Micellar reaction mechanism

Reversibly fonned micelles have long been of interest as models for enzymes, since tliey provide an amphipatliic environment attractive to many substrates. Substrate binding (non-covalent), saturation kinetics and competitive inliibition are kinetic factors common to botli enzyme reaction mechanism analysis and micellar binding kinetics. 

Figure 5a indicates the effect of the CTAB concentration on the rate constants of the complexes of 38b and 38c. In the case of the water soluble 38b ligand, the rate increases with increasing CTAB concentration up to a saturation level. This type of saturation kinetics is usually interpreted to show the incorporation of a ligand-metal ion complex into a micellar phase from a bulk aqueous phase, and the catalytic activity of the complex is higher in the micellar phase than in the aqueous phase. In the case of lipophilic 38c, a very similar curve as in Fig. 4 is obtained. At a first glance, there appears to be a big difference between these two curves. However, they are rather common in micellar reactions and obey the same reaction mechanism 27). 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

Ito s group [83] reported the micellar polymerization mechanism was operative during the radical polymerization of PEO macromonomers in cyclohexane and water under similar reaction conditions. The reaction medium has an important effect on the polymerization behavior of macromonomers. Cyclohexane was chosen as a nonpolar type of solvent. The polymerization was found to be independent of the lengths of p-alkyl group (R) and the PEO chain in benzene. On the other hand, the rate of polymerization in cyclohexane increased with increasing number of EO units. This may be attributed to the formation of aggregates (micelles) and/or compartmentalization of reaction loci,i.e., polymerization in distinct aggregates (polymer particles). The C12-(EO)14-MA macromonomer polymerized faster in bulk than in benzene but far slower than in water. 

Menger et al. synthesized a Ci4H29-attached copper(II) complex 3 that possessed a remarkable catalytic activity in the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) and the nerve gas Soman (see Scheme 2) [21], When 3 was used in great excess (ca. 1.5 mM, which is more than the critical micelle concentration of 0.18 mM), the hydrolysis of DNP (0.04 mM) was more than 200 times faster than with an equivalent concentration of the nonmicellar homo-logue, the Cu2+-tetramethylethylenediamine complex 9, at 25°C and pH 6 (Scheme 4). The DNP half-life is calculated to be 17 sec with excess 1.5 mM 3 at 25°C and pH 6. The possible reasons for the rate acceleration with 3 were the enhanced electrophilicity of the micellized copper(II) ion or the acidity of the Cu2+-bound water and an intramolecular type of reaction due to the micellar formation. On the basis of the pH(6-8.3)-insensitive rates, Cu2+-OH species 3b (generated with pK3 < 6) was postulated to be an active catalytic species. In this study, the stability constants for 3 and 9 and the thermodynamic pvalue of the Cu2+-bound water for 3a —> 3b + H+ were not measured, probably because of complexity and/or instability of the metal compounds. Therefore, the question remains as to whether or not 3b is the only active species in the reaction solution. Despite the lack of a detailed reaction mechanism, 3 seems to be the best detoxifying reagent documented in the literature. 

We shall now show how the reaction dynamics and reaction products can be affected by micellar environments, how the knowledge of the reaction mechanism in homogeneous solution can assist in determining the reaction mechanism in micelles and how understanding of the mechanism in a micelle can be employed to enhance our understanding of the structure and dynamics of micelles. 

One of the key features of this reaction mechanism is the particle nucleation beyond the first maximal Rp. Furthermore, the disappearance of monomer droplets in the conventional emulsion polymerization at ca. 30-50% conversion results from the transfer of monomer from the monomer droplets to the locus of polymerization (polymer particles). In mini-emulsion polymerization, the concentration of monomer decreases and monomer droplets may exist throughout the polymerization. In the context of the micellar nucleation model,both Np and the concentration of monomer in the particles in the constant reaction rate region contribute to Rp. Therefore, the first maximal Rp observed in the course of mini-emulsion polymerization does not necessarily correspond to the end of particle nucleation. This is because Np may increase in the course of polymerization, but the contribution of the increased Np to Rp can be outweighed by the decreased monomer concentration in the reaction loci. 

Based on the knowledge of micellar catalytic effects, an exclusive photochemical reaction mechanism in micellar solution possibly may be expected or designed to fulfill each particular environmental problem. Thomas J.K.. (1980) proposed four possible photochemical reaction mechanisms that might exist in micellar system (see Figure 2). 

The effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

In addition to a generalized medium effect, micelles also have a charge effect that seems to be related to the reaction mechanism. Most of the experiments were made by using CTAX as the cationic and SDS as the anionic surfactant, and the rate constants for reaction of fully micellar bound substrates are designated k+ and k that is, k+ and k are values of k M in cationic and anionic micelles, respectively (54, 55). 

