Benzimidazolium-based high temperature ionic liquid-in-oil microemulsion for regioselective nitration reaction

Article abstract of DOI:10.1016/j.molliq.2018.07.010

Owing to the fascinating applications of ionic liquids (ILs) based non-aqueous microemulsions (MEs) in the field of chemical reactions due to their high thermal stability compared to that of aqueous MEs and requirement of water-free environment, we design

Full text of DOI:10.1016/j.molliq.2018.07.010

Accepted Manuscript
Benzimidazolium-based high temperature ionic liquid-in-oil
microemulsion for regioselective nitration reaction
Barnali Kar, Prasanjit Ghosh, Kaushik Kundu, Soumik Bardhan,
Bidyut K. Paul, Sajal Das
PII:
DOI:
Reference:
S0167-7322(18)32662-X
doi:10.1016/j.molliq.2018.07.010
MOLLIQ 9331
To appear in:
Journal of Molecular Liquids
Received date:
Accepted date:
22 May 2018
2 July 2018
Please cite this article as: Barnali Kar, Prasanjit Ghosh, Kaushik Kundu, Soumik Bardhan,
Bidyut K. Paul, Sajal Das , Benzimidazolium-based high temperature ionic liquid-in-
oil microemulsion for regioselective nitration reaction. Molliq (2018), doi:10.1016/
j.molliq.2018.07.010
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ACCEPTED MANUSCRIPT
Benzimidazolium-based High Temperature Ionic Liquid-in-Oil
Microemulsion for Regioselective Nitration Reaction
Barnali Kar,a Prasanjit Ghosh,a Kaushik Kundu,b,†† Soumik Bardhan,a,† Bidyut K. Paul,b
Sajal Dasa*
a Department of Chemistry, University of North Bengal, Darjeeling 734 013, India
b Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute,
Kolkata 700 108, India
Present address: † Department of Biotechnology, Bhupat and Jyoti Mehta School of
Biosciences, Indian Institute of Technology Madras, Chennai-600036, India.
†† Department Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-
560012, India
Authors for correspondence
Dr. Sajal Das
Department of Chemistry
University of North Bengal
Darjeeling-734 013, India
E-mail: sajal.das@hotmail.com
Phone: (91) (0353) -2776 -381
Fax: ((91) (0353)-2699 -001
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Abstract
Owing to the fascinating applications of ionic liquids (ILs) based non-aqueous microemulsions
(MEs) in the field of chemical reactions due to their high thermal stability compared to that of
aqueous MEs and requirement of water-free environment, we designed a high-temperature
stable cationic IL-in-oil microemulsions using 1-ethyl-3-propylbenzimidazolium bromide
([EPbim][Br]) as a polar phase for the first time. In order to determine the thermodynamic
stability of such type of MEs, we measured the distribution of cosurfactant (herein, 1-octanol)
between bulk n-decane phase and interface covered by quaternary ammonium head group of
cetyltrimethylammonium bromide (CTAB) at elevated temperature. The spontaneous
formulation of high-temperature IL-in-oil MEs with organized IL-oil interface was identified
by the negative free energy and entropy change at studied temperature range (358-370 K).
Interestingly, we observed temperature-independent exothermic behavior at a specific
composition, which probably comes from the cation-π interaction between quaternary
ammonium head group of CTAB and aromatic moiety of ILs. We performed phase behavior
study to demonstrate a wide range of thermally stable of [EPbim][Br]/CTAB/decane MEs.
Surprisingly, the droplet size was decreased with progressive addition of IL, which may be due
to the difficulty of formation of strong and favorable H-bond between the polar head groups of
CTAB and [EPbim][Br] at higher temperature. Furthermore, the aggregation number and
molecular interfacial area at higher temperature were calculated, which have direct correlation
with the droplet size. Finally, the nitration reaction with three different aromatic compounds
was performed in this MEs media at the temperature-independent composition. The reaction
was completed with the highest regioselectivity of para isomer product over the ortho
one. Moreover, these results clearly highlight an efficient way towards the formulation of high
temperature stable MEs with ILs at ambient pressure and open a wide field of potential
applications.
Keywords: Ionic Liquid, Microemulsion, Thermodynamics, Dynamic Light Scattering,
Nitration.
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1. Introduction
Microemulsions are thermodynamically stable ‘nano-dispersions’ of water-in-oil (or oil-in-
water) stabilized by a surfactant film, frequently in combination with a cosurfactant [1]. There
has been considerable interest in the use of microemulsions as media for organic synthesis in
recent years. Not only can such a formulation be a way to overcome compatibility problems,
the capability of microemulsions to compartmentalize and concentrate reactants can also lead
to considerable rate enhancement compared to one-phase systems. A third aspect of interest for
preparative organic synthesis is that the large oil-water interface of the system can be used as
a template to induce regioselectivity [2]. The dynamic character of these nano-reactors is one
of the most important features, which has to be taken into account for a comprehensive
understanding of chemical reactions carried out in these media [3].
