Dynamics of micellar systems

Concerning the dynamics of block copolymers in solution, we have to consider on the one 

Kinetic studies of micelle formation and dissociation by direct methods are scarce as already mentioned by Tuzar and co-workers [7,114] and later on by Hamley [11]. Informations can be obtained by fast reaction techniques, such as stop-flow, temperature or pressure jump techniques, as well as by steady state methods, for example ultrasonic absorption, NMR, ESR. 

Stop-flow experiments have been performed by Tuzar and Kratochvil [7] and more recently by Kositza et al. [128]. In analogy to low molar weight surfactants, it could be shown that two relaxation processes have to be considered for block copolymer micellar systems the first in the time scale of tens of microseconds, associated to unimer exchange between micelle and bulk solution, and the second, in the millisecond range, attributed to the rearrangement of the micelle size distribution. Major differences were observed between A-B diblock and A-B-A triblock copolymers, which could be explained by the fact that the escape of a unimer, which has to disentangle from the micellar core, might be much easier in a diblock than in a triblock structure. 

The hybridization of micellar systems corresponding to the exchange of unimers between two micelle populations of a given type of copolymer, with formation of so-called mixed 

This kind of problem was approached in a very systematic way by Munk [113] and Tuzar [ 114] by using sedimentation velocity as experimental technique. According to these authors the mechanism of hybridization consists primarily in the transfer of unimers among the micelles of both type and the driving force for this phenomenon is the increase of entropy when the two types of imimers are mixed within the micelles. 

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The dynamics of micellar systems is a very important concern that is relevant at several levels. In this respect, the dynamics required for the establishment of the unimer-micelle equilibrium is one aspect. The so-called problem of micelle hybridization that deals with the exchange rate of unimers between different micelles is closely related. Finally, the chain dynamics of polymer blocks in either the core or the corona is another concern that can, however, be linked to some extent to the first two mentioned concepts. These different aspects have been scarcely studied and reviewed by Tuzar and Kratochvil [6,41], and... 

One aspect of the dynamics of micellar systems that has received a renewed interest during recent years is the translational motion of the micelles themselves. In the simplest approximation, the translational diffusion coefficient, D, of a spherical micelle is related to the hydrodynamic radius rM through the Stokes-Einstein relation... 

A wealth of information about the structure and dynamics of micellar systems has been obtained with fluorescence and phosphorescence molecules. The use of these... 

Zana, R. Dynamics of Micellar Systems. In Encyclopedia of Surface and Colloid Science Hubbard, A., Ed. M. Dekker, Inc. New York. 2002 1515-1528. and references therein. 

FIGURE 16.4. The presence of micelles in a two-phase reaction medium may produce several effects. A micelle with the same electrical charge as a dissolved reactant may slow its reaction with a solubilized component (path A), while one of opposite charge will usually enhance the reaction rate (path B). Alternatively, especially for nonionic surfactants, the micelle may provide an intermediate solvent environment that enhances the reaction rate (path C). Finally, the dynamics of micellar systems may provide a more readily accessible reservoir of insoluble reactant in the system thereby increasing the reaction rate (path D). 

The effect of alcohol concentration on the solubilization of brine has been studied in this laboratory (41). It was observed that there is an optimal alcohol concentration which can solubilize the maximum amount of brine and can also produce ultralow interfacial tension. The optimal alcohol concentration depends on the brine concentration of the system. The effect of different alcohols on the equilibrium properties and dynamics of micellar solutions has been studied by Zana (42). 

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. 

Equations (15) have been applied in a large number of NMR relaxation studies of micellar systems. It turns out that frequency-dependent relaxation studies are particularly useful to study small spherical micelles. These have a typical radius of about 2 nm. Using the water viscosity we obtain with Eq. (7) t, = 7 ns at room temperature. Including the effect of lateral diffusion, a typical value of the slow correlation time is t j = 5 ns. Thus relaxation dispersion from slow motion occurs around vo (2n Ti.) 5 30 MHz, which is well within the accessible Larmor frequency range for H. Examples of studies of micelle size and dynamics can be found in Refs. 58-61. 

The optimal alcohol concentration depends on the brine concentration of the system. The effect of different alcohols on the equilibrium properties and dynamics of micellar solutions was studied by Zana [30]. 

Additional information on potential alterations of chain conformations or core/ corona interface due to micellisation can also be gained. Similarly, the application of NMR spectroscopy for the study of chain dynamics in micellar systems is based mainly on the fact that block copolymer segment mobility is directly correlated to the intensity of respective NMR spectrum peaks. Thus, when the core of the micelles is formed and the mobility of the insoluble blocks is significantly reduced, the intensity of the corresponding NMR peaks is reduced accordingly. 

The effect of alcohol on the dynamic properties of micellar systems has been considered as a first approach toward the understanding of microemulsion systems. In mixed alcohol + surfactant micelles, the theory predicts the existence of three relaxation processes, which have been experimentally observed using chemical relaxation techniques a slow process associated with the formation/breakdown of mixed micelles and two fast processes associated with the exchange of the surfactant and alcohol, respectively, between the mixed micelles and the bulk aqueous phase. With g representing a mixed micelle with a alcohol (A) molecules and s surfactant (S) molecules, these two exchange reactions can be written in the form... 

The dynamics of micellar equilibria — that is, of surfactant exchange and micelle formation/breakdown processes — have been investigated a great deal. Indeed, in addition to providing a better knowledge of micellar systems, a good understanding of the dynamics of micelles is required for the interpretation of experimental results obtained in other areas of surfactant science. The most... 

Frese, C., Ruppert, S., Schmidt-Lewerkuhne, H., Witten, K.P., Eggers, R., Fain-erman, V.B., Miller, R. Adsorption dynamics of micellar solntions of a mixed anionic-cationic snrfactant system. Colloids Surf. A 2004, 239(1-3), 33-40. 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

Although RMs are thermodynamically stable, they are highly dynamic. The RMs constantly colhde with each other and occasionally a colhsion results in the fusion of two RMs temporarily. During this fusion surfactant molecules and the contents residing inside RMs may be exchanged. In AOT reverse micellar system, this dynamic behavior exhibits second-order kinetics with rate constants in the order of 10 to 10 M s [37]. This dynamic nature not only influences the properties of the bulk system but also affects the enzymatic reaction rates [38]. 

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