EFFECTS OF BUFFERS ON MICELLAR CATALYSIS IN THE BASIC HYDROLYSIS OF PARA-NITROPHENYL CARBOXYLATES

The effect of the kind and concentration of buffers on the basic hydrolysis of p-nitrophenyl carboxylates catalyzed by hexadecyltrimethylammonium bromide (CTAB) was measured at 25°C. The basic component of buffers (carbonate ions and triethylamine) catalyzed this reaction and was suggested to be incorporated on CTAB micelle. Since 2-amino-2-methyl-1,3-propanediol (AMP), however, was not incorporated because of its hydrophilicity, an AMP-HBr buffer is suitable for the present reaction system. Using this buffer, the effect of CTAB concentration on the pK value for thymol blue and on rate constants was investigated and analyzed in terms of the surface potential ψ0 of the micelle. An increase in pK values was explicable on the basis of a decrease of ψ0 with CTAB concentration. The shape of the rate-surfactant concentration profile markedly varied with the kind and concentration of buffers. The profile obtained in the AMP buffer was explicable, taking into account the surface hydroxide ion concentration estimated from pK shifts. The ratio of rate constants on the micelle and in bulk solution did not significantly depend on the chain length of the acyl group. © 1979 American Chemical Society.