Thermodynamic and aggregation properties of aqueous dodecyltrimethylammonium bromide in the presence of hydrophilic ionic liquid 1,2-dimethyl-3-octylimidazolium chloride

Article abstract of DOI:10.1016/j.molliq.2015.10.042

The effect of a trisubstituted imidazolium based hydrophilic ionic liquid (IL), 1,2-dimethyl-3-octylimidazolium chloride [odmim][Cl] on the thermodynamic and aggregation properties of dodecyltrimethylammonium bromide (DTAB) at temperatures 298.15-318.15 K has been studied by conductivity measurements in aqueous media. From the temperature dependence of critical micelle concentration (cmc), various thermodynamic parameters of micellization viz. standard free energy change (ΔG_m^°), standard enthalpy change (ΔH_m^°) and standard entropy change (ΔS_m^°) have been evaluated. Further fluorescence probe analysis has been utilized to obtain cmc value in order to validate the cmc values obtained from conductivity measurements. The micelles formed just like in aqueous medium and cmc values were found to be changing at different wt% of IL than in water. The cmc value of aqueous DTAB solution decreases until 2.0 wt% of IL in the medium. The properties of aqueous DTAB solution change in a different way at higher concentration of IL. The variation in chemical shifts of surfactant protons in the presence of IL revealed that the electrostatic interactions have a prominent influence on the DTAB micellization in IL/water systems. It is shown that the modification in aggregation properties of DTAB is related to the lipophilicity of interacting ions.

Full text of DOI:10.1016/j.molliq.2015.10.042

Journal of Molecular Liquids 212 (2015) 818–824
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Thermodynamic and aggregation properties of aqueous
dodecyltrimethylammonium bromide in the presence of hydrophilic
ionic liquid 1,2-dimethyl-3-octylimidazolium chloride
⁎
Amalendu Pal , Ankita Pillania
Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of a trisubstituted imidazolium based hydrophilic ionic liquid (IL), 1,2-dimethyl-3-octylimidazolium
chloride [odmim][Cl] on the thermodynamic and aggregation properties of dodecyltrimethylammonium bro-
mide (DTAB) at temperatures 298.15–318.15 K has been studied by conductivity measurements in aqueous
media. From the temperature dependence of critical micelle concentration (cmc), various thermodynamic pa-
rameters of micellization viz. standard free energy change (ΔG°m), standard enthalpy change (ΔHm° ) and standard
entropy change (ΔS°m) have been evaluated. Further fluorescence probe analysis has been utilized to obtain cmc
value in order to validate the cmc values obtained from conductivity measurements. The micelles formed just like
in aqueous medium and cmc values were found to be changing at different wt% of IL than in water. The cmc value
of aqueous DTAB solution decreases until 2.0 wt% of IL in the medium. The properties of aqueous DTAB solution
change in a different way at higher concentration of IL. The variation in chemical shifts of surfactant protons in the
presence of IL revealed that the electrostatic interactions have a prominent influence on the DTAB micellization in
IL/water systems. It is shown that the modification in aggregation properties of DTAB is related to the lipophilicity
of interacting ions.
Received 11 August 2015
Received in revised form 8 October 2015
Accepted 20 October 2015
Available online xxxx
Keywords:
Critical micelle concentration
Ionic liquid
Conductivity
1H NMR
Thermodynamic parameters
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Surfactants play a significant role in many applications such as
nanomaterial synthesis, drug delivery, chemical separations, pharma-
ILs comprise a class of compounds which is composed of bulky
organic cations and appropriate organic or inorganic anions, usually
present in molten state below 373 K. These have a wide spectrum of ap-
plications in the field of chemical synthesis, separation science, catalysis,
electrochemistry and many more [1–4]. The unusual and interesting
properties of ILs such as negligible vapour pressure, non-flammability,
high thermal and mechanical stability, low toxicity, wide electrochemi-
cal window, easy recyclability and excellent capacity to dissolve organic
and inorganic solutes makes these the central spot of many research
applications [5–9]. The physicochemical properties of ILs can be easily
modified by changing the N-alkyl substituents and/or anions [10]. The
term “task specific ILs” is used when certain functional groups are intro-
duced in their structure to impart a specific characteristic. Such as the
primary amine-functionalized IL shows superior ability for carbon diox-
ide capture if compared to other conventional ILs [11]. Different types of
ILs are also utilized to induce favourable alterations in properties of var-
ious surfactant systems.
ceutical formulations and other dispersant technologies [12–15]. The
surface and thermodynamic properties of surfactants in different
solvent systems are routinely probed by researchers, both in the
absence and presence of additives. This helps to understand the various
solute–solute and solute–solvent interactions of surfactants in solution
of different additives [16]. The related balance of solvophobic and
solvophilic interactions governs the aggregation properties of surfactant
solution in presence of different additives. These properties can also be
modified by variation in temperature and pressure.
