IL microemulsions

In general, one can say that a new type of nonaqueous microemulsions can be formed, consisting of very small IL droplets stabilized by the other more surfactantlike IL. By increasing the amount of IL, a transition to a bicontinuous phase range can be identified by conductivity measurements as well as a transition to oil-in-IL microemulsion droplets. 

In this review, we examine recent studies of microemulsions incorporating ILs and some recent applications of such microemulsions. We discuss studies wherein ILs are substituted for the oil component in water and IL microemulsions, for the polar or water component in IL and oil microemulsions, and for the surfactant component, along with new material applications. Taking as starting point the traditional water-in-oil microemulsion, in the new IL microemulsions, the IL can play the role of water, surfactant, or oil (see Scheme 13.1). If the IL substitutes dispersed phase, we have nonaqueous IL microemulsions. If the IL snbstitutes surfactant and/or continuous phase, we have aqueous IL mieroemulsions. We establish the following classifications ... 

Combinations of the methyl imidazolium cation with different lengths ([C mim]) and anion different from that of [C mim][BF ] can be involved in the preparation of new IL microemulsions. Although [C mim] [PFJ is normally taken as the IL in aqueous IL microemulsions, some nonaqueous IL microemulsions were also prepared. The aggregate size of [C mim][PFJ/TX-100/toluene [35] and [C mim][PFJ/TX-100/ ethylene glycol [36] microemulsions were characterized further by small-angle... 

Falcone et al. used the anionic sodium l,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant, benzyl-n-hexadecyldimethylammonium chloride (BHDC) to prepare IL microemulsions [48]. The ILs chosen were l-butyl-3-methylimidazolium trifluoromethanesulfonate ([C mim][CF3S03]) and l-butyl-3-methylimidazolium trifluoroacetate ([C mim][CF3COJ see Scheme 13.2). DLS experiments reveal the formation of microemulsions. Besides, the FTIR results suggest that the ionic interactions (with the surfactant polar head groups, surfactant counterions, or IL counterions) are substantially modified upon confinement. These interactions produce segregation of IL s ions, altering the composition of the microemulsion interfaces. 

Aqueous microemulsions with the IL as apolar phase are much more interesting than nonaqueous microemulsions. The reason is that both water and the IL are considered green solvents. The IL most commonly used in the preparation of aqueous IL microemulsions is l-butyl-3-methylimidazohum hexafluorophos-phate ([C mim][PF ]). The first aqueous IL microemulsion, water/TX-100/ [C mim][PFJ, was reported for Gao et al. showing water in [C mim][PFJ (w/IL), bicontinuous, and [C mim][PF ] in water (IL/w) subregions (see Fig. 13.1) [52]. These microregions were identified by cyclic voltammetry method using... 

Some remarkable applications for microemulsions constituted by ILs have been recently reported. For instance, aqueous IL microemulsions were used for hquid-liquid extraction. In particular, water/AOT/[C mim][PF ] system has been proved to entail selective extraction of hemoglobin from human whole blood [57] or to develop a synergic microextraction procedme for the preconcentration and determination of glucocorticoid hormones in water samples [58]. 

An interesting application of aqueous IL microemulsions is to develop IL polymer materials incorporating enzymes that can be used as active, stable, and reusable biocatalysts [65].The IL l-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl) amide ([veim][Tf2N] see Scheme 13.2) was used as the continuous phase. Incorporation of proteins in IL-based polymer frameworks is generally difficult because they are insoluble in most ILs.To overcome this limitation, the authors first employed water/Tween-20/[veim][TfjN] microemulsions to solubilize the enzyme in an IL phase and then incorporated the enzyme within these surfactant aggregates into IL polymer frameworks via polymerization in the presence of an IL-soluble cross linker and initiator. 

