Surfactant micelle dynamics exchange process

Micelles are not frozen objects. They are in dynamic equilibrium with the free (nomnicellized) surfactant. Surfactants are constantly exchanged between micelles and the intermicellar solution (exchange process), and the residence time of a surfactant in a micelle is fmite. Besides, micelles have a finite lifetime. They constantly form and break up via two identified pathways by a series of stepwise entry/exit of one surfactant A at a time into/from a micelle (Reaction 1) or by a series of frag-mentation/coagulation reactions involving aggregates A, and Aj (Reaction... 

Besides, micelles are dynamic objects. They constantly exchange surfactant with the bulk phase exchange process), and they form and break down by different pathways micelle formation / breakdown), which are reviewed in Chapter 3. 

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... 

Dynamics of Surfactant Self-Assemblies explains the dynamics of micellar equilibria, tracking surfactant exchange, and micelle formation/breakdown processes. Highlighting the structural similarities of amphiphilic block copolymers to surfactants, this volume elucidates the dynamics of more complex self-as.semblies that surfactants and amphiphilic block copolymers form in solutions. 

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... 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

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. 

Photoredox reactions at organized assemblies such as micelles and microemulsions provide a convenient approach for modeling life-sustaining processes. Micelles are spontaneously formed in solutions in the presence of surfactants above a certain critical concentration. In aqueous solutions, the hydrophobic tails of the surfactant form aggregates with the polar head facing toward the aqueous environment, as depicted in Fig. 9. The hydrophobic core in micelles is amorphous and exhibits properties similar to a liquid hydrocarbon. The polar heads are also randomly oriented, generating an electrical double layer around the micelle structure. In this respect, surface properties of micelles can be somewhat correlated with the polarized ITIES. The structure of micelles is in dynamic equilibrium, in which monomers are exchanged between bulk solution and the assembly. 

Likewise, water content, surfactant concentration, and polarity of organic solvent can be adjusted to yield different sizes and shapes of the nonreactor pockets. However, particle synthesis and growth is a dynamic process, with rapid exchange of micelle contents occurring, and so only a rough control of resultant nanoparticle size and shape can be expected. 

So far, all of the studies detailed have dealt primarily with nonequilibrium states, i.e micelle formation or breakdown stimulated by an external trigger, e.g., a jump in pressure, temperature, or concentration. It has of course been shown, however, that even at equilibrium, micelles are by their nature transient species with millisecond-scale lifetimes, and that dynamic processes exchange surfactant molecules between aggregates, interfaces, and bulk solutions. Studies of the kinetics of these processes in equilibrium solutions are much more scarce, mainly owing to experimental difficulties. 

Micelles are dynamic aggregates and exchange surfactant molecules with the surrounding bulk solution. The lifetime of a surfactant molecule in a micelle is in the order of 10 -10 s and depends upon various factors. The solubilized substances can also transfer from the micelles to the bulk phase and vice versa. The rate of such a process can be comparable with the rate of the excited molecule s deactivation [15-17]. The usual implication is that the photoexcitation does not lead to any considerable change of the binding constant. The binding constant for the products of the photoreaction may differ markedly from the ones of the reactants. So the reaction can result in some photoinduced flows of substance from the micellar phase to the bulk phase [18,19]. Such photoinduced transport processes can also be noticed where the... 

Thus far only processes involving motion of the surfactant as a whole have been mentioned. Other processes may occur in micellar solutions internal motion of the surfactant alkyl chains within the micelles exchange of cormterions between free and micelle-bound states and fast changes of micelle shape, among others. Also in the case of solubilized systems, i.e., micellar solutions that have solubilized compounds that are sparingly soluble in water, the solubilizate may exchange between micelles and the intermicellar solution. The dynamics of the exchange of counterions and of solubilizates are reviewed later. The dynamics of internal motions of the surfactant alkyl chains are not dealt with in this chapter, but some information and references can be found in Chapter 5, Section V. Some information on the fluctuations of micelle shapes can be found in Chapter 1, Section III.B. 

The chapter is organized as follows. Section II briefly recalls the theoretical aspects of micellar dynamics and the expressions of the relaxation times characterizing the main relaxation processes (surfactant exchange, micelle formation/breakdown). Section III reviews studies of micellar kinetics of various types of surfactants conventional surfactants with a hydrocarbon chain, surfactants with a fluorinated chain, and gemini (dimeric) surfactants. Section IV deals with mixed micellar solutions. Section V considers the d5mamics of solubilized systems. Section VI reviews the dynamics of sur-... 

The continuous exchange of the surfactant molecules (as well as cosurfactant molecules in case of mixed micelles) constitutes a major dynamic process in micellar systems. The situation in microemulsions, although more complex, directly derives from that in simple micelles. For this reason, we will briefly recall here the main conclusions that have been estab-hshed concerning the micellar dynamics, reviewed in detail in Chapter 3. 

The study of vesicle dynamics is now a mature area of research. The main processes involved in bilayer mobility and solute transport are quite well vmderstood, but this is not so with the spontaneous formation and breakdown of vesicles. There is a significant difference between the d5mamics of micelle forma-tion/breakdown and vesicle formation/breakdown. Micelle phenomena occur on a very short time scale, with processes for most micellar systems taking place in time scales less than 1 sec. For vesicles of synthetic surfactants like the alkylbenzene-sulfonates, the relevant processes are in the second to minute time range, although surfactant monomer exchange between vesicles and aqueous solution may well take place in the mil-... 

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. 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

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