Organic counterion-induced micellar stabilization in gemini surfactants: synergistic binding of azo dye anions

This study is influenced by the broad utilization of azo dyes in industry and their persistence in the environment. It investigates the interaction behavior of anionic sunset yellow (SSY) and tartrazine (TAZ) with gemini surfactants GS-5 and GS-6 at 25 °C by conductometric, surface tension, UV-visible, and particle size/zeta potential studies. The dyes considerably decreased the critical micelle concentration (CMC) of both surfactants, from 0.157 to 0.060 mmol L^-1 (SSY) and 0.083 mmol L^-1 (TAZ) for GS-5, and from 0.105 to 0.044 and 0.045 mmol L^-1 for GS-6, respectively, which implies strong dye-surfactant interactions. With increasing dye concentration, the micellar ionization (α = 0.230-0.298) decreased, and the total counterion binding (β = 1.034-1.254) increased owing to the efficient substitution of bromide ions by dye anions at the stern layer. Synergistic mixed-counterion binding was demonstrated using modified Corrin-Harkins and Gibbs-Duhem techniques. GS-6 displayed somewhat stronger interactions with spacer-length effects. UV-Visible experiments indicated increased intrinsic binding and partition constants of TAZ, suggesting more micellar incorporation. Particle size and zeta potential measurements indicated concentration-dependent aggregate expansion (102-285 nm) and a drop in surface charge (+50 to +17 mV), suggesting increased counterion condensation and stable mixed micelle formation. The findings demonstrate that the stability and organization of dye-gemini surfactant mixed systems are jointly modulated by electrostatic attraction, hydrophobic association, cooperative counterion binding and spacer architecture.