Micelles reverse/inverse

One of the major differences between micellar chromatography and standard reversed-phase chromatography is the selectivity of the separation. As the micelle concentration is increased, solute retention decreases as a result of increased solute-micelle interactions in the mobile phase. The rate of decrease varies from solute to solute, however, since different solutes will have a different affinity for the micelles thus, inversions in retention orders are produced.34... 

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

Unlike the aqueous micells just described (often referred to simply as micelles), there can also be micelles in which the apolar groups are in contact with the solvent at the exterior and the polar or ionic groups are associated with water in the interior. These are referred to as reverse or inverse micelles. Reverse micelles are particularly noted for their accelerating or retarding effect on biochemical reactions. 

As illustrated in Fig. 2, micelies can be classified as normal (tail groups forming the core and head groups coating this oil-like interior) in aqueous solution or reverse (inverse) where the tail groups are on the micelle exterior exposed to an organic solvent. There are fewer studies of reverse micelles compared to those on normal micelles, because the vast majority of surfactant applications are in aqueous solution, and normal Inicelles are much easier to form than reverse micelles. The focus in this article will be on surfactants in aqueous solution, due to this emphasis and space limitations. 

A modification of the micro-emulsion polymerization is pol5meriza-tion in reverse micelles, called inversed or micellar polymerization. This issue concerns the polymerization of hydrophilic monomers, from which one obtains the water-soluble pol miers. Therefore, as the phase diffuser mostly hydrocarbons such as isooctane, heptane, benzene or toluene are used. 

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap Hquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

A structure formed by the reversible association of am-phiphiles in apolar solvents. In inverted micelles, the polar portion of the amphiphile is concentrated in the interior of the macrostructure. Such association usually occurs with aggregation and is not typically characterized by a definite nucleation stage. Thus, inverted micelles (also referred to as inverse or reverse micelles) often fail to exhibit critical micelle concentration behavior. See Micelle... 

The parameter r2 is independent of the initiator type for the emulsion, however, and is slightly higher than that obtained in benzene (r2=1.23) (Table 3). This behavior results from good compatibility of the macromonomer with poly-BzMA. Therefore the reactivity of the macromonomer does not depend so much on the reaction medium type. In contrast, reversed apparent reactivity was observed in heptane in which the clear solution of monomer turned into a polymer suspension upon polymerization. Since BzMA is soluble in the medium, it has been suggested that the polymerization occurs preferentially on the (inverse) micelle surface which is enriched by the macromonomers. 

Hydrated electron probe inverse micelles. Hydrated electrons (e aq) are expected to be a very good probe to test the water pool of reverse micelles. The physical properties of hydrated electrons obtained by pulse radiolysis in AOT reverse micelles were experimentally determined (Calvo-Perezet al., 1981 Pileni,... 

Oriented aggregate of surface-active molecules formed in solution when the solubility limit for single molecules (monomers) has been reached. Such aggregates may contain from approximately 5 to 100 molecules. Thus, in water, the hydrophilic portion of the surfactant of molecules is on the outside and this aggregation is named a normal micelle. In organic low-polar solvents, the lipophilic moieties are on the outside and the aggregation is termed an inverse or reverse micelle. 

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. 

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