Micelle kinetic parameters

Table 11. Kinetic parameters for the hydrolysis of L- and D-ester in a CTAB micelle ...

DR. THOMAS The kinetic parameters of micelles are very well known, having been determined by temperature-jump relaxation methods and various other techniques. There are several kinetic events which can be described. First of all, the fastest event is the exchange of the counter ion (e.g., the sodium counter ion, in sodium lauryl sulfate). These ions exchange... 

From bisubstrate, kinetic analysis with a transferase from hen oviduct that, under the conditions of the assay, formed only GlcNAc-PP-Dol, it followed that both dolichol phosphate and UDP-GlcNAe have to he bound to the enzyme before release of the product occurs.52 However, the fact that only partially purified preparations have thus far been obtained (the preparations may also still be contaminated with substrates and product), together with experimental difficulties in handling both the substrate dolichol phosphate (which, furthermore, is not one compound, see the earlier discussion) and the unstable enzyme (enveloped in micelles of detergent), make difficult a sensible interpretation and comparison of the kinetic parameters detenuined for the different enzvme-preparations. The solubilized enzymes catalyzing reactions 1,2, and 3 have in common their alkaline pH optima and dependence on Mg2+ or Mn2+ ions. The latter fact makes (ethylenedinitrilo)tetraacetic acid (EDTA) a reversible inhibitor of enzyme activity and an important experimental tool. 

Use these values to criticize or defend the following proposition The smaller endothermic value for AH% in CTABr means the product molecules must be more readily expelled from these micelles, making the enthalpy contribution more favorable to the reaction in this case. The solubilized substrate has a higher entropy, so the decrease in entropy for the micellar reaction is larger. The last problem shows that the reaction occurs about twice as fast in water as in 0.01 M NaLS. The rate in water determines the kinetic parameters in the last case. 

Kinetic parameters for the hydrolysis in the presence of polyions and micelles at 3QOC and 20% EtOH-H20. 

If the substrate is preferentially soluble in the reversed micelles (Ps l), a comparison of the kinetic parameters observed for the overall system and the kinetic parameters intrinsic to the reversed micellar reaction medium gives... 

Thermodynamic and Kinetic Parameters for Protonation of Bromo Cresol Green Adsorbed on Brij 58 Micelles"... 

To account for the micelle effect, specific parameters of the respective surfactant micelles have to be known. Numerous papers on the determination of aggregation numbers and rate constants of micelle kinetics of many surfactants have been published (for example Aniansson et al. 1976, Hoffmann et al. 1976, Kahlweit Teubner 1980). Different micelle kinetics mechanisms exist, for example that summarised by Zana (1974). Three of these mechanisms are demonstrated in Fig. 4.12. 

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 efficiency of a block copolymer is fimited by the formation of micelles in bulk phases and by the kinetic factors. Consequently, the block copolymer used as a compatibilizer should be designed by taking thermodynamic and kinetic parameters into account to achieve the desired effects. Thus the stmcture and transitions in copolymers and homopolymer/copolymer systems is of great interest. 

Table 1 [53] shows the apparent kinetic parameters of HK in three kinds of reversed micellar solutions based on concentrations of substrates in the water pools and in the aqueous bulk solution. The apparent V aax in the AOT, HTAC, and C Eg reversed micelles, and in the aqueous bulk solution increased in that order. HK may become fairly denatured in the AOT and HTAC reversed micelles because the ionic surfactants in the aqueous bulk solution are usually known as strong denaturants for protein. 

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. 

Table 19.5. Kinetic parameters of association and dissociation of alkyl sulfates from their micelles...

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. 

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]. 

Table 10.3 Kinetic Parameters for Enzymes in AOT Hydrocarbon Reverse Micelles...

