Ionic liquid microemulsions and their technological...

Indian Journal of Chemistry Vol. 49A, May-June 2010, pp. 662-684

Ionic liquid microemulsions and their technological applications

S K Mehta* & Khushwinder Kaur Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160 014, India

Email: [email protected]

Received 18 February 2010; revised and accepted 22 March 2010

Ionic liquids are receiving much attention as environmentally benign media for reactions, separations, and multidisciplinary chemistry, due to their unique physicochemical properties which include non-volatility, high stability, high ionic conductivity, wide electrochemical window and easy recyclability. We summarize herein microemulsion formulations and applications where ionic liquids are used as oil substitutes, water substitutes, co-surfactants (additives) and surfactants.

Keywords: Ionic liquids, Microemulsions, Oil substitutes, Water substitutes, Surfactants

Chemistry is dominated by the study of species in solution. Although any liquid may be used as a solvent, relatively few are in general use. However, as the introduction of cleaner technologies has become a major concern throughout both industry and academics, the search for alternatives to the most destructive solvents is of the highest priority. Solvents are high on the list of damaging chemicals for two simple reasons: (i) they are used in huge amounts, and, (ii) they are usually volatile liquids that are difficult to contain.

Fused salts containing ions are known as ionic liquids (ILs). It is possible, by careful choice of starting materials, to prepare ionic liquids that are liquid at and below room temperature. Ionic liquids are not new; some of them have been known for many years, for instance [EtNH3]

-[NO-3], that has a melting

point of 12°C, was first described1 in 1914. For some time, it has been proposed that these ionic liquids provide a useful extension to the range of solvents that are available for synthetic chemistry. However, it is only in the past few years that significant literature has become available in this area.

Microemulsions are a unique class of thermodynamically stable isotropic dispersions of two or more immiscible liquids which are stabilized by an adsorbed surfactant film at the liquid–liquid interface2. Self-assembled structures of different types can be formed, ranging from spherical and cylindrical microemulsions to lamellar phases and bicontinuous microemulsions, which may coexist with predominantly oil or aqueous phases. Conventional microemulsions, which are composed

of water, oil and surfactant, can be classified as water-in-oil (w/o), oil-in-water (o/w) and bicontinuous phase microemulsions2. They have been extensively investigated in a variety of fields, such as chemical reaction and separation, drug delivery, material synthesis, solubilization, etc.3,4 Despite these features, solubility limitations for apolar solutes remain, which may be overcome by incorporation of hydrocarbon domains provided by normal micelles or formation of ionic liquid (IL)-in-oil (IL/o) microemulsions5. The significance is that these nanostructured surfactant assemblies provide hydrophobic or hydrophilic nanodomains, thereby expanding potential utilization of ILs as reaction and separation or extraction media. More recently, several studies have demonstrated that IL may substitute water or oil to form novel microemulsions in the presence of surfactant6-10. In this review, we examine recent studies of microemulsions incorporating ILs and applications of such microemulsions. Emphasis will be on the substitution of ILs for the oil component in water/IL microemulsions, for the polar or water component in IL/oil microemulsions, and for the surfactant component, along with new material applications.

Ionic Liquid Microemulsions

Triton X-100/1-butyl-3-methylimidazolium tetrafluoroborate

(bmimBF4) ionic liquid

The effect of successively adding organic solvents to the Triton X-100 (TX-100)/1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) binary system has been reported by Gao et al.6

MEHTA & KAUR: IONIC LIQUID MICROEMULSIONS AND THEIR TECHNOLOGICAL APPLICATIONS

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The addition of oils (cyclohexane, benzene, toluene, deuterium oxide) provides a strong hydrophobic environment for the hydrophobic groups of TX-100 molecules and induces them to aggregate into interface film. The oil and ionic liquid (IL) bmimBF4 two-phases are separated by the TX-100 interface film, and an oil-in-IL (o/IL) microemulsion is formed. The aggregation behavior of non conductive TX-100 leads to the increase of continuous bmimBF4

concentration and thus the solution conductivity. The hydrophobic interaction between the added organic solvents and hydrophobic groups of surfactant molecules is considered to play a crucial role in driving the formation of o/IL microemulsions. The formation mechanism is different from the IL/o microemulsions where the electrostatic attraction between the positively charged imidazolium cation of bmimBF4 and the electronegative oxygen atoms of TX-100 is the driving force for solubilizing bmimBF4 into the cores of TX-100 aggregates. The formation has been depicted in Scheme 1. The formed o/IL droplet microemulsion is successively swollen by the addition of organic solvents, and a bicontinuous microstructure is finally observed.

In another report by the same group7 the authors investigated the microemulsions consisting of bmimBF4, TX-100 and toluene for their phase behavior. The bmimBF4-in-toluene (IL/o), bicontinuous, and toluene-in-bmimBF4 (o/IL) microregions of the non-aqueous IL microemulsions have been initially identified by traditional electrical

conductivity measurements on the basis of percolation theory. Contrary to the earlier belief, that the microstructures of the aqueous IL microemulsions cannot be differentiated through conductivity method because ILs are essentially molten salts8, (i.e., no insulative media are present in these systems) and thus the percolation theory is not applicable, Goa et al.

7 have reported that if a hydrophilic IL such as bmimBF4 substitutes for water and builds an IL/o non-aqueous microemulsion in which IL is conductive and oil acts as an insulative medium, then the systems are in accord with the percolation theory according to its theoretical description. Three different microregions: IL/o, bicontinuous, and o/IL, are differentiated by electrical conductivity and cyclic voltammetry. The data obtained by electrical conductivity are the same as those by cyclic voltammetry, indicating that the simple electrical conductivity method is also feasible for identifying the microstructure of nonaqueous IL microemulsions.

Interesting, claims have recently been made on the formation of IL/o microemulsions in mixtures of bmimBF4 and cyclohexane, stabilized by the nonionic surfactant TX-100 by Anderson et al.9 Light scattering (LS) data from these systems indicate droplet sizes of the order of 0.1 µm. This is generally regarded as outside the range for microemulsions, which typically have nanometer-sized liquid domains. Owing to the higher resolution compared to LS, small-angle neutron scattering (SANS) is reported to be the ideal method to detect nanodroplet formation in such IL/o systems. Therefore, SANS provides prima facie evidence for the presence of compartmentalized liquid nanodomains10. The first reported SANS experiments on bmimBF4-in cyclohexane IL/o microemulsions, clearly demonstrate the formation of surfactant-stabilized dispersed nanodroplets with IL cores. The SANS data have been treated in terms of an ellipsoid form factor, showing a regular increase in droplet volume as micelles are progressively swollen with added IL, a behavior consistent with “classic” water-in-oil (w/o) microemulsions10.

Gao et al.11 delineated the phase behavior of the same microemulsion. The IL microemulsion has been found to be very sensitive to the addition of polymer PVP. The proposed structure of the IL microemulsion has been depicted in Fig. 1. However, no phase transition occurs on the basis of the analyses of conductivity and viscosity measurements. The addition of bmimBF4 destroys the dried reverse

Solution environment before and after adding organic solvents to the mixture of TX-100 and bmimBF4. [(Left) a molecular solution or micellar solution; (Right) an induced o/IL microemulsion]. [Reproduced from ref. 6].

Scheme 1

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micelles of TX-100; however PVP can slow down the destruction. The formation of reverse IL microemulsion is induced by the successive addition of bmimBF4 and size of the microemulsion gradually swollen by both bmimBF4 and PVP. FTIR and 1H NMR measurements show that there is no interaction between PVP and TritonX-100, i.e., the PVP is only solubilized into the reverse microemulsion polar cores, as confirmed by the 2D ROESY analysis.

IL microemulsions consisting of bmimBF4/ TX-100/toluene have been prepared and the phase behavior of the ternary system has been investigated by Li and coworkers12. The microregions of bmimBF4-in-toluene (IL/o), bicontinuous and toluene in- bmimBF4 (o/IL) have been identified by traditional electrical conductivity measurements. Based on the phase diagram, a series of samples in the IL/o microemulsion region have been chosen for DLS study. The DLS results support the formation of IL/o microemulsion. The UV-vis absorbance spectra of MO and MB are further used to investigate the micropolarities of the formulated IL/o microemulsions. The results indicate that, with the addition of the bmimBF4 into the TX-100/toluene reverse micelles, the polarity of the IL/o microemulsions increases, but when the IL pools began to form, the polarity of the IL pools is constant. The authors also demonstrated that biological molecule riboflavin and metal salts such as Ni-(NO3)2, CoCl2, and CuCl2 can be solubilized in the IL/o microemulsion droplets. It is suggested that when a small amount of bmimBF4 is added to the TX-100/toluene reverse micelle, bmimBF4 molecules

are bound to the EO groups of TX-100 in the polar core; the properties of the bound bmimBF4 molecules are different from that in the bulk IL. Therefore, the absorption spectra of CoCl2 are different from that in pure IL. However, after the IL pools form, the properties of the free bmimBF4 molecules in the IL pools are similar to that in the bulk IL. The absorption spectra show the same absorption band. The formation of IL pools at R > 1.58 has further been confirmed. In another investigation by Gao et al.