The addition of alcohol, as cosurfactant, to the [Cgmim][TfjN]/AOT/water system leads to stable w/IL microemulsions. DLS and protein solubilization experiments confirm the existence of an aqueous nanoenvironment in the IL phase of [C mirnTf N]/ AOT/l-hexanol/water microemulsions [67]. The kinetics of the enzymatic reactions were performed in this quaternary system. Specifically, lipase-catalyzed hydrolysis of p-nitrophenyl butyrate (p-NPB) was used as a model reaction [68]. In a similar way, the hpase-catalyzed hydrolysis of p-NPB was investigated to evaluate the catalytic efficiency in water/AOT/Triton X-100/[C mim][PFJ [69]. A large single-phase microemulsion region can be obtained from the combination of two surfactants in IL. 

Figure 13.2 Schematic depiction of w/IL microemulsion formation by SB-12 in the presence of ethanol. Reproduced from Rai et al. [66] with permission from John Wiley Sons. Copyright (2012).

The characterization of the following IL-based microemulsions has been presented as follows (i) IL-in-oil and oil-in-IL microemulsions, (ii) IL-in-water and water-in-IL microemulsions, (iii) IL-in-ethylene glycol (EG) and EG-in-IL microemul-sions, (iv) IL-in-IL microemulsion, and (v) n.-in-supercritical CO and CO -in-n. microemulsions. 

Han and coworkers [13] further studied the phase behavior of toluene/TX-100/1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF ]) system. As shown in Figure 16.2, the single-phase microemulsion area covers about 75% of the phase diagram at 25 °C. A transition from IL-in-oil microemulsion (marked as A) via a bicontinuous region (marked as B) to an oil-in-IL microemulsion (marked as C) occurs with the increase of weight fraction of TX-lOO and [bmim][PFJ. 

Figure 16.2 Phase diagram of toluene/TX-100/[bmim] [PFJ (in weight fraction) at 25.0 °C. A, IL-in-oil microemulsion B, bicontinuous region C, oil-in-IL microemulsion and D, biphasic region. Reproduced from Li et al. [13] with permission from Elsevier.

Han and coworkers [38] determined the phase behavior of the ternary system consisting of [bmim][PFJ,TX-100, and water at 25 °C. By cyclic voltammetry method using potassium ferrocyanide, K Fe(CN)g, as the electroactive probe, the water-in-[bmim][PFJ, bicontinuous, and [bmim][PFJ-in-water microregions of the microemulsions were identified (Fig. 16.7). The hydrodynamic diameter of the [bmim] [PFJ-in-water microemulsions is nearly independent of the water content bnt increases with increasing [bmim] [PF ] content due to the swelling of the micelles by the IL. Sarkar and coworkers [39-41] reported the solvent and rotational relaxation studies in [bmim][PFJ-in-water microemulsions and water-in-[bmim][PFJ microemulsions using different types of probes, coumarin 153 (C-153), coumarin 151 (C-151), and coumarin 490 (C-490). The solvent relaxation time is retarded in the IL-in-water microemulsion compared to that of a neat solvent. The retardation of solvation time of water in the core of the water-in-IL microemulsion is several thousand times compared to pnre water. Nozaki and coworkers [42] reported a broadband dielectric spectroscopy study on a microemnlsion composed of water. 

TX-lOO, and [bmim][PFJ. It was found that the phase behavior of the microemulsion could be easily identified by its dielectric response. The dielectric behavior of the IL microemulsion in the GHz range is consistent with that of TX-lOO/water mixtures with comparable water-to-TX-100 weight ratio. 

BFJ microemulsion. When TEOS was added to the microemulsion, it dissolved in the benzene core at the same time, some [bmim] [BF ] molecules were located in the palisade layers of the microemulsion. Subsequently,TEOS molecules at the interface of the core were hydrolyzed and polymerized, because the [bmim][BF ] molecules in the palisade layer could probably be employed as Lewis acid. So, the SiO polymerized and grew thickly around the interface of the benzene cores, forming the hollow silica spheres after calcination. Furthermore, Zheng and coworkers [54] synthesized silica products with two different morphologies using nonaqueous IL microemulsion droplets as templates. By adjusting the reaction conditions, ellipsoidal nanoparticles were formed under acidic condition, while hollow silica spheres were obtained under alkaline condition. 