Table 1 Kinetic parameters of MGDG synthase in mixed micelles. Mixed micelles were prepared as described by [5]. MGDG synthase activity was determined with various diacylglycerol molecular species as the varied substrate. Diacylglycerol concentration was expressed as mole fraction, i.e. [diacylglycerol]/([CHAPS] + [PG] -i- [diacylglycerol]). The Vmax and app Km values were respectively calculated from the IfV intercept and the 1/[substrate], expressed as mole fraction, intercept axes of double reciprocal plots. Diacylglycerol concentration within a micelle was also expressed as the number of diacylglycerol molecules per micelle, assuming that each 90 kDa-mixed micelle contains about 140 molecules of CHAPS, PG and diacylglycerol.

Aniansson and Wall (A-W) appear to be the first to develop a relatively more accurate and convincing kinetic model for micellization in conjunction with the multiple-equilibrium reaction scheme as shown by Equation 1.20. - The superiority of the A-W model over the others is that it predicts the presence of two discrete relaxation times (Xj and Xj) during the course of micelle formation in the aqueous solutions of a single surfactant above CMC — a fact revealed by many experimental observations in related studies. Although this model successfully predicts the presence of two discrete relaxation times, it is not fully tested in terms of (1) reproducibility of kinetic parameters derived from this model by using various chemical relaxation methods, and (2) kinetic parameters obtained from both relaxation times x, and Xj have reasonably acceptable values. 

Various experimental observations, obtained by studies of diverse nature, indirectly suggest that micellar pseudophase is not homogeneous in terms of micropolarity, water concentration, dielectric constant, and ionic strength (for ionic micelles). " This fact has not been considered in the classical pseudophase kinetic model hrst suggested by Berezin et al. " and Martinek et al. It is therefore logical for Davies et al. to suggest that the micellar pseudophase should be divided up into an arbitrary number of pseudophases, each with a different mean partition coefficient for the reactant or reactants and each with a different mean rate constant. This generalization of the classical (Berezin s) pseudophase model is referred to as the multiple micellar pseudophase (MMPP) model and leads to a kinetic equation similar to Equation 3.61 or Equation 3.11 with modified definitions of kinetic parameters such as kM (= (kMW]y,)KRKs) = E(kM iA Mr)KR iKs i with i = 1, 2, 3,. .., q Kr = S Kr, with i = 1, 2, 3,. .., q and Kg = X K i with i = 1,2, 3,..., q, where q represents an arbitrary number of micelle pseudophases. 

PP model of micelle. This model generally gives a satisfactory fit of observed data in terms of residual errors (= kobs i - kcaicd where kobs i and i are, at the i-th independent reaction variables such as [D ], experimentally determined and calculated [in terms of micellar kinetic model] rate constants, respectively). The model also provides plausible values of kinetic parameters such as micellar binding constants of reactant molecules and rate constants for the reactions in the micellar pseudophase. The deviations of observed data points from reasonably good fit to a kinetic equation derived in terms of PP model for a specific bimo-lecular reaction under a specific reaction condition are generally understandable in view of the known limitations of the model. Such deviations provide indirect information regarding the fine, detailed structural features of micelles. 

The currently accepted explanation for the effect of surfactant concentta-tion on micellar stability was proposed by Aniansson and coworkers in the 1970s and expanded by Kahlweit and coworkers in the early 1980s [13-16], Annianson s model [13-15] nicely predicts micelle kinetics at a low surfactant concentration based on stepwise association of surfactant monomers. Hence, the major parameters in this model are the critical micelle concentration (cmc) and the total concentration of the surfactant in solution. 

Table 15.2 The apparent kinetic parameters of UP in GCDE/TX-100 and ACT reverse micelles .

Figure 15.25 Activation of iipase by imidazolium cation in W/IL reverse micelles. Table 15.6 Kinetic parameters of the iipase-cataiyzed hydrolysis of p-NPB in two systems".

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

Finally, and apart from the importance of micelles in the solubilization of chemical species, mention should also be made of their intervention in the displacement of equilibria and in the modification of kinetics of reactions, as well as in the alteration of physicochemical parameters of certain ions and molecules that affect electrochemical measurements, processes of visible-ultraviolet radiation, fluorescence and phosphorescence emission, flame emission, and plasma spectroscopy, or in processes of extraction, thin-layer chromatography, or high-performance liquid chromatography [2-4, 29-33],... 

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