A new empirical kinetic method for the determination of ion-exchange constants for the counterions of cationic micelles: The rate of piperidinolysis and hydrolysis of anionic phenyl salicylate as the kinetic probes

Abstract

Pseudo-first-order rate constants ( k obs) for the nucleophilic reaction of piperidine (Pip) with anionic phenyl salicylate (S −) in aqueous solution, obtained at constant content (=2%, v/v) of CH 3CN, [Pip] T (=0.1 M), [S −] T (=2 × 10 −4 M), [CTABr] T (total concentration of cetyltrimethylammonium bromide), NaOH (≤0.10 M) and varying concentration of inert salts, MX (=2-NaOC 6H 4CO 2Na and 3-NaOC 6H 4CO 2Na), follow the relationship: k obs = ( k 0 + θK X/S[MX])/(1 + K X/S[MX]), derived from an empirical equation coupled with the pseudophase model of micelle. The empirical constants θ and K X/S provide the respective parameters F X/S and K X/S. The magnitude of F X/S gives the measure of the fraction of cationic micellized S − transferred to the aqueous phase by the limiting concentration of co-ions X − through the ion-exchange process X −/S −. In other words, the value of F X/S shows the cationic micellar penetration of ions X − relative to co-ions S −. The values of F X/S and K X/S have been used to determine usual thermodynamic ion-exchange constant ( K X Y ). The values of K X Y (Y = Br) have been calculated for X = 2- −OC 6H 4CO 2 − (2-OBz 2−) and 3- −OC 6H 4CO 2 − (3-OBz 2−). The mean values of K X Br are 44 ± 7 and 4.9 ± 0.7 for X − = 2-OBz 2− and 3-OBz 2−, respectively. The value of K X Br for 2-OBz 2− is nearly 9- and 16-fold larger than those for respective 3-OBz 2−and 4- −OC 6H 4CO 2 − (4-OBz 2−). These results are attributed to the presence and absence of viscoelasticity in CTABr aqueous solution with 2-OBz 2− and 3-OBz 2− as well as 4-OBz 2−, respectively. The mean values of F X/S are 0.98 ± 0.10, 0.24 ± 0.07 and 0.14 ± 0.02 for X − = 2-OBz 2−, 3-OBz 2− and 4-OBz 2−, respectively. The change in kinetic probe from piperidinolysis to hydrolysis of S − gives nearly same value, within the limits of experimental uncertainties, of K X Br for 2-OBz 2−. The mean value of F X/S × K X/S for 2-OBz 2− is decreased by 40-fold in the presence than in the absence of 0.05 M C 16E 20.