Determination of the Hydrolysis Kinetics of α-Naphthyl Acetate in Micellar Systems and the Effect of HPMC (catalyst present)

The change in the hexadecyltrimethylammonium bromide (CTAB) critical aggregation concentration (CAC) was studied in the presence of various concentrations and grades of hydroxypropylmethyl cellulose (HPMC) using surface tension measurement (duNoüy ring and Wilhelmy plate) and oil red O solubilization. According to the surface tension methods, the CAC was higher than the CTAB critical micelle concentration (CMC). CAC and CMC were not different when the solubilization method was used. Micellar solutions of CTAB have been found to accelerate the hydrolysis of α-naphthyl acetate (α-NA) by o-iodosobenzoic acid (IBA), a strong nucleophile. Pseudo-first-order kinetics were utilized for rate constant determination. The observed rate constants for the degradation of α-NA in the presence of varying CTAB concentrations with and without HPMC were analyzed according to the pseudophase model. The micellar rate constants and the micellar binding constants for the substrates were obtained. The presence of HPMC retarded the reaction rate, and the rate constant decreased as the polymer concentration increased. However, there was no obvious difference in the observed rate constants among the different grades of HPMC (Methocel E5®, Methocel E15®, Methocel E50®). The decrease in the rate constant was likely due to the polymer-micelle interaction interfering with substrate binding to the CTAB micelles. ©2006 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96:448-458, 2007