Reverse micelle model

It is sometimes argued that the reverse micelle terminology is an inappropriate comparison to aqueous micelles. Since water can be solubilized by these micelles, causing an increase in n, the reverse micelle model and vocabulary do seem useful for ternary systems. 

Figure 1. Schematic of the reversed micelle model of Nafion...

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... 

Dodecylpyridinium iodide-reversed micelles trapping chlorophyll a have been suggested as interesting photochemical model systems [22] and water/AOT/chloroethylene systems as peculiar dry-cleaning solvents [64]. 

FIG. 4 Onion model of spherical water-containing reversed micelles. Solvent molecules are not represented. A, surfactant alkyl chain domain B, head group plus hydration water domain C, hulk water domain. (For water-containing AOT-reversed micelles, the approximate thickness of layer A is 1.5 nm, of layer B is 0.4 nm, whereas the radius of C is given hy the equation r = 0.17R nm.)... 

As a result of their size and of specific interactions, hydrophilic macromolecules or solid nanoparticles cause strong changes in micellar size and dynamics, and their structural and dynamic properties are strongly affected. In these cases, the distribution among reversed micelles can be only described by ad hoc models [13,123]. 

In addition to solubilization, entrapment of polymers inside reversed micelles can be achieved by performing in situ suitable polymerization reactions. This methodology has some specific peculiarities, such as easy control of the polymerization degree and synthesis of a distinct variety of polymeric structures. The size and shape of polymers could be modulated by the appropriate selection of the reversed micellar system and of synthesis conditions [31,191]. This kind of control of polymerization could model and/or mimic some aspects of that occurring in biological systems. 

Moreover, as a consequence of their transient character, a hierarchy of clusters in dynamic equilibrium that may differ in shape and size can be hypothesized [253], Mass, momentum, and charge transport within a cluster of reversed micelles is expected to be strongly enhanced as compared to that among isolated reversed micelles. It has been shown that the dynamics of a network of interacting reversed micelles is successfully described by a model developed by Cates [35,69,254],... 

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. 

The protein-containing colloidal solutions of water-in-organic solvents are optically transparent. Hence, absorption spectroscopy, circular dichroism spectroscopy and fluorescence spectroscopy are found to be convenient for studying biocatalysis [53]. The reversed micelles are interesting models for studying bioconversion, since the majority of the enzymes in vivo act inside or on the surface of biological membranes. 

Figure 5 Model of a reversed micelle in a nonpolar organic solvent from which all possible water and impurities have been removed. (From Ref. 2 with permission.)...

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Reactivities of pentacyanoferrates(II) in micelles and reversed micelles have been studied. The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)] + CN. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation. 

Neuman RD, Ibrahim TH (1999) Novel structural model of reversed micelles The open water-channel model. Langmuir 15 10-12... 

Keywords. Downstream processing. Reverse micelles, Protein/enzyme extraction. Modeling, Recent developments... 

Other molecular thermodynamic models for protein-reverse micelle complexes have also emerged. Bratko et al. [171] presented a model for phase transfer of proteins in RMs. The shell and core model was combined with the Poisson-Boltzmann approximation for the protein-RM complex and for the protein-free RM. The increase in entropy of counterions released from RMs on solubilization of a protein was the main contribution to the decrease in free energy of com-plexation. Good agreement was found with SANS results of Sheu et al. [151] for cytochrome C solubilization and the effect of electrolytes on it. However, this model assumes that filled and empty RMs are of the same size, independent of salt strength and pH, which is not true according to experimental evidence available since then. 

The determination of the enzyme activity as a function of the composition of the reaction medium is very important in order to find the optimal reaction conditions of an enzyme catalysed synthesis. In case of lipases, the hydrolysis of p-nitrophenyl esters in w/o-microemulsions is often used as a model reaction [19, 20]. The auto-hydrolysis of these esters in w/o-microemulsions is negligible. Because of the microstructure of the reaction media itself and the changing solvent properties of the water within the reverse micelles, the absorbance maximum of the p-nitrophenol varies in the microemulsion from that in bulk water, a fact that has to be considered [82]. Because of this, the water- and surfactant concentrations of the applied micro emulsions have to be well adjusted. 

The hyperactivity of, for example, lipases at low w -values (shown in Fig. 5) is explained by the water-shell-model [2]. The activity of the enzyme at w -values higher than 5 corresponds to its activity in bulk aqueous solutions. There exist two aqueous regions within a reverse micelle, schematically shown in Fig. 6. One is located in the inner part of the reverse micelle and has the same physical properties as bulk water the other is attached to the polar head groups of the surfactant and differs in its physical properties strongly from bulk water. 

Fig. 5. Water-shell-model schematically drafted location of two water parts in a reverse micelle. One is located in the inner part of the reverse micelle and has the same physical properties as bulk water, the other is attached to the polar headgroups of the surfactant...

We are approaching the final part of the book, concerned with cellular models based on vesicles. The main keywords are now compartment and (if this word exists) compartmentation. The biological potential of these aggregates is closely related to their physical properties, and for this reason some of these basic characteristics will first be briefly considered. Also, to give a proper background to these properties, it may be useful to compare various kinds of compartments, such as micelles, reverse micelles, cubic phases, and vesicles. This will be useful to understand better biochemical reactions in vesicles, which will be dealt with in the next chapter. 

Caselli, M., Maestro, M., and Morea, G. (1988). A simplified model for protein inclusion in reverse micelles. SANS measurements as a control test. Biotech. Prog., 4,... 

Maestro, M. and Luisi, P. L. (1990). A simplified thermodynamic model for protein uptake by reverse micelles. In Surfactants in Solution, ed. K. L. Mittal. Plenum, vol. 9. 

Martinek, K., Levashov, A. V, Pantin, V. I., and Berezin, I. V. (1978). Model of biological membranes or surface-layer (active center) of protein globules (enzymes) - reactivity of water solubilized by reversed micelles of aerosol OT in octane during neutral hydrolysis of picrylchloride. Doklady Akademii Nauk SSSR, 238, 626-9. 

Zampieri, G. G., Jackie, H., and Luisi, P. L. (1986). Determination of the structural parameters of reverse micelles after uptake of proteins. J. Phys. Chem., 90, 1849. Zeleny, M. (1977). Self-organization of living systems formal model of autopoiesis. Int. J. Gen. Sysl, 4, 13-28. 

Reversed micelles can be used to concentrate water-soluble materials in the water pool of SCCO2. The extraction of water-soluble vitamins into reversed micelles has been examined. The efficiency of extraction was strongly affected by the extraction temperature and the concentration of reversed micelles, and the selectivity depended on the size of micelles. Water-soluble vitamins could be efficiently and rapidly extracted. The selective extraction of a model mixture of vitamins from pharmaceutical preparations was also demonstrated. Moreover, the usefulness of the proposed method for the determination of vitamins in various commercial tablets was also demonstrated. Using this method, the surfactant remains mixed with the extracted com-... 

Fig. 101. Idealized model for the CdS-sensitized water photoreduction by PhSH in AOT reversed micelles in isooctane. VB = valence band, CB = conduction band [611]...

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. 

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