Reddy's group first showed the effect of ILs of different chain lengths
on the aggregation properties of aqueous sodium dodecyl sulphate (SDS)
[17]. Then Pandey et al. highly explored the effect of hydrophilic and hy-
drophobic ILs on the aggregation behaviour of different surfactants
[18–22]. Inoue's group specifically studied the effect of ILs on
polyoxyethylene (POE)-type non-ionic surfactants [23–25]. Sarkar and
coworkers have studied the effect of a mixture of two different kinds of
ILs on the aggregation behaviour of non-ionic surfactant Triton X-100
[26]. Our own group has also studied the effect of different anions of IL
on the micellization properties of SDS [27,28]. Most of this work is limit-
ed to the use of 1,3-dialkyl substituted imidazolium based ILs. In our pre-
vious work, we have effectively demonstrated the effect of using
trisubstituted imidazolium based IL, 1-butyl-2,3-dimethylimidazolium
The self-assembly of surfactant molecules into a large number of
aggregate structures is a topic of basic and fundamental research.
⁎
Corresponding author.
E-mail address: palchem21@gmail.com (A. Pal).
http://dx.doi.org/10.1016/j.molliq.2015.10.042
0167-7322/© 2015 Elsevier B.V. All rights reserved.

A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
819
chloride, [bdmim][Cl] in altering the micellization properties of SDS [29]
and dodecyltrimethylammonium bromide (DTAB) [30] in aqueous
media. Therein we have shown the difference in the modulating effect
of IL [bdmim][Cl] on surfactant properties when the charge on the
head group of surfactant under study is different.
ether several times to get a solid, which was dried in vacuum to get
1,2-dimethyl-3-octylimidazolium chloride [odmim][Cl], with 85%
yield. Karl–Fisher examination of the IL indicated that the water content
reduced to less than 280 ppm. The product obtained was flushed with
N2 and stored in a dry place before use.
Further it will be interesting to study the effect of change of alkyl
chain length of these ILs on a surfactant. Therefore, as extension to our
previous work, herein we are reporting the modulations in the aggrega-
tion properties of aqueous DTAB (Scheme 1) solutions upon the addi-
tion of IL 1,2-dimethyl-3-octylimidazolium chloride [odmim][Cl]
(Scheme 1). Conductivity and fluorescence measurements have been
used to obtain the cmc value for different surfactant-IL solutions. The
thermodynamic parameters have been analysed to understand the driv-
ing force of micellization. It was possible to obtain the cmc values from
both conductometry and fluoremetry up to 1.0 wt% of added IL, after
that cmc determination has been done through fluorescence only. The
aggregation number has been determined through a fluorescence
quenching method. 1H NMR measurements have been done to under-
stand the preferential site of interactions between IL and surfactant
molecules. Here due to the addition of IL [odmim][Cl], highly unpredict-
able results have been obtained from our previous investigation on
DTAB [30]. The dependency of cmc and other properties of DTAB solu-
tion on the alkyl chain of investigated ILs have been observed. The aim
of this manuscript is to offer a comparative study and establish the
role of the alkyl chain length of the cation of the imidazolium IL on the
physicochemical properties of aqueous DTAB solution.
1H NMR (400 MHz, CDCl3, δ-ppm): 0.762(t, 3 H), 1.1745(m, 10 H),
1.696(m, 2 H), 2.476(s, 3 H), 3.659(s, 3 H), 3.997(t, 2 H), 7.206(d,
1 H), 7.235(d, 1 H).
2.3. Experimental procedure
Required amounts of materials were weighed using an A&D Co. Ltd.
electronic balance (Japan, model GR-202) with a precision of 0.1 mg.
All the experiments have been carried out in doubly distilled de-ionized
water obtained from a Millipore, Milli-Q Academic water purification
system having conductivity ≤5 μS cm−1. All experiments were per-
formed in triplicate and obtained values were averaged.
The conductivity measurements were taken with a digital conduc-
tivity meter CM-183 microprocessor based EC-TDS analyser having
ATC probe and conductivity cell with platinized platinum electrodes
purchased from Elico Ltd., India having cell constant 1.0021 cm−1. The
cell was calibrated with the standard aqueous potassium chloride solu-
tions in the concentration range of 0.01–1.0 mol kg−1 of known specific
conductance. The electrical conductivities were measured at different
temperatures with an uncertainty of 0.01 K in a water jacketed flow
dilution cell. This instrument works on the power supply of 90–260 V al-
ternating current and at frequency of 50–60 Hz. The surfactant conduc-
tance was measured after thoroughly mixing the solution and
temperature equilibration. The small conductance due to water was
subtracted from the measured data. The uncertainty of the measure-
ments was found to be less than 4%.
2. Experimental
2.1. Materials
DTAB (99%, AR) was obtained from Acros Organics and 1,2-
dimethylimidazole (98%) and pyrene (99.9%) were procured from
Sigma-Aldrich. Cetylpyridinium chloride (99%) was purchased from
Loba Chemie and methanol (99%) from Rankem. Deuterium oxide hav-
ing isotopic purity (≥99.9%) and 1-chlorooctane (N98%) was obtained
from SD Fine Chemicals. IL 1,2-dimethyl-3-octylimidazolium chloride
[odmim][Cl] used in the present study is synthesized using the same pro-
cedure as described in our earlier study [29].
