Micellar solutions kinetic effects

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

Little is known about the structures of these kinetically effective complexes, or even about the aggregates of the amphiphile. Both hydrophobic and coulombic interactions are important because these aggregates are much less effective than micelles at assisting reactions of hydrophilic nucleophilic anions. These observations are consistent with the view that the aggregates are much smaller than micelles. It is probable that the structures and aggregation numbers of these aggregates depend on the nature of the solutes which bind to them and Piszkiewicz (1977) has suggested that such interactions play a role in micellar kinetics. 

Three new macrocyclic ligands (187) when complexed with zinc(II) could promote ester hydrolysis and a kinetic study of the hydrolysis of 4-nitrophenyl acetate in Tris buffer at pH 8.63 in 10% (v/v) MeCN was earned out with these.153 The hydrolysis of lipophilic esters is also catalysed by zinc(H) in a complex of a long alkyl-pendant macrocyclic tetraamine (188) in micellar solution.154 A study with a copper chloride-containing micelle has compared its effectiveness in the hydrolysis of esters and amides.155... 

A kinetic study of the photosensitized oxidation of tryptophan-alkyl esters in Triton X-100 micellar solutions has been carried out by Criado et al. [24], The results obtained are presented in Table 6. These data show an important decrease in the relevance of the photo-oxidative pathway in the esterified compounds in the presence of the micelles. The magnitude of the effect seems to be extremely sensitive to the location of the probe, increasing as the length of the ester hydrocarbon chain increases. These results are interpreted in terms of the competition between the local oxygen concentration and the solvent micropolarity effect that... 

Another new approach combines MAE with the use of an aqueous surfactant solution as the extracting phase. This new technique is called microwave-assisted micellar extraction (MAME). This procedure is based on the well-known solubilization capacity of aqueous micellar solutions toward water-insoluble or sparingly soluble organic compounds. As a general rule, nonionic surfactants are usually the most effective, showing greater solubilization capacities that rapidly increase with the solubilization kinetics as the cloud-point temperature of the solution is raised. 

We may classify studies of the reaction kinetics in micellar solutions in the same manner as analogous studies in polyelectrolyte solutions, i.e., (a) reactions involving the micelles as reagents, (b) kinetic effects... 

This effect was observed by Oehme and co-workers for a variety of ionic and non-ionic surfactants. The surfactants were always added at a concentration of about twice the cmc but in substoichiometric ratio to the substrate. This significant effect of the amphiphiles above the cmc manifests the simultaneous solubilising effect of the micelles for the catalyst and the substrate. In kinetic studies of hydrogenation of MAC in micellar solutions of the anionic surfactant SDS and the non-ionic surfactant octa(ethylene glycol)monotridecyl ether (Ci3E8) micellar solutions the dissociation constant of the catalyst substrate complex was found to be considerably smaller than in methanol as solvent [56]. This indicates... 

Micellar media are formed from tensioactive molecules in aqueous solution. Mi-cellization is a manifestation of the strong self-association of water and water-like solvents [95]. Micelles are known to increase the solubilization of weakly polar substances in water and, as a consequence, their presence determines the magnitude of hydrophobic interactions. Micelles aggregate spontaneously in aqueous solution beyond a critical concentration which is a function of pressure [96]. As a result, pressure may induce an extra kinetic effect on the rate of organic reactions carried out in aqueous micellar systems. Representative ionic micelles are sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). Recent examples demonstrate the beneficial effect of the presence of surfactants in Lewis acid-catalyzed reactions, a kind of biactivation [97]. 

In the discussion of the adsorption kinetics of micellar solutions, different micelle kinetics mechanisms are taken into account, such as formation/dissolution or stepwise aggregation/disaggregation (Dushkin Ivanov 1991). It is clear that the presence of micelles in the solution influences the adsorption rate remarkably. Under certain conditions, the aggregation number, micelle concentration, and the rate constant of micelle kinetics become the rate controlling parameters of the whole adsorption process. Models, which consider solubilisation effects in surfactant systems, do not yet exist. 

Most of the traditional adsorption studies of surfactants correspond to dilute systems without aggregation in the bulk phase. At the same time micellar solutions are much more important from a practical point of view. To estimate the equilibrium adsorption, neglecting the effect of micelles can usually lead to reasonable results. However, the situation changes for nonequilibrium systems when the adsorption rate can increase by orders of magnitude when the of surfactant concentration is increased beyond the CMC. Current interest in the adsorption from micellar solutions is mainly caused by recent observations that the stability of foams and emulsions depends strongly on the concentration in the micellar region [1]. This effect can be explained by the influence of the micellisation rate on the adsorption kinetics. 

Numerous data on dynamic surface tension [77-93] and dynamic surface elasticity [94-103] of aqueous micellar solutions have been published until now. These data evidence the influence of micelles on the adsorption kinetics, although they are present only in the bulk phase. This effect can surprise on a first glance because it is well-known that the surface activity of micelles is negligible and hence their adsorption is almost zero. However, the influence of micelles can be easily explained if one takes into account that the adsorption kinetics of surfactants at fluid -fluid interface is determined by the diffusional exchange between the subsurface and the bulk phase [104, 105]. It is exactly the diffusion of monomers that changes in the presence of micelles. This point of view is widely accepted and difficulties arise only if one tries to obtain quantitative estimates of the observed effects. 

