Catalysis in Aqueous Solvent

FIGURE 16.4. The presence of micelles in a two-phase reaction medium may produce several effects. A micelle with the same electrical charge as a dissolved reactant may slow its reaction with a solubilized component (path A), while one of opposite charge will usually enhance the reaction rate (path B). Alternatively, especially for nonionic surfactants, the micelle may provide an intermediate solvent environment that enhances the reaction rate (path C). Finally, the dynamics of micellar systems may provide a more readily accessible reservoir of insoluble reactant in the system thereby increasing the reaction rate (path D). 

The abihty of a micellar system to solubilize a reactant may affect its action as a catalyst or inhibitor in a reaction. When a surfactant system serves as a reservoir for increasing the availability of one reactant, any change that increases the solubilizing capacity of the micelle should also increase its effectiveness as a catalysis. If, on the other hand, the reaction must occur in the bulk phase, increased solubiUzing power may remove reactant from the reaction medium and therefore decrease catalytic or increase inhibitor efficiency. 

In aqueous solution, the effectiveness of micellar systems as catalysts is quite often found to increase with the length of the alkyl chain. For example, the rate of acid hydrolysis of methyl o-benzoate in the presence of sodium alkyl sulfates increases in the order Cs Cio Cn Cu Cie. Such a result may be attributed to either electrostatic or solubilizing effects, or both. It might be expected that any effects due to electrostatic interactions would also increase. Alternatively (or additionally), the increasing aggregation number found in the series may result in a significant increase in the solubilizing power of the system. The importance of each mechanism will depend upon the specifics of the reaction. 

As might be expected, the structure of the reactive substrate can have as much influence on micelle-assisted rate enhancement as that of the surfactant. Since the catalytic effectiveness of the micelle can be related to the location and orientation of the substrate in the micellar structure, the more hydrophobic the substrate (and the surfactant), the more significant may be the catalytic effect. 

When nonsurfactant solutes (electrolytes, etc.) are added to the micellar reaction mixture, the results can be quite unpredictable. It is often found that the presence of excess surfactant counterions (common ions), when added to a system in which an ionic reactant is involved, retards the catalytic activity of the micelle, with larger ions being more effective in that respect. The effect can probably be attributed to an increase in ion pairing at the micelle surface and a reduction of its attractiveness to charged reactants. In contrast, the addition of neutral electrolyte has been found to enhance micellar catalysis in some instances. It has been proposed that the retardation effect of excess common counterions is due to a competition between the excess ions and the reactive substrate most closely associated with the micelle for the available positions or binding sites on or in the micelle. The enhancing effect, however, has been attributed to the more general effects of added electrolyte on 

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