Pseudophase exchange model

An attempt to quantify this phenomenon has led Bunton and Romsted to develop the pseudophase exchange model [27, 29] and alternative treatments [36]. 

Some examples of micellar rate enhancements of bimolecular reactions of electrophiles are shown in Table 5. Generally the surfactant was SDS with added electrophile, e.g. H30+ or a metal ion, but sulfonic acids were also used so that HaO+ was the counterion and there was no interionic competition. The maximum rate enhancements, knl, depend upon the specific conditions of the experiment, and, as predicted by the pseudophase ion-exchange model, generally decrease with increasing concentration of the electrophilic ion. In some cases the reactions were too fast for measurement... 

The acid hydrolysis of micellized alkyl sulfates (Kurz, 1962 Motsavage and Kostenbauder, 1963) has recently been very carefully reinvestigated (Garnett et al., 1983). For relatively dilute micellized alkyl sulfate, salt inhibition follows the predictions of the pseudophase ion-exchange model, with the expected salt order. But this order is not followed with more concentrated alkyl sulfate, and these results are a very interesting deviation from the widely observed pattern of micellar salt effects. 

A very careful analysis of the pseudophase ion-exchange model has been given by Romsted who reviewed the evidence up to 1982 and considered the limitations of the treatment (Romsted, 1984). 

An alternative approach to this problem is to assume that deprotonation of a weak acid in the micellar pseudophase will be related to the concentration of bound OH-, which should follow the ion-exchange model (Section 5). As a result much of the work has been based on the use of very weak acids which are deprotonated only at high pH. 

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

There is limited evidence for the applicability of the pseudophase kinetic model to microemulsions. Amines can act as cosurfactants and stabilize o/w microemulsions, and these amines are effective nucleophiles towards 2,4-dinitrochlorobenzene, and the rate constants in the microemulsion droplets are not very different from those in water [156]. Reactions in which bromide ion acts as a nucleophile also have similar second-order rate constants in a microemulsion aggregate as in a micelle, suggesting that it should be possible to apply an ion exchange model to microemulsions [157]. However, Mackay and coworkers have treated ion binding in microemulsions in terms of the surface potential of the aggregates [153,154], following an approach which has been applied to ionic micelles in water [158]. 

Micellar and microemulsion effects on reactivity in aquation and base hydrolysis reactions of iron(II)-diimine complexes have been much studied/ The latest contribution deals with the effects of added potassium chloride or bromide to micelles of the respective cetyltrimethylammonium halides. Effects on base hydrolysis of [Fe(phen)3] and its 4,7-diphenyl and 3,4,7,8-tetramethyl derivatives can be interpreted in terms of competitive binding to the micelles in a pseudophase-ion exchange model. In connection with these secondary effects of added halides it should be mentioned that further studies of kinetics of aquation of [Fe(bipy)3] and of [Fe(phen)3] in strong aqueous solutions of chlorides have been interpreted in terms of water and of chloride attack, with the postulation of transient diimine-chloride-iron(II) intermediates. ... 

Figure 3.17 Variation of the rate constant for escape from SDS micelles with the concentration of sodium ions in the aqueous phase from Reference 253 ( ) and Reference 254 (O). The solid line going through the data from Reference 253 is a fit based on the pseudophase ion exchange model. Reproduced from Reference 254 with permission of the American Chemical Society.

Neves, M. de F.S., Zanette, D., Quina, R, Moretti, M.T., Nome, F. Origin of the apparent breakdown of the pseudophase ion-exchange model for micellar catalysis with reactive counterion surfactants. J. Phys. Chem. 1989, 95(4), 1502-1505. 

Ruzza, A.A., Nome, F, Zanette, D., Romsted, L.S. Kinetic evidence for temperature-induced demixing of a long chain dioxolane in aqneous micellar solutions of sodium dodecyl sulfate a new application of the pseudophase ion exchange model. Langmuir 1995, 77(7), 2393-2398. 

The counterion binding with ionic micelles is generally described in terms of two alternative approaches the first one is the widely used pseudophase ion-exchange model (Chapter 3, Subsection 3.3.7) and the second one, less commonly used, is to write the counterion binding constant in terms of an ionic micellar surface potential (Q) (Chapter 3, Section 3.4). The value of K in Equation 6.16 is expected to remain independent of [CTACllj as long as the degree of association... 

The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyl-trimethylammonium bromide (TTABr) and sodium dodecyl sulfate (SDS) on the rates of alkaline hydrolysis of securinine (223) were studied at a constant [HO ] (0.05 m). An increase in the total concentrations of CTABr, TTABr and SDS from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, and 7, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelles. Cationic micelles of CTABr speed attack of hydroxide ion upon coumarin (224) twofold owing to a concentration effect. ... 

For the first case, one can use the so-called pseudophase ion exchange (PIE) model.The PIE model is based on the Menger-Portnoy model but additionally allows for ion exchange to occur in the micellar Stern region where a reactive counterion competes with nonreactive counterions (Scheme 5). 

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