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

Pseudo-first-order rate constants (kobs) for the nucleophilic reaction of piperidine (Pip) with anionic phenyl salicylate (S−) in aqueous solution, obtained at constant content (=2%, v/v) of CH3CN, [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-NaOC6H4CO2Na and 3-NaOC6H4CO2Na), follow the relationship: kobs = (k0 + θKX/S[MX])/(1 + KX/S[MX]), derived from an empirical equation coupled with the pseudophase model of micelle. The empirical constants θ and KX/S provide the respective parameters FX/S and KX/S. The magnitude of FX/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 FX/S shows the cationic micellar penetration of ions X− relative to co-ions S−. The values of FX/S and KX/S have been used to determine usual thermodynamic ion-exchange constant (KXY). The values of KXY (Y = Br) have been calculated for X = 2-−OC6H4CO2− (2-OBz2−) and 3-−OC6H4CO2− (3-OBz2−). The mean values of KXBr are 44 ± 7 and 4.9 ± 0.7 for X− = 2-OBz2− and 3-OBz2−, respectively. The value of KXBr for 2-OBz2− is nearly 9- and 16-fold larger than those for respective 3-OBz2−and 4-−OC6H4CO2− (4-OBz2−). These results are attributed to the presence and absence of viscoelasticity in CTABr aqueous solution with 2-OBz2− and 3-OBz2− as well as 4-OBz2−, respectively. The mean values of FX/S are 0.98 ± 0.10, 0.24 ± 0.07 and 0.14 ± 0.02 for X− = 2-OBz2−, 3-OBz2− and 4-OBz2−, respectively. The change in kinetic probe from piperidinolysis to hydrolysis of S− gives nearly same value, within the limits of experimental uncertainties, of KXBr for 2-OBz2−. The mean value of FX/S × KX/S for 2-OBz2− is decreased by 40-fold in the presence than in the absence of 0.05 M C16E20.