Reactions, microemulsion facilitation

Rh/tppts catalysed hydroformylation of 1-dodecene was carried out in highly stable microemulsions generated by conventional surfactants e.g. sodium dodecyl sulfate and co-surfactants (butanol) which breaks after the reaction into two phases facilitating catalyst recovery 4,4... 

Microemulsion is used as a special microreactor to limit the nano-sized particles growth. The shape of the microreactor depends on reaction conditions [9]. This method increases the homogeneity of the chemical composition at nano-level and facilitates the preparation of nano-particles with comparatively equal sizes [11]. The specific properties of the nano-particles make them suitable for microelectronics, ceramics, catalysis, medicine, cosmetics, as piezoelectric materials, conductors, etc. 

In emulsion polymerization, monomers are polymerized in the form of emulsions and polymerization in most cases involve free-radical reactions. Like suspension polymerization, the emulsion process uses water as the medium. Polymerization is much easier to control in both these processes than in bulk systems because stirring of the reactor charge is easier due to lower viscosity and removal of the exothermic heat of polymerization is greatly facilitated with water acting as the heat sink. Emulsion polymerization, however, differs from suspension polymerization in the nature and size of particles in which polymerization occurs, in the type of substances used as initiators, and also in mechanism and reaction characteristics. Emulsion polymerization normally produces polymer particles with diameters of 0.1-3//. Polymer nanoparticles of sizes 20-30 nm are produced by microemulsion polymerization (Antonietti et al., 1999 Ytldiz et al., 2003). 

Thermodynamically stable microemulsions and kinetically stable emulsions may be utilized to bring water and nonvolatile hydrophilic substances, such as proteins, ions, and catalysts, into contact with a SCF-continuous phase (e.g. CO2) for separation, reaction and materials formation processes. Reactions between hydrophilic and hydrophobic substrates may be accomplished in these colloids without requiring toxic organic solvents or phase transfer catalysts. CO2 and aqueous phases may be mixed together over a wide range in composition in w/c and c/w emulsions. The emulsion is easily broken by decreasing the pressure to separate the water and CO2 phases, facilitating product recovery and CO2 recycle. Reaction rates can be enhanced due to the considerably lower microviscosity in a w/c as compared to a water-in-alkane microemulsion or emulsion. 

Nucleophilic substitution reactions of a lipophilic substance, such as 2,4-dinitrofluoroben-zene (DNFB), with OH ions are facilitated when lipophilic substrates are incorporated into cationic micelles [23] and microemulsions [24] ... 

In addition to being a fundamental consequence of the nature of amphiphilic molecules, micelle formation also plays a significant part in the practical application of surfactants in various areas. Because they represent what might be considered a second liquid phase in solution, micelles are often found to facilitate the production of apparently stable, isotropic solutions of normally insoluble liquids and sometimes solids, quite distinct from the obviously two-phase emulsions and sols previously discussed. Depending on the system (and the observer), such solutions are said to result from either solubilization of a material in the continuous phase or from the formation of microemulsions. In addition, the unique character of the micelle makes it a potentially useful transition zone between phases in which the unique environment may facilitate (i.e., catalyze) chemical reactions difficult to achieve under normal two-phase conditions. The ability of a surfactant to carry out such functions is of great potential importance and warrants some closer attention. 

A microemulsion is a thermodynamically stable three-component system two immiscible components (generally water and oil) and a surfactant molecule that lowers the interfacial tension between water and oil resulting in the formation of a transparent solution. Water-in-oil microemulsions involve dispersion of the aqueous phase as nanosized droplets (5-25 nm in diameter) surrounded by a monolayer of surfactant molecules in the continuous hydrocarbon phase. These micellar droplets exhibit a dynamic exchange of their contents, which further facilitates the reactions between reactants dissolved in different droplets. One can synthesize size-controlled crystallites by carrying out a wide variety of chemical reactions in nanodroplets using this micellar exchange. Different types of microemulsions are... 

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). 

For the W/O microemulsion polymerization of acrylamide stabilized by sodium bis(2-ethylhexyl)sulfosuccinate and initiated by 2,2 -azobisisobutyro-nitrile, the initiation reactions take place predominantly in the acrylamide/ water-toluene interfacial layer, in which the encounter of initiator radicals with monomer molecules is facilitated [70-74]. On the other hand, as would be expected, free radical polymerization is initiated primarily within the acryl-amide/water cores of the microemulsion droplets when the water-soluble persulfate initiator is used. The technique of steady-state fluorescence of indoUc probes quenched by acrylamide and selectively located in different phases (the continuous toluene phase, the acrylamide/water-oil interface and the acryl-amide/water phase) of the W/O microemulsion system stabilized by sodium bis(2-ethylhexyl)sulfosuccinate was adopted to study the consumption of monomer during polymerization [79]. The experimental results show that acrylamide is consumed evenly from all parts of the microemulsion polymerization system, regardless of the initial microemulsion composition and the nature of initiator. 

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