Micellar nature

Micelle-forming copper complexes were found to effectively discriminate between enantiomers in the hydrolysis of a-amino esters (257). Hydrolysis of (.V)-phenylalanine p-nitrophenyl ester is 14-fold faster than its enantiomer, Eq. 223. Leucine affords 10-fold faster hydrolysis. The authors note that the micellar nature of these systems is extremely important for both rate of hydrolysis and selectivity (258). For example, the /V-mcthyl-dcrivcd ligand 419b leads to inhibition of the hydrolysis process, relative to catalysis by Cu(II) ion alone. 

Guetzloff,T. F., and Rice, J. A. (1996). Micellar nature of humic colloids. In Humic and Fulvic Acids Isolation, Structure, and Environmental Role, Gaffney, S., Marley, A., and Clark, S. B., eds., ACS Symposium Series, No. 651, American Chemical Society, Washington, D.C., pp. 18-25. 

Humic and fulvic adds are ill-defined and heterogeneous mixtures of naturally-occurring organic molecules that possess surface active properties The molecules that comprise this mixture are also known to form aggregates of colloidal dimensions Humic and fulvic adds are shown to be able to solubilize hydrophobic organic compounds (HOC) in a manner that is consistent with known, micelle-forming surfactants, but not at organic carbon concentrations that are environmentally relevant In addition, it is found that some HOCs are not solubilized to the same extent as other HOCs Some implications of the micellar nature of humic materials are briefly discussed ... 

GUETZLOFF RICE Micellar Nature of Humic Colloids... 

Fig. 3 C-NMR spin-lattice (T ) relaxation studies of guests, such as acetylsalisylic acid (14), encapsulated in polyamidoamine dendrimers (15) and the enhanced aqueous solubility of guests (i.e., 17) reveal the micellar nature of dendrimers.

An elegant experiment by Crooks showed that the hydrophobic modification of PAMAM dendrimers by noncovalent interactions could also result in macromolecules that behave like inverted micelles (118). The spontaneous assembly between the fatty acids and amino periphery of the PAMAM dendrimer was driven by ionic interactions (Fig. 32). These dendrimers were shown to be capable of extracting hydrophilic dyes such as methyl orange fi om water into toluene. Similarly, these dendrimers were also shown to be excellent molecular containers for catalytically active metal nanoparticles. The inverted micellar nature of various dendrimers have been used by Crooks and others for the preparation of a variety of nanoparticles (119-122). A related macromolecule, but an architecture with less of a control, is a hyperbranched polymer. Hydrophobically modified hyperbranched polymers have also been shown to be capable of acting as inverted micelles (123,124). 

One of the regulatory structures in living organisms is the cell membrane. The simple bimolecular leaflet structure for the lipid portion of the membrane probably still is considered to form the main bulk of the mammalian membrane, but the concept of a more flexible and mobile structure has been suggested by Lucy and others [340]. While the current views on membrane morphology are constantly shifting and recent texts should be consulted, it is salutory to consider work which has discussed the micellar nature of membrane lipids. Much of this is speculative but illustrates one approach to the problem. 

Solutions of fluorinated surfactants have been investigated and their micellar nature has been confirmed [58,59]. The substitution of the larger and highly electronegative fluorine atom for the smaller hydrogen increases the amphiphilic nature of the surfactant and lowers the surface tension and cmc. The alkali and ammonium salts of perfluoroalkanoic acids exhibit surfactant properties and form micelles for a chain length of four carbon atoms, whereas eight carbon atoms are needed for the nonfluorinated alkanoates. 

Phospholipases, responsible for the degradation of phospholipids, often have unique characteristics as enzymes. They usually operate best at the surface of immiscible solvents such as with lipid micelles. The micellar nature of their substrates makes the application of classical enzyme kinetics difficult. Many phospholipases (and lipases) are extremely stable molecules which exhibit activity in organic solvents and at remarkably high temperatures. 

This chapter focuses on three types of phenomenon that are closely related to the presence of amphiphUes and micelles in solution, and on the roles surfactant structures and other characteristics may play in their application. To exploit the micellar nature of surfactants and to realize their technological potential, it is necessary for the investigator to understand and very carefiilly control the many variables involved in the various phenomena. It is probable that in the near future we will see a dramatic increase in the use of micellar and related systems to produce better, more effective, more appealing, and (hopefully) cheaper products for the realization of a better world. 