13 the effect of the addition of small amounts of water to the bmimBF4-in-cyclohexane microemulsion has been studied. It has been found that the addition of water decreases the microemulsion droplet size and thereby increases the number of microemulsion droplets (Scheme 2). The water molecules are mainly hydrogen bonded to the peripheral second and third oxyethylene (OE) units of TX-100 and behaves like a chock to be inserted into the surfactant interface film. The IL/cyclohexane interface is bent toward the IL, and thus the microemulsion droplet size is decreased. A staggered arrangement of TX-100 at the IL/cyclohexane interface film is proposed for the original bmimBF4-in-cyclohexane microemulsion on the basis of 2D NMR spectroscopic analysis. The addition of water changes the elliptical droplet structure and leads to a more ordered arrangement of TX-100 molecules at the IL/cyclohexane interface. The structural change of the IL microemulsion induces a loss of entropy, while the enthalpy change derived from hydrogen bonding compensates for the entropy loss. The microstructural change can be interpreted as a transition from a large-size elliptical structure to a small-size spherical structure. Compared with the previously reported bmimBF4/benzene microemulsion by Zhul and Schelly14, the role of water is different in this microemulsion system.

Fig. 1 — Proposed molecular structure of a single droplet of the bmimBF4-in cyclohexane microemulsion, illustrating the position of PVP chains in the ionic liquid (IL) core. [Reproduced from ref. 11].

Schematic diagram of the bmimBF4-in-cyclohexane microemulsion. A large elliptical droplet structure can be assumed in the absence of water, while the addition of water results in small-size spherical microemulsion droplets. [Reproduced from ref. 13].

Scheme 2


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In the former case, no dried reverse micelles have been formed as TX-100 is also soluble in the highly polarizable benzene. The addition of bmimBF4 induces the formation of IL microemulsion by weak electrostatic attraction between the electropositive bmim+ and the electronegative oxygen atoms of the OE units of TX-10015. However, for the bmimBF4/cyclohexane microemulsion, dried reverse micelles of TX-100 are formed before the addition of bmimBF4

14-16. The micellar polar core consists of OE units of TX-100 via hydrogen bonding between OE units and the terminal OH group of TX-100. It is relatively difficult for water molecules to enter the total palisade layer of the bmimBF4/cyclohexane microemulsion. They are located mainly at the periphery of the microemulsion droplets which in turn leads to the microstructural transition. The special solubilization position of water molecules in the bmimBF4-in-cyclohexane microemulsion has been found to provide an aqueous interface film for hydrolysis reactions.

Gao and coworkers17 reported that bmimBF4 can substitute water to form a non-aqueous IL microemulsion, and the microemulsion droplets are of the same shape in contrast to “classic” droplets of w/o microemulsion. The group18 reported the microstructures and structural transition of bmimBF4/TX-100/toluene three-component nonaqueous microemulsion in which bmimBF4 is used as a substitute for water. Three different microenvironments, viz., IL/o, bicontinuous, and o/IL were differentiated by electrical conductivity. Apparently, the changes in the composition and microstructure of nonaqueous IL microemulsions result in the changes of the electric conduction mechanism and electrochemical reactive characteristic of the systems. It is firmly believed that the electrochemical system constructed by electrode and IL microemulsion is totally new; the interfacial structure and electrochemical properties of electrode-IL microemulsions are quite different from those of electrode-aqueous solution, electrode-ionic liquids, and electrode-traditional microemulsion (w/o or o/w). Also, IL microemulsions as electrolytes for electrochemistry study are also special and different from traditional aqueous and conventional microemulsion electrolytes. Electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) have also been applied to study the properties of nonaqueous IL microemulsions using potassium ferricyanide as electroactive probe.

Gao et al.19 prepared microemulsions consisting of bmimBF4, TX-100, benzene, and the phase behavior of the ternary system has been investigated. DLS studies reveal that with increasing amounts of bmimBF4 the size of the aggregates increased, as in the case of typical w/o microemulsions, thus suggesting the formation of an IL/o microemulsion. The micelles became swollen on successively adding bmimBF4 to the system, and IL pools formed when the bmim BF4 content is more than 1.11 wt% of the IL microemulsion system, as confirmed by UV-vis spectroscopic analysis with CoCl2 and MB as the absorption probes. The effect of added water on the microstructures of the IL/o microemulsions has been evaluated. First, it is found that the UV-vis absorption probe MO tended to locate in the interior of the IL pool. A constant polarity is obtained in the IL pool even if a small amount of water was added to the microemulsion, thus indicating that the water molecules are solubilized in the polar outer shells. These water molecules, through hydrogen-bonding interactions with the oxygen atoms of the OE units of TX-100 and electrostatic attraction to the electropositive imidazolium rings, act as glue that sticks the IL and OE units more tightly, thus making the microemulsion more stable. Scheme 3 shows the representation of water molecules in two different microenvironments. FTIR spectra revealed that the added water molecules are inclined to be first solubilized in the polar outer shells as bound and trapped water. The interaction between the added water and their surroundings is further confirmed by 1H NMR spectroscopic analysis. The

A schematic representation of water molecules in two different microenvironments. [a) IL/o microemulsion without IL pools; (b) IL/o microemulsion with an IL pool]. [Reproduced from ref. 19].

Scheme 3


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unique solubilization behavior of water provides an aqueous interface, and thus the IL/o microemulsion may be applied to the preparation of porous or hollow nanomaterials by hydrolysis reactions.

In an another report Gao et al.20 investigated microemulsions consisting of bmimBF4/TX-100/ toluene for 2D NMR spectroscopic analyses confirming the electrostatic interaction between the positively charged imidazolium cation of bmimBF4 and electronegative oxygen atoms of oxyethlyene (OE) units of TX-100. The reverse IL microemulsions thus formed are initially swollen by the added bmimBF4. However, a large sized microemulsion droplet cluster appears with successive addition of bmimBF4 (200–400 nm). The swelling process of the microemulsion droplets by increasing bmimBF4 content is detected by DLS, conductivity, UV-vis spectroscopy and freeze-fracture transmission electron microscopy (FF-TEM). In general, it has been found that there is no distinct interface between toluene and bmimBF4. The weak hydrophobicity and long OE chain of TX-100 and unique molecular structure of bmimBF4 and polar nature of toluene lead to the appearance of IL microemulsion droplet clusters. Such unique aggregation structures are interesting and may be useful for certain nano structural applications and processes. Fu and coworkers21 investigated the electrochemical properties of non-aqueous ionic liquid microemulsions such as electrical conductivity, electrochemical window, and solubility. It is reported that the nonaqueous IL microemulsions can be used as electrolyte for electrochemical research. The electrochemical properties of the non-aqueous IL microemulsions have been probed by CV and electrochemical impedance spectroscopy methods using potassium ferricyanide as electroactive probe. The reversibility is found better and the peak current densities of CV are higher for the [Fe(CN)6]

3−/[Fe(CN)6]4−

electrode reaction in the non-aqueous IL microemulsions than those in IL. However, the electrochemical behavior of the probe in the non-aqueous IL microemulsions with different microenvironments (o/IL, IL/o and bicontinuous) is different.

The phase behavior of 1-butyl-3-methylimidazolium tetrafluoroborate [bmimBF4]/TX-100 + n-butanol/ cyclohexane system has been investigated by direct visual observation and electrical conductivity by Cheng et al.22

Compared to the phase behavior

of bmimBF4/TX-100/cyclohexane system, it was demonstrated that addition of n-butanol favors the stabilization of the single-phase microemulsion and magnifies the single-phase region. UV-vis spectrum has been employed to characterize the formation of single phase microemulsions. In particular, the solubilization capacity of bmimBF4 increases as 1-buanol is added. Dynamic light scattering experiment verifies the increase in the hydrodynamic radius of the microemulsion droplet with the addition of n-butanol. The addition of n-butanol can strongly influence the behavior of the microemulsion and increase the droplet size depending on the hydrophilic/hydrophobic character of the alcohol. On the other hand, since the droplet becomes larger as more bmimBF4 is solubilized, the relative density of TX-100 on the surface of droplet decreases.

