Water solubilization in reverse

FACTORS AFFECTING WATER SOLUBILIZATION IN REVERSE MICELLES... 

In Section 3.3 we have briefly indicated that there are various factors that can affect water solubilization in reverse micelles. The importance of the oil-water interface in the context of water solubilization cannot just be overemphasized, and the entity that first comes to the forefront is the surfactant film that separates the two immiscible phases. At the very beginning, therefore, some basic and relevant points on the surfactant film are described below in brief [113, 114, 124, 125, 3]. The interfacial surfactant film can be described as a two-dimensional system in which one can consider a pressure term n [113] this term, characteristic of the film, defines the difference in the oil/water interfacial tension before and after addition of the surfactant to form an interfacial film ... 

Figure 10.19. Solid surface drying by water solubilization in reversed micelles (a) wet metal surface (b) drying surfactant micelles begin to absorb water (c) dry surface and solubilized waters.

The aqueous cores of reverse micelles are of particular interest because of their analogy with the water pockets in bioaggregates and the active sites of enzymes. Moreover, enzymes solubilized in reverse micelles can exhibit an enhanced catalytic efficiency. Figure B4.3.1 shows a reverse micelle of bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane with three naphthalenic fluorescent probes whose excited-state pK values are much lower than the ground-state pK (see Table 4.4) 2-naphthol (NOH), sodium 2-naphthol sulfonate (NSOH), potassium 2-naphthol-6,8-disulfonate (NSOH). The spectra and the rate constants for deprotonation and back-recombination (determined by time-resolved experiments) provide information on the location of the probes and the corresponding ability of their microenvironment to accept a proton , (i) NDSOH is located around the center of the water pool, and at water contents w = [H20]/[A0T] >... 

The amount of water solubilized in a reverse micelle solution is commonly referred to as W, the molar ratio of water to surfactant, and this is also a good qualitative indicator of micelle size. This is an extremely important parameter since it will determine the number of surfactant molecules per micelle and is the main factor affecting micelle size. For an (AOT)/iso-octane/H20 system, the maximum Wq is around 60 [16], and above this value the transparent reverse micelle solution becomes a turbid emulsion, and phase separation may occur. The effect of salt type and concentration on water solubilization is important. Cations with a smaller hydration size, but the same ionic charge, result in less solubilization than cations with a large hydration size [17,18]. Micelle size depends on the salt type and concentration, solvent, surfactant type and concentration, and also temperature. 

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. 

The phase boundary lines for supercritical ethane at 250 and 350 bar are shown in Figure 2. The surfactant was found to be only slightly soluble in ethane below 200 bar at 37 C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane binary system is a single phase. As pressure is increased, more water is solubilized in the micelle core and larger micelles can exist in the supercritical fluid continuous phase. The maximum amount of water solubilized in the supercritical ethane-reverse micelle phase is relatively low, reaching a W value of 4 at 350 bar. 

FIG. 3 a-Chymotrypsin solubility (protein concentration) in reverse micelles of 0.1 M AOT in octane, as a dependence on the degree of surfactant hydration (water-to-AOT molar ratio), for protein molecules with chemically modified surface groups. ( ) Acetyl-a -ch5miotrypsin (O) succinyl-a-chymotrypsin (—) maximal concentration of nonmodified native a -ch5miotrypsin solubilized in reverse micelles. 

Water solubilization in AOT reverse micelles has been studied by measurements of Wq at the phase boundary, Wq at a fixed temperature [16,20,21,48]. The data are in reasonable agreement for the heavy solvents octane, nonane, and decane and for the light solvents ethane, propane, and butane, but the results are not in agreement for the intermediate solvents pentane, hexane, and heptane [20]. To determine the location of for these solvents, phase boundary plots of the type shown in Fig. 3 were constructed, and a constant-temperature line was drawn across the plot to determine This is a much more reliable technique than the alternative procedure of adding water aliquots to a reverse micelle solution at a fixed temperature, because the phase boundary can be difficult to observe in such an experiment [19]. 

