Fusion and fission of catanionic bilayers

We herein report the reversible formation of a planar lamellar phase as an intermediate structure in a vesicle-micelle transition induced by temperature, for a salt-free catanionic surfactant-water system. Turbidity, small-angle neutron scattering, dynamic light scattering and microscopy data altogether demonstrate that the vesicles formed by hexadecyltrimethylammonium octylsulfonate (TA(16)So(8)) in water fuse at high temperature into planar lamellae before transforming into elongated micelles. On cooling the micelles first nucleate into lamellae, and only then vesicles appear through lamellae fission. This behavior is both intriguing and counter-intuitive, since a monotonous dependence of the spontaneous curvature of the surfactant film on temperature should be expected.