Li et al.23 demonstrated a novel ionic liquid

microemulsion, consisting of bmimBF4 and a nonionic surfactant, Triton X-100, prepared in triethylamine which is used either as an organic solvent or a Lewis base. The effects of small amounts of added water on the microstructure of the IL microemulsion have been investigated by various techniques. UV-vis spectroscopic analysis and FTIR spectra indicate that these water molecules are not solubilized into the ionic liquid pools of the microemulsions. 1H NMR spectra show that the added water binds with triethylamine to form a surrounding OH- base environment. Some of OH- ions enter the palisade layers of the ionic liquid microemulsions and a continuous base interface is created. The unique solubilization behavior of water reveals that it is possible to use the triethylamine microemulsions as a template to prepare metal hydroxides as well as metal oxides in the microemulsions, which is not possible when using traditional microemulsions. 1-n-Butyl-3-methylimidazolium hexafluorophosphate/

Triton X-100 ionic liquid microemulsions

Cheng and coworkers24 reported that at suitable conditions, bmimPF6/TX-100/EG systems can form bmimPF6-in-EG, bicontinuous, and EG-in-bmimPF6 microemulsions. TX-100 can form dry micelles in EG solution, as evidenced by surface tension measurement, but the surface activity of surfactant in EG is much lower than that in water, mainly because the hydrophobic interaction in the EG system is weaker than that in the water system. DLS study indicates that the hydrodynamic diameter of


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bmimPF6-in-EG microemulsions depends linearly on BmimPF6-to-TX-100 molar ratio. Solvatochromic study using MO as the probe demonstrates that micropolarity of EG domains in EG-in-IL microemulsions increases with increasing amount of EG, and MO molecules in the microemulsions exist mainly in the EG droplets. The waterless microemulsions consisting entirely of nonvolatile chemicals have some potential applications with advantages, especially when water should be avoided. At suitable conditions25 bmimPF6/TX-100/water system can form microemulsions, which can be divided into water-in-bmimPF6, bicontinuous, and bmimPF6-in-water sub-regions. DLS study shows that the hydrodynamic diameter (Dh) of the bmimPF6-in-water microemulsions is nearly independent of water content but increases with increasing bmimPF6 content due to the swelling of the micelles by the IL. Water domains exist in the water-in-bmimPF6 microemulsions, which can dissolve salts such as MO and K3Fe(CN)6, and the polarity of the water domains increases with increasing water content. This kind of clean microemulsion has potential applications in different fields, such as preparation of nanomaterials and chemical reactions, which is a benefit to environmental protection and may have other advantages. On comparision of the two studies, the difference between EG/TX-100/IL microemulsions and H2O/TX-100/IL microemulsions is that the water is replaced by EG. The surface excess (Γ2) value of TX-100 in pure EG is 1.25 mol m−2, while that in water is 3.84 mol m−2 at 30 °C. The Γ2 value in EG is three times smaller than that in water. This suggests that the surface activity of TX-100 in EG is considerably lower than that in water. The surface activity of the surfactant depends mainly on the molecular interaction and hydrophobic interaction. Firstly, the polarity of H2O is larger than that of EG, and thus the interaction between H2O and the polar head group of TX-100 is stronger than that between EG and the polar head group of TX-100, which is favorable to reducing the surface activity. On the other hand, hydrophobic interaction in the EG system is weaker than that in the water system, which is not favorable to reducing the surface activity. The reduction of the surface activity indicates that the contribution of the second factor is dominant.

In a study on the phase behavior of toluene/ TX-100/bmimPF6 by Li et al.26 the authors demonstrated that the single-phase microemulsion area

covered about 75% of the phase diagram at 25 °C. Electrical conductivities of the system with different R (bmimPF6-to-TX-100 molar ratio) values have been determined, and the results used to locate the sub-regions of the single-phase microemulsion. The results show that a transformation from bmimPF6/o microstructure via a bicontinuous region to an o/bmimPF6 microstructure occurred with the increase of F (weight fraction) of TX-100 and bmimPF6 in the system. The aggregate size of the reverse microemulsions of bmimPF6/o when determined using small-angle X-ray scattering shows that the size of the reverse microemulsions depends markedly on the R values.

The critical concentrations and the critical temperatures for the microemulsion systems of water/bmimBF4/sodium di(2-ethyl-1-hexyl)sulfo-succinate (AOT)/decane with various compositions of [bmim][BF4] have been measured by Lu and coworkers27. The coexistence curves for the quaternary microemulsion with the molar ratio of water to AOT being 40.8 and the mass fraction of [bmim][BF4] being 1.1×10-4 have been determined by measuring the refractive index at constant pressure in the critical region. The critical exponent, β, has been deduced from the coexistence curves. The value of β is very close to the 3D-Ising exponent in the critical region. It is found that the addition of bmimBF4 significantly reduced the lower critical temperature, which is contrary to the (water/KCl/AOT/decane) system, while the critical composition is almost kept unchanged. It has been attributed to the unique role of the ionic liquid which is capable of modifying the physicochemical properties of microemulsions.

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) (donor) to rhodamine 6G (R6G) (acceptor) has been studied in a room temperature ionic liquid 1-pentyl-3-methylimidazolium tetraflouroborate, (pmimBF4), in TX-100/benzene (RTIL) microemulsion employing picosecond and femtosecond emission spectroscopy by Adhikari and others28. It is considered that the acceptor being an ion, preferentially stays in the core of the ionic liquid pool. The donor is a neutral molecule and is distributed over different regions of the entire microemulsion. The microemulsion has been studied with and without water. This work demonstrates that femtosecond spectroscopy is a very sensitive tool to detect ultrafast FRET in an RTIL microemulsion. The authors have illustrated that


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the steady state spectra is dominated by non-FRET donors and does not reveal the time constant of FRET. From the rise of the acceptor emission they detected three different time constants of FRET in the RTIL microemulsion, i.e., 1, 250, and 3900 ps, in the absence of water and 1.5, 250, and 3900 ps in the presence of water. From the time constant of FRET, the donor-acceptor distances, RDA, were calculated with Eq. (1).

FRET

61 1 0A 0rise

Rk

RDD

= =

Γ Γ

.… (1)

This corresponds to three distinct D-A distances ~12±1, 29±1 and 45±2 Å. These distances are smaller than the sizes of the microemulsion. Comparing with other reports on DLS studies by the same group,29,30 the radius of the RTIL microemulsion in the present system is ~160 Å in the absence of water and ~43 Å in the presence of water (5 wt %). RDA±12 Å is very close to the sum of radii of the donor and the acceptor molecule. Since the ionic acceptor resides in the ionic liquid pool, the 12 Å D-A distance corresponds to a donor and an acceptor residing at close proximity in the RTIL pool of the microemulsion. The observed very large D-A distance of 45±2 Å may arise from a donor residing in the outer periphery of the microemulsion to an acceptor inside the polar RTIL pool. RDA = 45 Å may also involve diffusion of a donor from bulk benzene into the microemulsion. The 250 ps component (RDA ≈ 29 Å) corresponds to an intermediate situation where the acceptor is in the RTIL pool and the donor is residing in between the surfactant chain of the microemulsion and the RTIL pool. The most interesting observation is the multiple time scale of FRET and the λex dependence. Evidently, with increase in λex there is a marked increase in the contribution of the donor in the polar pool where FRET occurs very fast (because of short D-A distance). Also, the contribution of the ultrafast FRET component (~ 1ps) has been found to increase from 4 % to 100 % for the RTIL microemulsion and that for the water containing RTIL microemulsion increases from 12 % to 100 % as λex is increased from 375 to 435 nm. It is interesting to note that at λex = 435 nm there is no contribution of the slow FRET (in 3900 or 250 ps). This represents a situation where only the donors within the polar pool are excited. The contribution of the slow

FRET component (3900 ps) decreases with increase in λex from 375 to 435 nm for both systems.

The interaction of ionic liquid with water in bmimPF6/TX-100/H2O ternary microemulsions, i.e., “bmimPF6-in-water” microregions of the microemul-sions, has been studied by the dynamics of solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 151 (C-151) by Seth and coworkers31. In the case of C-153, with an increase in the bmimPF6 content in the microemulsions the change in the solvent relaxation time is small. The rotational relaxation time of C-153 also remains the same with an increase in R. This is due to the fact that with an increase in the bmimPF6 content in the microemulsions the position of C-153 remains the same, and this suggests that C-153 resides at the interface of these microemulsions. The most interesting characteristic in these microemulsions is that the solvent reorganization time is not very sensitive to an increase in R or an increase in the size of the droplet. In the case of C-151, with an increase in R the slow component of the solvation time gradually decreases, the fast component increases and the rotational relaxation time gradually increases. Therefore, with an increase in R the C-151 molecules gradually shifts to the core of the microemulsions, and in the case of RTIL-containing microemulsions the solvation time is very much dependent on the probe location.

Behera et al.32 elaborated that the addition of ionic

liquids to aqueous surfactant solutions can modify physicochemical properties of such systems in a favorable manner. They further assessed the changes in the properties of aqueous solutions of nonionic surfactant TX-100 upon addition of 2.1 wt% of popular ionic liquid bmimPF6. The solubility of ‘hydrophobic’ bmimPF6 in aqueous TX-100 increases with TX-100 concentration. This observation combined with the conductivity data strongly indicates partitioning of bmimPF6 into TX-100 micellar phase. Behavior of a variety of molecular absorbance (methyl orange, phenol blue, and N,N-diethyl-4-nitroaniline) and fluorescence (phenyl on the TX-100, pyrene, pyrene-1-carboxaldehyde, 2-(p-toluidino) naphthalene-6-sulfonate, and 1,3-bis-(1-pyrenyl)propane) probes further confirms this observation. The bmimPF6 (or more specifically, bmim+ or PF−

6) is distributed (most probably unevenly) throughout the micellar phase. This may be attributed to the presence of different types of interactions between TX-100 and bmim+ and/or PF−

6 ;


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the most pertinent being hydrogen-bonding between C2-H of bmim+ and ethoxy/hydroxyl –O–, between PF−

6 and H– of the TX-100 hydroxyl terminal, and dipole-induced dipole type of interaction between TX-100 phenyl π-cloud and bmim+, among others (Scheme 4 shows suggested possible interactions and locations of bmim+/PF−

6 within a TX-100 micelle). The presence of IL bmimPF6 within TX-100 micelles results in enhanced dipolarity of the micellar phase without significantly altering critical micelle concentration (cmc) and aggregation number. Statistically, insignificant increase in cmc and decrease in aggregation number (Nagg) of TX-100 micelles are observed upon addition of 2.1 wt% bmimPF6. The findings in the report are explained on the basis of bmimPF6 in aqueous TX-100 forming o/w microemulsions33. The formation of the o/w microemulsion has been confirmed by DLS, UV-vis spectroscopy and FFTEM. The report also discusses the possibilities of molecular and ionic interactions (H-bonding, dipole-induced dipole, etc.) between bmimPF6 and TX-100 which cannot be ignored. Limited solubility of bmimPF6 in many ‘oil-like’ organic solvents further corroborates the reported hypothesis34.

