CATALYSIS BY SURFACTANT AGGREGATES IN AQUEOUS-SOLUTIONS

Catalysis of organic reactions by unfunctionalized surfactant aggregates (micelles, vesicles) in aqueous solution is largely determined by medium effects induced at the micellar binding sites and by entropy effects due to compartimentalization. The efficiency of these catalytic effects responds to changes in size and shape of the surfactant assemblies. These effects are discussed for a series of 1-alkyl-4-alkylpyridinium halide surfactants using the highly medium dependent decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) as a model reaction. The decarboxylation of 6-NBIC is also strongly catalyzed in the presence of hydrophobically modified poly(alkylmethyldiallylammonium bromides), provided that the flexibility of the polymer main chain allows the formation of hydrophobic microdomains. A test for possible substrate orientation effects on the efficiency of micellar catalysis was performed for the reaction of the sulfonates R1SO2CH2OSO2R2 with OH- ions in the presence of CTAB micelles. Large variations in the hydrophobicities of R1 and R2 led to only small changes in the second-order rate constant for reaction in the micellar pseudophase. These results are in accord with recent insights into the structure of micellar aggregates.