Critical micelle concentration elevation

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

Temperature has a large effect on the mass transfer between the micelle and the stationary phase and can therefore be used to improve the efficiency of the separation. Micellar chromatography should be carried out at elevated temperature, typically around 40°C. At elevated temperatures, the effects of flow rate and surfactant concentration on the efficiency of the separation are minimized. For optimum efficiency, however, the flow rate should be minimized while still maintaining a reasonable elution time.33 Likewise, a surfactant concentration close to but above the critical micelle concentration should be used.33... 

Retention in Porous Media. Anionic surfactants can be lost in porous media in a number of ways adsorption at the solid—liquid interface, adsorption at the gas—liquid interface, precipitation or phase-separation due to incompatibility of the surfactant and the reservoir brine (especially divalent ions), partitioning or solubilization of the surfactant into the oil phase, and emulsification of the aqueous phase (containing surfactant) into the oil. The adsorption of surfactant on reservoir rock has a major effect on foam propagation and is described in detail in Chapter 7 by Mannhardt and Novosad. Fortunately, adsorption in porous media tends to be, in general, less important at elevated temperatures 10, 11). The presence of ionic materials, however, lowers the solubility of the surfactant in the aqueous phase and tends to increase adsorption. The ability of cosurfactants to reduce the adsorption on reservoir materials by lowering the critical micelle concentration (CMC), and thus the monomer concentration, has been demonstrated (72,13). 

The nature and limits of applicability of specific methods for determining critical micelle concentrations vary widely. Most methods have been developed for a relatively small set of pure surfactants involving very dilute electrolyte solutions and only ambient temperature and pressure. The determination of cmc at elevated temperature and pressure is experimentally much more difficult than for ambient conditions and comparatively little work has been done in this area. Most high temperature cmc studies have been by conductivity measurements and have therefore been limited to ionic surfactants. For example, erne s at up to 166 °C have been reported by Evans and Wightman [50]. Some work has been reported using calorimetry, up to 200 °C by Noll [5J ], and using F... 

On the basis of surface and bulk interaction with water. Small [85] classified bile acids as insoluble amphiphiles and bile salts as soluble amphiphiles. On account of the undissociated carboxylic acid group, the aqueous solubility of bile acids is limited [35] in contrast, many bile salts have high aqueous solubilities as monomers [33] and, in addition, their aqueous solubilities are greatly enhanced by the formation of micelles [5,6]. Because many bile salts are weak electrolytes, their ionization and solubility properties are more complicated than those of simple inorganic or organic electrolytes [5,35]. For example, the p/Tj, values of bile acids in water vary markedly as functions of bile salt concentration and, because micelles formed by the A (anionic) species can solubilize the HA (acid) species [5,35], the equilibrium precipitation pH values of bile acids also vary as functions of bile salt concentration. Finally, certain bile salts are characterized by insolubility at ambient temperatures [2,5,6,86,87], only becoming soluble as micelles at elevated temperatures (the critical micellar temperature) [6]. 

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