Surfactant micelle dynamics reactions

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

Extensive chemical relaxation data for solutions of ionic surfactants were first interpreted on the basis of this theory in 1976. The conclusions reported in this study still constitute the basis for the present rmderstanding of the dynamics of micelles. The treatment of Aniansson and Wall was later refined and extended to ionic surfactant micelles, taking into account the presence of the cormterions and of added electro-lyte.2 2 It was also extended to include fragmentation/coag-ulation (or fission/fusion) reactions (3.3) by which a micelle Ag can break into two daughter micelles Aj and Aj (with s = i + j), and conversely. ... 

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 purpose of this book is to present an up-to-date picture of the dynamics aspects of self-assemblies of surfactants and amphiphilic block copolymers, from micelles to solubilized systems, microemulsions, vesicles, and lyotropic mesophases. It is organized as follows. The first chapter introduces amphiphiles, surfactants, and self-assembhes of surfactants and examines the importance of dynamics of self-assembhes in surfactant science. Chapter 2 briefly reviews the main techniques that have been used to study the dynamics of self- assembhes. Chapters 3 and 4 deal with the dynamics of micelles of surfactants and of amphiphilic block copolymers, respectively. The dynamics of microemulsions comes next, in Chapter 5. Chapters 6 and 7 review the dynamics of vesicles and of transitions between mesophases. The last three chapters deal with topics for which the dynamics of self-assembhes is important for the understanding of the observed behaviors. The dynamics of surfactant adsorption on surfaces are considered in Chapter 8. The rheology of viscoelastic surfactant solutions and its relation to micelle dynamics are reviewed in Chapter 9. The last chapter deals with the kinetics of chemical reactions performed in surfactant self-assembhes used as microreactors. 

One of the most important characteristics of micelles is their ability to enclose all kinds of substances. Capture of these compounds in micelles is generally driven by hydrophobic, electrostatic and hydrogen-bonding interactions. The dynamics of solubilization into micelles are similar to those observed for entrance and exit of individual surfactant molecules, but the micelle-bound substrate will experience a reaction environment different from bulk water, leading to kinetic medium effects308. Hence, micelles are able to catalyse or inhibit reactions. The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentrations of the reacting species in or at the micelle. 

The formation mechanism of this family of materials is determined by two features [45], The first is the dynamics of surfactant molecules to shape molecular assemblies, which leads to micelle, and, ultimately, liquid crystal formation. The second is the capability of the inorganic oxide to undergo condensation reactions to form extended, thermally stable structures. 

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. 

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... 

Hirai et al. [97] used the AOT/isooctane/water system and titanium tetrabutoxide (TTBO). Particle diameters on the order of 3 nm were obtained by dynamic light scattering, a dimension that is smaller than the diameters of the reverse micelles (9-19.3 nm). Particle formation was strongly influenced by the water/surfactant molar ratio (R) and by the alkoxide concentration. The reaction kinetics was followed with UV-Vis absorption spectrophotometry. The absorbance of the reaction system increased monotonically with time for R = 9, whereas it went through a peak (after 400 min) for R = 30. These results were rationalized in terms of differences in availability of reactant water molecules. For... 

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). 

As with micelles, incorporation of reactants into an adsorbed surfactant layer on an electrode [6, 7] can lead to high reactant concentrations in a restricted reaction volume and enhanced rates of bimolecular reactions. Conversely, when reactants in a bimolecular r.d.s. reside separately in oil and water phases in microemulsions with insufficient interfacial area or slow partition dynamics, the reaction rate may be slower than in a homogeneous solution [27]. 

Alkaline hydrolysis of ethyl caprylate (itself insoluble in water) yields sodium caprylate, initially at a very slow rate bnt as soon as sufficient caprylate was formed for aggregation into micelles to take place, the authors observed an exponential increase in reaction rate owing to micellar catalysis. These self-assembling surfactant strucmres may consequently provide a model system for studies of pre-biotic chemistry. The possible relevance of this process to prebiotic chemistry was emphasized by their observation that the micelles can be converted into more robust vesicles by a pH change induced by dissolved CO2, and latter on, Luisi extended this approach to vesicular systems (see Section 3.3). Kinetic models for this kind of autocatalytic dynamic systems were also developed in the literature." ... 

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