In recent years, attempts were made to formulate and characterize waterless/non-aqueous
microemulsions, where other polar solvents like methanol, acetonitrile, glycerol, formamide,
ethylene glycol etc. were exploited. The first study of non-aqueous microemulsion with
ethylene glycol, lecithin and decane were reported by Friberg and Podzimek in 1984 [4]. Later,
Falcone et al. studied the properties of non-aqueous reverse micelles (RMs) using six polar
solvents, such as glycerol, ethylene glycol, propylene glycol, formamide, dimethyl formamide,
dimethylacetamide [5]. Very recently, Luna et al. also showed how the physicochemical
properties such as micropolarity, microviscosity, and H-bond interaction, of non-aqueous
AOT/n-heptane RM interfaces dramatically changed by simply using different non-aqueous
polar solvents [6]. Non-aqueous systems have distinct advantages over aqueous ones, such as
(i) they generally forms larger stable regions of isotropic solutions compared to the analogous
aqueous systems; (ii) a large variety of different surfactants can be used to give non-aqueous
microemulsions and (iii) these systems can be used as good reaction media, specifically for the
reactants which react with water [7]. These pioneer studies stimulated research on the
formulation of non-aqueous microemulsions containing room temperature ionic liquids
(RTILs), which also provide hydrophobic or hydrophilic nano-domains. Thus their applications
can be extended to fields of reaction and separation or extraction media. Earlier report has
shown that surfactant IL-based microemulsions can be used to produce polymer nanoparticles,
gels, and open-cell porous materials [8]. Also, IL-in-oil microemulsions were employed to
increase the solubility of a sparingly soluble drug to enhance its topical and transdermal
delivery and used as template to synthesize starch nanoparticles for investigation of the drug
loading and releasing properties [9-11]. ILs are considered suitable solvents for various
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chemical reactions and catalysis due to their characteristic properties like negligible vapour
pressure and tunability in structure by modifying cation and anion moieties [12, 13]. In
addition, ILs exhibit excellent chemical and thermal stability, wide polarity and are recyclable
[14, 15].
The IL-based microemulsions are studied by many groups [16-21], however, the above-
mentioned IL-based formulations are not stable at high temperatures because they contain non-
ionic surfactants. The effect of temperature on microemulsions with non-ionic surfactants is
very pronounced. Such microemulsions are not favourable for formulating systems that are
stable over a wide temperature range [22]. Therefore, to formulate high temperature stable IL
microemulsions, we have to look for charged surfactants based systems, which are rather scarce
in the literature [23-26]. Earlier, Kunz and his co-workers demonstrated the existence of high-
temperature stable microemulsions under ambient pressure, that showed a thermal stability
ranging from 30°C up to at least 150°C [22,27,28]. These systems are predestined for high
temperature applications such as reaction media, lubricant formulations and size controlled
nanoparticle synthesis.
Despite their great potential, reports on the performance of organic reaction in non-aqueous IL
microemulsions are still very scarce. In our previous work, we reported Heck reaction in w/o
non-ionic microemulsion system in the absence and the presence of an IL (1-Ethyl-3-
propylbenzimidazolium bromide), where the reaction ended with the highest yield (75%) in
presence of solute amount of IL, wherein the microemulsion forms spontaneously with the
highest stability [29]. Also, An and his co-workers reported the Diels-Alder reaction rate in the
microemulsion, which was enhanced in presence of IL and was faster than that in pure
isooctane and in generic AOT microemulsion [30]. The nitration of phenol is a fundamental
unit process of great industrial importance generating commercially valuable intermediates and
there is a great need for regioselective pollution free processes. In earlier work on nitration of
phenol in microemulsion, it was claimed that in an AOT [sodium bis(2-
ethylhexyl)sulfosuccinate]-based microemulsion, ortho nitration was favoured [31]. This was
explained as being due to the phenol orienting at the interface with the aromatic ring extending
into the organic domain and the hydroxyl group protruding into the aqueous domain. This
orientation would make the ortho positions more accessible than the para position to the
approaching nitronium ion. However, we have not been able to reproduce these results. A
survey of literature shows nitration of phenol lacks positional selectivity for para isomer with
majority of processes giving rise to o-isomer as the major product along with minor amounts
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of p-isomer. Among several nitrating agents employed which include mixed acid, super acids,
acyl nitrates and a variety of metal nitrates under different conditions, ferric nitrate in particular
either alone or supported on solid matrices such as clays has enjoyed considerable importance
[32, 33]. Herein, we formulated the non-aqueous microemulsions using
cetyltrimethylammonium bromide (CTAB) as cationic surfactant and n-octanol as cosurfactant
in n-decane with 1-ethyl-3-propylbenzimidazolium bromide ([EPbim][Br]) as a water-
substitute and finally, utilized as reaction media for regioselective nitration of substituted
phenol using nontoxic and inexpensive ferric nitrate. The current studies can help to understand
the microstructure of IL microemulsions and thus establish a better way of using them as a new
reaction medium.
2. Materials and Methods
2.1 Materials
Cetyltrimethylammonium bromide, CTAB (≥ 99%, CAS No. 57-09-0, Sigma), 1-octanol
(anhydrous, ≥ 99%, CAS No. 111-87-5, Sigma-Aldrich) and n-decane (anhydrous, ≥ 99%, CAS
No. 124-18-5, Sigma-Aldrich) were used without further purification.