Polyaniline/Silver Nanocomposites He and coworkers [56] employed chronopotentiometry to prepare polyaniline/silver (PANI/Ag) nanocomposite films in water-in-IL and IL-in-water microemnlsions, by simultaneous oxidative polymerization of aniline to PANI and rednction of silver nitrate to Ag nanoparticles. The PANI/ Ag prepared in water-in-IL microemulsion was nanofibrous, and the Ag nanocrystals with 5 nm diameter were dispersed homogeneously, whereas the PANI/Ag prepared in IL-in-water microemulsion exhibited dendritic structure, and the diameter of Ag nanocrystals was 50-100 nm. The special structures of the PANI/Ag nanocomposite resulted in more excellent electrochemical activity than that of the pure PANI. 

Mesoporous Metal-Organic Framework Zhang and coworkers [51] explored the apphcation of CO -in-IL microemulsions in fabrication of metal-organic frameworks (MOFs), which present great potential in gas storage, separation. 

Lipase-Catalyzed Hydrolysis Goto and coworkers [63] proposed an approach for carrying out enzymatic reactions in water-in-IL microemulsion. 

The lipase-catalyzed hydrolysis of p-nitrophenyl butyrate (p-NPB) was used as a model reaction. It was found that the hydrolysis rate was faster in the water-in-IL microemnlsions than in the water-in-isooctane microemulsions. Hie intrinsic activity of lipase in the IL microemulsion was about three times higher than that of water/ AOT/isooctane microemulsions of AOT under the given experimental conditions. The enhanced catalytic activity of lipase in water-in-IL microemulsions may be due to (i) aqueous microenvironmental changes, (ii) the partition of the substrate or other molecules involved in the reaction between water and IL phases, and (iii) the existence of 1-hexanol as a cosurfactant. 

Zheng et al. investigated how organic solvents (cyclohexane, p-xylene, toluene, and benzene) worked in the formation of [bmimjBF -based oil-in-IL nonaqueous microemulsion systems [12]. The added molecular solvents provided a nonpolar environment that resulted in the aggregation of the hydrophobic tails of the surfactant TX-lOO, so that the molecular solvents formed droplets dispersed in the continuous [bmim]BF phase. Results of 2D H-NMR confirmed that the solvophobic interaction between the molecular solvents and the hydrophobic tails of TX-lOO was the driving force in the formation of those oil-in-IL microemulsions. 

Han and coworkers [33] successfully developed the IL-in-IL microemulsion consisting of two immiscible ILs, [bmim]PF and propylammonium formate (PAF), and AOT. In the presence of surfactant AOT, the hydrophobic IL [bmim]PF (as the internal phase) was dispersed in hydrophilic IL PAF and led to form [bmim]PFj-in-PAF... 

IL-based nonaqueous microemulsion can also be used for preparation of silicon materials with different morphologies. By optimizing catalytic conditions, Zhao et al. synthesized two different morphologies of silica products using nonaqueous IL microemulsion ([bmim]BF -nTX-100-Hbenzene system) droplets as templates [45]. 

The product obtained under acid conditions was an ellipsoid nanoparticle, and that obtained under alkaline condition was hollow silica ensembles. The study revealed that [bmimJBF played a critical role in the process of formation of the mesoporous silica hollow structure. It was thus evident that nonaqueous IL microemulsions can contribute to the synthesis of materials with various structures by providing a reaction environment that is able to control the size and morphology of the materials. 

Nonaqueous IL microemulsions were also used as catalysts to improve reaction efficiency. Gayet et al. established an IL-in-oil microemulsion system with benzylpyridinium bis(trifluoromethanesulfonyl)imide ([BnPyrJNTfj), TX-lOO, and toluene, in which the Matsuda-Heck reaction between methoxybenzene diazotate and 2,3-dihydrofuran took place [46]. The reaction yield in this IL-in-oil microemulsion was twice as high as that in neat ILs. The results provided a basis for designing a nonaqueous IL microemulsion microreactor and also showed that nonaqueous IL microemulsion might have good prospects of applications in biocatalysis and nanomaterial synthesis. 

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