Whether changes in the relative importance of bond making and breaking, as in solvolyses of acyl chlorides, are to be regarded as changes in reaction mechanism is a matter of opinion, but clearly micelles, like any other reaction medium, can influence transition-state structure. Therefore, although values of k+/k can be considered as indicative of mechanism , the conclusions apply only to reactions taking place at micellar surfaces. However, these surfaces are water-rich, so the transition-state structures are expected to be similar to those in water. 

Depending on the relative rates of the chemical and diffusion steps, the reaction can proceed in the kinetic, diffusion, or mixed regime, the entire process being controlled by the rate of the chemical step, a diffusion process, or by both kinetics and diffusion. Thus, under very good hydrodynamic conditions, e.g., upon vigorous agitation, the influence of the diffusion can be substantially eliminated and the kinetic results can be used to discuss the reaction mechanism. This conclusion is not always true, and the use of typical surfactant micellar aqueous solutions with extractants dissolved (solubilized) in micellar pseudophase (micelles) and inorganic species dissolved in aqueous pseudophase mimic the extraction systems effectively and the diffusion processes are totally eliminated. 

Peggion et al. [76], however, concluded that the strong deviations from the micellar theory cannot be ascribed to the solution polymerization which competes with the micellar polymerization and suggest the following reaction mechanism The radicals produced by decomposition of the initiator react with the monomer dissolved in water. The polymer radicals separate from the... 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

The rate of polymerization and the final number of latex particles per unit volume of water are proportional to the 1.4 and 2.4 powers, respectively, of the NP-40 concentration. The polymerization system does not follow the conventional micellar nucleation model, and some possible reaction mechanisms responsible for this deviation are discussed. 

The coexistence of hydrophilic and hydrophobic nano-domains separated in space, with a local order and fluidity typical of liquids, confer to supramolecular surfactant structures remarkable properties, which are advantageous in applications involving molecular confinement within nanoscopic regions and reactivity in micro-heterogeneous media. Micelle-mediated reactions constitute the basis of the so-called micellar catalysis [62, 116], admicellar catalysis [117] or admicellar polymerisation [118] in which reaction mechanisms may be controlled at a molecular level to save energy and raw materials, as well as to avoid lengthy post-reaction purification and analytical steps. 

Bunton CA. The dependence of micellar rate effects upon reaction mechanism. Adv Colloid Interfile. 2006 123 333-343. 

Kinetic study has been one of the best mechanistic (reaction) tools to establish the most refined reaction mechanisms. In an attempt to establish such a reaction mechanism, kinetic experimental data on the reaction rate are obtained under a set of reaction conditions that could be explained by a kinetic equation derived on the basis of a proposed reaction mechanism. The study is repeated to obtain kinetic data under slightly or totally dilferent reaction conditions, and if these kinetic data fail to fit the kinetic equation derived on the basis of the earlier reaction mechanism, a further refinement in the mechanism is suggested so that the present and earlier kinetic data could be explained mechanistically. A similar approach has been used to provide quantitative or semiquantitative explanations for the micellar effects on reaction rates. Let us now examine the micellar kinetic models developed so far for apparent quantitative explanations of the effects of micelles on reaction rates. 

Experimentally determined rate constants for various micellar-mediated reactions show either a monotonic decrease (i.e., micellar rate inhibition) or increase (i.e., micellar rate acceleration) with increase in [Suifl CMC, where [Surf]T represents total micelle-forming surfactant concentration (Figure 3.1). Menger and Portnoy obtained rate constants — [SurfJx plots — for hydrolysis of a few esters in the presence of anionic and cationic surfactants, which are almost similar to those plots shown in Figure 3.1. These authors explained their observations in terms of a proposed reaction mechanism as shown in Scheme 3.1 which is now called Menger s phase-separation model, enzyme-kinetic-type model, or preequilibrium kinetic (PEK) model for micellar-mediated reactions. In Scheme 3.1, Kj is the equilibrium... 

The brief reaction mechanism for such a bimolecular reaction may be shown by Scheme 3.3. Subscripts W and M represent aqueous pseudophase and micellar pseudophase, respectively. The second-order rate constant k, stands for crossinterface reaction. The observed rate law rate = ko,JSub]x and Scheme 3.3 can lead to Equation 3.4 ... 

The reaction mechanism for such a micellar-mediated bimolecular reaction may be expressed by Scheme 3.4, which can lead to Equation 3.5. 

The reaction mechanism for such micellar-mediated reactions is shown by Scheme 3.5. 

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