Reverse micelle nanoreactors

A. Modification of Proteins (Enzymes) and Their Supramolecular Design in Reverse Micelles (Nanoreactors)... 

Increasing surfactant concentration at constant R corresponds to a higher population of reverse micelles, nanoreactors, in the continuous oil phase [43]. Despite the fact that the surfactant is nonreactive, it is anticipated that larger population of reverse micelles will accommodate higher concentration of stabilized nanoparticles leading to higher uptake. 

This review summarizes our findings on the effect of some operating and microemulsion variables on nanoparticle uptake in reactive and nonreactive microemulsion systems. Mixing was found to increase the rate of nanoparticle formation and shorten the time needed to reach the nanoparticle uptake for reactive and nonreactive systems. Temperature increased nanoparticle uptake for exothermic nanoparticle formation reactions in reactive nficroemulsions, given stable reverse-micellar structure is maintained. The overall effect of temperature, nevertheless, was dependent on the final particle size. Nanoparticle uptake increased linearly with the surfactant concentration, for reactive and nonreactive surfactant systems, most probably due to the increase in the population of reverse micelles, nanoreactors. The effect of surfactant counterion and water to surfactant mole ratio, R, was dependent on their effect on the stability of the reverse micellar structure... 

Lee, Y Lee, J., Bae, C.J., Park, J.G., Noh, H.J., Park, J.H. and Hyeon, T. (2005) Large-scale synthesis of uniform and crystalline magnetite nanoparticles using reverse micelles as nanoreactors under reflux conditions. Advanced Functional Materials, 15 (3), 503-509. 

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

Figure 6.34. Comparison of a traditional micelle used to entrain organic oils/dirt using an anionic surfactant, (a), and a reverse micelle used to stabilize aqueous nanoreactors within a nonpolar solvent, (b).

Direct micelles contain lyophilic component of surface-active substance, whereas the reverse micelles contain lyophobic one. The miceUes can be formed in the presence and absence of water. In the case of reverse miceUes, for instance, in the hydrocarbon medium, water is easily solubilized, forming a water pool . Its size is characterized by the ratio of the water and surfactant volumes. Thus, a limited amount of water inside the micelle determines the kinetics and thermodynamics of the nanoparticles formation in a small micro/nanoreactor volume. 

The basic nanoreactor in particle formation is a reverse micelle in most cases, with a generally accepted spherical shape. The particles generated from these micelles transforming into W/O microemulsions are also often spherical in shape. However, other surfactant architectures may also yield particles. Thus, vesicles have been instrumental in the formation of particles in many cases (not discussed in this book) [98] similarly, cylindrical micelles could also generate elongated nanoparticles with the required manipulations in the system. Unfortunately clear-cut evidence on this offshoot procedure of synthesis, i.e. rodlike particle formation from rod-like micelles is apparently not so extensively available. 

Uniformly sized silica-coated magnetic nanoparticles (magnetite-silica) synthesized in a simple one-pot process using reverse micelles as nanoreactors... 

In addition, polymer micelles have been demonstrated to be more stable and also have a significantly lower cmc than surfactant micelles. Further discussion of surfactant micelles is beyond the scope of this review, and, instead, the reader is directed to a recent review article by Armes. In fact, the polymer building blocks need not be amphiphilic and such phase-separated nanostructures can be formed from completely hydrophobic or lipophilic diblock copolymers that contain two segments with differing solubility (such as polystyrene- -polyisoprene) and hence can undergo phase separation in selective solvents. One example of such completely hydrophobic phase-separated micelles are those reported by Wooley and coworkers, which can be obtained from toluene and acetone solutions of a [polystyrene-a/f-poly(maleic anhydride)]-fc-polyisoprene Iriblock. Conversely, inverse structures are also accessible and are known as reverse micelles. These can be formed by adding a nonsolvent for the hydrophilic block to afford the opposite of a conventional micelle, for which the hydrophilic core is surrounded by a hydrophobic shell in a hydrophobic surrounding media. There have been a handful of reports on the application of these reverse micelles, for example, as nanoreactors and for the extraction of water-soluble molecules. ... 

Studies of water-in-scC02 reverse micelles derived from the surfactant ammonium carboxylate perfluoropolyether (PFPE) demonstrated that CO2 can partition into the PFPE reverse micelle [21], The important question related to the reaction rates and outcomes inside the nanoreactor is whether the pH within the reverse micelle water pool can be controlled by adjusting the CO2 pressure. In an attempt to answer this qnestion. Bright et al. quantitatively determined the pH within the PFPE water pools. Their results showed that the pH remains essentially constant as a fnnction of CO2 continnons phase pressure and micelle water loading [22]. 

Normal emulsions (ie, scC02-in-water emulsions, with water as the continuous phase) can also be obtained simply by manipulating the solubility of the surfactant (the phase in which the surfactant is most soluble will be the continuous phase), although examples for the formation of this kind of emulsion in literature are scarce and reverse micelles formed in SCFs have been used with much success as nanoreactors for a wide range of chemical syntheses and synthesizing various nanomaterials. 

V. Turco Liveri discusses structural aspects, the state of water and other solutes in reversed micelles, intermicellar interactions and percolation phenomena, solubilization of nonionic solutes, and the reversed micelles as nanoreactors. 

By employing metal 2-ethylhexanoate precursors which act as photo-reactive surfactants, de Oliveira et al. have developed new routes to metal oxide nanoparticles. This surfactant assembles into a reverse micelle in organic solvent, in effect forming a nanoreactor which promotes metal oxide nanoparticle formation within the micelle. The authors have demonstrated the synthesis of C03O4 and Bi metal nanoparticles using this route, but it is likely this clean approach could be employed to prepare a range of nanoparticles with a choice of surfactants. 

Figure 12.4 shows a schematic diagram of a water-in-oU microemulsion or reverse micelle. Reverse micelle-templated synthesis has the ability to control particle size and morphology. Reverse micelles are formed when the aqueous phase is dispersed as microdroplets, and those microdroplets of water that are stabilized in a nonaqueous phase by a surfactant act as a microreactor or nanoreactors in which reactions are conducted. These spatially and geometrically restricted. 

Ingert, D. and Pileni, M.P. (2001) Limitations in producing nanocrystals using reverse micelles as nanoreactors. Advanced Functional Materials, 11,136-9. 

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