Spontaneous curvature effects on spontaneous vesicles

In mixtures of cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate (FC₇) in aqueous solution, novel bilayer cylinders with hemispherical end caps and open, flat discs coexist with spherical unilamellar vesicles, apparently at equilibrium. Such an equilibrium between bilayer cylinders, spheres and discs is only possible for systems with a spontaneous curvature, r₀, and a positive Gaussian curvature modulus, κ We have measured the size distributions of the spherical vesicles, cylinders and discs using cryo-electron microscopy; a simple analysis of the size distributions then allows us to independently determine that the mean curvature modulus, κ ≈ 5±1k_BT and κ ≈ 2±1k_BT. The most probable spherical vesicle radius is 23 nm, giving a spontaneous curvature, r₀ =19±1 nm. This is one of the few situations in which r₀ , κ , and κ can be determined from the same experiment.