Water-to-surfactant ratio

Figure 2 schematically presents a synthetic strategy for the preparation of the structured catalyst with ME-derived palladium nanoparticles. After the particles formation in a reverse ME [23], the hydrocarbon is evaporated and methanol is added to dissolve a surfactant and flocculate nanoparticles, which are subsequently isolated by centrifugation. Flocculated nanoparticles are redispersed in water by ultrasound giving macroscopically homogeneous solution. This can be used for the incipient wetness impregnation of the support. By varying a water-to-surfactant ratio in the initial ME, catalysts with size-controlled monodispersed nanoparticles may be obtained. 

Surfactants having an appropriate hydrophobic/hydrophilic balance (sodium bis(-2-ethylhexyl)sufosuccinate, or AOT, for example) undergo concentration-dependent self association in apolar solvents to form reversed or inverted micelles (Fig. 33) [256-262]. Reversed micelles are capable of solubilizing a large number of water molecules (AOT reversed micelles in hexane are able to take up 60 water molecules per surfactant molecule, for example). Reversed-micelle-entrapped water pools are unique they differ significantly from bulk water. At relatively small water-to-surfactant ratios (w = 8-10, where w = [H20]/[Surfactant]), all of the water molecules are strongly bound to the surfactant headgroups. Substrate solubilization in the restricted water pools of reversed micelles results in altered dissociation constants [256, 257, 263-265], reactivities [256, 258, 266], and reaction products [267]. 

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). 

Effective pH values in soft-core RMs. Characterization of the acidity in the aqueous soft-core is important as ionizable compounds are solubilized in the water pool. The micellar core has a very high degree of organization of water. The water pool within reverse micelles is a different solvent than bulk water. The most interesting range of water content corresponds to rather small water pools (water-to-surfactant ratio of 3 to 10) in which peculiar properties of water cause the largest changes in behavior as compared to their behavior in bulk water. A water to surfactant ratio of 1 1 represents a very small, almost undetectable, quantity of... 

The amount of water added to a water-in-oil (w/o) microemulsion is defined by the molar water-to-surfactant ratio, W. Generally, the greater the W value, the larger the size of the nanometer-sized... 

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. 

Previous work, on the use of a reverse-micelle system for the production of tryptophan reported kinetic data obtained under various conditions (2). Both the water-to-surfactant ratio and the cosurfactant used influenced tryptophan production. The present work reports results from EPR studies of the effect of these parameters on both the water and the enzyme in the reverse micelle. EPR spectra of Mn(H20)g + were recorded to investigate the state of the water in reverse micelles. A nitroxide spin label that reacts with lysine residues was employed to probe the microstructure of tryptophanase in reverse micelles of different w0 values. 

Figure 4. Effect of water content on the amount of tryptophan produced in 20 hrs. w0 is the molar water-to-surfactant ratio. Experimental conditions 37°C, pH 9, 0.15 M Brij, 0.05 M indole, 0.83 mg enzyme/ml reactor (data from ref. 3).

Formation of microemulsion was carried out using a small quantity of ionic surfactant, CTAB (cetyltrimethylammonium bromide), and de-ionized water in excess dried toluene. Above critical micelle concentration (cmc) these three components form reversed micelles. As the size of such micelle system is related to the ratio of water/surfactant (W) added, tailored amount of water to surfactant ratios in order to control the sizes of the nano-composites were investigated. 

In the literature there is often no clear distinction between microemulsions and micellar systems. For instance, a system containing a small amount of water solubilized in hydrocarbon may be referred to as a W/O microemulsion (or an L2 microemulsion) or as a system of reverse micelles (or swollen reverse micelles). It has been suggested [2] that the borderline between reverse micelles and microemulsions droplets should be defined by the water-to-surfactant ratio above a molar ratio of 15, the system should be referred to as a microemulsion. In this chapter no such distinction is made. All systems containing oil and water together with surfactant are termed microemulsions, regardless of the relative component proportions. 

It was later demonstrated that JVq is not the only factor governing enzymatic activity in W/O microemulsions. Both a hydrophilic enzyme, a-chymotrypsin, and a lipophilic enzyme, hydroxysteroid dehydrogenase, varied in activity with surfactant concentrations at constant Wq [36]. Evidently, at least two parameters, molar water-to-surfactant ratio (Wq) and surfactant concentration, are decisive for enzymatic activity in these systems. 

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