Phase Behavior and Aggregate Formation for the Aqueous Monoolein System Mixed with Sodium Oleate and Oleic Acid

Abstract

The phase behavior and microstructure of the two ternary systems monoolein (MO)−sodium oleate (NaO)−water (2H2O) and MO−oleic acid (OA)−2H2O 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−2H2O 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−2H2O 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−2H2O system. The two types of bicontinuous cubic structures, gyroid (CG) and diamond (CD), formed by the binary MO−2H2O system are also prese...