Phase and rheological behavior of novel gemini-type surfactant systems

A novel anionic gemini-type surfactant with no spacer group, disodium 2,3-didodecyl-1,2,3,4-butanetetracarboxylate (GS)-was investigated for its phase behavior in water, water/decane, and water/cosurfactant systems in a wide range of compositions. At low surfactant concentration in the GS-water binary system, a micellar solution phase is formed which transforms to a hexagonal (H-1) phase, as in conventional ionic surfactant systems. At high GS concentration, however, the H-1, phase transforms to the rectangular-ribbon (R-1) liquidcrystal phase. Since the GS molecule has no spacer group, a small cross-sectional area of the headgroup and closely packed hydrophobic chain tend to increase the packing constraints of the lipophilic core with increasing surfactant concentration, thereby inducing the H-1-R-1 phase transition. In the presence of a normal hydrocarbon like decane, the H-1, phase is changed to a micellar cubic phase. On the other hand, the surfactant layer curvature becomes less positive upon addition of a lipophilic amphiphile because it is solubilized in the palisade layer of the aggregate. Addition of short poly(oxyethylene) chain nonionic surfactant (CmEOn, where m, n = 12, 3; 12, 4; and 16, 4) to the aqueous GS solution in a dilute region increases the viscosity by several orders and forms a transparent and viscoelastic micellar solution showing the rheological properties of typical wormlike micelles, with Maxwellian behavior in low oscillatory frequency. In the GS-C12EO3 and also in the GS-C16EO4 systems, the viscosity values are significantly higher than the reported values for dimeric surfactants of similar chain length.