Kinetic study on the effects of mixed nonionic-cationic micelles on the rate of alkaline hydrolysis of N-hydroxyphthalimide

Pseudo first-order rate constants (kobs) for the alkaline hydrolysis of N-hydroxyphthalimide (1) remain independent of the total concentration of CnE20, with n = 16,18 ([CnE20]T) at ≤ 0.17 M C16E20 and 0.10 M C18E20. The values of kobs, obtained at different [CnE20]T (with n = 16,18) in the presence of a constant post CMC value of [CTABr]T, follow the empirical relationship: kobs = (k0 + θ K [CnE20]T)/(1 + K [CnE20]T), where θ and K are empirical constants. The values of θ are not affected whereas the values of K decrease nonlinearly with increase in [CTABr]T at ≥ 0.003 M CTABr in a mixed CnE20-CTABr micellar system where n = 16,18. The effects of [CTABr]T on K and θ are explained in terms of the pseudophase model of micelles coupled with the empirical relationship: KS = KS0/(1 + KCnE20/S [CnE20]T) with n = 16,18, where KS and KS0 represent respective mixed CnE20-CTABr and pure CTABr micellar binding constant of anionic 1 (S−) and KCnE20/S is an empirical constant whose magnitude is the measure of the ability of the CnE20 surfactant to change the micellar affinity of S− from pure CTABr micelles to pure CnE20 micelles.