Zwitterionic surfactants critical micelle concentration

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

Shiloach, A., and D. Blankschtein. 1997. Prediction of critical micelle concentrations and synergism of binary surfactant mixtures containing zwitterionic surfactahtengmuirl3 3968-3981. 

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. 

The concentration at which this phenomenon occurs is called the critical micelle concentration (CMC). Similar breaks in almost every measurable physical property that depends on size or number of particles in solution, including micellar solubilization of solvent-insoluble material (Chapter 4) and reduction of surface or interfacial tension (Chapter 5), are shown by all types of surfactants—nonionic, anionic, cationic, and zwitterionic in aquecus media. 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

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). 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

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). 

As mentioned above, the most important adjuvants are surface active agents of the anionic, nonionic or zwitterionic type. In some cases polymers are added as stickers or antidrift agents. The production of spray droplets (from a spray nozzle) is determined by the adsorption of surfactants under dynamic conditions (with time in the region of 1 ms). The droplet adhesion to the target surface and its wetting and spreading is also determined by the dynamic contact angle which is also determined by the rate of surfactant adsorption to the surfeice. Above the critical micelle concentration (cmc), the supply of monomers is determined by the relaxation time of micelle formation and its breakdown. The dynamics of surfactant adsorption is determined by the monomer concentration and the diffusion coefftcient of the surfactant molecules to the interface. 

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