Another research focused on the influence of an ionic liquid (IL), i.e., ethyl-methylimidazolium hexylsulfate, on the spontaneous formation of microemulsions with ionic surfactants. Rojas et al.35 reported the influence of the ionic liquid on structure and formation in the optically clear phase region in water/toluene/pentanol mixtures in the presence

of the cationic surfactant, CTAB. The results show that replacing water by the ionic liquid increases the isotropic phase region in the water/toluene/ pentanol/surfactant system. Conductometric and DLS measurements in the CTAB-based microemulsion show that the droplet size in the L2 phase is decreased to 2 nm and the droplet-droplet interactions are drastically reduced. 1H NMR diffusion coefficient measurements clearly demonstrate that an inverse microemulsion still exists. The transitions from the L2 phase to a bicontinuous microemulsion, and finally to a L1 phase, by increasing the content of the IL-water solution, is well detected by considering the change of the diffusion coefficients. In combination with cryo-SEM micrographs, the bicontinuous sponge phase has been clearly identified. When a cationic polyelectrolyte poly(ethyleneimine) (PEI) is added, the bicontinuous phase range disappears, but an optically clear phase region in the oil corner can be still observed. Based on the conductometric and DLS data, the authors concluded that the PEI is incorporated into the individual small droplets. Rheological measurements confirm that the polymer is solubilized in the inner core of the droplet. A possible structure of the resulting PEI containing microemulsion droplets surrounded by a more fixed CTAB film with a IL-palisade layer, has been schematically given in Fig. 2.

Li et al.36 have used microcalorimetry method to obtain the second virial coefficient of the IL microemulsions. The second virial coefficient is found to be feasible for characterizing the interactions

Cartoon depicting a segment of the oblate ellipsoidal aqueous TX-100 micelle in the presence of IL bmimPF6. The proposed interactions between bmim+/PF6 and TX-100 are shown. Interactions between bmim+ and PF6, water and bmim+/PF6 and water and TX-100 are not shown for clarity. [Reproduced from ref. 32].

Scheme 4

Fig. 2 — Model of a micromulsion droplet in the CTAB/toluene-pentanol (1:1)/IL-aqueous PEI (1:1) system. [Reproduced from ref. 35].


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between IL microemulsion droplets. There are much stronger interactions between IL microemulsion droplets, which may be attributed to their relatively larger droplet size compared with that of traditional microemulsions. The validity of the method has further been confirmed by the percolation study and DLS. The method may also be used to investigate other ILs-containing colloids and may be helpful in understanding the microstructure and interaction mechanism of IL microemulsions. The effect of temperature on the bmimBF4/cyclohexane and bmimBF4/toluene ionic IL reverse microemulsions has been investigated by Gao et al.

37 The results show that the size of single microemulsion droplets is increased with increasing temperature. The IL microemulsions reveal a relatively high temperature-independence compared to the traditional aqueous systems, because in the IL microemulsions the electrostatic attraction which plays a role similar to that of hydrogen bonds or hydration in aqueous systems and is considered to drive the formation of IL microemulsions, is highly temperature independent. A low temperature results in the appearance of droplet clusters for the bmimBF4-in-toluene microemulsions. This is because the decrease in temperature leads to decreased size of microemulsion droplets and the solubilized bmimBF4 molecules are squeezed out of the palisade layer of the microemulsion droplets and therefore results in the appearance of droplet clusters. In other words, increasing temperature results in two different trends: (1) interaction between the hydrophilic OE units of the surfactant decreased, and, (2) the opposite result is obtained for interactions among the hydrophobic groups of TX-100 molecules. This finding reveals that the interfacial curvature of the TX-100 film between the bmimBF4 and cyclohexane decreased with increasing temperature. The interfacial curvature change in microemulsions with changing temperature is also shown in Scheme 5. The decreased interfacial curvature means that the microemulsion size increases, which is in accordance with the DLS and FFTEM measurements.

The concentration of a model ionic liquid bmimBF4

38 can be used as a control parameter for film curvature tuning in interfacially stiff microemulsions stabilized by cationic-anionic surfactants. The IL acts in a manner similar to electrolytes in standard systems, promoting ultralow interfacial tensions, (10-2 mN m-1) and evolution of the phase behavior from Winsor I→ Winsor III →

Winsor II MEs as a function of concentration of IL. The lower amount of IL needed as compared to that of normal NaCl makes ILs the most effective additives for curvature adjustment in ionic microemulsions, and ionic liquids including bmimBF4 used in this study can be employed to tune the microemulsion curvature. This enhanced efficiency is most likely a direct consequence of the unique structure of ILs, combining two ions of large volume, which can perform multiple functions at the interface, viz., structure breaking as well as binding and penetration into the stabilizing film to act as cosurfactants. These results point to new possibilities for ionic liquids as functional components in emulsions and microemulsions. Interestingly ILs such as bmimBF4 can act in two ways, simultaneously fulfilling the roles of polar phase as well as electrolyte. As such, IL microemulsions or emulsions represent flexible systems, offering approaches for forming structured fluids through a reduction in the number of components necessary to achieve the optimization of interfacial curvature.

Behera and Pandey39 reported that the addition of IL bmimBF4 can change the physicochemical properties of aqueous N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB-12). These changes depend on the concentration of bmimBF4. For many properties, the changes in aqueous zwitterionic surfactant solutions are observed to be different than those observed in aqueous nonionic and anionic surfactant solutions. Micellar or micelle- like aggregates exist even when as high as 30 wt %

Illustration of the bmimBF4/O IL microemulsion structure accompanied by the curvature change of TX-100 interfacial film at bmimBF4/oil two phases with changing temperature. [Reproduced from ref. 37].

Scheme 5


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bmimBF4 is present in aqueous SB-12. Addition of low concentrations of bmimBF4 results in changes in physicochemical properties that are similar to those expected from addition of an electrolyte or a cosurfactant. It has been proposed that the cation of the IL is either involved in electrostatic attractive interactions with the anionic part of the SB-12 zwitterion and/or the butyl chain on the IL cation orients along the SB-12 surfactants within the micellar pseudophase. Electrostatic attraction between anion of the IL and cation of the SB-12 zwitterion and vice versa results in decreased charge on the surfactant dipole and hence results in decrease in micellar size and cmc. Increasing the concentration of bmimBF4 beyond equimolar composition, however, results in IL acting more like a polar co solvent that reduces the extent of hydrophobic effect, thus reducing the efficiency of the micellization process. The outcome is the increase in cmc and decrease in Nagg. The decrease in Nagg, in part, results in increased water penetration into the micellar pseudophase. This is manifested through the increased dipolarity and microfluidity of the micellar pseudophase. In aqueous zwitterionic SB-12, micelles with desired physicochemical properties can be formed by addition of appropriate amounts of the hydrophilic IL, bmimBF4. The investigation has outlined the unique role of IL in modifying properties of aqueous zwitterionic surfactant solution. This may increase the potential applications of these neoteric and environmentally friendly solvents. The long term emphasis is on establishing a class of hybrid, environmentally benign system made up of IL-modified aqueous surfactants. Polarity estimations

On the basis of the response of solvatochromic probes Reichardt’s betaine dye, pyrene, and 1,3-bis-(1-pyrenyl)propane, Fletcher and Pandey40

investigated the aggregation behavior of common anionic, cationic and nonionic surfactants when solubilized within a low-viscosity room-temperature ionic liquid imide (emimTf2N) (1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)). They observed possible aggregate formation by all nonionic surfactants included in their study (Brij-35, Brij-700, Tween-20, and TX-100), while no aggregation was observed for the cationic surfactant cetyltrimethylammonium bromide. The anionic surfactant, sodium dodecyl sulfate (SDS) does not appear to solubilize within emimTf2N at

ambient conditions. This might be due to the fact that in the absence of water, the lack of hydration surrounding the SDS head group prevented the solid surfactant from dissolving in emimTf2N. This is contrary to the report by Pino and coworkers41 on SDS dissolved in bmimCl. Their study points to the possibility that the insolubility of SDS in emimTf2N may arise because of the nature of the salts formed (NaTf2N and emim dodecyl sulfate) by the statistical mixing rather than because of the absence of hydration water. The solvatochromic probe studies shows that ET(30) value remained constant indicating that the probe may reside primarily in the emimTf2N-rich region and is not affected to a measurable extent by any surfactant aggregation regardless of the surfactant structure. Also, due to the interaction between the cation moiety (-N+) on Reichardt’s dye and the lone pairs on many oxygen atoms present on these surfactants or the hydrogen bonding between the anionic site (-O-) of the dye and the surfactant headgroup, for each of the four nonionic surfactants investigated, a decrease in the optical density of Reichardt’s dye is observed as the surfactant concentration is increased. The lack of presence of any such functionality on the other two probes (i.e., pyrene and BPP) suggests the extent of any interaction between these probes and the nonionic surfactant monomers to be minimum or completely absent. Therefore, the authors believe these two probes are solubilized by the surfactant aggregates within emimTf2N micelles.

