Aqueous microemulsions bicontinuous microemulsion

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... 

In Winsor s type 3 systems (r = 1), the surfactant s affinity for the oil and the water phases is balanced. The interface will be flat. A type 3 nSOW system can have one, two or three phases depending on its composition. In the multiphase region the system can be (a) two phase—a water phase and an oleic microemulsion (b) two phase—an oil phase and an aqueous microemulsion (c) three phase—a water phase containing surfactant monomers at CMC, an oil phase containing surfactant at CMC and a surfactant phase . The surfactant phase may have a bicontinuous structure, being composed of cosolubilised oil and water separated from each other by an interfacial layer of surfactant. The surfactant phase is sometimes called the middle phase because its intermediate density causes it to appear between the oil and the water phases in a phase-separated type 3 nSOW system. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

Fatty alcohol- (or alkyl-)ethoxylates, CoE, are considered to be better candidates for LLE based on their ability to induce rapid phase separation for Winsor II and III systems. (Winsor III systems consist of excess aqueous and organic phases, and a middle phase containing bicontinuous microemulsions.) However, C,E,-type surfactants alone cannot extract biomolecules, presumably because they have no net negative charge, in contrast to sorbitan esters [24,26,30,31]. But, when combined with an additional anionic surfactant such as AOT or sodium benzene dodecyl sulfonate (SDBS), or affinity surfactant, extraction readily occurs [30,31]. The second surfactant must be present beyond a minimum threshold value so that its interfacial concentration is sufficiently large to be seen by... 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

Microemulsions are used as reaction media for a variety of chemical reactions. The aqueous droplets of water-in-oil micro emulsions can be regarded as minireactors for the preparation of nanoparticles of metals and metal salts and particles of the same size as the starting microemulsion droplets can be obtained [1-3]. Polymerisation in micro emulsions is an efficient way to prepare nanolatexes and also to make polymers of very high molecular weight. Both discontinuous and bicontinuous micro emulsions have been used for the purpose [4]. Microemulsions are also of interest as media for enzymatic reactions. Much work has been done with lipase-catalysed reactions and water-in-oil microemulsions have been found suitable for ester synthesis and hydrolysis, as well as for transesterification [5,6]. 

In addition to single phase microemulsions, several phase equilibria known as Winsor systems [4] are also shown at low surfactant concentrations. A Winsor I (WI) system consists of an 0/W microemulsion that is in equilibrium with an oil phase, while a Winsor II (WII) system is a W/0 microemulsion in equilibrium with an aqueous phase. A Will system has a middle phase (bicontinuous) microemulsion that coexists with both oil and aqueous phases. 

Bicontinuous microemulsions consisting of cationic surfactant AUTMAB (30-40 wt%), MMA (30-40 wt%), and 20-40 wt% of an aqueous 50 pM solution of water-soluble metal complexes such as Ru(dip)3Cl2 (dip=4,7-diphenyl-1,10-phenanthroline) have been investigated [98]. After polymerization, the microemulsion transformed into a transparent polymer film which showed a remarkable enhancement in lirminescence intensity. The emission hfetime also... 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

Figure (3) shows the solubilization parameters as functions of water concentration for SDS/2- entanol ratios of 0.25 and 0.40 at 25 C. The solubilization parameters are defined as Vo/Vs and Vw/Vs, where Vo, Vs and Vw are the volumes of organic phase, surfactant and aqueous phase in the microemulsions. The parameters are related to the drop size and also interfacial torsions f7.23). The bicontinuous phase is located around the composition range corresponding to equal values of solubilization parameters. The solubilization parameters are dependent on the initial surfactant and/or cosurfactant concentration. Similar dependence has been observed in other systems as a function of salinity and pH (7.231. Conductivity measurements performed as a function of water content indicate an S-shaped curve as shown in Figure (4). This is typical of microemulsions showing transition from oil-continuous to bicontinuous to water-continuous microstructure with increasing water content. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

Surfactants in solutions show a broad variety of microstructures caused by molecular selforganisation. The observed structures depend essentially on the physical interactions of the involved components and the composition of the mixtures. For the selection of a suitable type of reaction medium the required composition of the reaction mixture is more important than the question of whether a micellar solution, a bicontinuous microemulsion, a w/o- or an o/w-microemulsion is formed. For synthetic purposes high concentrations of reactants are indispensable in order to avoid high-energy cost for work-up procedures. Therefore, a reaction system that allows high-reactant concentrations needs to be chosen. For stoichiometric reactions involving reactant incompatibility, like nucleophilic substitution reactions with an inorganic nucleophile, often an aqueous solution of this reactant has... 

A complex system containing a branched anionic surfactant, non-ionic surfactants, rapeseed oil methyl ester and an aqueous calcium chloride solution was found to form bicontinuous microemulsions even at low temperatures [46, 90]. This type of microemulsion has been studied for DNAPL extraction on a large scale in an artificial aquifer and later in a joint project with different partners financed by the German Federal Ministry of Education and Research (BMBF) [91 ]. The project network applied an integrated concept regarding aspects of hydraulics, reuse and biodegradation [92]. Three large-scale experiments each with some hundreds of litres of preformed microemulsion were performed. Whereas extraction of perchloroethylene in the field-scale experiment was not successful... 

III microemulsion system in the presence of water and an appropriate oil, without requiring cosurfactant nor electrolyte. They play a dual role as [1] Oj generating catalyst and as surfactant stabilizing the bicontinuous microemulsion. From a physicochemical point of view, they must have an effective packing parameter close to one [73]. Bis(dimethyldialkylammonium)molybdates are typical examples of such balanced catalytic surfactants, which lead to a three-phase system as shown in Figure 22.6. In such reaction media, 02 is exclusively generated in the aqueous nanodomains of the middle-phase microemulsion where the reaction takes place. Under stirring, the excess oil phase transfers the substrate S to the... 

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