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 (C(11)pPHCNa) and cationic (oligoona)alkanediyl-alpha, omega-bis(dimethyldodecylammonium bromide) (C(12)-2-E(x)-C(12)center dot 2Br) gemini surfactants has been investigated by surface tension and pyrene fluorescence. The results show that the surface tension gamma drops faster with total surfactant concentration C(T), for alpha(1) = 0.1 or 0.3 than for alpha(1) = 0.7 or 0.9, where alpha(1) is the mole fraction of C11pPHCNa in the bulk solution on a surfactant-only basis. The fast drop in gamma for alpha(1) <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 gamma for alpha(1) >0.5 is attributed to the pre-aggregation in the solution before the critical micelle concentration cmc. A possible mechanism is proposed. (C) 2009 Elsevier B.V. All rights reserved.