2.2. Synthesis of ILs
The IL, 1-ethyl-3-propylbenzimidazolium bromide ([EPbim][Br]), is synthesized in
accordance with our reported method [34]. It is important to note that the synthesized pure IL
([EPbim][Br]) is white solid at room temperature [34], and it melts at 358 K. The 1H NMR of
synthesized IL has been provided in Supporting Information. In order to make non-aqueous IL
based microheterogeneous system, all the studies were carried out at the high temperature.
Nevertheless, the solubility of CTAB in the n-decane/1-octanol mixture was improved
drastically by increasing the temperature to 358 K. This is also the reason for the necessity of
tempering the system for getting optically clear solutions of IL based non-aqueous
microemulsion. However, these effects can principally not be assigned to electrostatic
interactions, which are most of all temperature independent in ionic surfactant-based
microemulsions in contrast to non-ionic-based ones.
2.3. Construction of phase diagram of ternary surfactant, oil and IL system
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The pseudoternary phase diagram comprising oil (n-decane/Dc), benzimidazolium IL (1-ethyl-
3-propylbenzimidazolium bromide, [EPbim][Br]), cationic surfactant (CTAB), and
cosurfactant (1-octanol/1-On) has been constructed by the method of titration and direct
observation. In a typical experiment, mixtures of oil and surfactant (including the cosurfactant)
with varying mass ratios of 4:1 are prepared in a series of stoppered test tubes. To construct the
pseudoternary phase diagrams for the IL based “water-free” microemulsions, the samples are
placed in a thermostatic water bath at (358 ± 0.5) K for 10 min, and then the mixture is titrated
by IL under moderate agitation. The IL volumes causing the solutions to turn to turbid from
clear transparent are noted to determine the phase boundaries. In each plotted phase diagram,
the upper part of the phase boundary represents a single-phase region (microemulsion, 1ϕ), and
the lower part is a multiphase region (2ϕ). The compositions in the phase diagrams are
represented in weight fractions. The same procedure was repeated for 2 to 3 times for each
mixture, and an average of these results was taken for the construction of phase diagrams.
2.4. Dilution method study for estimation of interfacial composition and spontaneity of
formation of microemulsion
In the dilution experiments, 0.00025 mole of surfactant (CTAB) is placed in a dry test tube,
followed by addition of fixed 2 ml of n-decane and different amount of [EPbim][Br] (at
different molar ratio of [EPbim][Br] to surfactant R = 1→5), respectively. The sample is placed
in a thermostated water bath, stirred constantly with a magnetic stirrer, and kept covered to
prevent loss due to evaporation. The cosurfactant (1-octanol) is then added slowly from an auto
pipette to the initially viscous and turbid mixture until the solution becomes clear, which is
indicative of the formation of the single phase. Sufficient time is given for equilibrium and then
the volume of cosurfactant is noted at that point. A known but small volume of oil is again
added to the system to destabilize it. A cloudy sample is made just clear by the addition of
cosurfactant. The quantity of cosurfactant is recorded. The experiment is monitored at fixed
temperature of 358 K. The entire experiment is then repeated for a second time to ensure
accuracy, and the average values obtained are used for data processing and analysis. In order
to estimate various thermodynamic parameters for the formation of IL-in-oil microemulsion,
the above-mentioned experiment is carried out at three other temperatures, such as 362K, 366K
and 370K.
2.5. Dynamic light scattering (DLS) studies
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DLS measurements are carried out using a Zetasizer Nano ZS90 (ZEN3690, Malvern
Instruments Ltd, U.K.). A 2.5 mmol of CTAB mixed with appropriate amount of 1-octanol
dissolved in 2ml of n-decane is used for such studies. The same set of CTAB/1-octanol ratio
(w/w), as used for the construction of phase diagram is employed for droplet size analysis.
Samples are filtered thrice before each measurement using MiliporeTM hydrophobic membrane
filter of 0.25 μm pore size to remove possible dust particles. A He–Ne laser operating at a
wavelength of 632.8 nm was used and the data were collected at a 90° angle. Temperature is
controlled to within ±0.05 K using a built-in Peltier heating-cooling device. Hydrodynamic
diameter (Dh) of the microemulsion solutions is estimated from the intensity autocorrelation
function of the time-dependent fluctuation in intensity. According to Stokes-Einstein equation,
Dh is defined as [35, 36]:
푘푇
퐷 =
(1)
⁄
3휋휂퐷ℎ
Where, k, T, D and η indicate the Boltzmann constant, temperature, diffusion coefficient and
viscosity of the solvent (herein n-decane) respectively. To check the reproducibility of the
results at least 6 measurements are performed.
2.6. General procedure for nitration reaction
Fe (NO3)3.9H2O (0.5 mmol) and the substrate (1 mmol) were placed in a 10 mL round bottom
flask containing the IL-in-oil (IL/O) microemulsion (3 ml). The mixture was placed on a
magnetic stirrer at 358 K and stirring continued for 15 min. Then the reaction mixture was
diluted with water and extracted with dichloromethane (3 x 10 mL). The combined organic
layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude
residue was purified by silica gel column chromatography using the mixture of petroleum ether
and ethyl acetate as eluent. The 1H and 13C NMR of all reaction products have been provided
in Supporting Information.
