Spontaneous formation of onion phases in a single surfactant system and their salt-induced transformation towards ordinary lamellar phases

Onion phases are lyotropic lamellar phases which contain regions in which the surfactant bilayers are concentrically stacked like the layers in an onion. We report the spontaneous formation of such phases containing the single surfactant AOT and water. Upon addition of salt, a spontaneous transition to an ordinary lamellar phase is observed and this is explained quantitatively in terms of electric contributions to the splay and saddle splay moduli of the bilayer.