EFFECT OF ALCOHOLS ON CATALYSIS BY DODECYL-SULFATE MICELLES

We studied the kinetics of the acid denitrosation of N-nitroso-N-methyl-p-toluenesulfonamide when catalyzed by sodium dodecyl sulfate (SDS) or hydrogen dodecyl sulfate (HDS) micelles in the presence of one of eight alcohols, with equimolar alcohol and surfactant concentrations. In preliminary experiments, the effects of the alcohols on micellar structure were investigated by conductimetric determination of micellar ionization and, in some cases, by fluoroscopic measurement of mixed micelle aggregation numbers. Both surfactants always catalyzed the reaction, but whereas catalysis by SDS peaked at a certain concentration of surfactant, catalysis by HDS increased with HDS concentration until reaching a limiting value that was maintained thereafter. The degree of catalysis decreased as the hydrophobic nature of the alcohol increased. The reaction kinetics were in all cases in keeping with a pseudophase ion exchange model in which the volume of the Stern layer varies explicity with the quantity of alcohol incorporated in the micelle. Micelle-substrate association constants, bimolecular rate constants in the micellar pseudophase and, for SDS, Na+/H+ exchange constants were calculated. All the kinetic and thermodynamic constants were independent of which alcohol was used and almost the same as those determined in the absence of alcohol. The change in reaction rate due to the alcohols therefore came about solely because the reagents were diluted in the micellar pseudophase. This finding is discussed in terms of the polarity of the Stern layer in single-component and mixed micelles.