3. RESULTS AND DISCUSSION
3.1. Distribution of 1-octanol for the formation of stable and spontaneous IL-in-oil
microemulsion
Typically a medium chain alcohol is required to stabilize a microemulsion comprising CTAB
as surfactant [37]. In IL/O microemulsion system, CTAB are considered to populate at the
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oil/water interface in partial association with the cosurfactant (herein, 1-octanol). On the other
hand, 1-octanol can distribute only between the interface and the bulk oil phase, because of
their negligible solubility in water [38]. Thus, a critical amount of 1-octanol is required for the
stabilization of the [EPbim][Br]/CTAB/Dc systems. In this report, we perform a simple
titrimetric technique (known as the dilution method) to estimate amount of 1-octanol
distributed in the interface and oil phase in order to form a stable microemulsion [39]. From,
this simple but informative method, one can derive many useful thermodynamic parameters
along with the partition coefficient/ distribution constant of cosurfactant between oil and
interface for the formation of IL-in-oil microemulsions [40-42]. However, similar studies
involving ILs as polar component instead of water are not common in literature. Thus the
dilution studies involving the evaluation of interfacial behaviour, thermodynamic and
structural parameters of IL-in-oil microemulsion are considered to be significant. The basic
mathematical formalism to determine the thermodynamics of microemulsion formation for the
presently studied IL -in-oil microemulsion systems can be found in earlier reports [41-43].
In the present report, this method is used for the estimation of different parameters concerning
to the formation of IL-in-oil microemulsion at 358 K with varying molar ratio of IL to
surfactant, R (= 1→5). The following equations are helpful to rationalize the distribution vis-
à-vis transfer process of 1-octanol from the continuous oil phase to the interfacial region:
푛푎0
⁄
푘표 =
(2)
(3)
푛표
ꢀ
푛
푛
푛
표
푎
=
푎 + 푘표
푛
푠 푠
푛
푠
푛
ꢀ
푛
푎
⁄
푋푎ꢁ
ꢀ
표
ꢀ
푛 (푛 +푛 )
푛 +푛
푎
표
푎
표
푠
푎
⁄
퐾푑 =
=
=
(4)
(5)
푋푎표
표
푎
ꢀ
푎
푛
푛 (푛 +푛 )
푠
푎
⁄
표
푛 +푛
표
푎
ꢀ
∆퐺푡0 = −푅푇푙푛퐾푑 = −푅푇푙푛 푋 = −푅푇푙푛 푆퐼((11++푆퐼))
푋
푎
표
푎
Where, na, nai and Gibbs free energy of transfer of cosurfactant from oil to interface (-∆G0 ),
t
nao, no, ns denote the total number of moles of cosurfactant, its number at the interface, in the
oil phase, the total number of moles of oil and the total number of moles of surfactant,
respectively. A plot of na/ns against no/ns at different R (Fig. 1A) according to equation (3)
o
yields the values of the slope (S) and the intercept (I). Slope (S) is actually ko and na can be
i
determined from equation (2). On the other hand, na can be calculated from the intercept (I),
which is equal to nai/ns. The partition of cosurfactant between the continuous oil phase and the
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interface of the droplet can be expressed in terms of the distribution constant, which is
i
represented by Kd. The values of Xa and Xao are the mole fraction of cosurfactant in the
i
interfacial layer and in the oil, respectively. It is evident from Fig. 1B that na or nai/ns values
increase with increase in R for all these systems. This indicates that with the increase in IL
content, more nai are required for the formation of stable IL-in-oil microemulsion systems at
constant temperature. With increasing IL content, in turn, the interfacial areas are extended
(which has been calculated in subsequent section with the help of droplet diameter measured
by DLS), and the requirement of cosurfactant at the interface to stabilize the droplets is
gradually increased. In other words, hydrophilic property of the system is strengthened as R
increases. Therefore, more cosurfactant is required to adjust the hydrophile-lipophile balance
i
of the microemulsion, which results in increase in na values [44]. Similar behaviour was
reported for [bmim][BF4]/[C12mim]Br/pentan-1-ol/octane [44] and [bmim][BF4]/Brij-35/1-
butanol/toluene [45] microemulsions.
The ΔG0 values for all studied compositions are negative at 358 K, and hence, spontaneous
t
formation of IL-in-oil microemulsion is suggested. It is also evident from Fig. 1B that the
values of -ΔG0 , which is indicative of spontaneity of the cosurfactant transfer process (1-
t
octanoloil→1-octanolint), decrease with increasing R (= 1→5) for the studied systems. In other
words, association between surfactant and cosurfactant molecules at the interface becomes less
favorable with increase in R. This type of variation was reported earlier by Hait et al., [42]
Zheng et al. [39] and Paul et al. [43] for water-in-oil microemuslion. From comparative study,
the -ΔG0 values obtained for the present system are comparable with those reported for
t
[bmim][BF4]/[C12mim]Br/pentan-1-ol/octane IL-in-oil microemulsions [44]; however, are
higher than [bmim][BF4]/Brij-35/1-butanol/toluene,[45] [bmim][BF4]/CTAB/alkanol (ethanol,
1-propanol, 1-butanol)/toluene [46] and [bmim][methanesulfonate] ± water/(Tween-20+n-
pentanol)/n-heptane microemulsion systems [35]. Thus, the present system possesses a
relatively higher thermodynamic stability at elevated temperature.
