Nonionic polymer-cationic surfactant interaction and its influence on micellar catalysis

In an attempt to investigate how a nonionic polymer, hydroxypropyl methylcellulose (HPMC), interacts with a cationic surfactant, cetyltrimethylammonium bromide (CTAB), a dialysis method was employed for directly measuring the HPMC-CTAB interaction. The result showed that an interaction existed between CTAB and HPMC, and the higher the HPMC concentration the greater the fraction of CTAB bound. The change of CTAB critical aggregation concentration, CAC, was studied in the presence of various concentrations and grades of HPMC using surface tension measurement, duNouy ring and Wilhelmy plate, and oil red O solubilization. According to the surface tension methods, CAC was higher than CMC. CAC and CMC were not different when the solubilization method was used.; The addition of HPMC altered reactions performed in micellar solutions due to the polymer-surfactant interaction. Micellar solutions of CTAB have been found to accelerate the hydrolysis of alpha-naphthyl acetate (alpha-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 alpha-NA in the presence of varying concentrations of CTAB were analyzed according to the pseudophase model. The micellar rate constant and the micellar binding constants for the substrates were obtained. These parameters were also determined in the polymer-micelle solutions. The presence of HPMC retarded the reaction rate, and the higher the polymer concentration the lower the rate constant. However, there was no obvious difference in the observed rate constant among the different grades of HPMC (Methocel E5,RTM Methocel E15,RTM Methocel E50RTM) at any given polymer concentration. The decrease in the rate constant was likely due to the polymer-micelle interaction interfering with substrate binding to the CTAB micelles.; The study of solubility as a function of CTAB concentration was another method used for determining the binding constants of alpha-NA and IBA. The addition of HPMC decreased the substrate binding constants to the micelles. The IBA binding constant was also determined using a potentiometric titration method. The results agreed well with those from both the solubility and kinetic studies.