The fluorescence spectra were recorded using a Shimadzu (model
RF-5301PC) spectrofluorimeter. It is provided with blazed holographic
grating excitation and emission monochromators fitted with a 150 W
Xenon lamp. Pyrene and cetylpyridinium chloride (CPC) were used as
fluorescence probe and quencher, respectively. Stock solution of pyrene
and CPC were prepared in methanol and water, respectively. The solu-
tion of DTAB and IL/water at various concentrations had been freshly
prepared in doubly distilled de-ionized degassed water. After stirring
the solutions for 12 h, these solutions were allowed to stabilize for
half an hour to record the spectra. The excitation wavelength was kept
at 334 nm and the emission was recorded in the wavelength region
350–600 nm. The excitation and emission slits were kept at (3 and
1.5) nm, respectively. The ‘II/IIII’ ratio has been obtained from fluores-
cence intensities of the first and third vibronic peaks of pyrene for
data analysis.
2.2. Synthesis of 1,2-dimethyl-3-octylimidazolium chloride [odmim][Cl]
For the synthesis of 1,2-dimethyl-3-octylimidazolium chloride
[odmim][Cl], initially a slight excess of 1-chlorooctane (16.1 g,
108 mmol) was added drop wise to 1,2-dimethylimidazole (10 g,
104 mmol) in a round bottom flask followed by refluxing the solution
at about 60–70 °C for 24 h under N2 atmosphere and then cooled to
room temperature for 12 h. The resultant compound was washed with
The NMR chemical shifts for various protons were observed with a
Brüker Avance III spectrometer operating at 400 MHz. In order to deter-
mine chemical shift δ for DTAB + [odmim][Cl] solution in water, deute-
rium oxide (D2O) was used as a solvent for all the NMR measurements.
The chemical shifts are reported as δ units (ppm). The chemical shift
values of peaks of interest were obtained using peak pick facility.
3. Results and discussion
3.1. Conductivity measurements
3.1.1. Effect of IL [odmim][Cl] on the cmc of DTAB
The conductivity, κ, values were obtained as a function of [DTAB]
concentration on the addition of different wt% of IL [odmim][Cl] in the
solutions at temperature range of 298.15–318.15 K with an interval of
10 K. The conductivity vs concentration plots of DTAB in the absence
and presence of different wt% of IL at 298.15 K are shown in Fig. 1.
Two linear regimes were observed in the conductivity graph for each
studied wt% of IL corresponding to the pre- and post-micellar regions.
The x-intersection of these two regions is identified as cmc. The slope
Scheme 1. Chemical structures of IL 1,2-dimethyl-3-octylimidazolium chloride
[odmim][Cl] and cationic surfactant dodecyltrimethylammonium bromide (DTAB).

820
A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
Fig. 1. (a) The conductivity, (κ) of aqueous DTAB solutions in the absence and presence of different wt% of IL [odmim][Cl] at 298.15 K. (b) The κ versus [DTAB] plots at 0 (black symbols) and
0.5 (red symbols) wt% of IL at temperatures (□) 298.15 K, (○) 308.15 K and (Δ) 318.15 K in aqueous media. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
ratio of post- to pre-micellar region provides the information about de-
gree of dissociation (α) of counter-ions from the micellar surface. We
have measured conductivity up to 1.0 wt% of added IL above this wt%
it was difficult to draw any meaningful conclusion from conductivity
data owing to very high conductivity of IL itself.
it can provide stabilizing hydrophobic interactions to DTAB molecules
than the butyl chain of IL [bdmim][Cl]. Thus [odmim][Cl] will solubilize
surfactant monomers to certain extent and decreases the solvophobicity
of surfactant molecules. This leads to a slight delay in the micellization
process. Consequently the cmc value decreases on the addition of
[odmim][Cl] but not as effectively as in the case of [bdmim][Cl]. Here
The cmc value for aqueous DTAB solution obtained from our data is
in good agreement with literature which is 15.7 mmol kg−1 [31]. It can
be clearly seen from Fig. 1 that the value of cmc gradually decreases
with the addition of IL [odmim][Cl] in the DTAB solution. The higher
the wt% of IL in the solution the lower the cmc value observed. The
cmc values obtained at different temperatures are reported in Table 1.
The IL [odmim][Cl] is constituted of two parts, bulky imidazolium cat-
ion, [odmim]+ and small anion, [Cl]−. The surfactant DTAB also has a
cationic head group with long dodecyl chain and bromide counter ion.
During micellization the interaction between the [DTA]+ ion of surfac-
tant and [odmim]+ ion of IL is restricted due to presence of similar
charge on these species, although the possibility of interaction between
alkyl chains cannot be ruled out. If one look at the anion of IL i.e. Cl− ion,
this can easily interact with the [DTA]+ ion of the surfactant. The elec-
trostatic interactions between IL anions and surfactant cations lead to
a decrease in repulsions among surfactant head groups. This allows sur-
factant monomers to undergo micellization at a lower concentration
and the cmc value of surfactant solution decreases in the presence of IL.
