Metal salts Microemulsion synthesis

This method involves formation of reverse micelles in the presence of surfactants at a water-oil interface. A clear homogeneous solution obtained by the addition of another amine or alcohol-based cosurfactant is termed a Microemulsion. To a reverse micelle solution containing a dissolved metal salt, a second reverse micelle solution containing a suitable reducing agent is added reducing the metal cations to metals. The synthesis of oxides from reverse micelles depends on the coprecipitation of one or more metal ions from... 

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]. 

As a solution-based materials synthesis technique, the microemulsion-mediated method [10-18] offers the unique ability to effect particle synthesis and particle stabilization in one step. The solubilized water droplets serve as nanosize test tubes, thus limiting particle growth, while the associated surfactant films adsorb on the growing particles, thereby minimizing particle aggregation. The purpose of this chapter is to review the literature on the microemulsion-mediated synthesis of metal hydroxides and oxides the definition of a metal is extended here to include the semimetal silicon. Since metal oxides are frequently produced by decomposing metal salts, aspects of the literature on microemulsion-derived metal salts are also considered. In principle, any previously established aqueous precipitation chemistry can be adapted to the microemulsion synthesis technique. Accordingly,... 

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 susceptibility of microemulsions to destabilization by electrolytes severely limits the highest metal concentrations that can be used for precipitation reactions. This, in turn, discourages the large-scale application of microemulsion-mediated materials synthesis. A possible approach to tackling this problem appears to lie in the judicious selection of cosurfactants for microemulsion formulations. Darab et al. [125] reported that addition of SDS to the AOT/isooctane/water microemulsion increased dramatically the tolerable concentration of metal salts in the water pools. According to Chhabra et al. [50], addition of -hexanol to the Triton X-lOO/cyclohexane/water microemulsion led to a significant improvement in the water-solubilizing capacity. 

The basic model as outlined above (often described as fusion and fission of droplets in W/O microemulsions) has been generally used for explaining reactions leading to, for example, polymerization of monomers and reduction of metal ions to metal particles. Natarajan et al. [160] who proposed a stochastic model for ultrafine metal particle synthesis from metal salts by the above method worked out some fusion-fission rules as summarized below ... 

Spinel ferrites can be synthesized in microemulsions and inverse micelles. For example, MnFejO nanoparticles with controllable sizes from about 4-15 nm are synthesized through the formation of water-in-toluene inverse micelles with sodium dodecylbenzenesulfonate (NaDBS) as suifactanP. This synthesis starts with a clear aqueous solution consisting of Mn(N03)2 and Fe(NOj)j. A NaDBS aqueous solution is added to the metal salt solution, subsequent addition of a large volume of toluene forms reverse micelles. The volume ratio of water and toluene determines the size of the resulting MoFCjO nanoparticles. 

Metallie nanoeatalysts ean be incorporated into the mesoporous structure by a variety of methods. Prefabricated nanopartieles can be incorporated into mesoporous solids by adding the partieles into the sol-gel mixture or, if the particles are formed by mieroemulsions (see Section 9.2.5), the microemulsion can be incorporated into the preformed mesoporous structure. Alternatively, metal salts can be added during gel formation or after the mesoporous structure has formed [12]. An example of the former is the synthesis of WO, and WO Pt films that were made by synthesizing W(OC2H5)e and H2PtCl6 sol-gel solutions, followed by aging and calcination [13]. A significant drawback associated with this method is that the catalytic nanoparticles may be buried within the structure rather than near the pores. If the partieles are not located near the pores they will not be accessible to reactants and therefore will not be efficient catalysts. 

Microemulsions have been used as confined reaction media during the past two decades, since, due to the very small size of the droplets, they can act as microreactors capable to control the size of the particles and at the same time to inhibit the aggregation by adsorption of the surfactants on the particle surface when the particle size approaches that of the microreactor droplet. The synthesis of nanoparticles using reactions in microemulsions was first described by Boutonnet and cowoikers They synthesized monodispersed metal particles of Pt, Pd, Rh and Ir by reduction of metal salts with hydrogen or hydrazine in water in oil (w/o) microemulsions. Since then, many different types of materials have been prepared using microemulsions, including metal carbonates, metal oxides, " metal chalcogenides, "" polymers," etc. 

Another interesting example of the use of microemulsions to overcome solubility problems is related to metal-ligand substitution. The synthesis of metalloporphyrins has received considerable attention because of their biological importance. Because most naturally occmring porphyrins are water insoluble, reaction with a metal salt requires some measure to overcome reagent incompatibility. To this end, water-in-benzene microemulsions, with... 

Unsupported Catalysts. - Most of the research work is focused on the preparation of noble metal catalysts using microemulsions in the metal particle synthesis. It is now well known that the early work of Boutonnet et al opened a new and simple way to preparing stable reduced noble metal particles from their salt precursors. In their work, metallic particles of platinum, rhodium, palladium and iridium were obtained from their salt precursors, HaPtCl, RhCh, PdCh and IrCh, respectively, in water in oil (w/o) microemulsions. For Pt and Pd, the most suitable reducing agent was hydrazine. In the case of Rh " and Ir the... 

To control the size and size distribution, synthesis of magnetic nanoparticles in a W/0 microemulsion has been reported. The presence of surfactant molecules results in the formation of different sizes (1-10 nm) of micelles. The surfactant molecules organize themselves with the polar end inside in the water phase and the non-polar end in the oil phase. The micelles/droplets contain the aqueous solution of iron salts. The concentration and type of surfactants and metal ions, the pH, reducing agents and co-surfactants can all affect the particle growth and, consequently, the particle size distribution (Fig. 4) [82, 83]. 

While the microemulsion method has been widely applied to the production and stabilization of spherical metal particles with various sizes and compositions, shape control of noble metallic particles using this procedure has only been demonstrated in a handful of studies to date. Pileni and co-workers demonstrated that it is possible to control nanocrystal shape to some extent within microemulsions.Although the shape of the templates plays a role during the growth of the nanocrystals, these authors showed that the particle shape can be controlled even if the microscopic structure of the self-assembled surfactant system used as a template remains unchanged and that addition of salt to the templates can induce drastic changes in the particle shape. Recently, the same group also reported the synthesis of silver nanodisks in reverse micellar solution by reduction of Ag(AOT) with hydrazine, with various sizes that depended on the relative amount of hydrazine, but with constant aspect ratio. 

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