usual meaning and its values were
obtained from conductivity measurements using Eq. 10.
Since the values of DGoad. are more negative than the cor-
responding DGoagg. values reveals that the adsorption pro-
cess is the primary process as compared to aggregation.
The negative values of DGaod. indicate that work is required
to shift the IL monomers at the interface to the aggregation
stage [31]. Sugihara et al. [32] introduced a thermody-
namic quantity, which gives a measure of the free energy
of a surface at equilibrium. From Table 3, we can see that
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the values of Gsmin. increase with increasing alcohol con-
centration, suggesting that the bulk phase is more
stable than the surface phase [32]. In general, organic
solvents help solubilize the IL and makes the bulk envi-
ronment more comfortable for the aggregates.
et al. [23]. An increase is observed in the electrical con-
ductivity of all the systems with the increase in the amount
of IL with an inflection in the curve. This change in the
curve indicates the reaching of the aggregation point and
the breakpoint gives the value of the CAC. Below the
CAC, the conductance increases quickly with increasing
concentration of IL but after the attainment of the CAC
point, the conductance increases more slowly. This change
in slope results from the aggregation of IL molecules.
These aggregates contribute to conductance to a lesser
extent due to their lower mobility [33].
Conductance Measurements
Determination of CAC
The conductance of IL in aqueous solutions and aqueous
solutions of alcohols of different weight percentages at
298.15 K are presented in Fig. 2a, b. The experimentally
determined CAC values of the IL in aqueous and aqueous
solutions of 1,2-propanediol and 1-propanol at 298.15 K
from conductance are given in Table 1. The CAC value of
IL in aqueous media obtained from conductivity mea-
surements are comparable with those reported by Vaghela
A close look at Table 1 reveals that for a particular
alcohol, the CAC increases with the increase in the con-
centration of alcohol. This is probably due to the reason
that the presence of organic additives weakens the solvo-
phobic power acting on the hydrocarbon part of the IL but
makes the interactions between the mixed solvents and
alkyl chains of IL stronger, which results in an unfavorable
formation of IL aggregates [34]. The aggregation depends
on the two opposing interactions. First, the electrostatic
repulsions among the charged head groups which delay the
aggregation process and second, the attractive hydrophobic
interactions among the long alkyl chain of IL which favors
aggregation. The electrostatic interactions can be reduced
by the adsorption of the anions of IL on the aggregates
surface as a result of which the CAC of the system gets
lowered. On the other hand, the hydrophobic interactions
stabilize the individual IL monomer, which delays aggre-
gation, and consequently CAC increases. The organic
additives disrupts the structure of water and the solution
becomes less hydrophobic in nature as a result of which it
solubilizes the IL molecules to a greater extent than water
and CAC increases. Here the alkyl chain of the alcohols
interacts with the alkyl chain of the IL because both are
hydrophobic in nature (like dissolves like). In addition, the
head group of the IL being polar in nature interacts with the
hydroxyl group present in the alcohols thereby delaying the
aggregation process. As compared to our earlier work using
1-butyl-3-methylimidazolium octylsulfate with alcohols,
the changes in the CAC are more evident in the present
study [25].
(a)
14
0%
10%
15%
20%
12
10
8
6
4
2
0
0
50
100
150
200
250
300
350
C /(mmol kg-1)
(b)
14
12
10
8
0%
10%
20%
30%
6
Figure 3a, b show the variation of the CAC with tem-
perature. In general the curves are U-shaped. This behavior
can be explained as a consequence of two opposing factors.
First, with the increase in temperature, there is a decrease
in the degree of hydration of head group. This results in the
increase in the hydrophobicity of solute, which supports
aggregation. Second, the structure of water molecules
surrounding the hydrophobic part of the IL gets disrupted
as the temperature increases, this inhibits the aggregation
process, and hence CAC increases. Thus at low tempera-
ture the first effect dominates while after reaching CAC
point the second effect dominates [35].
4
2
0
0
50
100
150
200
250
300
C /(mmol kg-1)
Fig. 2 Specific conductance showing variation in the CAC of
aqueous and aqueous-alcoholic mixtures of [C8mim]Cl at 298.15 K:
a aqueous-1,2-propanediol; b aqueous-1-propanol for varying com-
position of mixed solvents: (black filled squares) 0 % (w/w); (red
filled circles) 10 % (w/w); (green filled up-pointing triangles) 20 %
(w/w); (blue filled down-pointing triangles) 30 % (w/w)
123

J Surfact Deterg
charge on the aggregate surface, which disfavors the
aggregation process, and thus supporting the trend of
increasing CAC values [34]. The negative values of DGa0gg.
support the aggregation process but with the increasing
concentration of organic additives this value starts
increasing showing that the process is less spontaneous.
With the increasing amount of additives, the process seems
to be less spontaneous indicating the increased tendency of
the alcohols to solubilize the IL monomers. This suggests
that the aggregation ability of [C8mim]Cl is weaker in the
water–organic mixed solvents than in pure water.
