A new semiempirical kinetic method for the determination of ion exchange constants for counterions of cationic micelles: Study of the rate of pH‐independent hydrolysis of phthalimide as the kinetic probe

Abstract

AbstractPseudo‐first‐order rate constants (kobs) for pH‐independent hydrolysis of phthalimide (1), obtained at a constant total concentration of cetyltrimethylammonium bromide and hydroxide ([CTABr]T), 2.0 × 10−4 M 1, 0.02 M MOH (M+ = Li+, Na+ and K+) and various concentrations of inert salt MX (= LiCl, LiBr, NaCl, NaBr, KCl and KBr), follow a relationship derived from the pseudophase micellar (PM) model coupled with an empirical equation. This relationship gives empirical constants, FX/S and KX /S, with S representing anionic 1. The magnitude of FX/S is the measure of the fraction of micellized anionic 1 (S−M) transferred to the aqueous phase by the limiting concentration of X−. The value of KX/S is the measure of the ability of the counterions (X−) to expel the reactive counterions (S−) from the cationic micellar surface to the aqueous phase. The values of FX/ S are ∼ 1 for MBr (M+ = Li+, Na+ and K+) and in the range ⩽ 0.7 to ⩾ 0.5 for MCl (M+ = Na+ and K+) at 0.006, 0.010 and 0.016 M CTABr. For LiCl, the values of FX/S become ∼1 at 0.006 and 0.010 M CTABr and 0.8 at 0.016 M CTABr. The values of the empirical constants, FX/S and KX/S, have been used to determine the usual ion exchange constant (KClBr). The mean values of KClBr are 3.9 ± 0.5, 2.7 ± 0.1, and 2.6 ± 0.3 for LiX, NaX, and KX, respectively. These values of KClBr are comparable with those obtained directly by other physicochemical techniques. Thus, this new method for the determination of ion exchange constants for various counterions of cationic micelles may be considered as a reliable one. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 9–20, 2011