Abstract

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 (H1) phase, as in conventional ionic surfactant systems. At high GS concentration, however, the H1 phase transforms to the rectangular-ribbon (R1) liquid−crystal 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 H1−R1 phase transition. In the presence of a normal hydrocarbon like decane, the H1 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.

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