Reverse micelles containing water molecules

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

In reverse micelles, the water molecules in the internal pool can be divided into two subensembles. One of the ensemble consists of those water molecules which are near the charged head groups and involved in stronger polar interaction with head groups. This subensemble is termed the shell as these water molecules form the outer shell of the nano-pool water. The other suhensemble contains water molecules in the core of the reverse micelle and has dynamical character similar to those found in bulk water. This subensemble is termed the core (see Figure 17.6). 

Babu et al. (2003) studied encapsulating proteins in reverse micelles and dissolving it in a low-viscosity solvent that can lower the rotational correlation time of the protein. They examined the applicability of several strategies for the preparation and characterization of encapsulated proteins dissolved in low-viscosity fluids that were suitable for high-performance NMR spectroscopy. Ubiquitin was used as a model system to explore various issues such as the homogeneity of the encapsulation, characterization of the hydrodynamic performance of reverse micelles containing protein molecules, and the effective pH of the water environment of the reverse micelle. 

Eqnation 4 shows that, at constant , a change of the external parameter/ affects not only the radins but also the concentration of water-containing reversed micelles. It is also of interest that, by increasing R, the fraction of bulklike water molecules located in the core (or the time fraction spent by each water molecule in the core) of spherical reversed micelles increases progressively, whereas the opposite occurs for perturbed water molecules located at the water-surfactant interface, as a consequence of the parallel decrease of the micellar surface-to-volume ratio. 

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.)... 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

FIG. 6 Representation of spherical water-containing reversed micelles solubilizing a polar molecule (p) in the micellar core (A) or an amphiphilic molecule (a) in the palisade layer (B). 

It follows that in spite of the apolar coat surrounding water-containing AOT-reversed micelles and their dispersion in an apolar medium, some microscopic processes are able to establish intermicellar attractive interactions. These intermicellar interactions between AOT-reversed micelles increase with increasing temperature or the chain length of the hydrocarbon solvent molecule, thus leading to the enhancement of the clustering process [244-246], whereas they are reduced in the presence of inorganic salts [131]. 

Figure 14.23 Silica nanoparticles containing fluorescent dye molecules can be prepared using a reverse micelle suspension process (a) The water-in-oil emulsion is formed with the aqueous phase droplets containing TEOS and dye molecules in detergent, (b) The final particles contain entrapped dye within the silica particle matrix, creating highly fluorescent particles.

Oriented aggregate of surface-active molecules formed in solution when the solubility limit for single molecules (monomers) has been reached. Such aggregates may contain from approximately 5 to 100 molecules. Thus, in water, the hydrophilic portion of the surfactant of molecules is on the outside and this aggregation is named a normal micelle. In organic low-polar solvents, the lipophilic moieties are on the outside and the aggregation is termed an inverse or reverse micelle. 

A wide variety of organic solvents has been used to conduct bioconversions including nonpolar solvents such as isooctane, n-hexane, and toluene, in addition to methanol, acetone, and other water-miscible solvents. Dipolar aprotic solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO) are also compatible with many enzymes and are often used to enhance the solubility of substrates in combination with a nonpolar solvent. Tertiary alcohols such as f-butanol and t-amyl alcohol have been used for many lipase-mediated esterifications as the hindered tertiary alcohol is not typically a good substrate for most enzymes. It should be noted that the presence of small amounts of water is essential for the effective use of most biocatalysts in organic solvents. In some cases an enzyme may only require a monolayer of water molecules on its surface in order to operate. In other cases there may need to be enough water to form reverse micelles where the biocatalyst is contained within a predominantly aqueous... 

Some surfactants form reversed micelles in hydrophobic solvents containing a small amount of water. In these structures, the water is dispersed as microdroplets surrounded by a shell of polar head groups [12], When the water/surfactant ratio is less than 12, most of the water molecules are strongly hydrated to the ions or polar head groups of surfactant molecules, giving the aqueous core a viscosity and polarity very different from that of bulk water [13]. 

Reverse micelles are formed by the addition of a small volume of an aqueous solution to a surfactant-containing organic solvent. The surfactant molecules are orientated at the water-oil interface with the polar "head" groups in the aqueous phase and the nonpolar "tails" in the organic phase. Thus the reverse micelle can encapsulate an enzyme in the aqueous phase. The size of the reverse micelles can be controlled by varying the water-to-surfactant ratio, w0. 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

The enzyme molecules can be contained in the water cores of hydrated reverse micelles or microemulsion droplets. 

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