Micelle kinetics formation

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

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. 

Non-equilibrium kinetic processes typically involve monitoring a change in micellar structure or morphology over time, or following the formation of micelles from a molecular solution (unimers), i.e., micellization kinetics. Thus, in contrast to equilibrium processes a perturbation is required. Typically this is achieved by abruptly altering the thermodynamic conditions, which can be achieved either via extensive parameters like temperature and pressure, or by changing intensive parameters such as salt concentration or pH. 

Recently, the same group has presented also the micellization kinetics of a diblock copolymer, namely PEO-PDEAEMA [59]. Several samples of this type of copolymers have been synthesized having identical PEG blocks and PDEAEMA block with varying degrees of polymerization. The block copolymers tend to create micelles with PDEAEMA cores in basic conditions. Therefore, the micellization kinetics studies were performed using a stop flow technique in order to introduce micelles upon a pH-jump from 3 to 12. The observation of two processes, i.e. a fast one attributed to the formation of quasi-equilibrium micelles and a slow one attributed to the relaxation into final equilibrium micelles, were observed, as... 

The micellization of a polypeptide eontaining DHBC has been also studied under variable solution pH and temperature [30]. A sehizophrenic solution behavior was observed for a PNIPAM-PGA block copolymer. At room temperature and low pH values micelles having polypeptide cores were formed, while at elevated temperatures and increased pH values micelles with PNIPAM cores were observed. It has to be noted that the formation of PGA eoie micelles was accompanied by a coil-to-helix transition of the polypeptide sequence. The above transition had as a result a different micellization kinetic profile for the system in eomparison with other conventional pH responsive systems. 

Abstract Results of stochastic simulations of micellization kinetics are presented. The algorithm used was derived from the general Monte Carlo method introduced by D.T. Gillespie (1976, J. Phys. Chem. 22 403-434) and applied to micelle formation according to a mechanism that allows association and dissociation among n-mers of whatever aggregation number. With a careful choice of thermodynamic and... 

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]. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

Several studies have been performed to investigate the compatibalizing effect of blockcopolymers [67,158, 188,196-200], It is generally shown that the diblock copolymer concentration is enhanced at the interface between incompatible components when suitable materials are chosen. Micell formation and extremely slow kinetics make these studies difficult and specific non-equilibrium starting situations are sometimes used. Diblock copolymers are tethered to the interface and this aspect is reviewed in another article in this book [14]. 

The behavior of metal ions in reversed micelles may be more interesting, since the reversed micelle provides less solvated metal ions in its core (Sunamoto and Hamada, 1978). Through kinetic studies on the hydrolysis of the p-nitrophenyl ester of norleucine in reversed micelles of Aerosol OT and CC14 which solubilize aqueous cupric nitrate, Sunamoto et al. (1978) observed the formation of naked copper(II) ion this easily formed a complex with the substrate ester (formation constant kc = 108—109). The complexed substrate was rapidly hydrolyzed by free water molecules acting as effective nucleophiles. 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

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