Equilibrium constant for micellization

The equilibrium constant for micelle formation assuming ideality is given by... 

The rate of attack of water upon the tri-/>-anisylmethyl cation is unaffected by binding of this cation to anionic micelles of sodium dodecyl sulfate (SDS) (Bunton and Huang, 1972) and equilibrium constants for aldehyde hydration are only slightly reduced by binding to micelles (Albrizzio and Cordes, 1979). These observations are also consistent with substrate binding at a wet micellar surface rather than in the interior of the micelle. 

Although anation and aquation rates of vitamin B12 are not affected appreciably by aqueous micelles, the solubilized water in reversed micelles, in contrast, influences the rate and equilibrium constants for the formation and decomposition of glycine, imidazole, and sodium azide adducts of vitamin Bl2 (Fendler et al., 1974). A vitamin B12 molecule is conceivably shielded from the apolar solvent (benzene) by some 300 surfactant molecules. 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

The Menger-Portnoy model is closely related to the Berezin model employing partition coefficients instead of equilibrium constants.For the case where only two pseudophases (bulk water and micelle) are considered, the partitioning of the reactant is given by the partition coefficient P. This leads to Equation (4) describing observed rate constants as a function of surfactant concentration. 

In the mass action approach we use Reaction (B) as a prototype for the process of micellization. The equilibrium constant for this reaction is given by... 

Since k0 is known, Equation (29) allows km to be evaluated. Likewise, the equilibrium constant for the binding of the substrate to the micelle can be evaluated from Equation (31) if n is known from a separate experiment. This method of analysis of catalyzed reactions is called a Lineweaver-Burke plot after the corresponding technique in biochemistry. Example 8.5 illustrates the use of these relationships. 

Equilibrium constants for the binding between substrates and micelles — Reaction (G) — generally range from 103 to 106 for hydrophobic organic substrates. Furthermore, they are expected to increase as the hydrophobic character of the substrate increases. Figure 8.10b shows that this effect sometimes overshoots optimum solubilization. The figure shows, on a... 

Here M is a large number. Writing the equilibrium constant for 1 mol of micelles... 

Keto-enol equilibrium constants for simple /i-dicarbonyl compounds, RCOCH2COX (R = X = Me R = Me, Ph for X = OEt) have been measured in water1423 by a micelle perturbation method previously reported for benzoylacetone142b (R = Ph, X = Me). The results have been combined with kinetic data for nitrosation by NO+, C1NO, BrNO, and SCNNO in all cases, reaction with the enol was found to be rate limiting. 

The most straightforward approach to the micelle formation is through equilibrium constants. For an ionic amphiphile the association can be described through a number of equilibria... 

The equilibrium aspect of micelle formation can be considered by application of the second law of thermodynamics. The equilibrium constant for the process represented by Eq. (3) is given by... 

In quantitative investigations of micellar catalysis and interactions it is desirable to determine the binding (i.e. equilibrium) constant for the formation of the substrate-micelle complex, and, if feasible, to elucidate the nature of the environment of the substrate in the molecular aggregate. 

Suitable absorbance changes allowed the determination of the pseudo-first order rate of attainment of the equilibrium, k (equation 35), both in the presence and the absence of the surfactants. Fro m these values and the values of (the equilibrium constant for equation (36) in the presence of surfactants), y and were obtained. The results indicated that, in the case of 19 micelles exerted a greater effect on the second-order rate constants, y, than on y (Table 13). More specifically, is in creased considerably by CTAB and decreased by NaLS, while y is decreased by both surfactants but to a much greater extent by NaLS (Table 13). For the sulfonphthalein indicators 20, sodium dodecyl sulfate has no significant effect on k whereas CTAB decreases k for the equilibrium attainment. Qualitatively the effects of micelles on the equilibria for... 

An ionic chiral micelle is used as a pseudo-stationary phase it works as a chiral selector. When a pair of enantiomers is injected to the MEKC system, each enantiomer is incorporated into the chiral micelle at a certain extent determined by the micellar solubilization equilibrium. The equilibrium constant for each enantiomer is expected to be different more or less among the enantiomeric pair that is, the degree of solubilization of each enantiomer into the chiral micelle would be different for each. Thus, the difference in the retention factor would be obtained and different migration times would occur. 

The shift in Eyz value can be used to estimate the ratio of equilibrium constants for the binding of the oxidized and reduced forms of ions to DNA molecule. Similarly, for the detection of small molecules and micelles interactions this value was used [81]. 

Table 7.1 Water-Stationary Phase and Water-Micelle Equilibrium Constants for Sodium Dodecyl Sulfate Micellar Chromatographic System [1]...

UF has also been applied to the determination of stability constants for trace metal-HS complexes, equilibrium constants for the distribution of organic compounds between water and synthetic micelles, the binding of metal ions and small compounds to proteins and other macromolecules in biochemistry, etc. This type of application is essential to know precisely to what degree the complexing agent, the complexed species, and the free ions and molecules can pass through the membrane. 

It is important to note that at cjyj, which frequently appears as a (second) breakpoint in some physico-chemical properties vs. surfactant concentration plots, and is regarded as the saturation value of polymer with surfactant, the polymer is not necessarily saturated. In the present example the bound amount of surfactant at c is only 11% of the saturation value. As a result of the micelle formation the surfactant activity increases very little above cjyj, consequently IcompLJ cannot further increase due to the constancy of the mean activity of surfactant. The equilibrium constants for the complex formation can be chosen so that at the beginning of the micelle formation the amount of surfactant in complex is close to that for saturation. In this case the mean activity of surfactant as a function of the total surfactant concentration shows an inflexion below cjvi. ... 

An ionic achiral micelle [e.g., sodium dodecyl sulfate (SDS)] and a neutral CD are typically used as a pseudo-stationary phase and a chiral selector, respectively. When a pair of enantiomers is injected into this system, two major distribution equilibria can be considered for the solutes or enantiomers (a) the equilibrium between the aqueous phase and the micelle (i.e., micellar solubilization) and (b) the equilibrium between the aqueous phase and CD (i.e., inclusion complex formation). Each enantiomer may have a different equilibrium constant for the inclusion complex formation among the enantiomeric pairs due to the enantioselectivity of the CD. As a result, each enantiomer exists in the aqueous phase at a different time among... 

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