Pseudophase characteristics

As mentioned in Section 4, the analysis of rate data resulting from unimolecular reactions is considerably easier than the analysis of such data for bimolecular reactions, and the same is true for pseudounimolecular reactions. Kinetic probes currently used to study the micellar pseudophase showing first-order reaction kinetics are almost exclusively compounds undergoing hydrolysis reactions showing in fact pseudofirst-order kinetics. In these cases, water is the second reactant and it is therefore anticipated that these kinetic probes report at least the reduced water concentration (or better water activity in the micellar pseudophase. As for solvatochromic probes, the sensitivity to different aspects of the micellar pseudophase can be different for different hydrolytic probes and as a result, different probes may report different characteristics. Hence, as for solvatochromic probes, the use of a series of hydrolytic probes may provide additional insight. 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

The description given here is confined to the method developed by Abuin and Lissi demonstrating the use of fluorescence as a method for determining partition coefficients for solutes that are not by themselves fluorescent. The method is based on the observation that an additive changes the characteristics of the fluorescence of a micelle-incorporated probe such as pyrene. It is assumed that the fluorescence intensity of micelle-incorporated pyrene is determined only by the mole fraction of solute in the micellar pseudophase. The probe fluorescence intensity ratio f/I in the absence and presence of a solute is measured as a function of the solute concentration at different surfactant concentrations. From plots of the intensity ratio vs. the solute concentration at different surfactant concentrations we obtain a set of additive concentrations c,o, that corresponds to the same f/I value and thereby the same and K. Ctot is related to the concentration of micellized surfactant, through the following equation ... 

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. 

The use of solvatochromic compounds as polarity indicators may be extended to solvent mixtures and micellar systems. In this case, an additional difficulty is introduced in the assessment of systems that are not homogeneous from a microscopic point of view The microenvironment actaally seen by the sensor does not correspond to the bulk characteristics of the medium. In a binary solvent mixture, a solvatochromic probe may be more solvated by one of the components, thus reflecting through its spectrum a solvent composition that may be different from that of the bulk mixture. In micellar systems, the solvatochromic response of a probe reflects the nature of its microenvironment and is dependent on the relative solubility of the sensor in the aqueous or the organic pseudophases, or in the micellar interphase. 

The term water refers to a polar phase that is generally an aqueous solution containing electrolytes and other additives, provided that they behave reasonably as a pseudophase. In extreme cases it can be quite different from water, e.g., ethylene glycol or formamide. In the practical cases that are discussed here, the polar phase would be plain water or sodium chloride brine, the concentration of which would be a characteristic of the polarity of water. 

It seems reasonable to suggest that if the values of 0/G for respective ion-exchange X7Z- and Y7Z- are 0.5 and 0.8, then 50% and 80% of micellized Z ions, expelled by respective X and Y- ions from micellar pseudophase to the aqueous pseudophase, reside in the micellar environment of medium characteristic (such as polarity, hydrophobicity, and water concentration) similar to the micellar environment of entire micellized X " ions and Y ions at their respective limiting concentrations. This statement is graphically presented by Figure 3.3. 

The empirical definition of Kx/s shows that the magnitude of Kx/s should be directly proportional to the ionic micellar binding constant (Kx) of ion X (the counterion X) and inversely proportional to the ionic micellar binding constant (Kg) of ion S (another connterion). Thus, Kx/s = s Kx/K, where 5s represents the proportionality constant. The magnitude of 5s is assumed to depend only on the molecnlar characteristics of ion S (the ion that is expelled by another co-ion X from micellar psendophase to the aqueous pseudophase), and it is independent of the molecnlar characteristics of ion X (the ion that expels the co-ion S from... 

Pseudo-first-order rate constants, for methanolysis of PS show a decrease of three- to fivefold with the increase in CH3CN content from 2 to 60 or 70% v/v in mixed aqneons solvents containing 0.01 M LiOH and a constant content of CH3OH. However, at 0.01 M KOH, the rate constants k bs show a decrease of 15 to 20% and an increase of 7 to 130% with the increase in CH3CN content from 2 to 30% v/v and from 30 to 60 or 70% v/v, respectively. Snch characteristic difference of Li+ and K+ ions on k bs has disappeared for the rate of methanolysis of PS in the CTABr micellar pseudophase containing 2% v/v CH3CN. This shows that neither LP nor K+ ions entered the microenvironment of low polarity in which the micellar-mediated reactions occurred. The counterions for HOm and PS " in this micellar environment are perhaps cationic micellar head groups. 

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