One of the reasons of the insufficient reliability of micellisation kinetics data determined from dynamic surface tensions, consists in the insufficient precision of the calculation methods for the adsorption kinetics from micellar solutions. It has been already noted that the assumption of a small deviation from equilibrium used at the derivation of Eq. (5.248) is not fulfilled by experiments. The assumptions of aggregation equilibrium or equal diffusion rates of micelles and monomers allow to obtain only rough estimates of the dynamic surface tension. An additional cause of these difficulties consists in the lack of reliable methods for surface tension measurements at small surface ages. The recent hydrodynamic analysis of the theoretical foundations of the oscillating jet and maximum bubble pressure methods has shown that using these techniques for measurements in the millisecond time scale requires to account for numerous hydrodynamic effects [105, 158, 159]. These effects are usually not taken into account by experimentalists, in particular in studies of micellar solutions. A detailed analysis of... 

The transfer from CTAB and CPB (cmc 0.00085 M) to dodecylpyridinium bromide (DDPB cmc 0.018 M) results in a marked increase in the Fm value for substrate 1, followed by changes from the extremum type of kinetic dependences to S-shaped curves." While in the case of the more hydrophobic substrate 3 in the DDPB micellar solution, the maximum type kinetic curve and the negative effect of the micellar microenvironment (Fm < 1) are preserved (Table 15.1). 

The investigation of the kinetics of the solvolysis of p-nitrophenyl bis(chloromethyl) phosphinate 12 in the direct micelles of sodium dodecylsulfate (SDS) in ethylene glycol have demonstrated that the ratio of the factors Fc and Fn, in this system differ from that in aqueous micelles, namely, the positive predominant contribution of the factor of microenvironment to the micellar rate effect is observed (Table 15.1). In the solution of ethylene glycol the micellization is much less effective as compared to the aqueous solution (cmc of SDS in ethylene glycol is equal to 0.18M, while in water it is 0.0085 M) and the micelles formed have low aggregation numbers, a loose structure and a weak solubilization capacity (the binding constants of substrates are lower by two orders of magnitude as compared to those in aqueous micellar solutions). ... 

Micellar and microemulsion effects on reactivity in aquation and base hydrolysis reactions of iron(II)-diimine complexes have been much studied/ The latest contribution deals with the effects of added potassium chloride or bromide to micelles of the respective cetyltrimethylammonium halides. Effects on base hydrolysis of [Fe(phen)3] and its 4,7-diphenyl and 3,4,7,8-tetramethyl derivatives can be interpreted in terms of competitive binding to the micelles in a pseudophase-ion exchange model. In connection with these secondary effects of added halides it should be mentioned that further studies of kinetics of aquation of [Fe(bipy)3] and of [Fe(phen)3] in strong aqueous solutions of chlorides have been interpreted in terms of water and of chloride attack, with the postulation of transient diimine-chloride-iron(II) intermediates. ... 

The main effect of alcohol addition to a micellar phase is the increase of elution strength as reviewed in Chapters 7 and 8 and modeled by the MICHROM software (see Appendix I and attached CD-ROM). The decrease of the P values observed for the apolar compounds (Table 6.4) can be generalized to any solute. These effects are related to thermodynamics. The kinetics of the chromatographic process is also affected by the addition of alcohols to the micellar phases. 

Both coulombic and hydrophobic interactions of reactants with adsorbed surfactant on electrodes are important in determining electron transfer kinetics. Reactants in micellar solutions and microemulsions can be preconcentrated into adsorbed surfactant films on electrodes [30], yielding mixed layers of reactants and nonelectroactive surfactants. Coulombic effects in micellar solutions may result in small kinetic enhancements when ionic reactants interact with oppositely charged surfactant adsorbed on electrodes. Partial inhibition of electron transfer can occur by coulombic repulsion if the charge sign on the reactant and adsorbed surfactant are the same. Hydrophobic molecules and ions may show a small amount of preconcentration on the electrode. 

The pseudophase kinetic models for speeded or inhibited bimolecular, second-order, reactions are more complex. Here the focus is on reaction between a neutral organic substrate and a reactive counterion in micellar solutions in the absence of oil (d>o = 0, Scheme 4). Micellar effects on reactions of substrates with reactive counterions are important because they illustrate the general differences of micellar effects on spontaneous and bimolecular reactions and also how specific counterion effects influence the results. Pseudophase models also work for bimolecular reactions between two uncharged organic substrates and third-order reactions, reactions in vesicles and microemulsions, which may include partitioning into and reaction in the oil region, reactions of substrates with an ionizable (e.g., deprotonatable) second reactant, and the effect of association colloids on indicator equilibria. ... 

Until recently, a sound theoretical basis for the mechanistic interpretation of these kinetic measurements has been lacking. It is now recognised that several distinct factors can influence reactivity in micellar solutions. These include (i) partitioning coefficients for the reactants between the micelle and aqueous solution, (ii) kinetic effects due to a change in the environment in which the reaction takes place (either through a medium effect or direct involvement of the surfactant head-groups in the reaction) and (iii) the influence of the micelle surface potential on the etc. of the... 

Section 1 deals with the formal kinetics of the photochemical reactions in micellar solutions and its application to determine the rate constants of photoprocesses, the critical concentration of micellization (CMC) and the aggregation number of micelles from experimental data. In Section 2 the correlations of the rate constants of the photochemical charge separation and mass transfer processes with its thermodynamics, and also the microviscosity and effective polarity of organized assemblies are considered. 

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