The effect of successively adding organic solvents to the TX-100/bmimBF4 binary system has been investigated by Gao and coworkers42. The addition of oils provided a strong hydrophobic environment for the hydrophobic groups of TX-100 molecules and induced them to aggregate into interface film. The oil and IL bmimBF4 two-phases are separated by the TX-100 interface film, and an o/IL microemulsion is thus formed. The aggregation behavior of nonconductive TX-100 led to the increase of continuous bmimBF4 concentration and thus the solution conductivity. The hydrophobic interaction between the added organic solvents and hydrophobic groups of surfactant molecules is considered to play a crucial role in driving the formation of o/IL microemulsions. The formation mechanism is different from the IL/o microemulsions where the electrostatic attraction between the positively charged imidazolium cation of bmimBF4 and the electronegative oxygen atoms of TX-100 is the


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driving force for solubilizing bmimBF4 into the cores of TX-100 aggregates. The formed o/IL droplet microemulsion is successively swollen by the addition of organic solvents, and a bicontinuous microstructure is finally observed for the first time. This study helps in understanding the aggregation behaviors of ILs containing self-assemblies and thus building them as potential reaction media. Microemulsions consisting of bmimBF4/TX-100/p-xylene have been prepared and the phase behavior of the ternary system has been investigated43. The bmimBF4-in-p-xylene (IL/o), bicontinuous, and p-xylene-in-bmimBF4 (o/IL) microregions of the microemulsions have been identified by conductivity. Scheme 6 represents the probable structure of the microemulsion. Compared with the AOT based reverse microemulsions, where only a few initial water molecules are needed to solvate the interface and the remaining water molecules are available to form water pools, there is a higher demand for hydration water in TX-100 aqueous microemulsions. The inner core water molecules have a strong tendency to form hydrogen bonded bridged structure between two adjacent OE groups. An oblate form is observed in the TX-100 aqueous microemulsion due to the longer size of the hydrophilic groups of TX-100. The same structure is also observed in the bmimBF4/cyclohexane microemulsion. Gao et al.

44 have also provided, 1H NMR and FTIR spectra as reliable evidence for the presence of interaction between TX-100 and the solubilized bmimBF4. This interaction is considered as the driving force for solubilizing bmimBF4 in TX-100 aggregates, which play the same role as

hydrogen bonds between OE units and water. However, a significant difference exists in the fact that bmimBF4 being larger than water, results in a fussy conformation of TX-100 molecule in microemulsions, and thus resulting in a smaller curvature of surfactant interface film. Therefore, a larger droplet size of IL/o microemulsion compared to traditional microemulsions is obtained by DLS and FFEM. DLS results show that a larger bulk of microemulsion droplets appears as compared to the “classic” microemulsion system, and that the diameters of the aggregates increase with increasing solubilized bmimBF4. The DLS results are similar to those of typical w/o microemulsions, suggesting the formation of the IL/o microemulsions. FTIR spectra show that the OH stretching of the terminal hydroxyl group of TX-100 shifts to a high-frequency region after adding bmimBF4, indicating that the electronegative oxygen atoms of OE units bonded to the electropositive imidazolium ring. The same conclusion is further proved by 1H NMR spectra. The interaction is considered to be the driving force for solubilizing IL into the core of the TX-100 aggregates. The UV-vis absorption spectra of MO indicate that the polarity of the polar core in bmimBF4-in-p-xylene reverse microemulsions is lower than that of bulk bmimBF4, and gradually increases in a small range with increasing IL content. In addition, a larger droplet of the IL/o microemulsions as compared to traditional microemulsions has been explained and a plausible structure proposed.

Studies on long-chained imidazolium ionic liquids

The phase behaviors of three long-chained imidazolium ionic liquids, C12mimBr (1-dodecyl-3-methylimidazolium bromide), C14mimBr 1-(tetradecyl-3-methylimidazolium bromide) and C16mimBr (1-hexadecyl-3-methylimidazolium bromide), with p-xylene and water have been investigated by Li et al.45 There are three types of lyotropic liquid crystalline phases formed in the (C12mimBr) system, viz., lamellar phase, cubic phase and hexagonal phase. However, only lamellar phase and hexagonal phase have been observed in the C14mimBr and C16mimBr systems. The formation of lyotropic cubic phase has been reported for the first time. Lyotropic liquid crystalline phases are observed and structural parameters with different CnmimBr chain lengths are compared. The unique properties of CnmimBr, such as “π-π stacking” and “π-cation” interactions, play an

Possible structure of the bmimBF4-in-p-xylene microemulsion. [Reproduced from ref. 43].

Scheme 6


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important role in determining the internal structural parameters, which are different from conventional cationic surfactants. It is also found that for the samples with the same component molar content, both the rheological steady and dynamic moduli are increased with the increase of the carbon chain of CnmimBr. The results are essential to the use of IL liquid crystals as solvent systems or microreactors in the preparation and processing of semiconductors, nanomaterials enzyme-catalyzed reactions, and extraction applications. The formation46 of non-aqueous microemulsion by using EMIM-EtOSO3 and EMIM-HexOSO3 as polar phase in the presence of toluene as oil component and CTAB as surfactant is possible on heating the mixture up to 80 °C. The temperature treatment is necessary due to the better solubility of the surfactant and the ionic liquid in the oil phase. In addition, the isotropic phase range can be enlarged by heating up the mixture to 80 °C. Pentanol as co-surfactant may be used for tuning the range of microemulsion area. It has a stabilizing effect on the interfacial layer, which results in a widening of the microemulsion area, i.e., the solubilization capacity is increased by adding the co-surfactant. Therefore, pentanol is believed to be integrated into

the interfacial layer of the microemulsion droplets, schematically shown in Fig. 3. Conductometric titrations show IL/o microemulsion with a percolation boundary at 15 wt% of the ionic liquid as well as a second break point at 22 wt%. The first break point can be discussed by percolation according to a channel mechanism47,48. The second point of inversion, with a significantly smaller slope of conductivity, accompanied by increase in viscosity, may be related to a droplet coalescence mechanism. The fusion of two individual droplets to a larger one is seen by cryo-SEM micrographs. Moreover, the diffusion/fusion process strongly depends on the bending elasticity of the amphiphilic film, which can be determined experimentally by a combination of dynamic light scattering and neutron spin-echo spectroscopy. However, the change in the droplet-droplet interactions from a channel to a droplet coalescence mechanism can be of special importance for the formation of nanoparticles.

Safavi and group49 have formulated a microemulsion based on imidazolium ILs, 1-octyl- 3-methylimidazolium chloride [omim][Cl], as a surfactant, a hydrophobic IL as a substitute for traditional organic solvent, 1-bmimPF6 and water at

Fig. 3 — Insertion of pentanol into the interfacial layer of the microemulsion droplets. [Reproduced from ref. 46].


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25 °C. DLS study shows that the Dh value of the bmimPF6/w microemulsion is about 3 nm and is nearly independent of water content but increases with increasing bmimPF6 content due to the swelling of the micelles by the IL. An attractive feature of the proposed microemulsion system is its narrow size distribution, indicating the monodispersity of microemulsion droplets. The investigation of the phase behavior of the system at equal amounts of bmimPF6 and water at different temperatures indicates that when the weight fraction of [omim][Cl] is 0.6, a single phase, temperature insensitive microemulsion is formed. The phase behavior of the ternary system consisting of an ionic liquid (1-tetradecyl-3-methylimidazolium bromide [C14mim]Br, p-xylene, and water has been investigated by Li et al.50 The p-xylene, [C14mim]Br, and water can form hexagonal and lamellar lyotropic liquid crystals or microemulsions depending on the composition of the ternary system. The volume of the microemulsion droplets shows a linear increase as the microemulsions are progressively swollen with added p-xylene; this behavior is consistent with that of the classic o/w microemulsions. The xylene molecules are preferably solubilized in the palisade shells and the cores due to hydrophobic and lipophilic interactions. These o/w microemulsions can incorporate anthracenes with a polar 9-substituent and arrange the molecules at the interface of the microemulsion droplets in such a way that the anthryl moiety is located in the hydrophobic region with the polar substituent toward the water phase. Photoirradiation of such anthracene derivatives in the microemulsions results in great yields of the h-h cyclomers, which is contrasted with the case of the photocyclization in homogeneous solutions. The increase in the h-h cyclomer formation in the microemulsions may be attributed to the preorientation of the substrate molecules at the interface of the microemulsion droplets. The hydrophobicity of the anthrancene moiety, the cation-π interactions between the anthryl and the imidazolium of [C14mim]Br and the polar nature of the substituents of the substrates, indicates that the anthracene chromophore may be located in the interface of the microemulsion droplets and the substituent directed toward the water phase. Consequently, the photocyclization of two neighboring anthracene molecules favors the formation of h-h cyclomers.