Although this was also observed in case of our previous study with
DTAB + [bdmim][Cl]/H2O, but the decrease in cmc here is lesser as com-
pared to what is observed there. In that study lowering in cmc took
place to a greater extent [30]. There can be two probable reasons for
this observation. First, due to longer alkyl chain in IL [odmim][Cl] than
[bdmim][Cl] it will be less dissociated in the solution, so Cl− ions will
be available to lesser extent for interaction with surfactant head groups.
This fact is also supported by the low conductivity values of DTAB/
[odmim][Cl] systems than DTAB/[bdmim][Cl] systems at a fixed wt% of
IL (Fig. S1). Secondly due to the presence of octyl chain in [odmim][Cl],
Table 1
The critical micelle concentration (cmc) of the aqueous DTAB solutions in different wt% of
IL and the corresponding values of various thermodynamic parameters of micellization
obtained from Eqs. (1)–(3) at temperatures 298.15–318.15 K.
T (K)
cmc
α
ΔG°m
ΔH°m
−TΔS°m
(mmol kg−1
)
(kJ mol−1
)
(kJ mol−1
)
(JK−1 mol−1
)
DTAB + [odmim][Cl] (0 wt%)
298.15
308.15
318.15
15.8
16.4
17.0
0.268
0.274
0.280
−35.05
−35.94
−36.81
−8.48
−8.83
−9.19
−26.57
−27.10
−27.61
DTAB + [odmim][Cl] (0.05 wt%)
298.15
308.15
318.15
15.5
16.1
16.7
0.276
0.282
0.288
−34.97
−35.85
−36.72
−8.56
−8.91
−9.27
−26.41
−26.94
−27.45
DTAB + [odmim][Cl] (0.1 wt%)
298.15
308.15
318.15
15.3
15.8
16.4
0.284
0.292
0.299
−34.87
−35.73
−36.57
−8.99
−9.63
−10.27
−25.88
−26.10
−26.30
DTAB + [odmim][Cl] (0.5 wt%)
298.15
308.15
318.15
13.5
14.2
14.7
0.317
0.323
0.333
−34.72
−35.54
−36.32
−10.14
−11.19
−11.27
−24.58
−24.35
−25.05
DTAB + [odmim][Cl] (1.0 wt%)
298.15
308.15
318.15
11.6
12.2
12.9
0.355
0.362
0.369
−34.55
−35.35
−36.10
−10.67
−12.07
−12.55
−23.88
−23.28
−23.55

A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
821
the possibility of repulsions between [odmim]+ and [DTA]+ ions are
less than those between [bdmim]+ and [DTA]+ ions. The greater extent
of electrostatic interactions than hydrophobic interactions between the
additive and surfactant moieties lead to lowering in cmc of the system
studied here.
comparison to the previous study [30]. The value of entropy decreases
at higher wt% of IL. With the increase in temperature the value of en-
thalpy increases while that of entropy decreases. At high temperature
it is easy to break the three-dimensional water structure due to thermal
agitation so a high value of ΔH°m is obtained. So the micellization process
starts shifting from entropy-driven to enthalpy-driven with the increase
in temperature and wt% of added IL. This kind of behaviour is also
known for conventional ionic surfactants [34,35].
3.1.2. Effect of temperature on the cmc of DTAB
It is observed that for the studied temperature range the value of
cmc increases with an increase in temperature. It is well known in liter-
ature that ionic surfactants have minima in cmc values when studied
from low to high temperatures [32]. For DTAB + [odmim][Cl]/H2O sys-
tems as we studied from 298.15 K to 318.15 K, a continuous increase is
observed. The variation in cmc with temperature is usually analysed by
considering two opposite factors. First, with the increase in temperature
the degree of hydration of hydrophilic group of surfactant decreases,
this favours the micellization process. Secondly the increase in temper-
ature causes the disruption of water structure; this leads to the increase
in solubilisation of surfactant monomers and disfavours the micelliza-
tion. The cmc of DTAB solution has been found to increase with temper-
ature at all the studied wt% of IL. It indicates that the disruption of water
structure occurs to greater extent due to thermal agitation of molecules
in the system so cmc increases with temperature.
3.2. Fluorescence measurements
3.2.1. Determination of cmc
The fluorescence behaviour of fluorescent probes is highly used to
study the micellization behaviour of surfactants in different medium
[36,37]. We have studied the aggregation properties of DTAB +
[odmim][Cl]/H2O systems using the most common and popular probe
pyrene. The intensity ratio of first to third vibronic peak of pyrene is
highly sensitive towards the polarity of its microenvironment. This
ratio has a high value in a polar medium and vice-versa [38]. The polar-
ity sensed by pyrene molecules in a pre-micellar solution is high while
in a post-micellar solution pyrene molecules partition themselves near
the palisade layer of micelles. The environment inside micelles is a hy-
drocarbon like solvent so II/IIII ratio decreases. This drop in II/IIII ratio
marks the onset of micellization. Fig. 2 represents the pyrene II/IIII versus
log10[DTAB] in the presence of different amount of IL [odmim][Cl]. The
data were fitted to simplistic sigmoidal expression. The cmc values of
aqueous DTAB in the presence of different amounts of IL are reported
in Table 2. These have been found in good relevance to those obtained
from conductance measurements.
