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 (k_obs) for alkaline hydrolysis of phenyl benzoate (PB) show maxima in k_obs–[D_n] profiles at constant [NaOH] in the presence of cetyltrimethylammonium chloride (CTACl) micelles (D_n). 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 (K^OH_Cl) and in fractional micellar neutralization (β). The use of the PIE model for k_obs, obtained under varying concentrations of added NaCl salt at a constant [NaOH] and [CTACl]_T, gives a K_S value significantly different from the K_S value obtained from k_obs–[D_n]. These observed data (k_obs–[MX], where X = Cl and Br) were also treated in terms of the pseudophase micellar (PM) model coupled with an empirical relationship: K_OH = K_OH⁰/(1 + Ψ_X–OH[MX]) where K_OH is the CTACl micellar binding constant of HO⁻ in the presence of MX and Ψ_X–OH is an empirical parameter. This data treatment, which does not require the constancy of K^OH_X and β as needed in the PIE model, gives data fitting as good as the PIE model. The values of Ψ_X–OH for Cl⁻ and Br⁻ are explained with conceivable chemical reasoning.