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

Abstract

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 (^mK_X^Br or ^mR_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 °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.