Catalysis of Phosphate Triester Hydrolysis by Micelles of Hexadecyltrimethylammonium <i>anti</i>-Pyruvaldehyde 1-Oximate

Abstract

The hydrolysis reactions of 4-nitrophenyl diphenyl phosphate, 4-nitrophenyl di-4-tolyl phosphate, and diethyl 4-nitrophenyl phosphate (paraoxon) have been studied in hexadecyltrimethylammonium anti-pyruvaldehyde 1-oximate (C16TAOx) buffer solutions prepared by 1:2 mixing of hexadecyltrimethylammonium hydroxide with anti-pyruvaldehyde 1-oxime, thus allowing the avoidance of counterion inhibition effects observed when using conventional cationic surfactants. The reactions are strongly accelerated by the C16TAOx micelles, and the half-life for the persistent insecticide paraoxon is as low as 2 min at [C16TAOx] = 10-2 M. The data are discussed in terms of the pseudophase ion-exchange (PIE) model. However, this model is not sufficient to explain that the rate constants increase significantly when sodium oximate is added and that KCl and NaOAc salt effects exhibit S-shaped curves. Conductivity data suggest some minor variations in the degree of counterion association, β, but these variations are too small to account for the deviations from the PIE theory.