(a)
140
135
130
125
120
115
110
105
100
From the temperature dependence of CAC, the variation
of the standard enthalpy (DH0agg.) can be obtained using
Gibbs–Helmholtz equation [28]:
285
290
295
300
305
310
T (K)
(b)
150
140
130
120
110
100
ꢂ
ꢃ
d ln xCAC
dT
dð1 À aÞ
þ ln xCAC
dT
DHa0gg: ¼ ÀRT2 ð2 À aÞ
ð11Þ
And using the values of DG0agg. and DHa0gg., the standard
entropy change (DS0agg.) can be calculated from the equa-
tion [28]:
!
DH0agg: À DG0agg:
DS0agg:
¼
ð12Þ
T
The thermodynamic parameters obtained from these
equations are tabulated in Table 3. A glance at DH0agg. data
from Table 3 demonstrates that the aggregation process is
exothermic. Nusselder and Engberts stated that the main
driving force behind the formation of aggregates having
negative DH0agg. is the hydrophobic interaction [37]. The
values of entropy are large and positive indicating that the
aggregation is an entropy-driven process, which states that
the tendency of the hydrophobic group to shift from the
solvent to the inside of the aggregate provides the driving
force for the formation of aggregates [23]. From Table 3, it
285
290
295
300
305
310
T (K)
Fig. 3 Specific conductance showing variation in the CAC of
aqueous and aqueous-alcoholic mixtures of [C8mim]Cl at 288.15 K;
298.15 K and 308.15 K: (a) aqueous-1,2-propanediol; (b) aqueous-1-
propanol for varying composition of mixed solvent: (black filled
squares) 0 % (w/w); (red filled circles) 10 % (w/w); (green filled up-
pointing triangles) 20 % (w/w); (blue filled down-pointing triangles)
30 % (w/w)
Thermodynamics Parameters of Aggregation
is very clear that DG0agg. is mostly determined by DS0agg.
.
The conductivity measurements were used to calculate the
various thermodynamic parameters. By using the mass
action model, DGoagg. of IL in alcoholic solutions were
achieved by applying the following relation [36].
Fluorescence Measurements
Determination of CAC
DG0agg: ¼ ð2 À aÞRT ln xCAC
ð10Þ
In order to validate the results of aggregation obtained from
surface tension and conductance, fluorescence measure-
ments were made at room temperature. Figure 4a, b show
the variation of I1/I3 with IL concentration in various
alcohols in different compositions. Pyrene shows signifi-
cant vibrational bands in their fluorescent spectra therefore,
it is used as a fluorescent probe. The polarity scale of
pyrene is described by the ratio I1/I3 of the fluorescence
intensities bands at I1 (Ca. 376 nm) and I3 (Ca. 387 nm).
As pyrene is sensitive to the polarity of the medium, the
ratio I1/I3 of pyrene increases on moving from non-polar to
where a is the degree of counterion ionization, T is the
temperature and R is the gas constant. The values of a were
calculated using Carpena’s method by taking from the ratio
of slopes of post-micellar region to pre-micellar region of
the graph between specific conductance and concentration
and is given in Table 3. The value of a increases as the
amount of alcohol increases indicating a decrease in the
counterion binding in the stern layer of the aggregates. The
higher the a value, the weaker will be the binding of anions
on the aggregate surface resulting in a decrease in the
123

J Surfact Deterg
increased. This may be due to the reason that the molecules
of alcohols move into the stern layer of the aggregates and
pyrene sense the increased polarity of the medium. A few
molecules of 1,2-propanediol are known to penetrate into
the aggregates (due to the presence of a more hydroxyl
group density than 1-propanol) and make the environment
of the aggregates more polar than 1-propanol [39].
Therefore, the values of I1/I3 for 1,2-propanediol are
greater than 1-propanol. The CAC values obtained from
fluorescence agree well with those obtained from surface
tension and conductance and are given in Table 1.
(a)
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
0%
10%
20%
30%
0
50
100
150
200
250
300
Effect of Organic Solvent Addition on the Aggregation
Number of the Ionic Liquid
C /(mmol kg-1)
(b)
The aggregation numbers, Nagg., of [C8mim]Cl aggregates
in the presence of different weight percentages of different
alcohols were determined from fluorescence quenching of
pyrene using cetylpyridinium chloride (CPC) as a quencher
molecule. Nagg. is defined as the total number of amphi-
philic molecules in the aggregate. The aggregation number
of [C8mim]Cl in water-organic solvents were determined
using the following equation [40–42]:
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
0%
10%
20%
30%
ꢀ
ꢁ
Io
ln
IQ
ð13Þ
0
50
100
150
200
250
300
C /(mmol kg-1)
where Io and IQ are the intensity of pyrene fluorescence in
the presence and in the absence of the quencher, and [ag-
gregate]IL, Qaggregate (or [CPC]aggregate), and [IL] have their
usual meanings. This equation was derived by assuming
that the probe and the quencher are completely in aggre-
gate phase and the probe fluoresces only in the absence of
the quencher [42]. The obtained aggregation number of
[C8mim]Cl in aqueous and aqueous solution of alcohols are
included in Table 1. A decrease in the values of Nagg. was
observed with the increase in the amount of the additives
which demonstrates that the growth of aggregates is
unfavorable upon addition of organic solvents. This may be
explained further by considering that the attractive
hydrophobic interactions among the long alkyl chain of the
IL become weaker on adding organic solvents, which
definitely reduces the growth of aggregates.