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Figure 1. (A) Plot of na/ns vs. no/ns at varied molar ratio of IL to surfactant (R); (B) Plots of
i
ΔG0 and interfacial composition (na /ns) as a function of R for IL/CTAB/1-octanol/decane
t
microemulsion system at fixed temperature of 358 K.
3.2. Energetic parameters for IL/O microemulsion reveals organized oil/water interface
along with temperature-independent formulation at a critical R
The thermodynamic studies on the formation behaviour of IL-in-oil (IL/O) microemulsions
and the energetics for the interaction between the components are comparatively rare. In order
to prepare industrially acceptable microemulsion systems, extensive knowledge on formation
mechanisms along with their structural orientation is very much significant, which can be
gained from through the thermodynamic studies. The complex nature of microemulsions has
restricted rapid growth of the thermodynamic viewpoint and not been sufficiently explored
[47]. In this section, analysis of energetic parameters of the transfer of 1-octanol from n-decane
(Dc) to the IL-oil interface of CTAB microemulsion is presented in detail at higher temperature
range (358→370 K), which is not reported earlier. However, such reports for IL/O
microemulsions at the temperature range of 293 K to 323 K are available in literature, which
were recently reviewed by our group [48].
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Figure 2. (A) Plots of ΔG0 and (B) ΔH0 (inset: ΔS0 ) as a function of molar ratio of IL to
t
t
t
surfactant for IL/CTAB/1-octanol/decane microemulsion system at four different temperatures
(358 →370 K).
The following equations are helpful to justify the temperature dependent energetic parameters
for the transfer process of 1-octanol from the continuous Dc phase to the interfacial region for
stabilization of IL-droplets: the ΔH0 (standard enthalpy change of transfer process) can be
t
evaluated by the van’t Hoff equation. Thus,
0
∆퐺
푡
⁄
휕(
)
푇
[
]푝 = ∆퐻푡0
(6)
1
휕( ⁄ )
푇
Since the dependencies of (ΔG0 /T) on (1/T) are nonlinear for all these systems (plots are shown
t
for brevity), a two-degree polynomial equation of following form is used.
∆퐺푡0
2
1
1
⁄
(
) = 퐴 + 퐵1( ⁄ ) + 퐵2( ⁄ )
(7)
푇
푇
푇
The differential form of the relation helps to evaluate ΔH0 . Thus,
t
0
∆퐺
푡
⁄
휕(
)
푇
0
1
[
]푝 = 퐵1 + 2퐵2( ⁄ ) = ∆퐻푡
(8)
1
푇
휕( ⁄ )
푇
Where, A, B1 and B2 are the polynomial coefficients. Then the Gibbs-Helmholtz equation is
used to evaluate ΔS0 (standard entropy change of transfer process),
t
(∆퐻푡0 − ∆퐺푡0)
⁄
∆푆푡0 =
(9)
푇
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The dependence of ΔG0 along with ΔH0 and ΔS0 as a function of IL content (R) at four
t
t
t
different temperatures are illustrated in Figs. 2A and 2B. The negative ΔG0 value at all studied
t
temperatures implies that the successful formulation of “high-temperature stable IL/O
microemulsions” due to spontaneous transfer of 1-octanol from the oil phase to the interfacial
region. However, for all systems, the absolute values of ΔG0 decrease with increase in
t
temperature, which implies less favorable transfer of 1-octanol. In other words, with increase
in temperature, the interaction between surfactant and cosurfactant is greatly reduced, which
corroborated well with the degree of spontaneity of the transfer process. The lower spontaneity
at elevated temperature was earlier reported for microemulsions with the oil isopropylmyristate
(IPM), stabilized by trihexyl (tetradecyl) phosphonium bis 2,4,4-(trimethylpentyl)phosphinate
(surfactant) + isopropanol (IP as a co-surfactant) like the triisobutyl (methyl)phosphonium
tosylate/IPM/(IL-2+IP) system [49]. Further, overall transfer process is found to be exothermic
in nature at all experimental temperatures along with negative entropy change with progressive
addition of IL. So, 1-octanol causes release of heat during the cosurfactant transfer process.
Consequently, the negative entropy change is due to more organization of the interface and its
surroundings inside the IL/O droplets. Dissolution of IL into CTAB/1-octanol-Dc medium can
be demonstrated through four major processes. Absorption of heat occurs during (i) dispersion
and (ii) further penetration of IL in the interior of IL/O microemulsions (aggregates). Whereas
release of heat happens during (iii) the reorganization of the CTAB and 1-octanol at the oil/IL
interface, and (iv) organization of the penetrated IL. The total heat in this report shows
exothermicity, which means that the sum of the contributions of processes one and two are
lower than those of three and four. Such negative standard enthalpy and entropy changes for
w/o microemulsions stabilized by cationic cetyltrimethylammonium-based surfactant and 1-
octanol were also reported in literatures [50, 51]. However from comparative study, it can be
concluded that 1-octanol/cetyltrimethylammonium-based w/o systems showed much lower
free energy of formation than currently studied IL/O systems even at much higher temperatures
[50]. In this process, we found that TΔSmic° < ΔHmic°, which suggests that the reverse micelle
formation is an enthalpy-driven process, as was also found for the self-association of AOT in
nonpolar solvents [52]. It was also suggested earlier that the entrapment of isooctane molecules
into the surfactant tails during formation of w/o microemulsions is accompanied by the
negative entropy change [53].
