Apparent critical micelle

At surfactant concentrations above the equivalent point (>5 x 10 4 mol dm 3) the A curves in the presence of the polyion resemble the behavior of the curve in pine surfactant solutions. They show a break at the so-called apparent critical micellization concentration, cmc [16,42], This term is usually used for characterizing the formation of free surfactant micelles in the presence of the polymeric component. The cmc is higher than the ordinary cmc due to the formation of polyelectrolyte-surfactant complex at lower surfactant concentrations. The bound surfactant ions are not available for... 

Surfactant solutions for use in improved oil recovery can be of high (2.0-10.0%) or low (0.1-0.2%) surfactant concentration. In the low concentration systems, the ultralow interfacial tension occurs when the aqueous phase of the surfactant solution is at about the apparent critical micelle concentration (cmc). 

As seen, the typical equilibration time rapidly reaches extremely large values. This inspired Nyrkova and Semenov to define an apparent critical micelle concentration, cmcapp, corresponding an equilibration time of 3,600 s (1 h), i.e., r ,ic(cmcapp) = 3,600 s. An important conclusion from this work is therefore that the measured cmc will always be much larger (in Fig. 6, by about a factor of 80-90) compared to the real cmc equilibrium value. A real cmc will not be measurable on a typical experimental time scale. ... 

Fig. 6 Typical micellar equilibration time, Tnuc, as a function of the unimer concentration normalized by the equilibrium value (cmc). Dotted horizontal line corresponds to a time of 1 h and defines the apparent critical micelle concentration, cmcapp. Reproduced fi om [68]...

Scattered reports are available in the chemical literature concerning the interactions of surfactants and cyclodextrins. For instance, Ise and coworkers [4] have reported on the interaction of colloidal electrolytes and cyclodextrins. They observed that the apparent critical micelle concentrations (cmc) of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTABr) increase upon the addition of ACD and BCD. They concluded that the cyclodextrins form 1 1 complexes with the surfactants. More recently Satake et al. [5] determined the association constants of ACD with several ionic surfactants. For 1-alkanesulfonate... 

Apparent Critical Micelle Concentration (cmc) and Aggregation Number for Polymer-Bound Micelles. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

Very large solvent effects arc also observed for systems where the monomers can aggregate either with themselves or another species. For example, the apparent kp for polymerizable surfactants, such as certain vinyl pyridinium salts and alkyl salts of dimethylaminoalkyl methacrylates, in aqueous solution above the critical micelle concentration (cmc) are dramatically higher than they are below the cmc in water or in non-aqueous media.77 This docs not mean that the value for the kp is higher. The heterogeneity of the medium needs to be considered. In the micellar system, the effective concentration of double bonds in the vicinity of the... 

When the variation of any colligative property of a surfactant in aqueous solution is examined, two types of behavior are apparent. At low concentrations, properties approximate those to be expected from ideal behavior. However, at a concentration value that is characteristic for a given surfactant system (critical micelle concentration, CMC), an abrupt deviation from such behavior is observed. At concentrations above the CMC, molecular aggregates called micelles are formed. By increasing the concentration of the surfactant, depending on the chemical and physical nature of the molecule, structural changes to a more... 

Certain compounds are known to achieve higher absorption rates from the GI tract if they are taken with food, and this observation has been linked to their solubilization by bile salts [74], Bile salts, especially those of cholic and deoxycholic acids, have been used to solubilize steroid hormones [75], antibiotics [76], and nonsteroidal antiinflammatory drugs [77]. For example, amphotericin B (an antifungal agent) has been solubilized for parenteral use in micelles composed of sodium desoxycholate [78], As illustrated in Fig. 11, the degree of solubilization of carbamazepine by sodium desoxycholate is minimal below the critical micelle concentration but increases rapidly above this value [79]. At sufficiently high concentrations, when the micelles become saturated in carb-amezepine, the apparent solubility reaches a limiting value approximately seven times the true aqueous solubility in the absence of desoxycholate. 

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

Variation of the Critical Micelle Concentration and Apparent Charge of the Micelles in the Mixed Solvents... 

An apparently related effect due to membrane curvature is the phenomenon of immunosuppression induced by cationic surfactants. The cationic quaternary ammonium and pyridinium surfactants are widely used as sterilising (antibacterial) agents in an enormous variety of applications. Although the biochemical and genetic mechanism by which bacteria like streptococcus aureus develop immimity to these is not understood, it appears certain that the antiseptic effect is simply related to membrane disruption. At and above the critical micelle concentration ("cmc", discussed in Chapter 3),... 

Figure 6-9. Calculated concentration dependences of unimer and multimer concentrations as well as of the inverse apparent number- and mass-average molar masses for closed, molecule-based associations in the theta state (H.-G. Elias and J. Gerber). Calculations for (Mi) = (M )w = 200g/mol, yv = 21,and A f = 10 Cdm mol) Critical micelle concentrations are determined by extrapolation of dotted lines.

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