Micellar exit rates, solubilization

Micellar Exit Rates. In micellar solubilization, the dominant factors governing the exit and reentry rates of solubilizates are largely unknown. The exit rates for naphthalene, biphenyl, and 1-methylnaphthalene from ionic micelles are >5 X 104 s exit rates for anthracene and pyrene are reported as... 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

Figure 2 presents a schematic of one scenario in which nonionic surfactant may assist biomineralization. In this situation micellar nonionic surfactant has solubilized HOC from soil. As microorganisms deplete aqueous-phase HOC via mineralization, the micelle releases HOC to solution. HOC exit rates from micelles may be significantly faster than HOC desorption rates from soil, and this condition thereby potentially enhances the availability of HOC to the microorganism. Other researchers (25, 28) suggested that surfactants may make HOCs more available for microbial attack in soil by decreasing the interfacial tension between the compound and water. 

With the development of new dosage form technology in which control of drug release is achieved, it is conceivable that micellar systems will find some place because of the ability of the micellar phase to alter the transport properties of solubilized drug molecules. One can envisage the deliberate addition of surfactants to drug reservoirs to control the exit rate of drugs from polymeric devices. This will be explained in Chapter 7. 

For protolytic photodissociation of hydroxyaromatic compounds we may expect that in anionic micelles the excited anions of hydroxyaromatic compounds having the same sign of charge will leave the micelles, and in the cationic micelles - hydrogen ions will leave micelles. Exit rate constants of excited anions of hydroxyaromatic compound from the micelles were determined using the non-solubilized fluorescence quenchers. Simultaneously, we have proved that it is in the micellar phase that protolytic dissociation does proceed and not as a result of the preliminary exit of excited molecules of hydroxyaromatic compounds from the micellar phase to an aqueous one. 

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

While various techniques, such as stopped flow, have been used to follow substrate kinetics, many kinetic measurements have involved the photophysical properties of solubilized probes. Because of the luminescent properties of their excited states, the aromatic hydrocarbons provide opportunities for monitoring movement of such probes across the micelle boundary. For example, long-lived phosphorescence of aromatic hydrocarbons has been monitored in micellar solutions containing ionic quenchers that themselves are repelled by the surfactant head groups. Since quenching must take place in the aqueous phase, phosphorescence lifetimes may be interpreted to provide rate constants for exit of the probe from the micelle. Some typical values obtained by this technique are given in Table III. Fluorescence data have also been used to obtain such information. 

The relation between K and q depends on the different standard conditions for the two models in the microscopic model the standard concentrations of micelles, the solubilized A molecules and unsolubilized A molecules in overall volume of the solution are equal to Imole dm" in the pseudophase model the standard concentration of A molecules in each phase is equal to 1 mole dm The mono-molecular rate constants for exit from the micellar phase are independent of the model, but the rate constants for entrance into the micellar phase depend on the units in which the surfactant concentration is expressed as ... 

Solutes bound to micellar and microemulsion aggregates are in rapid equilibrium with free solute and aggregates. Rate constants for the exit of neutral solutes from ionic micelles are estimated at 10 -10 s ". Rates for recapture are near or at diffusion control. Rates of these processes are similar to equilibration rates between micelles and surfactant momomers. Solutes in micelles and microemulsions are most probably solubilized near the Stern layer or in the interfacial region. Hydrophilic or hydrophobic environments will be preferred depending on the properties of the particular solutie. There is little evidence for deep solute penetration into the core of the aggregate. Vesicles have much slower dynamics of solute equilibria. 

The equilibrium constant K for the formation of micelles as expressed by Equation 3.3 is independent of equilibrium constants Kg for micellar solubilization of different solubilizates, and rate constants k, for micellar-mediated reactions. In other words, the rates of formation and disintegration of micelle are independent of the corresponding rates of micellar intake and exit of a solubilizate kf k and k k where kf and k represent rate constants for micelle formation and micelle disintegration, respectively, and therefore kj /k = K (Scheme 3.1 and Scheme 3.2). In Equation 3.3,... 

The dynamics of micelle formation and disintegration as well as micellar solubilization and exit of an especially neutral solubilizate of moderate polar-ity/hydrophobicity indicate that assumptions 3 to 5 may easily break down for fast reactions and, under such conditions, the rate of a micellar-mediated reaction cannot be expected to follow a simple first- or second-order rate law. The rate law of such a reaction will constimte a transcendental kinetic equation because of the consecutive nature of the reaction. However, such an expected kinetic complexity has not been reported in any study on the effects of micelles on reaction rates. 

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