Micellar rate constant local

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

To what extent the reaction occurring in each of the two separate pseudophases contributes to the overall observed reaction depends on the local reaction rate constants and local concentrations. Typically, plots representing the rate of reaction or the (related) observed second-order rate constant obs,2 as a function of surfactant concentration go through a maximum. This maximum is related to the lowest surfactant concentration where both reactants are fully bound in the micellar pseudophase, that is, local concentrations in the micellar pseudophase are highest. 

With the exception of a few examples, bimolecular reactions in micelles are largely controlled by the local concentration (and pH) realized at the micellar pseudophase. The data reported in Table 2 [28, 31] give a comparison of the second-order rate constants measured for a series of functional derivatives in aqueous and micellar pseudophases. The ratios of the two rate constants (taking into account concentration and deprotonation effects in micelles) are all close to unity, confirming the above assertion. Finally, although the quantification of rate accelerations has been done mainly with micellar aggregates, the reactivity in vesicles appears to follow basically the same rules with minor differences due to the different lipophilicity and/or order of the membrane [37]. 

The situation is more complicated for nonspontaneous bimolecular reactions involving a second reactant, whose distribution between the two pseudophases has to be considered. The simplest situation is that for reaction of a hydrophobic species whose solubility in water is sufficiently low that it is incorporated essentially quantitatively in the association colloid. For example, for reactions of nucleophilic amines in aqueous micelles, second-order rate constants in the micellar pseudophase calculated in terms of local concentrations are lower than in water [103,104], because these reactions are inhibited by a decrease in medium polarity and micelle/water interfaces are less polar than bulk water [59,60,99101]. Nonetheless, these bimolecular reactions are generally faster in micellar solutions than in water because the nucleophile is concentrated within the small volume of the micelles. Similar results were obtained for the reaction of 2,4-dinitrochlorobenzene (5) with the cosurfactant -hexylamine in O/W microemulsions with CTABr and w-octane [99], again consistent with the postulated similarities in the interfacial regions of aqueous micelles and O/W microemulsions. 

The first theoretical model of surfactant adsorption from micellar solutions, proposed by Lucassen [142], uses the simplifying assumptions that the micelles are monodisperse and that the micellization happens as a single step, which is described as a reversible reaction of order n (the micelle aggregation number). Later, more realistic models, which account for the multi-step character of the micellar process, were developed [143-145]. The assumption for a complete local dynamic equilibrium between monomers and micelles makes possible to use the equilibrium mass action law for the micellization reaction [142,146,147]. In such a case, the surfactant transfer corresponds to a conventional diffusion-limited adsorption characterized by an effective diffusion coefficient, Deff, which depends on the micelle diffusivity, concentration, and aggregation number. Dgff is independent of the rate constants of the fast and slow demicellization processes and k. Joos et al. [146,147] confirmed experimentally that in some cases the adsorption from micellar solutions could be actually described as a diffusion-limited process characterized by an apparent diffusivity,... 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

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