MICELLAR EFFECTS ON THE KINETICS AND EQUILIBRIUM OF CHEMICAL-REACTIONS IN SALT-SOLUTIONS

Acid dissociation constants, Kaa, of thymol blue and first-order rate constants, k1, of the basic hydrolysis of p-nitrophenyl butyrate were measured spectrophotometrically at 25°C in the presence of hexadecyltrimethylammonium bromide (CTAB), sodium bromide, and a 2-amino-2-methyl-1,3-propanediol-HBr buffer. Two models, viz., electrostatic and ion-exchange models, which demonstrate the dependence of the kinetics and equilibrium of these chemical reactions on the concentrations of the surfactant and salt were developed and found to be similar under the experimental conditions employed. At high salt concentrations, the addition of CTAB increased the pKaa values and decreased the rate constants, and brought about opposite changes at low salt concentrations. This tendency of micellar effects is explicable on the basis of the two models. The differences between the bulk and micellar values of pKa's as well as rate constants are explained in terms of a lower dielectric constant of the micellar surface, viz., solvent effect. © 1979 American Chemical Society.