Basic hydrolysis of substituted nitrophenyl acetates in beta-cyclodextrin/surfactant mixed systems. Evidence of free cyclodextrin in equilibrium withmicellized surfactant
Ar. Alvarez et al., Basic hydrolysis of substituted nitrophenyl acetates in beta-cyclodextrin/surfactant mixed systems. Evidence of free cyclodextrin in equilibrium withmicellized surfactant, LANGMUIR, 15(24), 1999, pp. 8368-8375
The basic hydrolysis of o-, m-, and p-nitrophenyl acetates (NPA) in mixed s
ystems consisting of beta-cyclodextrin (beta-CD) and a surfactant--sodium d
odecyl sulfate (SDS), tetradecyltrimethylammonium hydroxide (TTAOH), or tet
radecyltrimethylammonium bromide (TTABr)--has been studied. beta-CD was fou
nd to catalyze the basic hydrolysis of NPA via the interaction of its hydro
xyl group, in deprotonated form, with the carbonyl group in the complexed s
ubstrate. In the presence of a surfactant, the rate of the reaction is init
ially decreased. This inhibitory effect is the result of the surfactant mon
omers being complexed by beta-CD and the NPA being in turn displaced from t
he aqueous medium, which cancels the catalytic effect of the cyclodextrin.
Once micellization starts, the system behaves Like a typical micellar syste
m in the hydrolysis of a hydrophobic substrate. Plots of the ratios of pseu
do-first-order rate constants in micelle-CD mixed systems and in micelles-k
appa(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) versus sur
factant concentration revealed the ratio to be greater than unity even at h
igh surfactant concentrations, which is consistent with a CD-catalyzed path
way. This result is in turn consistent with the presence of a substantial c
oncentration of free CD in equilibrium with the micellar systems. The const
ancy of k(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) at su
rfactant concentrations above the critical micelle concentration suggests t
he absence of interactions between CD and the micellar system once the latt
er has been established.