Micelle kinetics dissolution

The kinetic scheme used to describe micelle-facilitated dissolution is... 

Fig. 4.12 Micelle kinetics mechanisms 1- formation-dissolution, 2 - rearrangement, 3 aggregation-disintegration...

A numerical solution, based on the model presented for a formation-dissolution mechanism, was derived by Miller (1981). The following two Figs 4.13 and 4.14 demonstrate the effect of micelles on adsorption kinetics. The effect of the rate of formation and dissolution of micelles, represented by the dimensionless coefficient nkfC Tj /D, becomes remarkable for a value larger than 0.1. Under the given conditions (D /D, =1, c /c , =10, n=20) the fast micelle kinetics accelerates the adsorption kinetics by one order of magnitude. 

In the discussion of the adsorption kinetics of micellar solutions, different micelle kinetics mechanisms are taken into account, such as formation/dissolution or stepwise aggregation/disaggregation (Dushkin Ivanov 1991). It is clear that the presence of micelles in the solution influences the adsorption rate remarkably. Under certain conditions, the aggregation number, micelle concentration, and the rate constant of micelle kinetics become the rate controlling parameters of the whole adsorption process. Models, which consider solubilisation effects in surfactant systems, do not yet exist. 

The above picture shows that to describe the kinetics of adsorption, one must take into account the diffusion of monomers and micelles as well as the kinetics of micelle formation and dissolution. Several processes may take place and these are represented schematically in Fig. 4.9. Three main mechanisms may be considered, namely formation-dissolution (Fig. 4.9 (a)), rearrangement (Fig. 4.9 (b)) and stepwise aggregation-dissolution (Fig. 4.9 (c)). To describe the effect of micelles on adsorption kinetics, one should know several parameters such as micelle aggregation number and rate constants of micelle kinetics [25]. 

SANS has been recently used to study problems related to micelle preparation and kinetics, as reported by Bates and coworkers who have used time-resolved SANS to study molecular exchange and micelle equilibration for PEO-PB diblocks in water [71]. The authors have shown that the micellar structures initially formed upon dissolution were completely locked in up to 8 d after preparation. Fluorometry and DLS have also been used to monitor micelle equilibration [72],... 

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. 

Time resolved X-ray and neutron scattering have also been used to elucidate the kinetic behavior in many surfactant systems 14, 15), For the case of dissolution kinetics transitions between micelle and vesicle structures have been studied during homogeneous dilution of the solvent 16), Transient structures such as disks have been observed during such a transition. 

In essence, the compatibilization is a control of the interface of two immiscible PO phases, i.e., the interphase. In the simplest case, this is accompanied by partial dissolution of parts of the compatibilizer in the two phases. However, too much compatibilizer or using too high MW may form micelles then mesophases that reduce the blend performance. A compatibilizer must be designed by taking the thermodynamic and kinetic parameters into account. 

Solubilization as a bulk reaction Molecular dissolution and diffusion of oil into the aqueous phase takes place, with a subsequent uptake of oil molecules by surfactant micelles [156-161]. This mechanism is operative for oils (like benzene, hexane, etc.), which exhibit a sufficiently high solubility in pure water. Theoretical models have been developed and verified against the experiment [157,159-161]. The bulk solubilization includes the following processes. First, oil molecules are dissolved from the surface of an oil drop into water. Kinetically, this process can be characterized by a mass transfer coefficient. Next, by molecnlar diffnsion, the oil molecules penetrate in the water phase, where they react with the micelles. Thus, the concentration of free oil molecules dimmishes with the distance from the oil-water interface. In other words, solubilization takes place in a certain zone around the droplet [159,160]. 

Based on the above concepts, one would expect that the ratio of monomers Ci to micelles Cm, the aggregation number the rates of micelle formation (kf) and micelle dissolution (ka) will influence the rate of the adsorption process. Figure 11.20 gives a schematic picture of the kinetic process in the presence of micelles. 

The above picture shows that to describe the kinetics of adsorption, one must take into account the diffusion of monomers and micelles as well as the kinetics of micelle formation and dissolution. Several processes may take place (Figure 11.21). Three main mechanisms may be considered, namely formation-... 

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