Kinetic in micellar systems

The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)]3-+CN-. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation (135). 

The adsorption kinetics in micellar systems was studied first by Lucassen (1975). He defined a micelle relaxation time... 

Consequently, many features of kinetics in micellar systems are related to reactions in monolayers and polyelectrolytes surfaces. 

Romsted LS (1977) A general kinetic theory of rate enhancements for reactions between organic substrates and hydrophUic ions in micellar systems. In Mittal KL (ed) Micellization, Solubilization, Microemulsions. Plenum Press, New York... 

HRP-catalyzed steady-state oxidation of ferrocenes by H2O2 is fun to study by UV-vis spectroscopy because ferricenium ions generated are the only absorbing species at 500-700 nm (Fig. 3). A problem, actually solved by using micellar solutions, is the limited solubility of ferrocenes in water. The kinetics of oxidation of n-alkylferrocenes (alkyl = H, Me, Et, Bu and CsHn) (119) and later of larger variety of ferrocenes shown in Chart 1 (120) via Eq. (37) has been studied in detail in micellar systems of Triton X-100, , and SDS, mostly at pH 6.0 and 25 °C. Ferrocenes with longer alkyl radicals are oxidized immeasurably slow. 

The equilibrium and dynamics of adsorption processes from micellar surfactant solutions are considered in Chapter 5. Different approaches (quasichemical and pseudophase) used to describe the micelle formation in equilibrium conditions are analysed. From this analysis relations are derived for the description of the micelle characteristics and equilibrium surface and interfacial tension of micellar solutions. Large attention is paid to the complicated problem, the micellation in surfactant mixtures. It is shown that in the transcritical concentration region the behaviour of surface tension can be quite diverse. The adsorption process in micellar systems is accompanied by the dissolution or formation of micelles. Therefore the kinetics of micelle formation and dissociation is analysed in detail. The considered models assume a fast process of monomer exchange and a slow variation of the micelle size. Examples of experimental dynamic surface tension and interface elasticity studies of micellar solutions are presented. It is shown that from these results one can conclude about the kinetics of dissociation of micelles. The problems and goals of capillary wave spectroscopy of micellar solutions are extensively discussed. This method is very efficient in the analysis of micellar systems, because the characteristic micellisation frequency is quite close to the frequency of capillary waves. 

Fig. 1 Illustration of two kinetic processes in micellar systems, (a) Micelle formation, i.e., the kinetics associated with aggregation of single amphiphiles (unimers) into micelles and (i>) the equilibrium kinetics charactiaizing a dynamic equilibrium of unrmcas exchanging between micelles...

Fig. 2 Illustration of two important mechanisms involved in various kinetic processes in micellar systems, (a) Unimer exchange, single surfactant/block copolymer chains are interchanged one by one via the solvent medium, (b) Fusion/fission, where two micelles fuse or are fragmented to...

R(t) is the relevant function to be analyzed for extraction of the kinetics. However, in micellar systems where the micelles are not fully proteated/deuterated or there is residual contrast between core and shell, ncaiUnear interference scattering contributions are present. In order to take this into account, a more accurate description of the time-dependent scattering intensity is necessary. A scattering model, where the time-dependent hyrogen/deuterium composition of the core and shell of the micelles is built into a kinetic core-shell model, is described next. 

Despite its rather short history, TR-SAS techniques have helped to resolve many aspects of kinetic processes in micellar systems, in particularly the equilibrimn kinetics. However, many challenges remain for the future. For block copolymer micelles, these include studies of morphological transitions, drug encapsulation and... 

Kabalnov, A. and Weers, J., Kinetics of mass transfer in micellar systems surfactant adsorption, solubilization kinetics, and ripening, Langmuir, 12, 3442, 1996. Kanniah, N., Gnanam, F.D., and Ramasamy, F, Revert and direct Liesegang phenomenon of silver iodide Factors influencing the transition point, J. Colloid Interface Set, 94, 412, 1983. 

Nitrogen.—An indirect method of determining the electrophilic reactivity of nitrosyl chloride towards various substrates in aqueous solution has been proposed. The action of nitrosyl chloride, bromide, and thiocyanate or nitrosating agents in micellar systems has been studied, and the results were interpreted in terms of the competition of anions present in solution for sites at the surface of a cationic micelle. The kinetics of the oxidation of nitrite to nitrate by chlorine and bromine are complex and have been interpreted in terms of mechanisms involving the nitryl halides as intermediates ... 

This Report starts by considering various aspects of the effects of solvent properties and of solvation in inorganic kinetics. This is followed by a brief consideration of related medium effects in salt solutions and in micellar systems. The final section of the Chapter is devoted to bringing together references to medium effects on some of the more important types of inorganic substitution reactions. To some extent references relevant to this Chapter have been divided between those which concentrate on the solvent (first three Sections) and those which concentrate on the reaction (final Section), but a citation in the former sections does not preclude an additional mention in the final Section. 

Extrema in the enthalpy and entropy of activation for the substitution of 2,2 -bipyridyl into Ni q in a series of water/acetonitrile mixtures correlate welf with the extrema in the physical properties of the mixture which are related to sharp changes in the solvent structure. Further reports have appeared on rate enhancements for the reaction of Ni " with pyridine-2-azo-p-dimethylaniline (pada) in micellar systems,and the kinetics of complex formation with lasalocid (X-537A) in methanol " and with poly(4-vinylpyridine) in aqueous methanol " have been reported. 

In rheological experiments, that is, when the surface layer is periodically compressed and expanded around the equilibrium state, we also meet the situation that molecules desorb from the interface, increasing the local concentration of monomers such that micelles have to either take up molecules or form new micelles. In this case, a diffusion flux of monomers and micelles from and to the interface exists, depending on the respective situation at the interface [16, 25]. The peculiarities of the micellar kinetics in various systems are discussed in several papers however, the general principles hold [30-34]. 

Minero, C., Pramauro, E., Pehzzetti, E. Generahzed two-pseudophase model for ionic reaction rates and equihbria in micellar systems hexachloroiridate(IV)-iron(II) electron-transfer kinetics in cationic micelles. J. Phys. Chem. 1988, 92(16), 4670-4676. 

The situation is different for reactions of very hydrophilic ions, e.g. hydroxide and fluoride, because here overall rate constants increase with increasing concentration of the reactive anion even though the substrate is fully micellar bound (Bunton et al., 1979, 1980b, 1981a). The behavior is similar for equilibria involving OH" (Cipiciani et al., 1983a, 1985 Gan, 1985). In these systems the micellar surface does not appear to be saturated with counterions. The kinetic data can be treated on the assumption that the distribution between water and micelles of reactive anion, e.g. Y, follows a mass-action equation (9) (Bunton et al., 1981a). 

In the time-resolved quenching method, the decay kinetics of the monomer and the excimer emission are monitored in the presence of a micelHzed medium. If the micellar system is viewed as a group of individual micelles with probe occupancies 0, 1, 2, 3, etc., the probabihty of micelles with n probes, Pn, may be related to n, the average number of probes per micelle by Poissonian statistics through relation. 

Although RMs are thermodynamically stable, they are highly dynamic. The RMs constantly colhde with each other and occasionally a colhsion results in the fusion of two RMs temporarily. During this fusion surfactant molecules and the contents residing inside RMs may be exchanged. In AOT reverse micellar system, this dynamic behavior exhibits second-order kinetics with rate constants in the order of 10 to 10 M s [37]. This dynamic nature not only influences the properties of the bulk system but also affects the enzymatic reaction rates [38]. 

---