Microscopy, SAXD, and NMR Studies of Phase Behavior of the Monoolein−Diolein−Water System

The phase behavior of the ternary monoolein (MO)−diolein (DO)−water ( 2 H 2 O) system is presented. The experimental phase behavior and microstructure are studied by a combination of polarizing microscopy, small-angle X-ray diffraction, and NMR methods. Monoolein forms extensive reversed bicontinuous cubic liquid crystalline phases (C) that are in equilibrium with a lamellar liquid crystalline phase (L α ) on the water-poor side and with excess water on the other side. The presence of small amounts of DO in the MO−water system is sufficient to destabilize the C and L α liquid crystalline phases. Formation of a reversed hexagonal (H II ) phase from the cubic phase occurs at a lower transition temperature than that reported for the MO−water system. Within the cubic region, the diamond cubic phase, C D, is less stable than the gyroid type, C G . The solubility of DO increases within this phase when the MO content increases, and the phase reaches its maximum stability at 4 wt % DO. The large H II -phase formed in the ternary system is in equilibrium with water, and it solubilizes about 30 wt % DO within its stability range. A stable dispersion is formed at even higher DO concentrations. An ideal swelling of the H II -phase with increasing polar volume fraction is observed, whereas the length of the hydrocarbon chains along the hexagonal faces is constant. We measure a slight change of the average area per molecule in the H II -phase with DO concentration. The formation and stability of the liquid crystalline phases can be qualitatively understood from the self-aggregation model, using the geometrical packing parameter of the lipids.