Microemulsion-based synthesis

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

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),... 

Our synthesis is based on the hydrolysis of a silicon alkoxide (TEOS Si(OCH2CH3)4) in a diluted solution of nonionic polyethylene oxide-based surfactants. The hydrolysis is then induced by the addition of a small amount of sodium fluoride [5], Depending on the initial mixing conditions, the size of the solubilized objects leads to either a colorless or milky emulsion. Small particles ( 300 nm) with a 3D worm-hole porous structure or small hollow spheres with mesoporous walls, are usually obtained [6]. The synthesis we report herein after exhibits an apparently slight but actually drastic change in the preparation conditions. The main feature of this approach is an intermediate step that utilizes a mild acidity (pH 2 - 4), in which, prior to the reaction, a stable colorless microemulsion containing all reactants is... 

Figure 1 illustrates the use of Winsor I and Winsor II systems as reaction media for the synthesis of 1-phenoxyoctane from sodium phenoxide and 1-bro-mooctane. The reaction was performed in microemulsions based on various... 

Oh et al. [16] have demonstrated that a microemulsion based on a nonionic surfactant is an efficient reaction system for the synthesis of decyl sulfonate from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2). Whereas at room temperature almost no reaction occurred in a two-phase system without surfactant added, the reaction proceeded smoothly in a micro emulsion. A range of microemulsions was tested with the oil-to-water ratio varying between 9 1 and 1 1 and with approximately constant surfactant concentration. NMR self-diffusion measurements showed that the 9 1 ratio gave a water-in-oil microemulsion and the 1 1 ratio a bicontinuous structure. No substantial difference in reaction rate could be seen between the different types of micro emulsions, indicating that the curvature of the oil-water interface was not decisive for the reaction kinetics. More recent studies on the kinetics of hydrolysis reactions in different types of microemulsions showed a considerable dependence of the reaction rate on the oil-water curvature of the micro emulsion, however [17]. This was interpreted as being due to differences in hydrolysis mechanisms for different types of microemulsions. 

Fig. 12. Effect of addition of ionic surfactant to a microemulsion based on a nonionic surfactant on the rate of synthesis of decyl sulfonate from decyl bromide and sodium sulfite (from [60])...

The synthesis of nanosized nickel sulfide particles through a microemulsion-based method is reported in this paper. We used a UV-spectrophotometer to monitor the growth process of the particles and used TEM to confirm the sizes of the particles. By changing W, Ni/S molar ratios and the concentration of reactants, the effects of reaction conditions on particles have been investigated and we put forward the mechanism of growth of NiS particles. 

Monitoring of the particle size of MoS nanoparticles by a new microemulsion-based synthesis... 

The microemulsion-based route has already provided encouraging results, for instance in the synthesis of ZnS or CdS nanoparticles [4]. Their success is due to the easiness of finding simple cationic forms for Zn and Cd, which is of course much more difficult in the case of molybdenum. The main idea developed by Pileni and coworkers [3], which consists in forcing the supersaturation of the aqueous medium by using functionalized surfactants like (AOT)2Zn instead of (AOT)Na, cannot be applied to molybdenum either. An attempt to apply the microemulsion-based route to molybdenum sulfide nanoparticles has been carried out by Boakye et al. [2], but sizes obtained were in the size range 10-30 nm, which is still too large for catalytic applications to hydrotreating reactions. 

At higher surfactant concentration liquid crystalline phases may be formed. Surfactant liquid crystals can also solubilise appreciable amounts of oil into the non-polar regions made up of the surfactant tails. Thus, both binary surfactant-water systems and ternary systems with oil included can be formulated into liquid crystals. Such systems can also be used as media for organic synthesis. In fact, a reaction in a surfactant liquid crystal often runs very rapidly, considerably faster than in a microemulsion based on the same surfactant [19]. Figure 5.1 shows the reaction profiles of a typical substitution reaction of... 

The rate constant at the interface (kA) can be obtained as the slope of the straight line by plotting A ma(l — a) (1 — y )2 versus a (Fig. 5.3). This approach was used for describing the kinetics of the synthesis of 1-phenoxyoctane from sodium phenoxide and 1-bromooctane in a microemulsion based on the non-ionic surfactant Triton X-100, which is an octylphenol ethoxylate [27]. The total interfacial area was calculated from known values of the head group area of the non-ionic surfactant. As shown in Fig. 5.3, straight lines were obtained from which the rate constants could be obtained. From the values of kA determined at the three different temperatures, an activation energy of 85 kj mol-1 was calculated. This is a typical value for an SN2 reaction, as usually determined in homogeneous reaction media. 

Microemulsion-based synthesis of nanodimensional materials provides a physical means to control particle size and shape while still exploiting the advantages of sol-gel processing. The principle limitation of emulsion methods is that... 

Esterification. The lipase-mediated esterification is promoted by microwave (90°, 15 min.). Four lipases immobilized in microemulsion-based gels retain their activity in catalyzing esterification. Resolution by selective acetylation of ethyl 6-hydroxy-1-cyclohexenecarboxylate gives access to a chiral intermediate for a synthesis of the spirocyclic alkaloids (+)-nitramine, (+)-isonitramine, and (-)-sibirine. 

