Micelles lifetime

For conventional surfactants with a long alkyl chain (m > 16) and dimeric (gemini) surfactants with m>8, the entry of a surfactant in a micelle is slower than for a diffusion-controlled process. The surfactant residence time and the micelle lifetime can become long with respect to the values found for conventional surfactants. 

The fluctuations of the electrical potential between two electrodes placed in two cells filled with a micellar solution and connected by a capillary tube are expected to be affected by the micelle lifetime. Indeed, the fluctuations should differ... 

The last contribution of Aniansson to micellar d3mamics before his untimely death in 1984 was the derivation of the relationship between the micelle lifetime T and x, in the case of dilute micellar solutions ... 

As stated by Aniansson, except close to the cmc a/[l + (a /N)a] is of the order of one so that generally the order of magnitude of the micelle lifetime is determined by the product Nx2. Since N is often close to 100, the micelle lifetime is therefore much longer than X2. 

In Equation 3.26, T is the equilibrium surface excess, C the bulk concentration, t the time, and D the surfactant monomer diffusion coefficient. Eastoe et al. have measured the time dependence of the DST and the relaxation time %2 for solutions of many surfactants nonionic, dimeric, and zwitterionic. In all instances the fitting of the data to Equation 3.26 with the experimentally determined value of %2 was poor. The authors concluded that the micelle dissociation may have an effect on the measured DST only if the concentration of monomeric surfactant in the subsurface diffusion layer is limiting or when the micelle lifetimes are extremely long. No surfactant for which this last condition is fulfilled was evidenced by the authors. They also concluded that the rapid dissociation of monomers from micelles present in the subsurface was not likely to limit the surfactant adsorption and thus the DST. 

The book first discusses. self-assembling processes taking place in aqueous surfactant solutions and the dynamic character of surfactant self-assemblies. The next chapter reviews methods that permit the. study of the dynamics of self-assemblies. The dynamics of micelles of surfactants and block copolymers,. solubilized systems, microemulsions, vesicles, and lyotropic liquid crystals/mesophases are reviewed. successively. The authors point out the similarities and differences in the behavior of the.se different self-as.semblies. Much emphasis is put on the processes of surfactant exchange and of micelle formation/breakdown that determine the surfactant residence time in micelles, and the micelle lifetime. The la.st three chapters cover topics for which the dynamics of. surfactant self-assemblies can be important for a better understanding of observed behaviors dynamics of surfactant adsorption on surfaces, rheology of viscoelastic surfactant solutions, and kinetics of chemical reactions performed in surfactant self-assemblies used as microreactors. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

FT-EPR spectra of the ZnTPPS/DQ system in a solution of cetyltriinethylaininonium chloride (CTAC), a cationic surfactant, are shown in figme BE 16.21. As in the TX100 solution, both donor and acceptor are associated with the micelles in the CTAC solution. The spectra of DQ at delays after the laser flash of less than 5 ps clearly show polarization from the SCRP mechanism. While SCRPs were too short-lived to be observed in TXlOO solution, they clearly have a long lifetime in this case. Van Willigen and co-workers... 

Resolution at tire atomic level of surfactant packing in micelles is difficult to obtain experimentally. This difficulty is based on tire fundamentally amoriDhous packing tliat is obtained as a result of tire surfactants being driven into a spheroidal assembly in order to minimize surface or interfacial free energy. It is also based upon tire dynamical nature of micelles and tire fact tliat tliey have relatively short lifetimes, often of tire order of microseconds to milliseconds, and tliat individual surfactant monomers are coming and going at relatively rapid rates. 

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

By flourescence techniques, it was observed that the fluorescence yield and lifetime of 1,8-anilinonaphthalenesulfonate decrease with an increase in the aqueous core of AOT-reversed micelles, while the position of the emission maximum shifts to longer wavelengths [64], These changes in the electronic properties were attributed to the peculiar effective polarity and viscosity of the micellar core and to their evolution with R. 

It has been reported that while in aqueous solution the lifetime of optically excited nile red is 0,65 ns inside AOT-reversed micelles it is 3,73 ns, becoming 2,06 ns at/ = 30 [150],... 

The fluorescence lifetime of trans-4-[4-(dimethylamino-styryl]-l-methylpyridinium iodide trapped in water-containing AOT-reversed micelles has been found to be markedly influenced by R, implying a significant effect on its excited-state twisting motion [122],... 

Solutions of surfactant-stabilized nanogels share both the advantage of gels (drastic reduction of molecular diffusion and of internal dynamics of solubilizates entrapped in the micellar aggregates) and of nonviscous liquids (nanogel-containing reversed micelles diffuse and are dispersed in a macroscopicaUy nonviscous medium). Effects on the lifetime of excited species and on the catalytic activity and stability of immobilized enzymes can be expected. 

The termination constants kt found previously (see Table XVII, p. 158) are of the order of 3 X10 1. mole sec. Conversion to the specific reaction rate constant expressed in units of cc. molecule" sec. yields A f=5X10". At the radical concentration calculated above, 10 per cc., the rate of termination should therefore be only 10 radicals cc. sec., which is many orders of magnitude less than the rate of generation of radicals. Hence termination in the aqueous phase is utterly negligible, and it may be assumed with confidence that virtually every primary radical enters a polymer particle (or micelle). Moreover the average lifetime of a chain radical in the aqueous phase (i.e., 10 sec.) is too short for an appreciable expectation of addition of a dissolved monomer molecule by the primary radical prior to its entrance into a polymer particle. 

Moss and coworkers provided an early example of the way in which micellization can control the stereochemical course of a reaction. Deamination of chiral primary aliphatic amines in water proceeds with net inversion and extensive racemization, and the extent of racemization depends upon the lifetime of the carbocation-like intermediate. The situation changes dramatically if the salts of the primary amine can self-micellize, because now the nucleophile, typically water, is directed in from the front-side so that there is extensive retention of configuration (Moss et al., 1973). 

There is no reason to believe that these surfactant effects upon regioselecti-vity are related to differences in molecularity. In this respect they differ from micellar control of elimination to substitution ratios. The controlling factors may be the orientation or conformation of reactants or intermediates in micelles or similar aggregates and changes in the lifetimes of intermediates (cf. van der Langkruis and Engberts, 1984). 

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