Perpendicular and lateral equations of state in layered systems of amphiphiles

For colloidal solutions containing any type of surfactant inducing the formation of locally flat interfaces, we show here that two equations of state are required to understand phase behaviour and stability: the perpendicular and the lateral equations of state. This applies to lamellar phases, locally lamellar connected or sponge phases, uni- and multilamellar vesicle phases formed by detergents, lipids, extractants and theta-shaped molecules in all solutions showing optical anisotropy.