Microemulsion media

Gonzalez and Holt (1981) have shown how macrocyclic lactones can be made conveniently in microemulsion media. Thus, intramolecular e.sterification of (o-hydroxy alkanoic acids can be carried out with a very small amount of acid catalysts like p-toluene sulphonic acid, and rates are markedly higher as compared to simple media. 

Microemulsion media seem to be very useful in getting monodisperse CaCOj particles of 30 A via carbonation of calcium phenates (Marsh, 1987) this process is relevant in making lube additives. The mechanism of reaction crystallization in such systems has hardly received attention. 

Microemulsion media can be adopted for biotransformations lipases have been used and large difference in rates of esterification, e.g. I-pentanol vs 2-pentanol with oleic acid, have been observed (Stamatis et al., 1993). 

We have examined the photosensitized reduction of a series of + 4,4 -bipyridinium salts, CnV2+, (2), (where n=l-16) with Ru(bpy)l as sensitizer and (NHi jEDTA as electron donor in a water-in-toluene microemulsion media. Under steady state illumination the quantum yield of CjjVt" formation strongly depends on the alkyl chain length of the electron acceptor (Figure 4). It improves as the hydro-... 

The majority of organic reactions that have been performed in microemulsions are substitution reactions, most commonly bimolecular nucleophilic substitutions, i.e. Sn2 reactions. Figure 2 shows representative examples of such reactions that have been investigated in microemulsion media. Some of the reactions will be discussed below. Previous reviews can be consulted for other examples [13-15]. 

It should also be noted that while very few Interfaclad inorganic reactions have been examined in microemulsion media, these studies have shown that this is a fertile field for future investigations. 

As yet o kinetic data have been reported for the metallation of pheopbytinates or for the hydrolysis of metal pheophytinates or metalloporphyrins in well-characterized microemulsion media. 

Figure 3. A typical plot of the change in concentration of chlorophyll a as it is hydrolyzed to pheophytin a in microemulsion media.

Both micellar and microemulsion media have been investigated as candidates for solubilization and detoxification of nerve agents. Normal micelles, however, do not appear to bind Sarin well (J ) and oximate functionalized surfactants are severely limited by their low water solubility (2 ). These oximate functionalized surfactants have been shown to be excellent... 

In this preliminary study, we have investigated the coagulation rate of silver chloride sols in microemulsion media. The results are intimately related to structural properties of the microemulsions. 

Assuming the changes of A and V on various microemulsion media are negligible, and N is proportioRal to the volume fraction of water in a microemulsioR sample, the following equation is then used to obtain a relative rate constant for a microemulsion sample,... 

It has been shown that the stability of colloidal suspensions can also be influenced by a pure alcohol-water mixture, without the addition of any surface active agent. In a study of the flocculation of polystyrene emulsions in ethanol-water mixtures (42), the concentration of sodium chloride required to produce rapid flocculation increases with increasing ethanol concentration up to 0.09 molar fraction, beyond this composition, the concentration of sodium chloride required for flocculation decreases rapidly. It will be very informative, therefore, to compare our coagulation rate obtained in the microemulsion media to that in pure IPA + water mixture. The results can be used to further delineate the role of inverted micellar structure on the enhancement of coagulation. 

This example (supplemented by the additional results of Anderson et al., 1998a,b) shows that a microphase can greatly influence the nucleation and growth rates of crystals, which in turn can drastically alter the crystal size distribution of the precipitate. In other words, the average precipitate size can be controlled to enhance the precipitation and therefore the reaction rate. A parallel example is the use of microemulsion media to obtain monodispersed calcium carbonate particles via carbonation of calcium phenate (Marsh, 1987). 

Microemulsion media of low water activity enable hydrolytic enzymes to catalyze condensation as opposed to hydrolysis. This has opened the possibility to perform lipase-catalyzed ester synthesis, an area of considerable practical interest. Lipase-catalyzed transesterification in microemulsion of low water content is another area of industrial relevance, in particular for synthesis of triglycerides with unusual fatty acid composition. 

Tsang SC, Zhu J, Yu KMK. A new oxidation catalyst system using fluorous cobalt(ll) species in water-supercritical carbon dioxide (C-H free environment). Catal Lett 2006 106 123-6. Martin CA, McCrann PM, Angelos GH, Jaeger DA. Reactions in microemulsion media nucleophilic displacement reaction of benzyl chloride with bromide ion. Tetrahedron Lett... 

Tafesse, F. and Mndubu, Y. 2007. Iron promoted decontamination studies of nitrop-henylphosphate in aqueous and microemulsion media A model for phosphate ester decontamination. Water Air Soil Pollut, 183, 107-113. 

Gratzel, C. K., Kira, A., Jirousek, M., and Gratzel, M., Dimer cation formation in microemulsions media duroquinone-sensitized photooxidation of 2,6-dimethylnaphthalene and tetrathioful-valene, /. Phys. Chem., 87, 3983, 1983. 

Although the synthesis of inorganic particles in microemulsions is already widespread, only polymer nanoparticles have been synthesized in microemulsion media as far as the organic particles are concerned. In this chapter, it will be shown that it is also possible to synthesize organic particles by a direct precipitation reaction in the microemulsions. 

In the numerous studies concerning the synthesis of nanoparticles in microemulsion media, the location of water after the nanoparticle synthesis has never been determined. Two models can be proposed (Eig. 13). In the first one the particles are surrounded by a layer of water, and in the second the surfactant molecules (the AOT) are directly adsorbed onto the particles and only a small amount of water molecules is present. 

FIG. 13 Two models of the nanoparticles stabilized in the microemulsion media, (a) The particle is surrounded by a layer of water, (b) AOT is directly adsorbed onto the particle. 

TABLE 6 Thermal Decomposition Range of the Synthesized Poly(C i2 Alkyl Acrylates and Methacrylates) in Emulsion and Microemulsion Media... 

The polymerization of Cg-ia alkyl acrylates and methacrylates in microemulsion media produced translucent and stable latexes with final particle sizes in the range 30-70 mn. The molecular weights of the microemulsion products were found to be higher than those of the emulsion products synthesized under similar conditions. The microemulsion products were also more effective as viscosity index improvers in the paraffin base oil SN 500 than the emulsion products. 

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