3.1.3. Effect of IL and temperature on the thermodynamic parameters of
micellization of DTAB
To understand the driving force of micellization process various
thermodynamic parameters of micellization such as standard free ener-
gy change, ΔG°m, the standard enthalpic change, ΔH°m and the standard
entropy change, ΔS°m, in DTAB solutions have been obtained by using
the following equations [33]:
The cmc of aqueous DTAB solution decreases from 15.9 mmol kg−1
to 10.5 mmol kg−1 with the addition of 2 wt% [odmim][Cl], but the fur-
ther increase in concentration of IL up to 10 wt% causes the increase in
cmc up to 38.5 mmol kg−1. The cmc value at 5 wt% of IL increases but
remains lower than the value of DTAB in pure water. So the titled IL is
able to enhance the surface activity of DTAB upon addition of up to
5 wt% of IL after this it highly decreases the surface activity as the cmc
value gets almost doubled at 10 wt% of IL. Interestingly, in the case of
[bdmim][Cl], only the decrease in cmc takes place upon the addition
of 10 wt% of IL [30]. The inset in Fig. 2 clearly indicates the more effec-
tiveness of [bdmim][Cl] in lowering the cmc of DTAB. So the dissimilar
effect of the two ILs, [odmim][Cl] and [bdmim][Cl] is noticeably ob-
served by the variation in cmc of DTAB on the addition of ILs. Our results
clearly demonstrate that the two ILs [bdmim][Cl] and [odmim][Cl] pro-
duce different effects on the aggregation properties of aqueous DTAB so-
lutions. At a lower concentration (i.e., ≤2 wt%) both ILs show the same
trend in modifying properties of aqueous DTAB solution although to dif-
ferent extent. But at the higher concentration of ILs, the properties of
aqueous DTAB solutions start changing in other way.
To account for these differences we need to understand the structur-
al difference between the two ILs. Due to the long octyl chain,
[odmim][Cl] can interact like a co-surfactant and penetrate in the mi-
celle of DTAB, whereas [bdmim][Cl] will be unable to properly act like
a co-surfactant. IL [bdmim][Cl] has more tendencies to behave like an
electrolyte. It was expected that IL [odmim][Cl] can show mixed micell-
ization with DTAB. So, we studied the aggregation behaviour of pure IL
[odmim][Cl] in aqueous media, it has two distinct break points in con-
ductivity graph corresponding to critical aggregation concentration
(cac) at 24.4 mmol kg−1 and the cmc value at 230.7 mmol kg−1
(Fig. S2). It is reported in literature [39] that surfactant molecular inter-
action parameter, β has a value near to zero if the two ionic surfactants
have similar charge on head groups as in our study (DTAB +
[odmim][Cl]). This is due to the negligible change in the magnitude of
self-repulsion before mixing and mutual repulsion after mixing with
each other. The addition of IL increases the ionic strength of surfactant
solution this decreases the electrostatic self-repulsion. Here at low
wt% [odmim][Cl] decreases the cmc of aqueous DTAB solution. This
ΔGom ¼ ð2−αÞRT lnXcmc
ð1Þ
ð2Þ
ꢀ
ꢁ
d lnXcmc
dð1−αÞ
ΔHom ¼ −RT2 ð2−αÞ
þ lnXcmc
dT
dT
ΔHom−ΔGom
ΔSom
¼
ð3Þ
T
where α is the degree of dissociation, R is gas constant, T is temperature
and Xcmc is the cmc expressed in terms of mole fraction. The calculated
values of these parameters for all the studied systems at different tem-
peratures have been presented in Table 1. If one analyses the trend ob-
served for α with the increase in temperature and wt% of IL, it is found to
increase in both the cases. The increase in α with temperature is due to
greater dissociation of counter ions from micellar surface as a conse-
quence of high thermal agitation in the system. While the increase in
α with high wt% of IL is due to interaction of [Cl]− ions of IL near micelle
surface which interacts with –N+(CH3)3 head groups. It causes some of
the [Br]− ions of surfactant to dissociate from micellar aggregate. With
the increase in concentration of IL the number of interacting [Cl]− ions
also increases.
A perusal of data from Table 1 reveals that the negative values of
ΔG°m and ΔH°m are observed at all the studied temperatures. This indi-
cates the spontaneous and exothermic nature of micellization process.
The negative values of ΔG°m have been found to be slightly dependent
of temperature and the wt% of IL added. The value of ΔG°m itself is obtain-
ed by the contribution of two terms, enthalpic (ΔH°m) and entropic
(−TΔS°m). It is observed that the entropy change for micellization pro-
cess, ΔS°m is large and positive for all the studied systems. This contrib-
utes to greater extent to ΔG°m values at lower wt% of IL. At higher wt%
of IL the enthalpic contribution increases towards ΔG°m, as depicted in
Table 1. If compared to DTAB + [bdmim][Cl]/H2O system [30], there
enthalpic contribution increases more significantly with higher wt% of
IL. Here the lesser interaction of IL anions near micellar surface as
discussed earlier leads to a less exothermic micellization process in

822
A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
Fig. 2. Pyrene II/IIII versus log10[DTAB] in the absence and presence of different wt% of IL [odmim][Cl] in the aqueous media. Inset shows the variation in cmc of DTAB in presence of different
wt% of ILs, [bdmim][Cl] [27] and [odmim][Cl].
result reduces the possibility of mixed micellization between DTAB and
[odmim][Cl]. As [odmim][Cl] has very high cmc value so the decrease in
surfactant cmc value indicates electrolyte behaviour of IL at lower wt%.