Role of IL in modifying the properties of aqueous zwitterionic

surfactant solution

The changes in the physicochemical properties of aqueous SB-12 by addition of IL bmimBF4 have been reported by Behera and Pandey51. These changes depend on the concentration of bmimBF4. For many properties, the changes in aqueous zwitterionic surfactant solutions are observed to be different than those observed in aqueous nonionic and anionic surfactant solutions. Micellar or micelle-like aggregates exist even when as high as 30 wt% bmimBF4 is present in aqueous SB-12. Addition of low concentrations of bmimBF4 results in changes in physicochemical properties that are similar to those expected from addition of an electrolyte or a cosurfactant. It is proposed that the cation of the IL is either involved in electrostatic attractive interactions with the anionic part of the SB-12 zwitterion and/or the butyl chain on the IL cation orients along the SB-12 surfactants within the micellar pseudophase. Electrostatic attraction between anion of the IL and cation of the SB-12 zwitterion and vice versa results in decreased charge on the surfactant dipole, and hence, results in decrease in micellar size and cmc. Increasing the concentration of bmimBF4 beyond equimolar composition, however, results in IL acting more like a polar cosolvent that reduces the extent of hydrophobic effect, thus reducing the efficiency of the micellization process. The outcome is the increase in cmc and decrease in Nagg. The decrease in Nagg, in part, results in increased water penetration into the micellar pseudophase. This is manifested through the increased dipolarity and microfluidity of the micellar pseudophase. In aqueous zwitterionic SB-12, micelles with desired physicochemical properties can be formed by addition of appropriate amounts of hydrophilic IL, bmimBF4. This investigation has outlined the unique role of IL in modifying the properties of aqueous zwitterionic surfactant solution.

Liquid crystal-in-ionic liquid microemulsion

The conditions during evaporation in a liquid crystal-in-ionic liquid microemulsion (LC/µEm) have been estimated by Friberg52 using phase diagram. The equations for selected tie lines have been established and the coordinates calculated for the sites at which the evaporation lines cross the tie lines. These values combined with the coordinates for the phases connecting the tie lines have been used to calculate the amounts and the composition of the fractions of the two phases present in the emulsion during the


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evaporation. The emulsion system is shown in Fig. 4. It consists of a lamellar liquid crystal, marked LC in Fig. 4, and a microemulsion, marked L, that reaches from the water corner to homogeneous surfactant in a continuous region. Hence, the liquid part of the emulsion changes from a water continuous liquid to a surfactant continuous liquid with no phase change. The evaporation line that is given in the figure emanates from the water corner because the ionic liquid does not evaporate to a degree influencing the direction of the line. Also, one of the emulsion phases is a lamellar liquid crystal and high energy emulsification would lead to the liquid crystal being disrupted to form vesicles. Such a system tenders a unique opportunity to study the interaction between vesicles and normal micelles, which gradually change to inverse micelles over bi-continuous structures. The amount of vesicles in the liquid phase versus the fraction liquid crystal has been calculated for two extreme cases of vesicle core size and shell thickness. The limit of evaporation while retaining the vesicle structure is calculated for emulsions of different original compositions assuming the minimum continuous liquid phase to be 50 % of the emulsion.

Studies on phase behavior and microstructure

The ternary phase behavior of microemulsions consisting of water, the hydrophobic ionic liquid bmimPF6 and the nonionic surfactant TX-100 have been studied as a function of temperature and surfactant concentration at different ionic liquid mass fractions by Anjum et al.53 The microstructure of some selected monophasic samples, investigated by SANS and polarizing microscopy at different temperatures, confirms the presence of bicontinuous microemulsions. A lamellar phase is observed in the 1-phase region of TX-100 microemulsions. The

coordinates of the X˜ points (point of highest efficiency γ˜ at temperature T˜) in this system stay nearly constant over the entire studied R-range, where only a slight increase of γ˜ (surfactant mass fractions) with IL content has been observed. Very high γ˜ values have been measured, indicating a low efficiency of TX-100. These large γ˜ values are reflected in the measured short length scales of the microemulsions, i.e., short mean repeat distances and correlation lengths. The fish-shaped three- phase region is distorted at low and high R values, whereas it has a symmetric form at intermediate values of R. These observations are qualitatively similar to the classical water/n-alkane/CiEj (alkyl oligoethyleneoxide) and to the novel IL/n-alkane/CiEj

microemulsion. As far as the efficiency is concerned, a comparison of CiEj with TX-100 microemulsion shows that the water/bmimPF6 system gets less efficient with increasing chain length. An explanation can be the solvophobicity of IL towards increasing alkyl chain length of the surfactant. These findings are in agreement with recent investigations of the liquid-liquid phase diagrams for mixtures of bmimPF6 and n-alkanes, where it is shown that the temperature of the miscibility gap increases with increasing the length of the hydrocarbon alkyl chain. The results show that the knowledge gained from the classical water/oil/CiEj systems cannot be transferred directly to aqueous microemulsion containing hydrophobic ionic liquids. Therefore, previous concepts for formulating and characterizing microemulsion must be studied in more detail. For example, the low efficiency of the TX-100 microemulsion cannot be enhanced by increasing the hydrophobicity of the surfactant.

The phase behavior and microstructure of ternary mixtures of nonionic alkyl oligoethyleneoxide (CiEj) surfactants, alkanes and the ionic liquid ethylammonium nitrate (EAN) investigated by Atkin and Warr54 closely parallel observations in water. In general, the surfactant chain length must be ~4-6 CH2 groups longer in EAN to produce effects similar to those observed in aqueous systems such as a strongly structured microemulsion or an intruding lamellar phase. Increasing the amphiphilicity initially increases, and then decreases, the dimensions of the “fish” body. As the fish begins to decrease in size, the microemulsions undergo transition from weak to strong structure. For EAN-dodecane systems, this occurs between C12E3 and C14E4, as shown using both SAXS and conductivity. EAN microemulsions

Fig. 4 — The emulsion system. The liquid part of the emulsion, L, is a microemulsion and the liquid crystal, LC, has a lamellar structure. [Reproduced from ref. 52].


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respond to changes in the length of the surfactant headgroup, tail and alkane in a manner broadly consistent with aqueous systems. A lamellar phase is often present at lower temperatures for surfactants with longer alkyl tails. Variation in the dodecane-to-EAN ratio for constant surfactant concentration reveals that this phase has the highest melting point when the solvent volumes are equal. Where present, lamellar phases followed the expected one dimensional swelling behavior with surfactant concentration at constant molecular area. The effective area of interaction for each ethoxy unit is found to be significantly higher in EAN than in water. This is attributed to the increased size of the species that hydrogen binds to the headgroup in the ionic liquid (EAN) as compared to that in water. In microemulsion phases, a single broad scattering peak is observed reminiscent of aqueous systems. This microemulsion peak is modeled using the Teubner-Strey model. The values of the periodicity of the oil and water domains (d), and correlation length (ξ) indicate that EAN microemulsions are generally more highly structured than their aqueous counterparts. For the most part, variation in the length of the surfactant tail group and alkane oil produced changes in d and ξ, consistent with aqueous systems. Both the lamellar and microemulsion scattering peaks shifted to lower peak position as the temperature is increased above phase inversion temperature. This is due to a decrease in the interfacial area of the surfactant layer as temperature is increased; a consequence of the increased level of thermal fluctuations and the lower area of interaction of the surfactant headgroup with EAN.

The nonionic surfactant, TX-100, is shown to form micellar aggregation in ILs, bmimBF4 and bmimPF6 by Gao et al.55 The surface tension measurements reveal that a relatively high cmc is obtained, which is attributed to the low hydrophobicity of TX-100 in ILs in comparison with aqueous system. FFTEM shows that the micellar shape is not regular spherical, similar to aqueous micelles, but a larger micellar aggregation is observed (Fig. 5). 1H NMR and two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (2D ROESY) further indicate that ion pairs of ILs have been destroyed by the addition of TX-100, and there is strong interaction between the positively charged imidazolium ring and the electronegative oxygen atoms of OE units of TX-100. This interaction is similar to the hydrogen

bonds or hydration that happens to traditional aqueous aggregations to balance the hydrophobicity. Solvatophobicity between TX-100 and ILs is deduced to drive the formation of micelles in ILs, and surface tensions of solvents can be also used to determine micellar formation to a certain extent.

The phase diagram and microstructure of the ternary system ionic, liquid benzylpyridinium bis(trifluoromethanesulfonyl)imide)/nonionic surfactant (octylphenol ethoxylate)/toluene, have been studied by Gayet and et al.56 using conductivity measurements, dynamic light scattering, pulse field gradient spin-echo NMR and SANS. Three microregions have been identified by conductivity measurements according to the percolation theory. The sizes of IL/o microemulsions with various IL fractions have been then determined by NMR and DLS and were found to be in accordance with the radii of gyration (approximately 2 or 3 nm) determined by SANS. The reverse IL/oil microemulsions are used as nanoreactors to perform a Matsuda-Heck reaction between p-methoxyphenyl diazonium salt and 2,3-dihydrofurane in the presence of a palladium catalyst. The reaction yields obtained are greater in microemulsions (67 %) than in bulk IL (33 %), highlighting a strong effect of confinement. Moreover, a direct correlation between the quantity of IL and the reaction yield is observed.

