Aqueous dispersions of cubic lipid–water phases

Inverse lipid–water phases such as cubic phases can form kinetically stable dispersions by fragmentation in water. Cubic lipid phases can be dispersed by polar lipids favoring lamellar phases or by block copolymers, which can close the bilayer at the surface so that the hydrocarbon chain core is not exposed to water. Monodisperse particles based on glycerol monooleate, with their bilayer curved as the P-, D- or G-minimal surface, have been prepared in this way. Their inner bilayer conformation and outer shape have been examined, mainly by X-ray diffraction and cryo transmission electron microscopy. There is also a different type of cubic lipid bilayer particles with a periodicity in the micrometer range, which have been identified in phospholipid–water dispersions and in cell membrane assemblies. The mechanism behind formation in vivo of such cubic membranes, which also follow the P-, D- and G-surfaces, is discussed. Other lipid–water dispersions with lower symmetry are finally considered; dispersions formed by the inverse hexagonal phase and the dispersed state of a tetragonal bilayer structure formed by lung surfactants.