Vesicle formation and other structures in aqueous dispersions of monoolein and sodium oleate

Abstract

The macroscopic appearance and microstructure in the dilute corner of the ternary monoolein (MO)–sodium oleate (NaO)–water ( 2H 2O) system have been investigated by visual inspection and by using direct structural imaging with light microscopy and cryogenic transmission electron microscopy. The microstructural transformations that take place between the micellar phase (binary NaO 2H 2O axis) and the dispersed cubic phase (binary MO 2H 2O axis) upon increasing the ratio of MO to NaO are micelles, ruptured multilamellar vesicles together with flexible threads, various vesicle structures, vesicles in equilibrium with densely packed layers that either represent bilayers or domains of H II tubes visible from the side, and finally a pattern that may be either a cubic phase or domains of H II tubes visible in a cross section. Spontaneously formed uni- and multilamellar vesicles that show long-term stability are found to be the dominant structure for mixed dispersions over almost the entire concentration range. The addition of NaOH to the non-bilayer-forming system, the ternary MOOA (oleic acid)– 2H 2O system, leads to the formation of vesicles. Vesicles were also observed in other ternary MO–aqueous-based systems with potassium oleate, cetyltrimethylammonium bromide, sodium taurodeoxycholate, or dipalmitoylphosphatidylcholine instead of NaO.