Reverse water pool

The strong interactions between the water molecules also become obvious from NMR measurements by Tsujii et al..57) 13C-NMR experiments were used for determining the microviscosity of water in reversed micelles of dodecylammonium-propionate with 13C glycine cosolubilized. It was found that the apparent viscosity of the water-pool corresponds to the viscosity of a 78 % aqueous glycerol solution, obviously as a consequence of the extended network formation by strong hydrogen bonding. 

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. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

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] >... 

Fig. B4.3.1. Schematic illustration of the average residence sites of the probes NOH (1), NSOH (2), NDSOH (3) in AOT reverse micelles. Length of the surfactant 11 A. Diameter of the water pool 18A at w = 3, 36A at w = 9. Largest dimension of the naphthol derivatives 9 A (adapted from Bardez et al.a ).

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). 

Effect of salt type and concentration The ionic strength of the aqueous solution in eontaet with a reverse micelle phase affects protein partitioning in a number of ways [18,23]. The first is through modification of electrostatic interactions between the protein surface and the surfaetant head groups by modifieation of the eleetrieal double layers adjacent to both the eharged inner mieelle wall and the protein surface. The second effect is to salt out the protein from the mieelle phase because of the inereased propensity of the ionie speeies to migrate to the micelle water pool, reduee the size of the reverse mieelles, and thus displace the protein. 

Reverse micelles of CTAB in octane with hexanol as cosurfactant were reported to be able to lyse whole cells quickly and accommodate the liberated enzyme rapidly into the water pool of surfactant aggregates [50,51]. In another case a periplasmic enzyme, cytochrome c553, was extracted from the periplasmic fraction using reverse micelles [52]. The purity achieved in one separation step was very close to that achieved with extensive column chromatography. These results show that reverse micelles can be used for the extraction of intracellular proteins. 

Surfactants can aggregate in nonpolar solvents in the presence of small amounts of water with the tails oriented towards the bulk nonpolar solution and head groups interacting with water in the center (Fig. 2). The water pool formed in reverse micelles has been used as a medium to study chemical and biological reactions [22]. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Reverse micelles are small (1-2 nm in diameter), spherical surfactant aggregates huilt in an apolar solvent (usually referred to as oil), whereby the polar heads form a polar core that can contain water - the so-called water pool. The connection with autopoiesis is historically important, because it was with the collaboration with Francisco Varela that the work started (in fact it began as a theoretical paper - see Luisi and Varela, 1990). The idea was this to induce a forced micro-compartmentalization of two reagents, A and B, which could react inside the boundary (and not outside) to yield as a product the very surfactant that builds the boundary (Figure 7.13). The product S would concentrate at the membrane interface, which increases its size. Since reverse micelles are usually thermodynamically stable in only one given dimension, this increase of the size-to-volume ratio would lead to more micelles. Thus the growth and multiplication would take place from within the structure of the spherically closed unit, be governed by the component production of the micellar structure itself, and therefore (as will be seen better in... 

Figure 7.13 The first self-reproduction scheme conceived for reverse micelles, (a) A reverse micelle, (b) Two reagents, A and B, penetrate inside the water pool and react with each other inside the boundary, yielding the very surfactant S that makes the micelles. The S thus produced migrates to the boundary and induces growth and eventually multiplication of the micelle. (Adapted from Luisi and Varela, 1990).

Reverse micelles form in aprotic organic solvents, such as hydrocarbons or CCI4, and can be seen as a core containing water (the water pool) solubilized in an oily environment (for example hydrocarbons) by the hydrophobic tails. Figure 9.9 also shows the structure of AOT (from aerosol octyl), which is the most popular surfactant for reverse micelles. A typical reverse micellar system appears as a clear... 

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.)...

Figure 9.11 A case of selective compartmentation in reverse micelles, permitting the synthesis of a peptide by the reverse protease action. The product C, produced in the water pool, is expelled into the outside hydrocarbon environment due to its insolubility in water. (Adapted from Barbaric and Luisi, 1981.)...

The importance of the particular compartmentation in this field is made apparent by a series of interesting and partly still unexplained effects. For example, when the amount of water is varied in the reverse micellar solution, the maximum enzyme activity - even in the case of hydrolases - is not observed with higher water-content values, but with relatively low amounts of water. In addition, the local pH - due to the constraints of the water pool - is anomalous with respect to the pH value in water (El Seoud, 1984 Luisi and Straub, 1984). 

Reverse micelles are well known to be spherical water in oil droplets stabilized by a monolayer of surfactant. The phase diagram of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate, called Na(AOT), with water and isooctane shows a very large domain of water in oil droplets and often forms reverse micelles (3,23). The water pool diameter is related to the water content, w = [H20]/[ AOT], of the droplet by (23) D(nm) = 0.3w. From the existing domain of water in oil droplets in the phase diagram, the droplet diameters vary from 0.5 nm to 18 nm. Reverse micelles are dynamic (24-27) and attractive interactions between droplets take place. 

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-... 

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]. 

Proteins and enzymes have been successfully entrapped in surfactant-solubilized water pools in organic solvents [268-278]. Furthermore, many reversed-micelle-entrapped enzymes retained their activity and could be used for peptide synthesis [273,274]. That the water pools corresponding to very small w-values exhibited freezing points Mow — 50°C enabled both the enzyme structures and the rates of enzyme-catalyzed reactions to be investigated at low temperatures. These studies much aided the development of cryoenzymology [279, 180],... 

A higher level of size and morphology control in the incipient semiconductors has been accomplished in reversed micelles prepared from cadmium AOT [614] and from mixtures of cadmium AOT and sodium AOT [615] or, alternatively, by arresting particle growth by surface derivatization [592, 621, 622]. Indeed, surface derivatization of semiconductor clusters was first reported for particles in reversed micelles [621] the reversed micelles act to confine precursor ions and to control the growth of the semiconductor particles. Conditions are typically arranged so that, initially, there is no more than one metal ion (say Cd2+) per water pool. Addition of a heptane solution of bis(trimethylsilyl) selenium resulted in the formation of size-quantized metal selenide particles (say CdSe) in the reversed micelles. This solution could be evaporated to dryness and the resultant particles could be reconstituted in a hydrocarbon solvent Alternatively, addition of metal (say Cd2+) ions to the reversed-micelle-entrapped metal selenide particles, followed by the addition of alkyl(trimethylsilyl)selenium, RMSiMe3, led to the formation of alkyl-capped... 

Most recently, Fox, Johnston, and co-workers (65) showed how an environmentally-sensitive fluorescent species could be used to probe the water pool of AOT reverse micelles in supercritical alkanes. The interesting observation was that the emission spectra were not shifting with fluid density. These results were consistent with the postulate of the micelle interior not changing with the supercritical fluid density paralleling conclusions reached using other spectroscopic techniques (61-64). 

Sodium bis-2-ethylhexylsulfosuccinate (AOT) was used to form reversed micelle-entrapped water pools in isooctane. Platinized CdS, in situ generated in those water pools, sensitized water photo reduction by thiolphenol dissolved in the organic phase. 

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