An apparent weakness of the pseudophase ion-exchange (PIE) model for micellar catalysis by cationic surfactants with nonreactive counterions†

Abstract

Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of phenyl benzoate (PB) show maxima in kobs–[Dn] profiles at constant [NaOH] in the presence of cetyltrimethylammonium chloride (CTACl) micelles (Dn). These observed data have been explained in terms of the pseudophase ion-exchange (PIE) model. But the quality of the data fit to the kinetic equation derived from the PIE model approach remains essentially unchanged with a large change in ion-exchange constant (KOHCl) and in fractional micellar neutralization (β). The use of the PIE model for kobs, obtained under varying concentrations of added NaCl salt at a constant [NaOH] and [CTACl]T, gives a KS value significantly different from the KS value obtained from kobs–[Dn]. These observed data (kobs–[MX], where X = Cl and Br) were also treated in terms of the pseudophase micellar (PM) model coupled with an empirical relationship: KOH = KOH0/(1 + ΨX–OH[MX]) where KOH is the CTACl micellar binding constant of HO− in the presence of MX and ΨX–OH is an empirical parameter. This data treatment, which does not require the constancy of KOHX and β as needed in the PIE model, gives data fitting as good as the PIE model. The values of ΨX–OH for Cl− and Br− are explained with conceivable chemical reasoning.