Anionic–cationic mixed surfactant systems: Micellar interaction of sodium dodecyl trioxyethylene sulfate with cationic gemini surfactants

The mixed surfactant systems of anionic sodium dodecyl trioxyethylene sulfate (SDES) and gemini surfactants (cationic) of the series N,N’-bis-(dimethyldodecyl)-α,ω-dialkanediammoniumdibromide, 12-s-12 (s = 2, 4, 6) at different molar ratios were studied by surface tension measurements of aqueous solution as a function of total concentration under standard condition. Various parameters like critical micelle concentration (CMC), surface excess concentration (Гmax), minimum area per molecule (Amin), interaction parameter of mixed micelle and adsorption monolayer (βm, βo) as well as thermodynamic and micellar properties have been determined using Clint, Rubingh, Maeda and Rosen approach. The strong interaction showing very low CMC and large negative interaction parameter β were due to weakening of the electrostatic head group repulsion which favors the mixed micelle formation. The results are discussed in terms of the structural characteristics and spacer chain of gemini surfactants and also in terms of the presence of EO group in SDES molecule. Gemini surfactants bind tightly with SDES by electrostatic, hydrophobic and ion–dipole interactions. Thermodynamic parameters for all three cationic–anionic mixed systems were evaluated. From excess free energy of micellization we can conclude that thermodynamically stable micelles are formed with strong synergistic interaction.