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 is now clear enough that in a surfactant-oil-water system (with or without additional components), there should be at least two microemulsion phases that are relevant for our work water solubilized in oil (W/O droplet microemulsions, commonly called the L2 phase) and oil solubilized in water (O/W droplet microemulsions, commonly called the Li phase). Indeed, the situation is much more complex than this [3,41,47, 104]. Figure 3.2 is indicative of some common phases that can be identified in a water/oil/surfactant system under selected conditions. This, however, does not concern us much, as most of the present book deals with spherical micelles and microemulsions (generally reverse, but also normal in some specific cases) as they are the most popular templates for synthesis. 

Fig. 5. a, b Influence of water in the reverse micellar system on enzyme activity, a) Enzyme activity profile in the water pool of chymotrypsin. The substrate is glutatyl-phenyl alanine p-nitro anilide, b) Dependence of the first-order rate constant for peroxidase oxidation of pyrogallol on the content of water solubilized in 0.1 M Aerosol OT plus aqueous buffer (0.02 M phosphate-borate-acetate, pH 7.0) plus octane at 26°C. For comparison, the value of kcat in the same buffer (dashed line) is shown. The initial steady-state rate of formation of purpurogallin was followed spec-trophotometrically (420 nm) (from Ref. [105])... 

Water Solubilized by Reverse Aggregates of Dodecyltrimethylammonium Bromide Didodecyidimethylammonium Bromide, and Their Mixtures in Organic Solvents... 

Durchschlag H, Zipper P Volume, Surface and Hytotion Properties of Proteins 19 Bering T -) Kettler E El Seoud OA, Pires PAR FTIR and H NMR Studies on the Structure of Water Solubilized by Reverse Aggregates of Dodecyltrimethyl-ammonium Bromide Didodecyl-dimethylammonium Bromide, and Their Mixtures in Organic Solvents 101... 

Horse liver alcohol dehydrogenase (HLADH) is an enzyme known to stereo-selectively oxidize and reduce a wide range of alcohol and ketone substrates. In a study that appeared in 1987, HLADH was solubilized in reverse micelles formed with AOT/cyclohexane and the oxidation of ethanol and reduction of cyclohexanone in a coupled substrate/coenzyme recycling system was investigated [147]. Activity and stability studies showed that the enzyme remains active and stable for at least 2 weeks while the charged coenzyme is retained within the dispersed water droplets. 

NH4 or Na 8-anilino-l-naphthalenesulfonate, ANS), and a complex probe (pyrene in the presence of MA-dimethylaniline). The I h ratio (the ratio of the fluorescence intensity of the first and third vibronic peaks of pyrene), which indicates the polarity of the probe environment, increases with increasing FHA concentration and the formation of reversed micelles. The formation of reversed micelles is driven by interactions of TCFE, which are lyophobic with the carboxyl group of the surfactant and favorable with the fluorocarbon chain of the surfactant. The aggregation numbers and the amount of water solubilized in the reversed micelles increase with increasing FHA concentration. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

The interest and success of the enzyme-catalyzed reactions in this kind of media is due to several advantages such as (i) solubilization of hydrophobic substrates (ii) ease of recovery of some products (iii) catalysis of reactions that are unfavorable in water (e.g. reversal of hydrolysis reactions in favor of synthesis) (iv) ease of recovery of insoluble biocatalysts (v) increased biocatalyst thermostability (vi) suppression of water-induced side reactions. Furthermore, as already said, enzyme selectivity can be markedly influenced, and even reversed, by the solvent. 

Obviously, water, aqueous solutions of salts, and mixtures of highly hydrophilic solvents have also been found to be solubilized in the micellar core [13,44]. The maximum amount of such solubilizates that can be dissolved in reversed micelles varies widely, strongly depending on the nature of the surfactant and the apolar solvent, on the concentrations of surfactant and of additives, and on temperature [24,45-47]. 

The small and positive values of enthalpy of solution of water in AOT-reversed micelles indicate that its energetic state is only slightly changed and that water solubilization (unfavorable from an enthalpic point of view) is driven mainly by a favorable change in entropy (the destructuration of the water at the interface and its dispersion as nanodroplets could be prominent contributions) [87],... 

The cobalt, nickel, and copper bis(2-ethylhexyl) phosphate surfactants dissolved in n-heptane lead to quasi-one-dimensional association microstructures, i.e., rodlike reversed micelles that increase in size via water solubilization [111],... 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

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],... 

By IR spectroscopy it was emphasized that the solubilization of amino acids or ohgopeptides in water-containing lecithin-reversed micelles involves structural changes in the aqueous micellar core [159]. 

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