The synthesis of sodium decanesulfonate from the alkyl halide and Na2S03 can be carried out very conveniently in microemulsions based on nonionic surfactants [108]. The rate data fit a pseudophase model, and the nonionic surfactants make this method preparatively very useful because phase separation with a change of temperature allows recovery of the surfactants. This approach is very useful because surfactants have high molecular weights and large amounts are needed in preparative reactions and the solubilizing ability of surfactants complicates product isolation. Therefore, more research is needed... 

In order for microemulsion-based materials synthesis to be feasible, surfactant/oil/water formulations that give stable microemulsions must be identified. Phase diagrams already available in the literature [122-124] provide a useful starting point. Frequently, however, these published diagrams do not extend to conditions directly relevant to materials synthesis, e.g., in terms of the specific metal salt, base, acid, and temperature. Of important consideration, therefore, are investigations into the effects of the reactants... 

The first report of microemulsion-mediated synthesis of an iron oxide phase was by Inouye et al. [46]. The microemulsion-plus-reactant method (C of Fig. 3) was used by bubbling molecular oxygen into a ferrous ion-containing microemulsion. Microemulsions based on either AOT (i.e., AOT/hydrocarbon/water, where the hydrocarbons were cyclohexane, hexane, or heptane) or potassium oleate (i.e., potassium oleate/ -hexane///-hexanol/water) were used the aqueous pseudophase consisted of a ferrous ammonium sulfate solution (5x10 and 10 M). Electron microscopy revealed needle-shaped primary particles that aggregated into cubic clusters ( 80 nm). It was concluded from the particle shape and elec-... 

Only one study was found on microemulsion-based preparation of zinc oxide [103]. The two-microemulsion synthesis protocol was used by Hingorani et al. [103] to prepare zinc carbonate that was then calcined ( 220 C) to produce zinc oxide. Working with the CTAB/butanol/octane/water microemulsion system and the two-microemulsion protocol, one aqueous pseudophase contained zinc nitrate while the other contained ammonium carbonate. X-ray diffraction identified the resulting calcined particles as ZnO with an average particle size of 14 nm. 

Figure 13 Effect of (a) calcium ions and (b) buffer pH on yield of hydrolysis and synthesis of phosphatidylcholine. The reactions were carried out in W/O microemulsions based on AOT-isooctane-aqueous buffer and run at optimum conditions except for the varied parameters. (Redrawn from Ref 78.)...

The microemulsion process for material synthesis and other applications has sometimes been termed an emergent technology , and therefore, new developments are also taking place to increase the suitability of the process. One such important development that has taken place in microemulsion-mediated particle synthesis is the use of water-supercritical CO2 systems [225-227, 229, 425,445]. Though this system is also valid for macro- and miniemulsions, reports of particle synthesis (metals, sulfides, halides, as above) are so far essentially microemulsion-based. As the system is environment-friendly and follows some of the important requirements of green chemistry [452], this process is expected to be further matured into a technology for nanoparticle synthesis. The present scale of operation, however, is apparently very small. 

MICROEMULSION-BASED ORGANOGELS CONTAINING LIPASE APPLICATION IN THE SYNTHESIS OF ESTERS Pastou A Stamatis H Xenakis A National Hellenic Research Foundation... 

Rees, G.D., Nascimento, M.G., Jenta, T.R.J., Robinson, B.H. 1991. Reverse enzyme synthesis in microemulsion-based organo-gels. Biochim. Biophys. Acta 1073, 493-501. 

Pastou, A., Stamatis, H., Xenakis, A. 2000. Microemulsion-based organogels containing lipase Application in the synthesis of esters. Prog. Colloid Polym. Sci. 115, 192-195. 

Microemulsions also facilitate and control heterogeneous chemical reactions. Examples include, hypochlorination reactions of water insoluble liquid reactants in microemulsion reaction media produced industrially useful epoxides and epoxide derivatives in high yields (107). Many other microemulsion-based reaction systems were described by Hager (108), including bioorganic reactions in microemulsions (109), metal nanoparticle synthesis in water-in-oil microemulsion (110), as well as polymerization within microemulsions and other self-organized media (111). 

Reverse-enzyme synthesis in microemulsion-based organo-gels has been reported. > Chromobacterium flsco-sum (CV) lipase was used as the catalyst, and ester S3uithesis was reported for a wide range of primary and secondary alcohols with n-decanoic acid. Some t5q)ical data are given in Table 10.4. With primary alcohols the reaction was quite fast and the conversion to the ester was close to 100%. For the secondary alcohol octan-2-ol reaction proceeded to 45%, but some days were needed to reach equilibrium. Selective esterification of the (-) enantiomer was found to occur with enantiomeric excess values of > 90%. No reaction was observed with the tertiary... 

Ganguli AK, Ganguly A, Vaidya S. Microemulsion-based synthesis of nanocrystalline materials. Chem Soc Rey 2010 39(2) 474-85. 

He, T., and Yu, X.Q. (2012) Enhancement of activity and stability of lipase by microemulsion-based organogels (MBGs) immobilization and application for synthesis of arylethyl acetate. ]. Mol. Gated. B Enzym., 78, 65-71. 

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