Only few IL molecules penetrated and interacted inside the micelle
while most of them present near outer corona. If the size of the alkyl
chain attached to IL is considered octyl chain is longer than butyl
chain, stearic repulsion increases with increased bulkiness of the
group. It is also known that stearic effect is greater at convex mixed mi-
cellar interior than electrostatic effect [33]. In the study of mixed micell-
ization of cationic surfactants among themselves when taken in 1:1
ratio it was seen that the cmc value lies closer to the surfactant cmc
which has a lower cmc value [40]. Accordingly, the cmc value should
be intermediate of the surfactant cmc values under study. It is interest-
ing that, in our study the cmc value is below pure DTAB cmc, indicating
IL effects on DTAB micellization rather doing mixed micellization. As
van der Waals interactions will always be positive and depend on length
and closeness of packing of hydrophobic groups; so [odmim][Cl] will be
interacting more with DTAB due to longer octyl chain. These interac-
tions will substantially increase at higher concentration of IL. At
10 wt% IL [odmim][Cl] solubilizes surfactants molecules due to promi-
nent hydrophobic interaction of alkyl chains, this indicates the co-
solvent nature of IL at high concentration. The asymmetric size of IL
head group as compared to DTAB will also hinder the micellization pro-
cess at such high concentration. If we compare the II/IIII ratio at each wt%
studied its value is less for [odmim][Cl] than [bdmim][Cl] [30]. The low
value clearly indicates a less polar environment due to interaction of
alkyl chains of surfactant and IL. The trend in cmc of aqueous DTAB so-
lution shows that at lower concentration [odmim][Cl] majorly acts like
an electrolyte. The anion of IL undergoes coulombic attractive interac-
tion with the cationic head groups of DTAB, lowers the electrostatic
repulsion among head groups and decreases the cmc. However at high
concentration it acts like a solvent and solubilizes surfactant molecules
thus delays the aggregation. At higher wt%, due to the low
solvophobicity of the environment surfactant molecules undergo mi-
cellization at a higher concentration than cmc. While in the case of
[bdmim][Cl], it specifically behaves like an electrolyte even at 10 wt%
and lowers the cmc of aqueous DTAB solution. It is noteworthy that at
concentrations greater than 10 wt% of IL the fluorescence measure-
ments here become unreliable rendering it impossible to extract any
meaningful conclusion.
3.2.2. Determination of micellar aggregation number
The aggregation numbers (Nagg) of aqueous DTAB micelle in the
presence of different amounts of IL [odmim][Cl] is obtained by observ-
ing the fluorescence quenching behaviour of pyrene by CPC. The follow-
ing equation is used for the determination of micellar aggregation
number [41–43]:
Table 2
The cmc values and aggregation number obtained from fluorescence measurements in the
aqueous solution of DTAB on the addition of different wt% of IL [odmim][Cl] at 298 K.
[odmim][Cl] (wt%)
cmc (mmol kg−1) fluorescence
Nagg
ꢂ
ꢃ
ꢀ
ꢁ
0
15.9
15.6
15.2
13.8
11.9
10.5
11.4
38.5
49
52
54
59
62
67
51
42
Io
IQ
Qmicelle
½micelleꢀ ½micelleꢀDTAB
½CPCꢀmicelle
Nagg
½DTABꢀ−cmcDTAB
ln
¼
¼
¼ ½CPCꢀmicelle
ð4Þ
0.05
0.1
0.5
1.0
2.0
5.0
10.0
where IO and IQ are fluorescence intensities of pyrene in the absence and
presence of quencher CPC, respectively. Qmicelle (or [CPC]micelle),
[micelle]DTAB, and [DTAB] are the concentrations of quencher CPC with-
in the micellar phase, micelles, and DTAB surfactant, respectively.

A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
823
Aggregation number is determined from the slope of the ln(Io/IQ) versus
[CPC]micelle plot for all the systems. The plots of ln(IO/IQ) versus [CPC] for
quenching of pyrene by CPC within IL-added 50 mmol kg−1 aqueous
DTAB solution is presented in Fig. 3. It is suggesting a good linear behav-
iour on the addition of IL. The obtained slope increases up to 2.0 wt% of
IL and then starts decreasing from 5.0 wt% of IL. The aggregation number
thus obtained for aqueous DTAB micelles in the presence of different
amount of IL [odmim][Cl] are listed in Table 2.
Our result of Nagg for aqueous DTAB solution is in good agreement
with that reported in the literature [44]. The aggregation number of
DTAB increases from 49 to 67 with the addition of 2 wt% of IL, but the
further increase in concentration of IL up to 10 wt% causes a decrease
of aggregation number to 42. It is well known in literature that the ag-
gregation number increases with the decrease in cmc and vice-versa.
