Surfactant micelles, exchange kinetics

The kinetics of surfactant exchange between gemini surfactant micelles and intermicellar solution was investigated. Gemini surfactants with short alkyl chains, 8-6-8, 2Br" and 8-3-8, 2Br", were found to behave similarly to their monomeric counterparts [35]. Gemini surfactants associate to, and dissociate from, their micelles in a single step (the two chains at a time). The association reaction is nearly diffusion controlled, whereas the rate of dissociation (exit) depends strongly on the surfactant hydrophobicity. 

In Equation 6.2, R is a quantitative measure of the resistance to flow in the critical region between monomers and micelles. Later, Kahlweit proposed a different equation to explain his kinetic data obtained at higher surfactant concentrations and/or in the presence of added salt, where a maximum in X2 is observed.24 26 pj g literature associated with micelle exchange/breakdown has been recently reviewed. The most useful derived data are the residence time of a... 

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. 

In principle, what has just been stated for surfactant micelles also holds for the larger and more complex self-assembhes that surfactants and amphiphific block copolymers can form microemulsion droplets, vesicles, and mesophases. The lifetimes of these assembhes are much longer than for micelles, mainly when they involve block copolymers. Nevertheless, exchanges and other processes can also take place. Vesicles and lyotropic mesophases can be considered as permanent objects. However, vesicles can be transformed into micelles, and vice versa. Likewise, a lyotropic mesophase may be transformed into another mesophase or in a micellar solution by an appropriate change brought to the system. The kinetics of these transformations is of basic as well as of practical interest. 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

In practice computer simulation has generally been used to predict the variation of with concentration of reactant, surfactant, or added electrolyte in terms of various values of the parameters, k, and This simulation procedure has been used as an indirect method for the determination of the ion exchange constant K, and, for example, for the competition between various counterions for micelles, there is reasonable agreement between the values obtained kinetically and by other methods [25,72-79],... 

Numerous data on dynamic surface tension [77-93] and dynamic surface elasticity [94-103] of aqueous micellar solutions have been published until now. These data evidence the influence of micelles on the adsorption kinetics, although they are present only in the bulk phase. This effect can surprise on a first glance because it is well-known that the surface activity of micelles is negligible and hence their adsorption is almost zero. However, the influence of micelles can be easily explained if one takes into account that the adsorption kinetics of surfactants at fluid -fluid interface is determined by the diffusional exchange between the subsurface and the bulk phase [104, 105]. It is exactly the diffusion of monomers that changes in the presence of micelles. This point of view is widely accepted and difficulties arise only if one tries to obtain quantitative estimates of the observed effects. 

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. 

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