Applications of IL Microemulsions

Biocatalysis/enzymatic activity in ionic liquid microemulsions

The technological utility of enzymes can be enhanced greatly by their use in ILs rather than in conventional organic solvents or in their natural aqueous reaction media due to their unusual solvent characteristics. Studies on enzymatic reactions in ILs

Fig. 5 — Typical FFTEM image of TX-100/bmimBF4 micellar solution at 15 CMCs. [Reproduced from ref. 55].


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over the last 8-9 years have revealed not only that ILs are environmentally friendly alternatives to volatile organic solvents, but also that in such a solvent, enzymes exhibit excellent selectivity including substrate, regio- and enantioselectivity. Besides these, many enzymes maintained very high thermal and operational stability in ILs. Therefore, ILs are of growing interest as a new and highly efficient reaction medium for biocatalytic reactions. The first successful report on an enzyme catalyzed reaction using IL as a medium was published in 2000 by Russell and coworkers57. Since then, a wide number of enzymes have been subjected to ILs to test their catalytic activity, e.g., lipases58,59, alcohol dehydrogenases60-63

and oxidoreductases, etc.

In a report by Moniruzzaman’s group64, the first results on the enzymatic activity of horseradish peroxidase (HRP) microencapsulated in w/IL microemulsions composed of AOT /hydrophobic IL [C8mim][Tf2N] (1-octyl-3-methyl imidazolium bis (trifluoromethylsulfonyl)amide)/water/1-hexanol using pyrogallol as the substrate has been published. In this system, the catalytic activity of HRP is measured as a function of substrate concentrations, ω (molar ratio of water to surfactant), pH, and 1-hexanol content. The curve of the activity-ω profile is found to be hyperbolic for the new microemulsion. The stability of HRP solubilized in the newly developed w/IL microemulsions has been examined, and it has been found that HRP retained almost 70 % of its initial activity after incubation at 28 °C for 30 h. Apparently, it is observed that HRP-catalyzed oxidation of pyrogallol by hydrogen peroxide in IL microemulsuions is much more effective than in conventional AOT/water/isooctane microemulsion. According to the authors, the reason for the enhanced catalytic activity of HRP in w/IL microemulsions may be the following: firstly, the partition of the substrate, products, or other molecules involved in the reaction between the aqueous pseudophase and the IL continuous phase; secondly, a change in the enzyme microenvironment, and thirdly, the presence of 1-hexanol in the reaction medium. The solubility studies show that both substrate (pyrogallol) and product (purpurogallin) are significantly soluble in IL [C8mim][Tf2N] whereas they are poorly soluble in isooctane. In the case of w/IL microemulsions, it is reasonable to assume that the product concentration in the aqueous pseudophase is reduced significantly because of the favorable

partitioning to the IL continuous phase. Consequently, the product inhibitory effect may be less effective in the IL systems than that in the organic solvents systems. Another reason for the high effectiveness of HRP oxidation in AOT-based w/IL microemulsions, apparently, is the change of the microenvironment (medium). In fact, the properties of new microemulsion inner cavity, such as the dynamics of the trapped water pool should differ from the nature of water pool in AOT-based w/o microemulsions.

Pavlidis et al.65 considered w/IL microemulsion systems formulated with a non-ionic surfactant such as Tween-20 and TX-100, in bmimPF6 to be a promising medium for biocatalytic processes. The lipases from C. rugosa, C. viscosum and T. lanuginosa exhibit higher catalytic performance and operational stability in these novel systems in comparison to other micro-heterogeneous media used so far for various biocatalytic reactions. The retention of catalytic activity is due to the entrapment of enzyme molecules into aqueous microdroplets formed in w/IL microemulsions, indicating that these IL-based reaction systems provide a protective environment for the enzymes. The observation is confirmed by spectroscopic studies which indicate that enzymes entrapped in w/IL microemulsions tend to retain their native structure or adapt a more rigid structure compared to that observed in other reaction media. As compared to conventional organic solvents, the enhanced stability of enzymes in w/IL microemulsions, the ability of easy separation of products from the reaction system and good enzyme reusability, as well as the unique solvent properties of ionic liquids indicate that these novel micro-heterogeneous media can be efficiently used as reaction media for various biocatalytic reactions. In order to compare structural changes of lipases after their entrapment in w/IL microemulsions with those in other microheterogeneous media, the secondary structure of the lipases entrapped in w/o microemulsions and ternary surfactantless systems has been investigated by far UV-CD spectroscopy. Inconsistency in secondary structure of lipases in water is observed between the results obtained from CD and those from FT-IR spectroscopy. This has been attributed to the enzyme concentration used in the two experimental procedure. CD spectroscopy shows that the secondary structure of lipases is significantly altered from their native forms when entrapped in w/o microemulsions or surfactantless systems. Specifically, in all cases, a substantial


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decrease in α-helix (up to 32 %) and an increase in β- sheet content (up to 35 %) are observed with respect to the structure of the lipase in water, while these variations are more prominent in the case of the surfactantless microemulsions. These extended structural changes in w/o microemulsions and surfactant-free microemulsion-like ternary systems may explain the lower stability of lipases in microheterogeneous media as compared to that observed in w/IL microemulsions.

Polymerization

Lade and coworkers66 have reviewed the salient features of microemulsion polymerization (chain radical polymerization). Very efficient polymerizable microemulsions result when the curvature of the surfactant film is carefully balanced. Close to the balanced state of efficient microemulsions, lamellar phases exist either alone or in coexistence with other phases, e.g., the microemulsion or an excess oil or water phase. While polymerizations in the one-phase microemulsion follow the usual well-understood kinetics, phase separation or transitions into lamellar phase region during the polymerization process result in severe systematical changes of the kinetics, which are not well understood. Shaomei67 have recently reported some interesting new materials synthesized by microemulsion polymerization of methylmethacrylate microemulsions stabilized with reactive IL surfactants described above67,68.

The obtained IL/polymer composites show high conductivity at both room temperature and elevated temperature. In the water/IL-Br/MMA system very stable nanolatexes have been synthesized (close to the water corner; Fig. 6, left) that retained stability in concentrated NaBr (0.1 M). However, when these same latexes are exposed to aqueous BF4

− or PF6−,

they undergo coagulation. At higher surfactant concentration in a bicontinuous domain, a transparent gel is obtained by thermally initiated polymerization at 60 °C. At 15 % (w/w) IL surfactant (IL-Br), thermally initiated microemulsion polymerization of this system produces a transparent gel, illustrated in Fig. 7. Treatment of this soft gel with aqueous PF6

− produces an open cell microporous material by a mechanism based on spinodal decomposition69. Further soaking of the open cell material in aqueous NaBr essentially reverses the transformation back to a transparent gel. The other pseudo-ternary system illustrated in Fig. 6 (right), aqueous propanol/IL-BF4/MMA, produces similar materials that can be transformed to nanoporous open cell materials upon treatment of gels obtained by microemulsion polymerization70.

The first example of an effective approach to prepare the nonaqueous proton conducting membranes via the polymerization of microemulsions comprising surfactant (1-(2-methyl acryloyloxy-undecyl)-3-methyl imidazolium bromide) (MAUM-Br)

Fig. 6 — (Left) Ternary phase diagram for water/IL-Br/MMA system at 21 oC and 60 oC. The filled squares denote the boundary at 21 oC and the dotted line denotes the boundary at 60 oC. The dashed filled domain is the single-phase microemulsion domain and the multiphase domain is unshaded below the illustrated boundary curves and above the water/MMA axis. (Right) Ternary phase diagram for water-propanol/IL-BF4/MMA system at 21 oC and 60 oC. The filled squares of the upper curve denote the boundary at 21 oC, the shaded domain is the single-phase microemulsion domain, and the multiphase domain is unshaded below the illustrated boundary curve and above the water/MMA axis. The corresponding boundary at 60 oC is the lower of the two. In both diagrams the protrusion of the single-phase microemulsion domains was not investigated above 75 % (w/w) surfactant. [Reproduced from ref. 68].


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stabilized protic ionic liquid (PIL) dispersed in a polymerizable oil, and a mixture of styrene and acrylonitrile has been given by Yan group71. The produced PIL-based polymeric composite membranes are shown in Fig. 8.

The resultant polymer electrolyte membranes are uniform, transparent and flexible even though the resulting vinyl polymers are immiscible with PIL cores. These PIL-based polymer membranes have quite a good thermal stability, chemical stability, tunability and good mechanical properties. Under non-humidifying conditions, these polymer electrolyte membranes have conductivity up to the order of 1 × 10-1 S cm-1 at 160 °C because of the well-connected PIL nanochannels formed in the samples. The results demonstrate that the PIL-based membranes combine the high proton conductivity and good mechanical properties in a unique way. This type of composite membranes is expected to have an impact on further investigations in the field of proton conducting membranes for PEMPCs. The methodology presented can be extendable to other liquid monomers and PILs with desirable properties for the preparation of non-fluorinated polymer electrolyte membranes. However, the long-term operation of the PIL-based membranes may be affected by a progressive release of the PIL.

