Phase behavior and aggregate formation for the aqueous monoolein system mixed with sodium oleate and oleic acid

The phase behavior and microstructure of the two ternary systems monoolein (MO)-sodium oleate (NaO)-water ((H2O)-H-2) and MO-oleic acid (OA)-(H2O)-H-2 are studied by a combination of optical microscopy, cryo-transmission electron microscopy, small-angle X-ray diffraction, and NMR methods. The results demonstrate significant differences in phase behavior between the two systems. The isothermal phase diagram of the MO-NaO-(H2O)-H-2 system is dominated by a large lamellar liquid crystalline phase that shows an ideal swelling up to high water contents. Stable vesicles are the dominant aggregates at water concentration > 90 wt%. The existence of a lamellar phase is, however, absent from the MO-OA-(H2O)-H-2 system, where the largest single-phase region is a reversed hexagonal liquid crystalline phase, HII, at low water content. A similar water-poor Hii phase is also identified for the MO-NaO-(H2O)-H-2 system. The two types of bicontinuous cubic structures, gyroid (C-G) and diamond (C-D), formed by the binary MO-(H2O)-H-2 system are also present in the ternary systems. Part of the single C-G phase initially formed by the ternary system with NaO is found to be metastable and becomes destabilized within a few weeks, leaving the rest of the C-G phase which is stable like other thermodynamically stable phases for the system. A cubic phase with a reversed micellar type structure is characterized for the oleic acid system. The experimentally determined phase diagrams and the phase structures can be qualitatively understood in terms of the geometry of the lipid molecule in combination with electrostatic effects.