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

Pseudo‐first‐order rate constants (k obs ) 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, F X/S and K X /S , with S representing anionic 1 . The magnitude of F X/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 K X/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 F X/ 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 F X/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, F X/S and K X/S , have been used to determine the usual ion exchange constant (K Cl Br ). The mean values of K Cl Br are 3.9 ± 0.5, 2.7 ± 0.1, and 2.6 ± 0.3 for LiX, NaX, and KX, respectively. These values of K Cl Br 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