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Both the ΔH0 and ΔS0 values are found to be decreased with increase in temperature for all
t
t
these systems (Fig. 3B and inset). That means the cosurfactant transfer process (1-
octanoloil→1-octanolint) alters from less exothermic to more exothermic with increase in
temperature. With increase in temperature, the extent of hydrogen bonding of IL molecules
with the CTAB decreases, and therefore, the energy required to break their association
diminishes i.e., ΔH0 decreases with increase in temperature [54]. Subsequently, it also affects
t
the significance of the entropic term due to decrease in IL-hydration around the head group of
CTAB [54]. It is also interesting to note that both -ΔH0 and -ΔS0 values increase with increase
t
t
in IL content (R) before reaching a maximum at R = 3 and after that they gradually decrease at
higher IL content. Most importantly, the enthalpic processes are quantitatively identical at the
maximum, i.e., at R = 3; which indicates that the transfer events of cosurfactant from oil to the
interface are physicochemically same and isenthalpic at R = 3. The mixing fraction at the
interface is not greatly changed in ionic surfactant-cosurfactant systems with increasing
temperature, however, the monomeric solubility of cosurfactant in oil increases monotonically.
In order to attain a temperature-insensitive microemulsion, the monomeric solubility of
cosurfactant in oil has to be reduced by using an appropriate cosurfactant [55]. Earlier, Maiti
et al. reported a couple of isenthalpic formulations for cationic w/o microemulsion composed
of octadecyltrimethylammonium bromide (C18TAB), n-butanol, and n-heptane [56]. The origin
of specific interactions probably comes from cation-π interaction between quaternary
ammonium head group of CTAB and aromatic moiety of ILs [57, 58], which gave the
temperature-independent exothermic contribution to excess molar enthalpy. Based on the
above results, the different affinity of CTAB surfactant to IL at different temperatures should
be interpreted in terms of hydration effects in the palisade layer rather than in terms of
counterion binding effect.
3.3. Determination of extent of single phase microemulsion region
From the application point of view, construction of the phase diagram is a primary task towards
a microemulsion formulation. Herein, Fig. 3 illustrates the pseudoternary phase diagram of 1-
ethyl-3-propylbenzimidazolium bromide ([EPbim][Br])/(CTAB + 1-octanol)/n-decane
systems at fixed surfactant-cosurfactant ratios (1:4, w/w). The shaded areas under the curves
represent the two-phase turbid region, where the stable microemulsions are not formed. The
unshaded portions correspond to the microemulsion zone. In the currently studied systems, oil-
rich (IL-in-oil microemulsion), IL-rich (oil-in-IL microemulsion) as well as bicontinuous states
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are observed. However, for simplicity, all these three different regions are represented as a
single phase (microemulsion zone; the unshaded portion in the pseudoternary phase diagram).
All further experiments are carried out using the oil-rich (i.e., IL-in-oil) microemulsion
systems. The areas under the microemulsion region (unshaded portion) and the turbid regions
are calculated by simply weighing the individual areas. Similar type of phase characteristic is
also reported by Zech et al. for dodecane/([C16mim][Cl]/decanol at 1:4, w/w)/ethylammonium
nitrate (EAN) microemulsion [59].
CTAB : Octanol
(S:CS= 1: 4)
0.00 1.00
0.25
0.75
0.50
0.50
0.75
0.25
1.00
0.00
0.00
1.00
0.25
0.50
0.75
Decane
1-Ethyl-3-Propyl-Bezimidazoliumbromide
Figure 3. The pseudoternary phase diagram of IL/CTAB/1-octanol/decane microemulsion
system at fixed surfactant-cosurfactant ratio (1:4, w/w) and temperature (358 K).
3.4. Anomalous behaviour of aggregated droplet size as a function of IL content (R)
In order to confirm the successful encapsulation of ILs for the formation of IL-in-oil
microemulsion, droplet size measurement by DLS is well-known powerful technique. To
identify and characterize the [EPbim][Br]-in-oil microemulsions formed by CTAB/1-octanol
(1:4, w/w), the droplet size is measured as a function of [EPbim][Br] content (i.e., molar ratio
of IL to surfactant, R). Variation in droplet diameter as a function of content of IL (R) is
measured at 358 and 362 K and subsequently, presented in Fig. 4A. Size distribution plot
clearly revealed fairly monodispersed IL droplet, which are not shown here for brevity. Earlier
reports showed increase in droplet hydrodynamic diameter as a function of content of
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encapsulated IL for IL-in-oil reverse micelles stabilized by ionic surfactant [59-63]. However
in the present study, it is observed from Fig. 4A that the size of microemulsions decreases
almost linearly with increase in R (= [EPbim-Br]/[CTAB]) at both temperatures. This
anomalous behaviour for presently studied system can be explained in following manner. Both
the [EPbim][Br] and n-decane are immiscible with each other. As a result, [EPbim][Br] prefers
to sit deep down in the nano-cage of IL/O microemulsion rather than the interfacial region. At
the elevated temperature, IL molecules cannot form strong H-bond with the cationic polar head
group [N(CH3)3+] of CTAB after gradual addition of IL, which keep IL molecules to stay away
from the interface and much near to the reverse micellar core. This fact is manifested by the
lowering of hydrodynamic diameter of the microemulsions [64]. Earlier, Ghosh noticed similar
behaviour for non-aqueous reverse micelles formed by Brij surfactants in benzene and EAN
[65].
