Quantitative Correlation between Counterion (X) Affinity to Cationic Micelles and X-Induced Micellar Growth for X = 2,4-; 2,5-; 2,6- and 3,4-Dichlorobenzoate Ions

Abstract

A semi empirical kinetic (SEK) method has been used to determine the ratio of cetyltrimethylammonium bromide (CTABr) micellar binding constants of counterion X^– and Br^– (a reference counterion), i.e. K_X/K_Br (=R_X^Br). The values of K_X and K_Br have been derived from the kinetic parameters obtained in the presence spherical/non-spherical and spherical micelles, respectively. This rather new method gives the respective mean values of R_X^Br as 45±2, 25±3, 4.7±0.6 and 119±10 for X=2,4-, 2,5-, 2,6- and 3,4-Cl₂C₆H₃CO₂^– (Cl2Bz’Na). Literature lacks the report on the values of R_X^Br for all X except for X=2,6-Cl₂C₆H₃CO₂^– (2,6-Cl₂Bz’^–) for which the reported value is 5.0. Rheological properties, such as shear thinning behavior, reveal indirectly the presence of wormlike micelles (WM) in the CTABr micellar solutions containing MX for all X except X=2,6-Cl₂Bz’^–. The micelles remain spherical within [2,6-Cl₂Bz’^–] range 0.01–0.34 M at 0.015 M CTABr. The maxima of the plots of zero shear viscosity, η₀, (obtained from the initial plateau region of flow curves, i.e. η vs. γ curve) vs. [MX] (MX=2,4-, 2,5- and 3,4-Cl₂Bz’Na) at 0.015 M CTABr also support indirectly the presence of linear, entangled and branched WM.