Application, reverse micelles

It is of particular interest to be able to correlate solubility and partitioning with the molecular stmcture of the surfactant and solute. Likes dissolve like is a well-wom plirase that appears applicable, as we see in microemulsion fonnation where reverse micelles solubilize water and nonnal micelles solubilize hydrocarbons. Surfactant interactions, geometrical factors and solute loading produce limitations, however. There appear to be no universal models for solubilization that are readily available and that rest on molecular stmcture. Correlations of homologous solutes in various micellar solutions have been reviewed by Nagarajan [52]. Some examples of solubilization, such as for polycyclic aromatics in dodecyl sulphonate micelles, are driven by hydrophobic... 

Studies of reversed micelles dispersed in supercritical fluids have shown their ability to solubihze hydrophihc substances, including biomolecules and dyes, opening the door to many new applications [60,61]. In particular, solutions of reversed micelles in liquid and supercritical carbon dioxide have been suggested as novel media for processes generating a minimum amount of waste and with a low energy requirement [62]. 

Other applications are based on the use of solutions of reversed micelles as templates. For example, solutions of reversed micelles have been employed as a matrix to control the porosity of cross-linked polymer resins. The pore size of the polymers was controlled by varying the amounts of water in the AOT-reversed micelles [67]. 

It has been observed that whereas the catalytic activity of malic dehydrogenase in water is not influenced by pressure, in reversed micelles it shows a bell-shaped dependence, suggesting regulation of the enzymatic activity by pressure application, which cannot be realized in aqueous solutions [180],... 

The next two chapters concern nanostructured core particles. Chapter 13 provides examples of nano-fabrication of cored colloidal particles and hollow capsules. These systems and the synthetic methods used to prepare them are exceptionally adaptable for applications in physical and biological fields. Chapter 14, discusses reversed micelles from the theoretical viewpoint, as well as their use as nano-hosts for solvents and drugs and as carriers and reactors. 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

As mentioned earlier, reversed micelles have different properties from normal micelles. These properties have the potential to favorably affect the sensitivity and other analytical aspects of CL reactions. Thus, reversed micelles have been used to prolong the duration of the observed CL of various oxalate ester (or acid)-hydrogen peroxide-sensitizer reaction systems for application as chemical light sources [62],... 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Berthod et al. [325] employed countercurrent chromatography with diethyl-hexyl phosphoric acid (DEHPA) reverse micelles in heptane as a stationary phase to extract metaUic cations such as Fa +, Ce, Pr, and Nd + (lanthanide series). This technique was suggested for the application of ion filtering and concentration or for deionization of aqueous phases. Ashrafizadeh et al. [326] re-... 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

Older compilations about the state of the art can be found in several review articles [41 -47]. It is surprising that most work is carried out with the surfactant bis-ethylhexyl-sulfosuccinate (tradename AOT or Aerosol OT). The reasons seem to be the variability of the obtained reverse micelles (from very low up to high water concentrations) and the well-known phase behaviour of AOT with water and several oils [48,49]. AOT is approved for medical application, e.g. as an additive in suppositories, but not for food engineering. 

Application of reverse micelles for the extraction of amino acids and proteins. Chimia, 44, 270-82. 

Some of the topics we discuss in this chapter are essential for understanding processes such as MEUF. The same ideas can also be used for other separation processes (e.g., protein separation in reverse micelles) and in genetic engineering, as mentioned in Vignette 1.3 in Chapter 1. We also see in this chapter other applications such as using micelles as microreactors, i.e., using the unique environment inside micelles for catalysis and material synthesis. 

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. 

Use of reverse micelles in synthetic chemistry to improve the rate and the yield of reactions seems likely to be a fruitful area of research in the future. In addition to catalysis, several other applications of reverse micelles can be cited. Just as nonpolar dirt is solubilized in aqueous micelles, so, too, polar dirt that would be unaffected by nonpolar solvents may be solubilized into reverse micelles. This plays an important role in the dry cleaning of clothing. Motor oils are also formulated to contain reverse micelles to solubilize oxidation products in the oil that might be corrosive to engine parts. 

Hydrates have further applications in bioengineering through the research of John and coworkers (Rao et al., 1990 Nguyen, 1991 Nguyen et al., 1991, 1993 Phillips et al., 1991). These workers have used hydrates in reversed micelles (water-in-oil emulsions) to dehydrate protein solutions for recovery and for optimization of enzyme activity, at nondestructive and low-energy conditions. 

N. Pena, G. Ruiz, A.J. Reviejo and J.M. Pingarron, Graphite-teflon composite bienzyme electrodes for the determination of cholesterol in reversed micelles. Application to food samples, Anal. Chem., 73(6) (2001) 1190-1195. 

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