Influence of mixed CTABr–C16E20 nanoparticles on relative counterion binding constants in aqueous solutions of inert salts (2-NaOC6H4CO2Na and NaBr): kinetic and rheometric study

Abstract

A semi-empirical kinetic technique has been used to obtain the ratios of pure CTABr (cationic nanoparticle) and mixed CTABr–C₁₆E₂₀ (cationic–nonionic nanoparticle) micellar binding constants of counterions X and Br, K_X/K_Br (=K^Br_X or R^Br_X) for X = salicylate ion. The values of K^Br_X or R^Br_X for the salicylate ion remain typically independent of the concentration of pure micelles, [CTABr]_T (0.006–0.015 M). These values are also independent of mixed [CTABr]_T–[C₁₆E₂₀]_T micelles under similar conditions. The presence of C₁₆E₂₀ decreases the average values of K^Br_X or R^Br_X from 42 to 16 for salicylate ions. Increase in concentration of cationic and cationic–nonionic nanoparticles at a fixed amount of CTABr reveals the monotonic increase in the value of k_obs, which was explained using the PPM model together with an empirical equation. Rheological characteristics of 0.015 M CTABr solutions in the presence of disodium salicylate indirectly show the presence of branched elongated nanoparticles at 25 °C and 35 °C and [C₁₆E₂₀]_T = 0. Contrarily, such findings reveal the existence of spherical micelles at 25 °C and 35 °C in the presence of ≥0.006 M [C₁₆E₂₀]. The values of K^Br_X or R^Br_X and rheological observations clearly demonstrate that 0.015 M CTABr/M₂X/H₂O (M₂X = disodium salicylate, 2-NaOC₆H₄CO₂Na) solutions contain wormlike and spherical micelles when K^Br_X or R^Br_X values are ≈42 and 16, respectively.