Interaction between anionic and cationic gemini surfactants at air/water interface and in aqueous bulk solution

The mixture of the anionic O,O′-bis(sodium 2-lauricate)-p-benzenediol (C11pPHCNa) and cationic (oligoona)alkanediyl-α, ω-bis(dimethyldodecylammonium bromide) (C12-2-Ex-C12·2Br) gemini surfactants has been investigated by surface tension and pyrene fluorescence. The results show that the surface tension γ drops faster with total surfactant concentration CT for α1 = 0.1 or 0.3 than for α1 = 0.7 or 0.9, where α1 is the mole fraction of C11pPHCNa in the bulk solution on a surfactant-only basis. The fast drop in γ for α1 < 0.5 indicates strong adsorption at the air/water interface owing to the interaction between oppositely charged components, resulting in the formation of the adsorption double layers in the subsurface. The slow descent in γ for α1 > 0.5 is attributed to the pre-aggregation in the solution before the critical micelle concentration cmc. A possible mechanism is proposed.