Kinetics and Mechanism of Counterionic Salt-Catalysed Piperidinolysis of Anionic Phenyl Salicylate in the Presence of Cationic–Nonionic Mixed Micelles

The quantitative correlation of counterion-affinity to aqueous hexadecyltrimethylammonium bromide (HDAB, cationic micelles/nanoparticles) and the counterion-induced HDAB micellar growth, in the presence of different amounts of poly(ethylene glycol hexadecyl ether) (C16E20, nonionic surfactant), was achieved by the use of a semi-empirical kinetic (SEK) method. The values of the ratio of cationic HDAB, as well as mixed HDAB-C16E20, micellar binding constants of X and Br, K-X/K-Br (= K-X(Br) or R-X(Br)) for X = 4-ClC6H4CO2-, were obtained by the SEK method. The concentration range (0.006-0.015 M) of pure HDAB was found to have no influence on the values of K-X(Br) or R-X(Br). These observations were also recorded upon addition of a nonionic surfactant, C16E20, in an aqueous solution of HDAB. The mean value of K-X(Br) or R-X(Br) obtained in the presence of pure HDAB (K-X(Br) or R-X(Br) = 50.3) is 2.3 times larger than that in the presence of mixed HDAB-C16E20 (K-m(X)Br or R-m(X)Br = 21.7). From rheometric measurements of aqueous HDA(+)/4-ClC6H4CO2- with 0.015 M HDAB, single symmetric maxima (at both 25 and 35 degrees C) were obtained at [4-ClC6H4CO2Na] = 0.03 M. This is evidence for the existence of wormlike micelles/nanoparticles. However, the absence of a maximum in rheometric data for aqueous HDA(+)/C16E20/4-ClC6H4CO2- with 0.015 M HDAB and 0.006 M C16E20 at various [4-ClC6H4CO2Na] revealed the existence of spherical micelles.