Intramicellar processes

In the emulsion-polymerization of methyl methacrylate by carbonyl initiators, intramicellar processes have been found to reduce the conversion rate. On the other hand, the application of weak magnetic fields has been found to increase the conversion of emulsion-polymerized vinyl monomers using triplet ketone sensitizers. Here external magnetic fields will decrease the efficiency of triplet-to-singlet radical-pair intersystem crossing within the micelles and thus increase the function of radical pairs that escape without terminating the polymer chains. 

In the scheme proposed by Frey-Wyssling intramicellar processes are those occurring inside the solid framework structure and intermicellar ones those occurrii in its interstices. It would seem, however, that there are good arguments to maintain these concepts in the sense originally proposed by Katz, since they are then subject to an unequivocal criterion which may be read from the X-ray digram. 

An automated vapor pressure method has been used to obtain highly precise values of the partial pressure of benzene as a function of concentration in aqueous solutions of sodium dodecylsulfate (at 15 to 45 C) and 1-hexadecylpyridinium chloride (at 25 to 45 C). Solubilization isotherms and the dependence of benzene activity on the intramicellar composition are inferred from the measurements and related to probable micellar structures and changes in structure accompanying the solubilization of benzene. Calculations are made to determine the efficiency of micellar-enhanced ultrafiltration (MEUF) as a process for purifying water streams contaminated by benzene,... 

The primary results in Table I may also be processed to yield values of the benzene activity coefficient in the Intramicellar solution, Tg, defined as fg/(fg Xg), where fg is the fugacity of benzene in equilibrium with the aqueous surfactant solution, and fg is the fugacity of pure benzene at the given temperature. Figures 1-4 are plots of the solubilization constant (K) and the benzene activity coefficient (Tg) against the intramicellar mole fraction of benzene (Xg) for the surfactants CPC and SDS at the Indicated temperatures. 

CMC = 9 mM) was used. The nonluminescent cationic surfactant seemed to play a noninnocent role. Indeed for concentration of CTAB above the CMC, both 1 and 2 displayed an increase of their emission intensity as well as an elongation of their excited-state lifetimes, suggesting an incorporation of the metallosiufactant in the CTAB-based micelles. In Fig. 8 are depicted the emission spectra of equimolar mixtures of the metallosiufactants 1 and 2, upon variation of the concentration of CTAB. These experiments clearly showed the dependence of the intramicellar energy transfer process occurring between the two amphiphilies due to the micellization equilibrium of CTAB. 

Fig. 4.7. Kinetic features of elementary processes in the intramicellar electron transfer reaction...

On the other hand, the kind of irreversible (i.e. chemical) processes that is favored by the micellar solution structure deserves scrutiny. There is a growing body of information on the regioselectivity of photoreactions in surfactant solutions. Selectivity is intimately connected with details of the micelle structure that are mostly unknown. Over SO years after Hartley s work [26], the structure of micelles is still in debate. Micelles are known to have a fleeting existence which renders it difficult to predict the stereochemistry of intramicellar photoreactions. 

An explanation of the discrepancies was offered by the results from some experiments made by Johannsson et al. [30], employing a more long lived probe, Crfbpy) (t 25 /is), in the AOT-alkane-water system. From the observation of two decay processes, well separated in time as shown in Fig. 7, the authors concluded that small clusters of reverse micelles were present in the microemulsions. The initial fast process, the intramicellar quenching, occurs on the submicrosecond time scale and appears only as an initial drop since it is not resolved on the time scale used with Cr(bpy)3. It is this part of the deactivation that is possible to monitor in normal TRLQ measurements with short-lived probes. The initial drop is followed by a second decay with a characteristic time of a few microseconds before the final, very slow deactivation occurs. The results suggest that the fast exchange... 

In connexion to the study of swelling phenomena and chemical reactions in cryptocrystalline macromolecular systems, the terms intramicellar and intermicellar processes were introduced by Katz and have become very popular. The former refers to processes occurring between the crystallites, the latter to occurrences inside them. They can be discriminated by X-ray examination. (See section 6b. 8, p. 577). 

True catalysis of esterolysis, (as opposed to rapid acyl transfer and formation of a stable intermediate) requires a second rapid step in which the acyl group is transferred to water or to an oxygen nucleophile. This has presented micellar histidines and imidazoles with a second role, in experiments first carried out separately by Tagaki, Moss, and Tonellato. In the simplest experimenta mixture of (49) with myristoylhistidine (72) reacts with p-nitrophenyl acetate first by nucleophilic attack of imidazole and then transfer of the acyl group to (49) in a rapid step. In the bifunctional surfactant (73) this process was shown to be intermolecular (but presumably intramicellar). The acylimidazole can be observed spectroscopically when the substrate is p-nitrophenyl hexanoate. Similar conclusions are recorded in a study by Tonellato, who additionally... 

Photochemically-induced electron transfer from N-methylphenothiazine in anionic micelles to cupric ions occurs in the Stern layer. This process is complete within the 15 ns duration of the 347 nm ruby laser excitation. The primary photochemical event is formation of the Tj state of N-methylphenothiazine and both this and the subsequent electron transfer occur with unit quantum yield completely suppressing competitive photochemical pathways. Added Co and Ni deactivate the triplet state. A related system in which Eu " replaced Cu was examined, and electron transfer was again observed. In this case back reaction occurs in the ground state and intermicellar events may be kinetically distinguished from the much faster intramicellar events. ... 

The role of micellar dimensions and what has been called the spatial extent of species in intramicellar kinetic processes has been considered [63]. Three qualitatively different types of reaction were studied (i) the diffusion of a confined excited species to a reactive surface (ii) energy transfer between two separated reactants and (iii) chemical reaction between species which are restricted in their diffusion to the surface of the micelle. Table 11.3 summarizes the main findings when r and D are fixed for these three cases. 

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

The first is a radiative decay, the second a nonradiative decay, and the third a quenching decay. According to the form given for (12.4), the kinetics of intramicellar quenching is assumed to be first order and the presence of other quenching molecules in the micelle is assumed not to disturb the process. The constant fc, is the rate constant of the process in the case where one Q and one P coexist in a micelle (Fig. 12.6). The concentration of micelle incorporating one P and I Q is given by the product of [PJ and (12.1), and the stationary radiation from these micelles becomes... 

Micelles incorporating n probe molecules (MP ) obey the Poisson distribution with respect to w, and the following are the four fundamental intramicellar photochemical processes ... 

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