Figure 4. (A) Variation in the hydrodynamic diameter with the molar ratio of IL to surfactant
(R) at two different temperatures (358 and 362 K), and (B) Variation of molecular interfacial
area of the surfactant as a function of hydrodynamic diameter for IL/CTAB/1-octanol/decane
microemulsion system.
3.5. Mathematical model for the determination of aggregation number and molecular
interfacial area
The efficient use of newly formulated microemulsions requires a sound knowledge on the basic
physicochemical properties of the specific system. For example, in order to model the
distribution of various chemical species within the system, parameters such as droplet size,
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aggregation number, and surfactant molecular interfacial area play important role [66]. Recent
work of Lemyre and coworkers confirmed that hydrodynamic diameter of reverse micelles is
directly proportional with aggregation number [67]. According to the authors, the relationship
between hydrodynamic diameter (Dh) and aggregation number (Nagg) for IL-in-oil
microemulsions can be expressed using the following equation;
1
3푁
푉
퐷
푎푔푔
퐼퐿,푡표푡푎푙
ℎ
3
= [
(
푛
푠푢푟푓,ꢀꢂ 푀퐸푠
+ 푉 )]
(10)
푠
2
4휋
Where, VIL,total, nsurf, in MEs, and Vs are the total volume of water, number of surfactant molecules
in microemulsions (MEs) and molecular volumes of surfactant(s), respectively. This equation
is based on a model assuming the reverse micelles as a compact sphere and neglects presence
of any polar solvent molecules between the surfactant hydrophobic tails which would be the
ideal case if two perfectly immiscible solvents are used. This assumption holds true for current
system. Molecular volume of CTAB (VCTAB) can be calculated as;
(
)
푉퐶푇퐴퐵 = 푉 + 푉ℎ푒푎푑
(11)
푡푎ꢁꢃ
Where, Vtail and Vhead are the molecular volume of hydrocarbon chain and polar head of CTAB.
Vhead of CTAB has been determined from head group area of CTAB [42], whereas Vtail can be
expressed as;
푉
푡푎ꢁꢃ
= 푉퐶퐻3 − (16 − 1)푉퐶퐻2
(12)
Group molar volumes (i.e. VCH3, and VCH2) used for the estimation of molecular volume of
CTAB are taken from Preu et al. [67]. A representative plot of Nagg as a function of R (i.e.,
[IL]/[CTAB]) for these systems is depicted in inset of Fig. 4A. Therefore, as hydrodynamic
diameter decreases with increasing R, aggregation number also decreases. Since surfactant
concentration is fixed, a decrease in aggregation number with R is indicative of an increase in
the number of IL droplets which provides additional support to the decreased droplet size as
evidenced from DLS measurements.
Once the micellar composition has been known, equation (13) can be used to determine the
molecular interfacial area of the surfactant if the volume occupied by the hydrophilic part of a
surfactant molecule (Vsurf, hydrophilic or Vhead) and the effective length of the hydrophobic chain
of the surfactant (lhydrophobe) are known. This leads to following equation [66];
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푉
퐷ℎ
퐼퐿,푡표푡푎푙
3
⁄
⁄
=
(
+ 푉푠푢푟푓,ℎ푦푑푟표푝ℎꢁꢃꢁ푐) + 푙ℎ푦푑푟표푝ℎ표푏푒
(13)
휎
2
푛
푠푢푟푓,ꢀꢂ 푀퐸푠
Where, σ is the surfactant molecular interfacial area and lhydrophobe can be estimated from
Tanford’s formula [68] for the maximum extension of the alkyl chain of surfactant. Fig. 4B
demonstrates the molecular interfacial area of the surfactant as a function of droplet size. It is
clearly evident that, the interfacial area is not constant. A limiting value is reached for droplet
size larger than 5 nm, but the interfacial area increases sharply with increasing IL content (R)
and decreasing droplet size. The primary reason for the variation of the molecular interfacial
area is the dependence of the conformation of the anchored surfactant chains on the radius of
curvature [66]. The variation of these parameters with droplet size of microemulsion indicates
that the decrease in thermodynamic stability (as evidenced from dilution method) with
decreasing size can be attributed to the steric confinement of the hydrophilic quaternary
ammonium chain of CTAB and 1-ethyl-3-propylbenzimidazolium cation ([EPbim]+) of IL.