The increase in aggregation number is due to the lesser head group re-
pulsions among surfactant monomers which allow greater a number of
surfactant molecules to form the micellar aggregate. The decrease in ag-
gregation number at higher concentration of IL evidences the co-solvent
nature of IL. The asymmetric head group of [odmim][Cl] will prevent a
greater number of surfactant monomers to assemble for the micellar ag-
gregate. As co-solvents like glycerol [45] and ethylene glycol [46] also
decrease the aggregation number of cationic surfactants when present
in high concentration.
Fig. 4. The change in chemical shifts, Δδ for the protons of DTAB against different wt% of IL
in aqueous media.
the conclusion derived earlier that interaction of IL anions occurs at the
micellar surface. The greater dissociation of counter-ions from micellar
surface leads to slight deshielding of surfactant protons so the δ values
shifts downfield as shown in Fig. 4. At wt% of IL ˃ 1.0, the peaks of surfac-
tant protons get diminished and unaccountable, so NMR study has been
done up to 1.0 wt% of IL.
3.3. 1H NMR measurements
The 1H NMR technique was applied to reveal the intermolecular in-
teraction in the IL-DTAB aqueous solutions. The hydrogen atoms on var-
ious carbons of surfactant are labelled as shown in Scheme 1. The
observed chemical shifts reflect the changes in surrounding environ-
ment of surfactant protons in the micelles upon the addition of IL. The
chemical shift, δ of protons of DTAB (50 mmol kg−1) aqueous systems,
upon the addition of different IL concentration up to 1.0 wt% of IL are
4. Conclusions
In conclusion, we can say that trisubstituted imidazolium based
ionic liquids of varying N-alkyl chain length can be used to modify the
thermodynamic and aggregation properties of cationic surfactants in a
controlled fashion. Here the anion of IL acts as oppositely charged coun-
terion for cationic DTAB micelles and supports the micellization process
at low wt% of IL. Nevertheless, a clear change is observed in the physico-
chemical behaviour of aqueous solution of DTAB at high concentration
of [odmim][Cl] and [bdmim][Cl] [27]. The longer alkyl chain that is
present on the [odmim][Cl] allows it to participate with micelle at
lower concentration and as a co-solvent at high wt% of IL. This decreases
the solvophobicity of DTAB monomers and micellization gets delayed.
While [bdmim][Cl] mainly behaves like an electrolyte even at a concen-
tration as high as 10 wt%. These surfactant-IL mixed systems may have
enormous future applications in conducting interfacial reactions and
micellar catalysis. The micellization of DTAB in the presence and ab-
sence of IL [odmim][Cl] was found to be entropy driven at low temper-
atures and low wt% of IL while enthalpy driven at high temperatures
and high wt% of IL. The results obtained from conductivity measure-
ments are well-supported by fluorescence and NMR techniques.
presented in Table S1. The 1H NMR spectrum of DTAB (50 mmol kg−1
)
shows five characteristic peaks corresponding to the terminal −CH3
(a) protons of the dodecyl chain at 0.8006 ppm, bulk −CH2
(b) protons at 1.2767 ppm, β–CH2 (c) protons at 1.6868 ppm, α–CH2
(d) at 3.253 and –CH3 (e) protons attached to ammonium group
resonates at 3.0408 ppm (Scheme 1). The chemical shift of the protons
(a and b) of tail the region of DTAB varies to small extent while that of
the head region (c, d and e) varies more at high concentration of IL.
The overall trend is a downfield shift for all the protons but more prom-
inent for protons near head group. This result is in good agreement with
Acknowledgements
One of the authors, Ankita Pillania would like to thank University
Grants Commission (UGC), India for a JRF fellowship. Authors gratefully
acknowledge the financial support for the work by Government of India
through the UGC, New Delhi (Letter No. F. 41-328/2012 (SR)). Authors
also wish to acknowledge Department of Chemistry, Guru Jambheshwar
University of Science and Technology, Hisar for providing NMR research
facilities.
Appendix A. Supplementary data
Fig. 3. Pyrene fluorescence quenching by CPC in aqueous DTAB in the absence and pres-
ence of different wt% of [odmim][Cl]. Solid lines represent the results of linear regression
analysis.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2015.10.042.

824
A. Pal, A. Pillania / Journal of Molecular Liquids 212 (2015) 818–824
[23] T. Inoue, Y. Iwasaki, J. Colloid Interface Sci. 348 (2010) 522–528.
References
[24] T. Inoue, Y. Higuchi, T. Misono, J. Colloid Interface Sci. 338 (2009) 308–311.
[25] T. Inoue, T. Misono, J. Colloid Interface Sci. 337 (2009) 247–253.
[26] V.G. Rao, S. Mandal, S. Ghosh, C. Banerjee, N. Sarkar, J. Phys. Chem. B 116 (2012)
13868–13877.
[27] A. Pal, S. Chaudhary, Fluid Phase Equilib. 352 (2013) 42–46.
