Basic Hydrolysis ofm-Nitrophenyl Acetate in Micellar Media Containing β-Cyclodextrins

The alkaline hydrolysis of m -nitrophenyl acetate ( m -NPA) was studied in sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium (TTAX; X = Br - or HO - ) micellar systems containing β-cyclodextrin, CD. CD catalyzes the alkaline hydrolysis of m -NPA through reaction of the complexed substrate with the hydroxyl group of the cyclodextrin. The presence of aggregates at concentrations below the cmc exert an inhibitory effect. Above their cmc's, SDS or TTAX micelles exhibit the typical inhibitory and catalytic effects, respectively, on the alkaline hydrolysis of hydrophobic substrates. This behavior is explained in terms of two competing equilibria involving complexation of the substrate and the surfactant monomers by CD. The kinetic model combines the pseudophase model, including ion exchange, with a competitive binding model for the CD-catalyzed hydrolysis of m -NPA. In this model, counterions do not bind to the surfactant ion in the surfactant·CD complex and neither CD nor the surfactant·CD complex bind to micelles. Competitive complexation by CD and self-association (micellization) of the surfactant monomers results in the presence of a substantial amount of free (uncomplexed) CD. Once micelles are formed, no effect of cyclodextrin addition is observed on their properties.