Kinetics of micellization

The observed relaxation times, obtained experimentally from the kinetic study of rates of formation and breakup of micelles under a variety of experimental conditions, can lead to the mechanism for micelle formation as expressed by Equation 1.20, which involves the addition of a monomer surfactant molecule to the existing aggregates of different aggregation numbers. 

In Equation 1.19 and Equation 1.20, Aj represents the monomer of surfactant molecule, A is an aggregate (micelle) made up of n monomers of surfactant molecules, and k and are respective n-th and second-order rate constants for aggregation of n monomers and single monomer with aggregate A . whereas and represent respective first-order rate constants for the exit of 

Although the mechanism of micelle formation is still incompletely understood, a generally accepted mechanism for such a complex dynamic process is represented by Equation 1.20. The stability of aggregate A (n = 2, 3, 4,. ..) is governed by both polar/ionic and van der Waals attractive as well as repulsive forces. The rate constants, kj - , stand for diffusion-controlled association processes and, hence, the magnitudes of lie within 10 to 10 M sec for all values of n 2. But the values of k are expected to decrease with 

As the multiple-equilibrium model (Equation 1.20) contains a large number of equilibrium constants, drastic simplifying assumptions about the relations between them must be made in order to derive a relationship between experimentally determined relaxation times and rate constants of micelle formation as expressed by Equation 1.20. Kresheck et al. assumed that the rate-determining step is the loss of the first monomer from the micelle. In other words, the micelle reluctantly parts with one monomer molecule and then explodes. The role of equilibria involving the associations of intermediate species such that 

An alternative assumption put forward by Muller is that micelle formation and breakup may be expressed by Equation 1.19 with equal forward and backward rate constants for each step. This leads to Equation 1.23, which fails to account for the observed surfactant concentration dependence of x. 

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Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

There is a substantial body of theoretical work on micellization in block copolymers. The simplest approaches are the scaling theories, which account quite successfully for the scaling of block copolymer dimensions with length of the constituent blocks. Rather detailed mean field theories have also been developed, of which the most advanced at present is the self-consistent field theory, in its lattice and continuum guises. These theories are reviewed in depth in Chapter 3. A limited amount of work has been performed on the kinetics of micellization, although this is largely an unexplored field. Micelle formation at the liquid-air interface has been investigated experimentally, and a number of types of surface micelles have been identified. In addition, adsorption of block copolymers at liquid interfaces has attracted considerable attention. This work is also summarized in Chapter 3. 

There have been very few studies on the kinetics of micellization in block copolymer solutions. Micellization in aqueous surfactant systems close to equilibrium occurs on a time-scale far below one second. Experimental results obtained by fast reaction techniques, such as temperature jumps or pressure jumps or steady-state methods such as ultrasonic absorption, NMR and ESR, show that at least... 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

The kinetics of micelle formation and the dynamics of the micelle-unimer equilibrium are considerably slower for polymeric surfactants. 

The degradation kinetics of micelles is assessed by following the area under micelle curve. 

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. 

Miktoaim stars consisting of one thermoresponsive PNIPAAM arm and four pH-responsive PDMAEMA arms were synthesized and their micellization behavior in aqueous solutions was compared with the corresponding linear PNIPAAM-b-PDMAEMA block copolymers.PNIPAAM-core micelles were obtained in acidic solutions at elevated temperatures, whereas PDMAEMA-core micelles were formed in slightly alkaline solutions at room temperature. Furthermore, the kinetics of pH-induced micellization of the AB4 miktoarm stars and the linear block copolymers was studied by the stopped-flow LS technique upon a pH jump from 4 to 10. The data of both types of copolymers could be fitted with double-exponential functions yielding a fast (xj) and a slow (T2) relaxation process. For both copolymers xj decreased with increasing polymer concentration. However, xj was independent of polymer concentration for the AB4 stars, whereas it decreased with increasing polymer concentration for the linear block copolymer. This result indicates that the macromolecular architecture may greatly influence the kinetics of micellization. 

J. Lang and R. Zana, Effect of Alcohols and Oils on the Kinetics of Micelle Formation-Breakdown in Aqueous Solutions of Ionic Surfactants, J. Phys. Chem., 90 5258 (1986). 

Since the shape of an oil droplet is an indication of IFT as measured by sessile drop method, the oil droplet flattening time reflects the rate of change in IFT. The results clearly show that the presence of alcohol increases the rate of achieving the final value of interfacial tension. This implies that the surfactant molecules come to the interface much faster in the presence of alcohol. Zana (20) has shown that the kinetics of micellization is more rapid in the presence of alcohol. This is presumably due to loose packing of mixed micelles containing surfactant and alcohol. Thus, it appears that the kinetics of micellization could influence the rate at which molecules saturate the surface by the breakdown of micelles to provide monomers for adsorption. 

As has been shown above, the kinetics of micelle formation, breakdown, and associated dynamic processes has been documented. However, much less is known about the kinetic processes involved with transformations between other aggregate structures. 

By several research groups the kinetics of micelle formation... 

Stop flow techniques Kinetics of micelle formation and dissociation... 

The theoretical developments [95,129,148] revealed that the exponential relaxation is influenced by the kinetics of micellization, and from the data analysis we could determine the rate constant of the fast process, k. The observation of different kinetic regimes for different surfactants and/or experimental methods makes the physical picture rather complicated. 

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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