Basic hydrolysis of substituted nitrophenyl acetates in β-cyclodextrin/surfactant mixed systems.: Evidence of free cyclodextrin in equilibrium with micellized surfactant

The basic hydrolysis of o-, m-, and p-nitrophenyl acetates (NPA) in mixed systems consisting of beta-cyclodextrin (beta-CD) and a surfactant--sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium hydroxide (TTAOH), or tetradecyltrimethylammonium bromide (TTABr)--has been studied. beta-CD was found to catalyze the basic hydrolysis of NPA via the interaction of its hydroxyl group, in deprotonated form, with the carbonyl group in the complexed substrate. In the presence of a surfactant, the rate of the reaction is initially decreased. This inhibitory effect is the result of the surfactant monomers being complexed by beta-CD and the NPA being in turn displaced from the aqueous medium, which cancels the catalytic effect of the cyclodextrin. Once micellization starts, the system behaves Like a typical micellar system in the hydrolysis of a hydrophobic substrate. Plots of the ratios of pseudo-first-order rate constants in micelle-CD mixed systems and in micelles-kappa(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) versus surfactant concentration revealed the ratio to be greater than unity even at high surfactant concentrations, which is consistent with a CD-catalyzed pathway. This result is in turn consistent with the presence of a substantial concentration of free CD in equilibrium with the micellar systems. The constancy of k(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) at surfactant concentrations above the critical micelle concentration suggests the absence of interactions between CD and the micellar system once the latter has been established.