Synthesis of nano and micro particles

The microemulsion with ionic liquid bmimPF6 as the continuous phase has been used to fabricate porous silica micro rods using TX-100 as surfactant

by Li and coworkers72. The diameter of the obtained microrods increases with increasing ω , while the amount of tetraethylorthosilicate (TEOS) on the diameter of the prepared silica microrods is not considerable. In the traditional microemulsion formed by Triton surfactant using apolar solvent of cyclohexane as continuous phase, only silica nanoparticles are obtained, which is different from those microrods obtained by Li et al.72 This indicates that the IL (bmimPF6) plays an important role in the formation of the final morphology of synthesized silica. When TEOS is added to the solution, it hydrolyzes at the interface of the water droplet to form silica. The IL serves as a directing agent for the formation of the silica microrods. In addition, PF6

−

can hydrolyze and the released F− may also play a role in the formation of silica microrod due to the reaction of HF with silica. However, in another report by Zhao et al.73 silica products with two different morphologies have been synthesized using non-aqueous ionic liquid microemulsion benzene, TX-100/bmimBF4 droplets as templates and TEOS as silica source. The morphologies of the obtained products have been characterized by both transmission electron microscopy and scanning electron microscopy. By adjusting the reaction conditions, ellipsoidal nanoparticles were formed under acidic conditions, while hollow silica spheres are obtained under alkaline conditions. It has been demonstrated that the size distribution of hollow silica spheres is narrower than that of the ellipsoidal nanoparticles. The various vibration modes of different functional groups in the silica materials have been revealed by FTIR spectroscopy. The two samples are both shown to be amorphous, and not crystalline by X-ray diffraction. A simple diagram of the formation process including the hydrolysis

Fig. 7 — SEM images of copolymers produced by microemulsion polymerization in water/IL-Br/MMA system at 60 oC at a composition of 75 % (w/w water, 15 % (w/w) IL-Br, 10 % MMA. (A) gel obtained after microemulsion polymerization; (B) gel in (A) treated with aqueous 0.1M KPF6; (C) microporous material in (B) treated with aqueous 0.1 M NaBr solution. [Reproduced from ref. 70].

Fig. 8 — Photograph of PIL-based polymer composite membranes. [Reproduced from ref. 71].


680

and condensation reactions is given in Scheme 7. Furthermore, a probable mechanism for the formation of silica materials under acidic or alkaline conditions is presented, which may be helpful for better understanding of the different silica materials obtained under different conditions.

In another report by the same group74, hollow silica spheres have been successfully synthesized by using the ionic liquid microemulsion droplets, BmimBF4/TX-100/benzene, as the template. The morphology and microstructures of the silica spheres have been investigated by SEM, HRTEM, and nitrogen adsorption-desorption measurements. The images show that the average size of the silica spheres is almost between 150 and 300 nm. The nitrogen adsorption-desorption investigation on the silica spheres indicated the amorphous structure on the interface. A possible formation mechanism of the observed hollow silica spheres with irregular structures on the surface is shown in Scheme 8. When TEOS is added to the microemulsion system, it dissolves in the benzene core of the microemulsion droplets. At the same time, some bmimBF4 molecules are located in the palisade layers of the microemulsion. Then the TEOS molecules at the interface of the core may be hydrolyzed and polymerized, because the bmimBF4 molecules in the palisade layers can probably be employed as the Lewis acid. Therefore, the SiO2 polymerizes and grows thickly around the interface of the benzene cores, forming the hollow silica spheres after calcining. [C4mim]+ and several other surfactant molecules usually adhere to the surface of the spheres, and the removal of the ionic liquid and surfactant molecules by calcining may lead to the formation of irregular structures on the surface.

Zhoua et al.75 further demonstrated that for

hydrophobic ionic liquid, bmimPF6, a water-in-

bmimPF6 microemulsion can be formed in the presence of nonionic surfactant TX-100. In such a medium, both lignin peroxidase (LiP) and laccase can express their catalytic activity with the optimum ω of 8.0 for LiP and >20 for laccase, and the optimum pH of 3.2 for LiP and 4.2 for laccase, respectively. High expression of the catalytic activities of both enzymes in bmimPF6

based microemulsion may be attributed to the TX-100 interfacial membrane which separates the enzymes from bmimPF6, though high levels of TX-100 also had obvious negative effect on the activities of both enzymes. As compared with pure or water saturated bmimPF6 in which the two oxidases have negligible catalytic activity due to the strong inactivating effect of bmimPF6 on both enzymes, the use of the bmimPF6 based microemulsion has some advantages. Not only was the catalytic activities of both fungal oxidases

The formation mechanism of silica materials under both acidic and alkaline conditions. [Reproduced from ref. 73]

Scheme 7

Proposed formation mechanism of the hollow silica spheres. [Reproduced from ref. 74].

Scheme 8


681

greatly enhanced, but the apparent viscosity of the medium was also decreased. Yan and Texter76 found that cell dimensions can be controlled by cross-link density, by adding monomers such as ethyleneglycol dimethacrylate (EGDMA) to the microemulsions. Polar solvents such as DMSO, alcohols, etc. that can solvate the imidazolium-PF6 ion pairs, induce spinodal decomposition back to a gel state. The production of the open cell structure from the polymerized gel is again induced by PF6

− induced collapse of the copolymer. Aqueous

DMSO (25 % water) suffices to resolvate the imidazolium-PF6 ion pair to recover the gel state. It is reported that water is a poor solvent and induces spinodal decomposition back to the open cell structure. Figure 9 suggests that some ripening or annealing occurs on cycling back and forth. Each of the illustrated micrographs in Fig. 9 is from a freshly prepared fracture surface. The slight pitting in frames A and C has been attributed to evaporation under the SEM chamber vacuum and partial opening of some pores.

Fig. 9 — SEM images of gels and open cell copolymer systems produced by microemulsion polymerization at 60 oC of a composition 35 % water, 35 % propanol, 15 % ILBF4, 15 % MMA, and 2 % EGDMA, and various subsequent solvent treatments. (A) after polymerization of the microemulsion; (B) gel in (A) treated with aqueous 0.1 M KPF6; (C) gel in (B) treated with aqueous DMSO; (D) gel in (C) treated with water; (E) after three cyclic treatments of aqueous DMSO/water; (F) after seven cyclic treatments of aqueous DMSO/water. [Reproduced from ref. 76].


682

Electrochemical properties

Microemulsion electrokinetic chromatography (MEEKC) using bmimBF4 as additive has been developed by Zhang et al.77 for the analysis of baicalin, wogonin and baicalein in Scutellariae radix. The bmimBF4 enhances the separation. Figure 10 shows schematically the MEEKC separation with bmimBF4. MEEKC is a separation technique providing good selectivity and high separation efficiency of anionic, cationic as well as neutral solutes. In some cases, changes in the nature of surfactant have been used to improve the separation of analytes with MEEKC. Organic modifiers can change the distribution of analytes between microemulsion droplets and continuous pseudophase, and improve the separation accordingly. Imidazolium cations interact with microemulsion droplets through electrostatic and hydrophobic interactions. The interaction between imidazolium cations and microemulsion droplets alters the distribution of the analytes. The flavones may associate with imidazolium ions or distribute to the swollen micellar phase, which are driven by the electrostatic interaction, hydrophobic interaction, hydrogen bonding, etc. Bryning, in a

patent document78, has illustrated the use of ionic liquid microemulsion systems for separation of biomolecules.

Conclusions

The chemistry of room temperature ionic liquid microemulsions is at an incredibly exciting stage of development. No longer mere curiosities, ionic liquids microemulsions are beginning to be used as solvents for a wide range of synthetic procedures. The advent of systems that are easy to handle allows those without specialist knowledge of the field to use them for the first time. Ionic liquids have also been used as additives in microemulsion formulation. A number of reactions investigated so far show the potential of the ionic liquids. The retention of catalytic activity in IL microemulsions is due to the entrapment of enzyme molecules into aqueous microdroplets formed in w/IL microemulsions, indicating that these IL-based reaction systems provide a protective environment for the enzymes. Also, due to the unique solubilization behavior of the added water molecules, the IL/o microemulsion system may be used as a medium to prepare porous or hollow nanomaterials by hydrolysis reactions. The solvent environment

Fig. 10 — Mechanism of the flavones separation using 1-butyl-3-methylimidazolium-based ionic liquid as additive in MEEKC. [Reproduced from ref. 77].


683

that is provided by the ionic liquids is unlike any other available at, or close to, room temperature. Already, startling differences have been seen between reactions in ionic liquids microemulsions and normal microemulsions/molecular solvents. The enhanced stability of enzymes in w/IL microemulsions, the ability of easy separation of products from the reaction system and good enzyme reusability, as well as the unique solvent properties of ionic liquids compared to conventional organic solvents, indicate that these novel micro-heterogeneous media can be efficiently used as reaction media for various biocatalytic reactions. Significant advanced materials applications are already being developed, but much more work remains to be done in exploring the use of ionic liquid-based microemulsions in advanced materials synthesis.

Acknowledgement

SKM is thankful to DST, New Delhi (100/IFD/11145/2009-2010) and CSIR, New Delhi, (01(2306)/09/EMR-II) for research grants. KK is thankful to CSIR for award of a fellowship (09/135/(0532)/2008/EMR-I ).

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Ionic liquid microemulsions and their technological...

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