Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1): Kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

Abstract

Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1) decrease nonlinearly with increasing total concentration of nonionic surfactant CmEn (i.e. [CmEn]T where m and n represent the respective number of methyl/methylene units in the tail and polyoxyethylene units in the headgroup of a surfactant molecule and m/n=16/20, 12/23 and 18/20) at constant 2% v/v CH3CN and 1.0mM NaOH. The kobsvs. [CmEn]T data follow the pseudophase micellar (PM) model at ⩽50mM C16E20, ⩽1.4mM C12E23 and ⩽2.0mM C18E20 where rate of hydrolysis of 1 in micellar pseudophase could not be detected. The values of kobs fail to follow the PM model at > ∼50mM C16E20, > ∼1.4mM C12E23 and > ∼2.0mM C18E20 which has been attributed to a micellar structural transition from spherical to rodlike which in turn increases CmEn micellar binding constant (KS) of 1 with increasing values of [CmEn]T. Rheological measurements show the presence of spherical micelles at ⩽50mM C16E20, ⩽1.4mM C12E23 and ⩽3.0mM C18E20. The presence of rodlike micelles is evident from rheological measurements at > ∼50mM C16E20, > ∼1.4mM C12E23 and > ∼3.0mM C18E20.