Finally, in Chapter 5, micellar catalysis of Diels-Alder reactions is discussed. In view of the nonpolar nature of most Diels-Alder reactants, efficient micellar catalysis of this reaction was anticipated However, this has not been observed. The results for the Diels-Alder reaction between cyclopentadiene and substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-one dienophiles, discussed in... 

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

Petroleum sulfonates are widely used as solubilizers, dispersants (qv), emulsifiers, and corrosion inhibitors (see Corrosion and corrosion inhibitors). More recentiy, they have emerged as the principal surfactant associated with expanding operations in enhanced oil recovery (66). Alkaline-earth salts of petroleum sulfonates are used in large volumes as additives in lubricating fluids for sludge dispersion, detergency, corrosion inhibition, and micellar solubilization of water. The chemistry and properties of petroleum sulfonates have been described (67,68). Principal U.S. manufacturers include Exxon and Shell, which produce natural petroleum sulfonates, and Pilot, which produces synthetics. 

These effects appearance is explained by the nature of hydrophobic interactions and the stixrctural organization of micellar solutions. The host-guest phenomena determine the increase of the selectivity of analytical reactions in the surf actant-based organized systems. 

The above enantioselectivities are obviously complex functions of many factors, perhaps even more complex than in natural enzymes. Complexity is partly due to the present co-micellar system in which it is difficult to analyze separately the interaction of the substrate with the achiral micelle, and that of the substrate with the catalyst complex. 

The former investigation was motivated, in part by the fact that in a previous study (7) there had been a marked difference on the rates of reactions of e (aq) and U(VI) between homogeneous solutions and those containing micellar material. When the rate of disappearance of the hydrated electron is measured over a range of concentrations from 2 x 10-5 M to 8 x 10-lt M at pH = 9.7 in solutions formally 0.003 M Si02, the calculated second order rate parameter is 1.4 x 109 M-1s-1. This is a marked decrease from any of the previous measurements and emphasizes the point that the prediction of Pu chemistry in a natural water system must take cognizence of factors that are not usually deemed significant. 

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 hydrolysis of zeaxanthin esters by a carboxyl ester lipase indeed enhanced both the incorporation of zeaxanthin in the micellar phase and uptake of zeaxanthin by Caco-2 cells. As mentioned earher, carotenoids can also be linked to proteins by specific bindings in nature and these carotenoid-protein complexes may slow the digestion process and thus make their assimilation by the human body more difficult than the assimilation of free carotenoids. Anthocyanins are usually found in a glycosylated form that can be acetylated and the linked sugars are mostly glucose, galactose, rhamnose, and arabinose. 

MEKC is a CE mode based on the partitioning of compounds between an aqueous and a micellar phase. This analytical technique combines CE as well as LC features and enables the separation of neutral compounds. The buffer solution consists of an aqueous solution containing micelles as a pseudo-stationary phase. The composition and nature of the pseudo-stationary phase can be adjusted but sodium dodecyl sulfate (SDS) remains the most widely used surfactant. 

Much effort has been directed at developing aqueous Diels-Alder reactions toward the syntheses of a variety of complex natural products. Grieco employed micellar catalysis and pure water as the solvent for the Diels-Alder reaction of dienecarboxylate with a variety of dienophiles. For example, when the Diels-Alder reaction in Scheme 12.3 was carried out in water, a higher reaction rate and reversal of the selectivity were observed, compared with the same reaction in a hydrocarbon solvent (Scheme 12.3).81 Similarly, the reaction of 2,6-dimethylbenzoquinone with sodium ( )-3,5-hexadienoate (generated in situ by the addition of 0.95 equiv sodium bicarbonate to a suspension of the precursor acid in water) proceeded for 1 hour to give a 77% yield of the adduct... 

Surfactant Transport in an Adsorbent Porous Medium. Chromatographic Aspects A first observation was made in all the tests in Table III. The breakthrough of both surfactants from the micellar slug always occurs simultaneously without any chromatographic effect (Figures 5 and 6). This stems both from the chemical nature of the two products selected and also from the fact that the injected concentration is much greater than the CMC of their mixtures. 

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