Micellization zwitterionic surfactants

MEKC is also performed using cationic, nonionic, and zwitterionic surfactants. Widely employed are cationic surfactant consisting of a long chain tetralkylammonium salt, such as cetyltrimeth-ylammonium bromide, which causes the reversal of the direction of the EOE, due to the adsorption of the organic cation on the capillary wall. Other interesting options include the use of mixed micelles resulting from the simultaneous incorporation into the BGE of ionic and nonionic or ionic and zwitterionic surfactants. Chiral surfactants, either natural as bile salts [207] or synthetic [208] are employed for enantiomer separations. 

Few studies exist for ionic silicone surfactants. Several trisiloxane anionic, cationic and zwitterionic surfactants have been found to form micelles, vesicles and lamellar liquid crystals. As would be expected, salt shifts the aggregates toward smaller curvature structures [40]. 

Zwitterionic Micelle-Forming Surfactants of General Formula... 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

Surfactants or emulsifiers help stabilize the emulsion and are classified into four broad categories anionic, cationic, nonionic, and zwitterionic. Surfactants are dissolved in water at low concentrations, where they form aggregates or micelles. At a concentration greater than their critical micelle concentration (CMC), all excess molecules form micelles. 

Bunton and his co-workers55 have shown that the decarboxylative ring opening of 6-nitroindoxazene-3-carboxylic acid is strongly catalyzed by cationic micelles and by micelles of zwitterionic surfactants such as N,N-dimethyl-JV-dodecylglycine. Later studies by other workers indicate that the decarboxylation is also catalyzed by polysoaps,56 modified polyethylene-imines,57 cycloheptaamylose,9 and by poly(vinylbenzo-18-crown-6).58 The... 

Cloud-point extraction (CPE) and micelle-mediated extraction are methods that utilize aqueous solutions of nonionic or zwitterionic surfactant concentrations above their critical micelle concentrations. The addition of ions or solvents as well as increasing temperature of the solution... 

Similar non-polymeric catalysts were prepared by Tagaki et al., who owed that zwitterionic surfactants 14 possessed much eiflianced nucleophilicity toward PNPA above die critical micelle concentration 101). 

Micellar mobile phases of anionic, cationic, nonionic and zwitterionic surfactants are used in conjunction with different bonded stationary phases (including C8, Cl8 and cyano). Considerably less surfactant (usually <0.2 M) is used compared to the organic modifier content in an analogous traditional separation. A variation in the concentration of surfactant is translated into an increase in the concentration of micelles in the solution, whereas the number of monomers of surfactant remain constant. As a consequence, the characteristics of the stationary phase modified by the adsorption of surfactant are very stable, and usually, a regular retention behavior is observed as a function of the concentration of surfactant. 

Zwitterionic smfactants are not widely used in MEKC. However, zwitterionic surfactants will be interesting if they are used in mixed micelles or as a modifier of the micelle because they should show... 

Liquid phase extraction of the analyte by cloud point extraction (CPE) is a versatile technique in separation chemistry for the purpose of extraction, purification and preconcentration of numerous organic compounds in different samples (Saitoh and Hinze 1991 Shi et al. 2004). The cloud point refers to a certain temperature above which an aqueous homogeneous solution of nonionic and zwitterionic surfactant becomes turbid and separates into two phases (i) a surfactant-rich phase which contains most of the surfactant and (ii) a diluted aqueous phase which contains water and surfactant at a concentration close to the critical micelle concentration (Quina and Hinze 1999 Stalikas 2002). 

Note that rheochaos has also been observed in solutions of mixed anionic-zwitterionic surfactants [220] and in other systems including shear-thickening wormlike micelles [239], lamellar, onion, and sponge surfactant phases [156,240-243] and dense colloidal suspensions [244]. 

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