Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles

Mixtures of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzene sulfonate (SDBS) in water form a fluid lamellar phase at ≤40 wt % water but surprisingly turn into viscous gels at higher water fractions. The gels are characterized by spherulite and other bilayer defects consistent with a low bending elasticity, κ ∼ k_BT, and a nonzero spontaneous curvature. Caillé analysis of the small-angle x-ray line shape confirms that for 7:3 wt:wt CTAT=SDBS bilayers at 50% water, κ = 0.62 ± 0.09 k_BT and κ = -0.9 ± 0.2 k _BT. For 13:7 wt:wt CTAT:SDBS bilayers, the measured bending elasticity decreases with increasing water dilution in good agreement with predictions based on renormalization theory, giving κ_o = 0.28 k_BT. These results show that surfactant mixing is sufficient to make κ ∼ k_BT, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles formed at even higher water fractions to be equilibrium structures.