Fig. 4 Pyrene (1 lM) I1/I3 versus the log of the concentration of
[C8mim]Cl in presence of various organic mixed solvents at
298.15 K: a aqueous-1,2-propanediol; b aqueous-1-propanol for
varying composition of mixed solvents: (black filled squares) 0 %
(w/w); (red filled circles) 10 % (w/w); (green filled up-pointing
triangles) 20 % (w/w); (blue filled down-pointing triangles) 30 % (w/
w). (kex = 334 nm and slit widths = 3, 1.5 nm)
the polar medium [33]. This very particular feature of
pyrene makes it a suitable probe for determining the CAC
of IL in various solvent systems as the polarity of solution
generally differs from the microenvironment of the core of
aggregates. From the fluorescence curves, it is clear that the
ratio I1/I3 remains constant up to a particular concentration
and then decreases very quickly and then afterwards
becomes stable. This steep change in the curve indicates
the onset of aggregation. The mid-point of the curve marks
the CAC of the system. This type of the behavior of the
pyrene is due to the incorporation of hydrophobic pyrene
molecules into the inner core of the aggregates. The value
of I1/I3 in the curve decreases because in the inner core of
the aggregate the pyrene molecule sense non-polar envi-
ronment [38]. However, an increase in the value of I1/I3 is
observed as the concentration of alcohol in the solution
Conclusion
The present work deals with the effect of organic solvents
on the aggregation and interfacial behavior of [C8mim]Cl
by employing surface tension, conductivity and fluores-
cence at room temperature. The results obtained from these
techniques are in good agreement with each other. An
123

J Surfact Deterg
15. Armstrong DW, Henry SJ (1980) Use of an aqueous micellar
mobile phase for separation of phenols and polynuclear aromatic
hydrocarbons via HPLC. J Liq Chromatogr 3:657–662
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behavior of imidazolium based ionic liquids in organic solvent-
water mixtures. J Colloid Interface Sci 333:548–556
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Aggregation behavior of long-chain imidazolium ionic liquids in
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increase in the CAC values was observed which is due to
the presence of organic additives that make the interactions
between the mixed solvents and alkyl chain of ionic liquid
much stronger. This disfavors the formation of IL aggre-
gates. The other thermodynamic and surface-active
parameters also favor the above mentioned interactions.
This investigation shows the pivotal role of different
alcohols in modulating the aggregation properties of sur-
factant like IL. The main picture of the work is that the
alkyl chain of the IL can be better dissolved by the aqueous
organic solvents than water resulting in the increase in the
CAC of the IL, which would serve for tremendous potential
applications of SAIL.
Acknowledgments One of the authors Alka Yadav gratefully
acknowledge the financial support for the work by the Government of
India through the University Grants Commission, New Delhi (Letter
No. F. 41-328/2012 (SR)).
21. Alexanandridis P, Lindman
B (2000) Amphiphilic block
copolymers: self-assembly and applications. Elsevier, Amster-
dam, p 3
22. Patel R, Khan AB, Ali M, Malik NA, Ali A (2013) Role of
1-methyl-3-octylimidazolium chloride in the micellization
behavior of amphiphilic drug amitriptyline hydrochloride. Col-
loids Surf B: Biointerfaces 112:460–465
23. Vaghela NM, Sastry NV, Aswal VK (2011) Surface active and
aggregation behavior of methyl imidazolium based ionic liquids
of type [Cnmim][X], n = 4,6,8 and [X] = Cl–, Br- and I- in
water. Colloid Polym Sci 289:309–322
24. Kumar A, Singh T (2007) Aggregation behavior of ionic liquids
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anions. J Phys Chem B 111:7843–7851
25. Pal A, Yadav A (2015) Modulations in the aggregation behavior
of ionic liquid 1-butyl-3-methylimidazolium octylsulfate in
aqueous alcohol solutions. J Mol Liq 212C:569–575
26. Jaycock MJ, Parfitt GD (1981) Chemistry of interfaces. Wiley,
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Amalendu Pal is an Emeritus Scientist (CSIR, New Delhi) in the
Department of Chemistry, Kurukshetra University, Kurukshetra,
India. He obtained his Ph.D. from Kalyani University, Kolkata, India
in 1984 and Postdoctoral from University of Blaise Pascal, France
from 1985–1987. His research interests in physical chemistry cover
the study of polymers, drugs, polyelectrolytes, ionic liquids, surfac-
tants, electrochemistry and thermodynamics.
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based ionic liquid 1-butyl-2,3-dimethylimidazolium chloride on
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Alka Yadav is a Ph.D. research scholar in the Department of
Chemistry, Kurukshetra University, Kurukshetra, India under the
supervision of Dr. Amalendu Pal. She was awarded her M.Sc. in 2010
from the same department. Her area of research involves the study of
aggregation behavior of different surface active ionic liquids (SAIL).
123