Reverse micelles water properties

In summary, these authors (70) have proposed that the transparent, isotropic, clear, stable systems prepared from oil/water/emul-sifier can be classified into one of three main categories Normal or reversed micelles, water-in-oil or oil-in-water microemulsions or cosolubilized systems. One can distinguish these classes of structures by using a combination of physical techniques to study the properties of such systems. 

C. Physical Properties of Water in AOT Reverse Micelle Water Pools... 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

Given their radio-frequency electrical properties and nuclear magnetic resonance chemical shift components, solutions of reversed micelles constituted of water, AOT, and decane have been proposed as suitable systems to test and calibrate the performance of magnetic resonance imagers [68]. 

A. Structural and Dynamic Properties of Water-Containing Reversed Micelles... 

In contrast, thermodynamic as well as spectroscopic properties of core water in AOT-reversed micelles are similar to those of pure water. Together with electrostatic considerations, this suggests that the penetration of counterions in the micellar core is negligible and that a relatively small number of water molecules are able to reconstruct the typical extended H-bonded structure of bulk water. 

Incidentally, it is of interest to note that solutions of water-containing reversed micelles could be employed to study the physicochemical properties of nanosize solid water. 

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

The microenvironment in water-containing AOT-reversed micelles has a marked effect on the spectral properties of flnorescein. The absorption peaks are red-shifted by about 10 nm from the corresponding positions in aqueous solution, the absorption extinction coefficient increases with R, and the fluorescence is more effectively quenched in AOT-reversed micelles than in aqueous solution [149],... 

The observation of slow, confined water motion in AOT reverse micelles is also supported by measured dielectric relaxation of the water pool. Using terahertz time-domain spectroscopy, the dielectric properties of water in the reverse micelles have been investigated by Mittleman et al. [36]. They found that both the time scale and amplitude of the relaxation was smaller than those of bulk water. They attributed these results to the reduction of long-range collective motion due to the confinement of the water in the nanometer-sized micelles. These results suggested that free water motion in the reverse micelles are not equivalent to bulk solvation dynamics. 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

The acido-basic properties of water molecules are greatly affected in restricted media such as the active sites of enzymes, reverse micelles, etc. The ability of water to accept or yield a proton is indeed related to its H-bonded structure which is, in a confined environment, different from that of bulk water. Water acidity is then best described by the concept of proton-transfer efficiency -characterized by the rate constants of deprotonation and reprotonation of solutes - instead of the classical concept of pH. Such rate constants can be determined by means of fluorescent acidic or basic probes. 

Figure 1 shows a reversed micelle where the bulk solvent is a hydrocarbon and the core is a water pool surrounded by surfactant. These systems possess unique features as the physical properties of the water pools only start to approach those of bulk water at high water content when the pool radii are >150 pools with radii as small as 15 can be constructed (1, 25). These systems have been used to investigate the nature of several inorganic reactions by stopped flow methods (26, 27). They have also been used to produce so-called naked ions, i.e., ions that possess a minimum of aqueous solvation (28). The system strongly promotes many reactions, a fact attributed to the unusual nature of the water in this system. 

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

It appears from a survey of the literature that the essential properties of micelles in nonpolar solvents are understood, namely their stability and variations of size, the dissociation behavior, and their solubilizing capacities. Reverse micelles can dissolve relatively large amounts of water (1-10% w/v depending on emulsion formula) as well as polar solutes and, of course, water-soluble compounds. Consequently, they can be used as media for a number of reactions, including enzyme-catalyzed reactions. Very few attempts to investigate such reverse micelles at subzero temperatures are known, in spite of the fact that hydrocarbon solutions present very low freezing points. 

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. 

Many investigations have been undertaken regarding the effect of the water concentration in the microemulsion on the catalytic behaviour of enzymes. The surfactant concentration of the microemulsion defines the size of the internal interface but it often has no measurable influence on the enzyme kinetics. On the other hand, the physical properties of the water located inside the reverse micelles differ from those of bulk water, and the difference becomes progressively smaller as the water concentration, expressed in the w -value, increases. 

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...

Figure 9.10 Some structural details and dynamic properties of reverse micelles 50 irtM AOT/isooctane, Wo = 11.1 (= 10 p lHoOperml), 25°C 3.2% AOT (w/w), 1.4% H2O (w/w) mean water pool radius 20 A, mean hydrohynamic radius 32 A concentration of micelles 400 (xM, monomer AOT concentration 0.6-0.9 mM aggregation number 125 total interfacial area 14 m mC (Adapted from Fletcher and Robinson, 1981, and Harada and Schelly, 1982.)...

Although supercritical CO2 is an effective solvent for oils, fats, and similar substances, it is a poor one for nonvolatile hydrophilic (water-loving) substances such as proteins or metallic salts. Adding water as such to the supercritical CO2 is of little help, as the solubility of water in it is limited. Johnson and co-workers216 overcame the latter limitation by forming water-in-C02 emulsions with the aid of an added nontoxic perfluoropolyether surfactant that forms reverse micelles around the water microdroplets, in effect combining the special properties of supercritical CO2 with the solvent power of water. These emulsions can dissolve a variety of biomolecules at near-ambient temperatures, without loss of their biological activity. 

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). 

Molecules that self-assemble into reverse micelles with low surfactant properties are generally efficient extractants (such as HDEHP, TBP, malonamides, etc.). Their adsorptions at the interface permit the complexation of the aqueous solute and their low surfactant properties permits the avoidance of the formation of very stable emulsion. Hence, ions are extracted, but typically there is less than one water molecule per ion extracted. Exact determination of coextracted water is still important, however, for interpreting the conductivity values and for evaluating the polar core volumes. Typical values are found for the Hamaker constant, because polar cores are supersaturated salt solution. 

Much interest has been focused on solubilizing various amounts of water in reverse micelles. The micellar solutions can solubilize considerable amounts of water this is bound to the polar groups of the surfactant molecules by ion-dipole or dipole-dipole attraction. The properties of water solubilized by RMs are different from those of bulk water and are sensitive to the water pool parameter, Wo = [H20]/[Surfactant]. Assuming the water molecules in the oil droplets are spherical, the radius of the sphere is expressed as (Luisi et al., 1988) ... 

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