Micelle, inverse

The traditional association colloid is of the M R" type where R" is the surfactant ion, studied in aqueous solution. Such salts also form micelles in nonaqueous and nonpolar solvents. These structures, termed inverse micelles, have the polar groups inward if some water is present [198] however, the presence of water may prevent the observation of a well-deflned CMC [198,199]. Very complex structures may be formed in nearly anhydrous media (see Ref. 200). 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Figure C2.17.2. Transmission electron micrograph of a gold nanoneedle. Inverse micelle environments allow for a great deal of control not only over particle size, but also particle shape. In this example, gold nanocrystals were prepared using a photolytic method in surfactant-rich solutions the surfactant interacts strongly with areas of low curvature, thus continued growth can occur only at the sharjD tips of nanocrystals, leading to the fonnation of high-aspect-ratio nanostmctures [52].

Wilcoxon J P, Williamson R L and Baughman R 1993 Optical properties of gold colloids formed in inverse micelles J. Chem. Phys. 98 9933... 

Use of a nonpolar solvent such as hexane or benzene in place of water should give rise to the formation of inverse micelles. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

Figure 5. Synthetic steps for preparation of monodispersed Au nanoparticles by the inverse micelle method and digestive ripening.

Figure 6. TEM micrographs representing the transformations of (a) polydispersed nanoparticles upon (b) alkanethiol addition at room temperature and (c) after digestive ripening (inverse micelle system). (Reprinted with permission from Ref [49], 2002 American Chemical Society.)...

Figure 9. TEM micrographs of nanocrystal superlattices of Au nanoparticles prepared by the inverse micelle method and digestive ripening, (a) and (b) low-magnification images (c (f) regularly-shaped nanocrystal superlattices (g) magnified image of a superlattice edge. Note the perfect arrangement of the Au nanoparticles. (Reprinted with permission from Ref. [30], 2003, American Chemical Society.)...

Figure 17. Different steps involved in the digestive ripening procedure of colloids prepared by the inverse micelle method.

Inverse least squares, 539-41 Inverse micelles, 25487 Inverse microemulsion polymerization, 20 461... 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

Recently, Mecking et al. reported the synthesis of inverse micelles based on a hy-perbranched polyglycerol polymer. Terminal -OH groups were modified with palmi-toyl chloride and gave a polymeric catalyst soluble in organic solvents with hydrophilic core to immobilize water-soluble guest molecules such as PdCl2 or Pd(OAc)2. 

It can therefore be concluded that the aggregation of the monofunctional molecules in dilute solution of a nonpolar solvent leads to the formation of star-type inverse micelles of narrow molecular weight distribution. In addition, the extremely small CMC shows that the relative amount of material dissolved as unimers can be neglected at concentrations above 1%. 

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