Reverse polar pool

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

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... 

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) ... 

For small amounts of solubilized water, as a polar additive, the stability of the micelle is markedly increased, as shown by a decrease in the CMC. On the other hand, large amounts of water as a polar additive decrease the stability of the micelle. It is known that a solution of AOT in iso-octane solubilized up to 50 moles of water per mole of surfactant. As the concentration of water increases, the isotropic reverse micellar solution changes to a water-in-oil microemulsion. A clear understanding of the complex analyte-micelle-water pool interactions, especially analyte concentration and pH at the head group interfacial region, is under intensive study (Cline Love and al., 1984 Little and Singleterry, 1964 Luisi and Straub, 1984 Mclntire, 1990). 

Water solubilized by RMs in hydrocarbon solvents by different surfactants (anionic, cationic and zwitterionic) exhibits two absorption bands in the near infrared spectral region (5200-4700 cm 1) due to water populations (Sunamoto et al., 1980), one assigned to the surfactant polar heads and the other to water dispersed in the bulk phase. Three methods have been proposed to evaluate the acidity in the water pool of soft-core of reverse micelles ... 

An inverse (or reverse) micelle, which forms in a non-polar solvent, will have the hydrophilic head groups oriented toward the inside of the sphere, where a water pool is formed and a hydrophilic probe can become associated [10]. Some surfactants commonly employed to stabilize reverse micelles include sodium diisooctylsulfosuccinate (AOT), benzylhexadecyldimethylammonium chloride (BHDC), and dodecylammonium propionate (DAP). Ionic surfactants induce formation of a larger water pool than non-ionic surfactants, but the size of the hydrophilic core also depends on temperature and on the ratio of water to surfactant. 

To avoid this difficulty, one technique is to use reverse micelles. These materials can host a protein in a small water pool. Reverse micelles are spherical aggregates formed by dissolving amphiphiles in organic solvents. The polar head of the amphiphilic molecule is in the interior of the aggregate and the hydrophobic tail is in the organic phase. The micellar suspension is transparent, and controlled amounts of water can be added. 

In this chapter, the recent progress in the understanding of the nature and dynamics of excess (solvated) electrons in molecular fluids composed of polar molecules with no electron affinity (EA), such as liquid water (hydrated electron, and aliphatic alcohols, is examined. Our group has recently reviewed the literature on solvated electron in liquefied ammonia and saturated hydrocarbons and we refer the reader to these publications for an introduction to the excess electron states in such liquids. We narrowed this review to bulk neat liquids and (to a much lesser degree) large water anion clusters in the gas phase that serve as useful reference systems for solvated electrons in the bulk. The excess electrons trapped by supramolecular structures (including single macrocycle molecules ), such as clusters of polar molecules and water pools of reverse micelles in nonpolar liquids and complexes of the electrons with cations in concentrated salt solutions, are examined elsewhere. 

Hence, the exiplex has a sandwich structure which promotes efficient back e transfer at the water pool, and the ion yield is very small. However, a sandwich reactant pair of this sort is not formed on a micelle surface and back reaction is slower than the escape of the cation from the surface. Hie swollen micelle and microemulsion systems lead to both randomly organised ionic products and sandwich pairs, to varying extents, which are reflected in the observed yield of ions, with polar derivatives of pyrene, e.g. pyrene sulfonic acid, etc., the reactants are kept on the assembly surface where reaction occurs, giving rise to ions from a non-sandwiched type of configuration. In the reverse micellar system, these ions although they are formed, nevertheless have a short lifetime, as they cannot escape to any great distance in the small water pool. Huts, micelles are far superior to microemulsions in various aspects of... 

Dlpeptide formation in the reversed micellar medium is influenced by the localization and concentration of the various species in the reaction medium (Figure 3). a-Chymotrypsin is a hydrophilic protein and resides within the water pool of the reversed micelle. Glyclnamide is a polar molecule and as such partitions into the reversed micellar water pools. Consequently, in the immediate environment of the enzyme, there is a locally high glyclnamide concentration (ca. 0.8 M), accelerating the rate of dlpeptide formation. The dlpeptide product that is formed is hydrophobic and will partition away from the water pool, isolating it from back reaction and thus forcing the equilibrium towards synthesis. 

Amphiphilic molecules, when dissolved in organic solvents, are capable of self-assembly to form reversed micelles. The reversed micelles are structurally the reverse of normal micelles in that they have an external shell made up of the hydrocarbon chains of the amphiphilic molecules and the hydrophilic head-groups localized in the interior of the aggregate. Water molecules are readily solubilized in this polar core, forming a so-called water pool. This means that reversed micelles form microcompartments on a nanometer scale. The reversed micelles can host all kinds of substrate molecules whether hydrophilic, hydrophobic, or amphiphilic due to the dynamic structure of the water pool and the interface formed by the surfactant layer, in contrast with a liposome system. The properties of water molecules localized in the interior of reversed micelles are physicochemically different from those of bulk water, the difference becoming progressively smaller as the water content in the micellar system increases [1,2]. The anomalous water at low JVo =[water]/[surfactant] obviously influences the chemical behavior of host molecules in the water pools. 

JVo is a key parameter which significantly affects the physical properties of AOT reversed micelles. In the case of an AOT/oil solution, discontinuity of several physical properties of the solubilized water is observed at IVg 10 [16]. Below IV 10, the water is bound to the AOT polar head-groups and counterions, and further addition of water leads to the appearance of free water in the core of the water pools. However, the state of the water in the AOT reversed micelles, especially below Wg 2, appears unusual. We found that the solution enthalpy of the water in AOT/various organic solvents solutions indicated a great change in the state of the solubilized water [17,18]. 

Sizes of reversed micelle iid water pool Polarity of water pool " " ... 

In contrast to micelles, the core region of a reverse micelle can encapsulate a fairly large amount of water to form what is known as a microemulsion . Up to 50 water molecules per surfactant molecule can be incorporated inside AOT reverse micelles. Such a surfactant-coated nanometer-sized water droplet dispersed in a non-polar liquid is called a water pool . The radius (r ) of the water pool varies linearly with the water-to-surfactant mole ratio, Wq. In n-heptane, (in A) is approximately equal to 2 IFo. Thus, one can form reverse micelles with varying water pools and tune the extent of confinement by varying Wo [4]. 

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). 

In two-phase systems, however, where surfactant and water can partition between a fluid and a liquid phase, significant pressure effects occur. These effects were studied for AOT in ethane and propane by means of the absorption probe pyridine N-oxide and a fluorescence probe, ANS (8-anilino-l-naphthalenesulfonic acid) [20]. The UV absorbance of pyridine A-oxide is related to the interior polarity of reverse micelles, whereas the fluorescence behavior of ANS is an indicator of the freedom of motion of water molecules within reverse micelle water pools. In contrast to the blue-shift behavior of pyridine N-oxide, the emission maximum of ANS increases ( red shift ) as polarity and water motion around the molecule increase. At low pressures the interior polarity, degree of water motion, and absorbance intensity are all low for AOT reverse micelles in the fluid phase because only small amounts of surfactant and water are in solution. As pressure increases, polarity, intensity, and water motion all increase rapidly as large amounts of surfactant and water partition to the fluid phase. The data indicate that the surfactant partitions ahead of the water thus there is a constant increase in size and fluidity of the reverse micelle water pools up to the one-phase point. An example of such behavior is shown in Fig. 4 for AOT in propane with a total fVo of 40. The change in the ANS emission maximum suggests a continuous increase in water mobility, which is due to increasing fVo in the propane phase, up to the one-phase point at 200 bar. 

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