Reverse micelles nonaqueous

In spite of the potentialities of reversed micelles entrapping nonaqueous highly polar solvents [34], very few investigations on the solubilization in such systems are reported in the literature. An example is the study of the solubilization of zinc-tetraphenylporphyrin (ZnTPP) in ethylene glycol/AOT/hydrocarbon systems by steady-state and transient... 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

An overview of other forms of micellar systems follows in the next three sections. Formation of reverse micelles, in nonaqueous media, is discussed briefly in Section 8.8. Sections 8.9 and 8.10 present an introduction to microemulsions (oil, or water, droplets stabilized in water or oil, respectively) and their applications. 

Another striking difference between aqueous and anhydrous, nonaqueous systems is the size of the aggregates that are first formed. As we have seen, n is about 50 or larger for aqueous micelles, while for many reverse micelles n is about 10 or smaller. A corollary of the small size of nonaqueous micelles and closely related to the matter of size is the blurring of the CMC and the breakdown of the phase model for micellization. Instead, the stepwise buildup of small clusters as suggested by Reaction (D) is probably a better way of describing micellization in anhydrous systems. When the clusters are extremely small, the whole picture of a polar core shielded from a nonaqueous medium by a mantle of tail groups breaks down. 

Some surfactants undergo an aggregation process in hydrocarbon and other nonpolar solvents. Th forces involved in surfactant aggregation with nonaqueous solvents must differ considerably from those already discussed for water-based systems. The orientation of the surfactant relative to the bull solvent will be opposite to that in water therefore, these systems are referred to as reverse micelles, These micelles will not have any signiLcant electrical properties relative to the bulk solvent (Luisi etal., 1988). 

Setua, R, Chakraborty, A., Seth, D., Bhatta, M.U., Satyam, P.V., and Sarkar, N. 2007. Synthesis, optical properties, and surface enhanced Raman scattering of silver nanoparticles in nonaqueous methanol reverse micelles. Journal of Physical Chemistry C, 111 3901. 

The reactions of organic compounds can be catalyzed markedly in micellar solution. Catalysis by both normal micelles in aqueous medium and by reversed micelles in nonpolar solvents is possible (Fendler and Fendler, 1975 Kitahara, 1980). In normal micelles in aqueous medium, enhanced reaction of the solubilized substrate generally, but not always, occurs at the micelle-aqueous solution interface in reversed micelles in nonaqueous medium, this reaction occurs deep in the inner core of the micelle. 

The tendency of decreasing the contribution of the concentration factor and increasing the contribution of the factor of the micellar microenviromnent is also observed for the reverse nonaqueous micellar solutions. For the case of aminolysis of phosphonates 1-4 in the polyethylene glycol-600-monolaurate (PM) reverse micelles, both factors are shown to contribute positively to the micellar rate effect, with F higher than Fc (Table 15.1). In nonaqueous systems micellization is mainly contributed by dispersion interactions and results in the formation of small aggregates with a low solubilization capacity towards the substrates. ... 

The fundamental principles controlling activity in nonaqueous systems are the same as those for aqueous solutions, except that the specificity of the micellar core for the solubilization of polar substrates is much greater than for the aqueous situation. The popularity of reversed micelles as models for enzyme catalysis stems from the fact that the micellar core is capable of binding substrates in concentrations and orientations that can be very specific to certain functionalities, much as an enzyme would do. As a result, reaction rate enhancements can be obtained comparable (with luck) to those of the natural systems, and far in excess of what can be explained on the basis of partitioning or availability of substrate. 

The properties of ionic polymers in nonaqueous media have only recently become the subject of systematic studies. In solvents of low dielectric constant, salt groups resist dissociation and are poorly solvated. Thus, ionic moieties promote intra- and inter-polymer association in organic solvents. The tendency of ionic groups to aggregate or cluster resembles the coalescence of such groups in reversed micelles. Similar considerations underly the formation of ionic "cross-links" that modify the behavior of ionomers in the solid state. Solutions of polyions in nonaqueous media thus provide systems in which a powerful array of experimental techniques can be used to probe phenomena that are important to the bulk properties of a commercially important group of materials. The article by Teyssie and Varoqui in Part IV describe significant explorations in this novel field. 

What have we learned about the structure and composition of colloidal metal particles in the 1-10 nm size range from the above techniques There is a consensus in those reports in which structure measurements were made that above a certain size, the colloid particles adopt the structure of the bulk phase. Thus palladium colloids with a 6.5 nm mean diameter were found to have the fee structure found in bulk palladium. [113] Similar bulk-like structures were found for platinum colloids of varying sizes, [185, 200] and for colloidal platinum, palladium, rhodium, and iridium produced in reverse micelles. [151] In fact it seems that regardless of whether the colloids are prepared in aqueous or nonaqueous media, by chemical or physical means, a crystalline elemental structure can usually be identified in particles above ca. 2.5 nm. Below this size, at which the use of diffraction methods becomes problematic, there is evidence from reports... 

Countercurrent chromatography is based on the distribution of substances in two liquid phases [128,129]. The liquid is fed into a coiled tube that is moved along an orbital trajectory. Due to centrifugal power, the liquids move in a counter-current. For proteins and many other biomolecules, this method is not practical because of denaturation in a nonaqueous phase. In aqueous two-phase systems, at least one phase exhibits high viscosity and, therefore, mass transfer between the two phases is limited. Similar problems occur with reversed micelle extraction as were observed with the aqueous two-phase extraction [130]. CCC has not been used for large-scale purification of proteins and other biopolymers. 

Riter, R. E., Kimmel, J. R., Undiks, E. P., and Levinger, N. E. 1997. Novel reverse micelles partitioning nonaqueous polar solvents in a hydrocarbon continuous phase. 

The evolution of the two-phase system may, however, be more complex. Solubilization of some hydrocarbons in the micellar aqueous phase can take place. Surfactant molecules can migrate across the water-liquid hydrocarbon interface and form structures that have been called reversed micelles, providing the surfactant concentration in the whole system is high enough to reach the critical aggregation concentration in the considered hydrocarbon solvent. Reversed micelles have an aqueous core ensuring the hydration of hydrophilic head group, whereas hydrophobic tails orient toward the nonpolar liquid. It is not our purpose to discuss surfactant behavior in nonaqueous media. 

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. 

In this chapter, latest advancements in solvent engineering in bioreductions and greener needs for bioreaction media have been discussed in depth with recent examples. Solvents for bioreductions may be categorized as (i) aqueous (ii) water/water-miscible (monophasic aqueous-organic system) (iii) water/ water-immiscible (biphasic aqueous-organic system) (iv) nonaqueous (mono-phasic organic system, including solvent-free system) and (v) nonconventional media (e.g., ionic liquids, supercritical fluids, gas-phase media, and reverse micelles). 

R. E. Riter, J. R. Kimmel, E. P. Undiks, and N. E. Levinger, /. Phys. Chem. B, 101, 8292 (1997). Novel Reverse Micelles Partitioning Nonaqueous Polar Solvents in a Hydrocarbon Continuous Phase. 

One of the first reported instances of catalysis by reversed micelles in the early 1970s concerned the hydrolysis of p-nitrophenyldodecanoate in hexanol-water systems containing cetyltrimethylammonium bromide (CTAB). Since that time, nonaqueous systems have gained greater attention as models that mimic the catalytic activity of natural enzymatic reactions. The fundamental principles controlling activity in nonaqueous systems are basically the same as those for aqueous solu-... 

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