[28] A. Pal, S. Chaudhary, Fluid Phase Equilib. 372 (2014) 100–104.
[29] A. Pal, A. Pillania, Colloids Surf. A 452 (2014) 18–24.
[30] A. Pal, A. Pillania, Fluid Phase Equilib. 389 (2015) 67–73.
[31] S. Chauhan, M. Kaur, K. Kumar, M.S. Chauhan, J. Chem. Thermodyn. 78 (2014)
175–181.
[1] P. Wasserscheid, T. Welton, Ionic Liquids in Syntheses, Wiley, Weinhein, Germany,
2003.
[2] Y. He, T.P. Lodge, J. Am. Chem. Soc. 128 (2006) 12666–12667.
[3] J.L. Anthony, E.J. Maginn, J.F. Brennecke, J. Phys. Chem. B 106 (2002) 7315–7320.
[4] L.A. Blanchard, D. Hancu, E.J. Beckman, J.F. Brenneck, Nature 399 (1999) 28–29.
[5] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398.
[6] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers,
Green Chem. 3 (2001) 156–164.
[7] G.A. Baker, S.N. Baker, S. Pandey, F.V. Bright, Analyst 130 (2005) 800–808.
[8] R.D. Rogers, K.R. Seddon, Ionic Liquids: Industrial Applications for Green Chemistry,
ACS Symposium Series, vol. 818, American Chemical Society, DC, 2000.
[9] L.G. Chen, H. Bermudez, Langmuir 28 (2012) 1157–1162.
[10] F. Maier, T. Cremer, C. Kolbeck, K.R.J. Lovelock, N. Paape, P.S. Schultz, P.
Wasserscheid, H.P. Steinrueck, Phys. Chem. Chem. Phys. 12 (2010) 1905–1915.
[11] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 926–927.
[12] R. Savic, L. Luo, A. Eisenberg, D. Maysinger, Science 300 (2003) 615–618.
[13] C. Allen, D. Maysinger, A. Eisenberg, Colloids Surf. B 16 (1999) 3–27.
[14] Y. Morai, Micelles: Theoretical and Applied Aspects, Springer, New York, 1992.
[15] J.H. Fendler, Membrane Mimetic Chemistry: Characterizations and Applications of
Micelles, Microemulsions, Monolayers, Bilayers, Vesicles and Host–Guest Systems,
Wiley, New York, 1983.
[16] V. Srinivasan, D. Blankschtein, Langmuir 19 (2003) 9946–9961.
[17] A. Beyaz, W.S. Oh, V.P. Reddy, Colloids Surf. B 35 (2004) 119–124.
[18] K. Behera, P. Dahiya, S. Pandey, J. Colloid Interface Sci. 307 (2007) 235–245.
[19] K. Behera, M.D. Pandey, M. Porel, S. Pandey, J. Chem. Phys. 127 (2007)
184501–184510.
[32] A.G. Perez, J.L. Castillo, J. Czapkiewicz, J.R. Rodrıguez, J. Phys. Chem. B 105 (2001)
1720–1724.
[33] M.J. Rosen, Surfactant and Interfacial Phenomenon, 2nd ed. John Wiley and Sons,
New York, 1988.
[34] K.H. Kang, H.U. Kim, K.H. Lim, Colloids Surf. A 189 (2001) 113–121.
[35] S.K. Mehta, K.K. Bhasin, R. Chauhan, S. Dham, Colloids Surf. A 255 (2005) 153–157.
[36] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, Berlin, 2006.
[37] K.P. Ananthapadmanabhan, E.D. Goddard, N.J. Turro, P.L. Kuo, Langmuir 1 (1985)
352–355.
[38] D.S. Karpovich, G.J. Blanchard, J. Phys. Chem. 99 (1995) 3951–3958.
[39] Q. Zhou, M.J. Rosen, Langmuir 19 (2003) 4555–4562.
[40] N. Dubey, Fluid Phase Equilib. 368 (2014) 51–57.
[41] M. Tachiya, J. Chem. Phys. 76 (1982) 340–348.
[42] K.K. Karukstis, P. del Burgo, O. Llorea, E. Junquera, Langmuir 22 (2006) 4027–4036.
[43] N.J. Turro, A. Yekta, J. Am. Chem. Soc. 100 (1978) 5951 (5152).
[44] S.P. Moulik, M.E. Haque, P.K. Jana, A.R. Das, J. Phys. Chem. 100 (1996) 701–708.
[45] G. Errico, D. Ciccarelli, O. Ortona, J. Colloid Interface Sci. 286 (2005) 747–754.
[46] M.A. Rodrıguez, M. Munoz, M. Graciani, M.S.F. Pachon, M.L. Moya, Colloids Surf. A
Physicochem. Eng. Asp. 298 (2007) 177–185.
[20] K. Behera, S. Pandey, J. Colloid Interface Sci. 316 (2007) 803–814.
[21] R. Rai, G.A. Baker, K. Behera, P. Mohanty, N.D. Kurur, S. Pandey, Langmuir 26 (2010)
17821–17826.
[22] K. Behera, S. Pandey, Langmuir 24 (2008) 6462–6469.