Micelles effect upon reaction rate

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Quantitative treatments of micellar rate effects in aqueous solution The development of quantitative models of micellar effects upon reaction rates and equilibria was based on the concept that normal micelles in aqueous, or similar associated, solvents behave as a separate medium from the body of the solvent. 

The general principles which govern the effects of normal, aqueous, micelles upon reaction rates and equilibria are considered first, and then we discuss some specific reactions and the relation of micellar effects to mechanism. We also briefly consider some non-micellar species generated by amphiphiles which can also mediate reactivity. 

Provided that equilibrium is maintained between the aqueous and micellar pseudophases (designated by subscripts W and M) the overall reaction rate will be the sum of rates in water and the micelles and will therefore depend upon the distribution of reactants between each pseudophase and the appropriate rate constants in the two pseudophases. Early studies of reactivity in aqueous micelles showed the importance of substrate hydropho-bicity in determining the extent of substrate binding to micelles for example, reactions of a very hydrophilic substrate could be essentially unaffected by added surfactant, whereas large effects were observed with chemically similar, but hydrophobic substrates (Menger and Portnoy, 1967 Cordes and Gitler, 1973 Fendler and Fendler, 1975). 

This hypothesis is satisfactory for nucleophilic reactions of cyanide and bromide ion in cationic micelles (Bunton et al., 1980a Bunton and Romsted, 1982) and of the hydronium ion in anionic micelles (Bunton et al., 1979). As predicted, the overall rate constant follows the uptake of the organic substrate and becomes constant once all the substrate is fully bound. Addition of the ionic reagent also has little effect upon the overall reaction rate, again as predicted. Under these conditions of complete substrate binding the first-order rate constant is given by (8), and, where comparisons have been made for reaction in a reactive-ion micelle and in solutions... 

The quantitative treatment of micellar rate effects upon spontaneous reactions is simple in that the overall effect can be accounted for in terms of distribution of the substrate between water and the micelles and the first-order rate constants in each pseudophase (Scheme 2). The micelles behave as a submicroscopic solvent and to a large extent their effects can be related to known kinetic solvent effects upon spontaneous reactions. It will be convenient first to consider unimolecular reactions and to relate micellar effects to mechanism. 

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. 

Micelles exert large rate effects upon organic reactions and can in principle discriminate between different reactions, depending upon their charge type or molecularity. There are a number of examples of this type of discrimination in the literature, and they are easily explained in terms of the generally accepted models of kinetic micellar effects. 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

The data in Tables I and II, together with extensive additional evidence, allow several generalizations to be made about micellar effects upon bi-molecular reactions (5). First, overall rate constants follow the distribution of both reactants between water and micelles. Second, second-order rate constants for reactions of nonionic nucleophiles are lower in micelles than in water. Third, second-order rate constants for reactions of anionic nucleophiles are similar in water and micelles except for some reactions of azide ion (37). 

Second-order rate constants in the micelles (k2m) do not depend in any obvious way upon the hydrophobicities of the reactants or, for anionic nucleophiles, upon the surfactant counterion. The relatively small inhibitory micellar medium effect on reactions of nonionic nucleophiles is readily explained by the lower polarity of the micellar surface relative to water (32, 33). The generally small effects of the medium upon the ionic reactions are also understandable because water activity and ionic hydration are similar at the micellar surface and in water (34, 35, 38). 

The importance of proximity effects upon bimolecular reactions is evident from the example of micellar rate effects shown in Table 1. The volume of micelles in dilute aqueous surfactant solution is much less than that of the aqueous component, so that concentration of both reactants into the small volume of the micelles should increase the rate of a bimolecular reaction [1-6,25,61]. 

Micellar effects upon chemical equihbria in aqueous solution were recognized many years ago, and Hartley [12] explained them in terms of the ability of ionic micelles to attract counterions and repel coions. This general explanation was subsequently applied to micellar effects upon chemical reactivity in aqueous solution [13]. A very important monograph outlined the state of knowledge up to 1974, and also noted other associated species which could influence the rates of thermal, photochemical and radiation induced reactions [1]. The initial studies of micellar effects were made in water, but subsequently micelle-like aggregates were observed in non-aqueous solution. These aggregates can also influence chemical reactivity. In some respects the effects of micelles on reactivity are similar to those of cyclo-dextrins or synthetic polyelectrolytes. 

There is little effect of micelles upon the rate of an intramolecular nucleophilic reaction. Micelles of hexadecyltrimethylammonium bromide catalyse, by factors of 10 —10, the arenesulphinate anion-induced hydrolysis of 4-tolylsulphonyl-methyl perchlorate. There is no relationship between the rate acceleration and hydro-phobicity of the sulphinate anion and catalysis is attributed to the concentration of the reactants in the micellar phase.The rate constants for the reaction of nucleophiles with carbonium ions and those for the addition of cyanide ion to the A -alkylpyridinium ions are similar in the micellar and aqueous phases, and the rate enhancement is due to the concentration of reactants in the micellar pseudophase. Similarly, although micellar catalysed dephosphorylation by nucleophiles may show rate enhancements of up to 4 x 10 -fold, the second-order rate constants may be slightly smaller in the micellar pseudophase lowing to its lower polarity. However, the reaction of fluoride ion with 4-nitrophenyldiphenyl phosphate is very rapid in micelles of cetyltrimethylammonium fluoride, but the rate constant continues to increase when the substrate is fully bound with increasing cetyltrimethylammonium fluoride or sodium fluoride. The failure of the micellar pseudophase model is also apparent in the reaction of hydroxide ion with 2,4-dinitrochlorobenzene. It is suggested that reaction occurs between reactants in the aqueous and micellar pseudophases and also between hydroxide ion in water and substrate in the micelle. ... 

The effect of cetyltrimethylammonium bromide on the rate constants of hydrolysis of ethyl glycinate hydrochloride has been reported. " Cationic and non-ionic micelles inhibited and anionic micelles accelerated the acid hydrolysis of A-p-tolyl-benzohydroxamic acid (189). The rates of acid hydrolysis of acetohydroxamic acid, MeCONHOH, benzohydroxamic acid, PhCONHOH, and A-phenylbenzohydroxamic acid, PhCON(OH)Ph, were increased by perfiuorooctanoic acid and by sodium 1-dodecanesulfonate and sodium dodecyl sulfate (SDS). " The effects of micelles of SDS upon the rates of reaction of ionized phenyl salicylate with Bu"NH2, piperidine and pyrroiidine have been reported. 

Some examples of rate and binding constants for these micelle-assisted reactions are in Table 6. There are very large differences in k j /k y/ for these reactions, but the rate effects on decarboxylation are large and depend upon the charge on the head group. Reaction of 2,4-dinitrophenyl phosphate is often written as generating intermediate metaphosphate ion, but this species is so short-lived that reaction follows an enforced association mechanism (Buchwald and Knowles, 1982). 

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. 

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