Micellar solvents

In some cases even the use of micellar solvent phases can be avoided and the reactions can be carried out in an entirely aqueous medium. For water-soluble reagents, catalytic reactions such as hydrogenations and hydroformylations may be carried out homogeneously in the aqueous phase with water-soluble ligands such as triphenylphosphinotrisulfonate (Sheldon et al., 1998). 

Singlet acenaphthylene (64) stereospecifically gives the syn dimer 65a, while in the T, reaction the anti dimer 65b predominates. The syn-anti ratio can be influenced by solvents with heavy-atom effect (Cowan and Drisco, 1970b), as well as by micellar solvents (Ramesh and Ramamurthy, 1984). 

The rates or efficiencies of bimolecular photoreactions such as excimer formation, photodimerization, and photoaddition can be strongly affected by using micellar solvents when the reactants associate with the micelles When low concentrations of reactants are solubilized in solutions of high concentrations of micelles, the reactants will be separated by association to different micelles and the reaction will be inhibited. Under opposite conditions high local concentrations will cause an increase of quantum yields compared to homogeneous solutions of equal analytical concentration. Thus in micellar solution the efficiency of bimolecular... 

In conclusion, the best amount of alcohol to add depends on the surfactant concentration used. It means again that efficiency enhancements cannot be dissociated from micellar solvent strength. 

The micellar solvent system used in this assay has several advantages, in addition to rapid separation SDS is an effective-denaturing agent and, as such, can be used to stop reactions without the need for protein precipitation with trichloroacetic acid or heating. Since SDS also has a unique solubilizing power, it can be used for direct injection of concentrated protein solutions into the RPLC system, without time-consuming steps to remove protein precipitates or extraction of folate analogues. Therefore, the assay is simple, rapid, inexpensive, and applicable to crude hydrolase preparations. 

These disadvantages were overcome by providing for a more selective elution of the extraction column [48]. This was achieved in the following stepwise manner (i) the physiological sample was injected under micellar conditions (the micellar solvent and column were selected to ensure... 

This protocol resulted in the following ordering of mobile phases flowing through the extraction column (i) extraction solution (the micellar solvent tlmt washes endogenous material off the colunm while permitting analyte retention), (ii) recovery solution (the solvent that elutes the analyte off the extraction column onto the analytical column) and (iii) wash solution (a strong solvent employed to clean-up the extraction column). The recovery... 

See also Capillary Electrochromatography. Liquid Chromatography Micellar. Solvents. 

De Maria, P., Fontana, A., Gasbarri, C., Siani, G., Zanirato, P. Kinetics of the Z-E isomerization of monosubstituted azobenzenes in polar organic and aqueous micellar solvents. Arkivoc 2009(8), 16-29 (2009)... 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

Mass-action model of surfactant micelle formation was used for development of the conceptual retention model in micellar liquid chromatography. The retention model is based upon the analysis of changing of the sorbat microenvironment in going from mobile phase (micellar surfactant solution, containing organic solvent-modifier) to stationary phase (the surfactant covered surface of the alkyl bonded silica gel) according to equation ... 

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