3.6. Nitration of aromatic compounds in IL/O microemulsion offers regioselectivity
Nitration reaction of three different aromatic compounds was performed in the cationic
surfactant-based IL/O microemulsion system and their selectivity (ortho/para ratio) was also
examined. The results are summarized in Table 1. We began with the nitration reaction of
phenol which get completed within 15 minutes. Upon isolation, we observed two different
products, e.g., p-nitrophenol, and o-nitrophenol in 81 and 19%, respectively.
Table 1. Regioselective nitration in IL/O microemulsion medium.
R
R
R
NO2
Fe(NO3)3.9H2O
+
IL microemulson, 900 C, 15 min
NO2
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Reaction Yield (%)
Entry
Reactant
Ortho
Para
1.
19
81
[Phenol]
2.
3.
23
30
77
70
[Aniline]
[Anisole]
The key feature for effective nitration reaction of aromatic compounds using ferric nitrate is
the in situ formation of nitronium ions [69]. Herein, two more aromatic compounds such that
aniline and anisole other than phenol have been used for the electrophilic substitution reaction
(e.g., nitration) using ferric nitrate as the nitrating agent in reverse micellar gallows. The IL-
based microemulsion systems shows regioselectivity in product formation that means para
product is favoured over the ortho one, which is listed in Table 1. The extent of regioselectivity
towards the formation of para isomer is found to be higher for phenol as starting reagent in
microemulsion and followed the order; anisole < aniline < phenol. Earlier, Chhatre et al.
reported favourable formation of ortho nitration product, when nitration reaction was carried
out for phenol in negatively charged AOT-based microemulsion [70]. This may be due to the
difference in nature of surfactant charge interface for the present system. However, Holmberg
and his co-workers revealed similar type of para directing effect for the nitration reaction of
phenol and anisole in mixture of the cationic single-tailed DTAB and the double-tailed DDAB
surfactant based microemulsion [33]. However, they found that the para-directing effect is
stronger for the anisole than for the phenol. Thus it can be concluded that the interfacial charge
of formulated microemulsions play an important role during orientation of aromatic
compounds near the surfactant interface. Mainly, electronegativity of hydroxyl, methoxyl and
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amino groups for substituted aromatic compounds pulled the π-electron cloud of aromatic ring.
In order to obtain favourable cation-π interaction between surfactant and starting aromatic
compounds, the aromatic ring will be positioned in such a way that the cationic head group
remains closer to the ortho positions than to the para position. This will make the approach by
the positively charged attacking species (the nitronium ion) more favourable at the para
position than at the ortho position. The reactions went to completion in around 15 min. It is
noteworthy that the reactions proceeded fast in the microemulsions despite the fact that the
surfactant used carried the same charge as the reacting nitronium ion. Evidently, the positive
effect of the large oil-IL interface dominates over the charge repulsion between the surfactant
head groups and the attacking nitronium ion.
4. Conclusion
Present study is focused on the formulation and characterization of a high-temperature stable
quaternary IL-in-oil microemulsions comprising of IL ([EpBzim][Br]), CTAB, 1-octanol (1-
On) and n-decane with a detailed description of the interfacial composition and energetic
parameters for the transfer of 1-On from bulk oil phase to the interface as a function of
molar ratio of IL to surfactant (R) and temperatures. Spontaneous formation IL-oil interface
was evidenced with organized interface. Interestingly, the present system produced a
temperature-independent exothermic formulation at a particular composition, which believed
to arise due to the specific cation-π interaction between CTAB and IL. An anomalous behaviour
was observed in terms of reduction of droplet size with progressive addition of IL from DLS
measurement at two elevated temperatures. The difficulty in formation of H-bonding ability
between surfactant and IL at elevated temperatures might be the possible reason. Both the
estimated aggregation number and the molecular interfacial area were found to be directly
correlated with the droplet size. Finally, nitration reaction of some aromatic compounds
was performed in this formulated IL/O microemulsion. The reaction ends up with the
highest regioselectivity of para isomer product over the ortho isomer. The confinement
of IL-in-decane MEs imparts the regioselectivity on the reaction product. It should be
stressed that here we have chosen only one model system. We believe that this concept
can be extended to other ILs and, depending on the system, the thermal stability range can
probably be enlarged much more by appropriately choosing the components. These high-
temperature stable MEs open a wide field of potential applications, such as for
nanoparticle synthesis, new reaction media, extractions, or lubricant formulations.
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Acknowledgements
We are thankful to the CSIR, New Delhi for financial support [02(0022)/11/EMR-II].
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Graphical Abstract
In Situ NO2
Ionic Liquid
Core
HO
Regioselective nitration reaction of aromatic compounds in novel high-temperature
stable ionic liquid-in-oil microemulsion
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Research Highlights
 High-temperature stable microemulsion forms using 1-ethyl-3-propylbenzimidazolium
bromide.
 Temperature-independent exothermic behaviour notice due to cation-π interaction
between surfactant and ionic liquid.
 Droplet size measurement reveals anomalous trend as a function of ionic liquid content.
 Aggregation number and molecular interfacial area of microemulsion are calculated.
 Regioselectivity in nitration reaction for aromatic compounds evidences in
microemulsion media.
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