Critical micelle concentration organic additives

MeutralSoluble Salts. So dium sulfate [7757-82-6] and, to a considerably lesser extent, sodium chloride [7647-14-5] are the principal neutral soluble salts used in laundering compositions. They are often considered to be fillers although they perform an important standardizing function enabling the formulator to manufacture powders of a desired, controlled density. Sodium sulfate, in addition, lowers the critical micelle concentration of organic surfactants and thus the concentration at which effective washing can be achieved. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

The surface tension of a solution of a surfactant is lower than that of the pure solvent. Surface tension is roughly a linear function of ln(surfactant concentration) up to the critical micelle concentration (CMC) (Figure 3). Above the CMC the thermodynamic activity of the surfactant does not increase with the addition of more surfactant, and the surface tension remains constant. Interfacial tension also decreases with the concentration of an emulsifier dissolved in one of the phases. In Figure 4 the decrease in y does not level off, because the emulsifier (PGMS) does not form micelles in the organic solvent phase (heptane). The changes in the slope of the plot are attributed to changes in orientation of emulsifier molecules at the interface (7). 

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