On the microstructures of ethyl acetate/monoolein/water

A phase diagram, describing the behavior of the polar lipid monoolein (MO), water, and ethyl acetate (EtAc), is here presented as well as results from small angle X-ray scattering. MO is found to have a solubility of 60 wt.% in EtAc at 20 °C. No macroscopic aggregation of MO can, initially, be detected in the binary MO/EtAc solution even though MO forms solid crystals in concentrated samples when times goes by. In case of the ternary system small amounts of water, mainly bound to the lipid head groups, can be incorporated in the liquid EtAc/MO phase as water has a limited solubility in EtAc. For EtAc/water mass ratios below 2/3 EtAc is present into the reversed bicontinuous cubic and lamellar phases present in the binary MO/water system. To conclude, EtAc is mainly partitioned to the lipid membranes, with minor effects on spontaneous curvature. Hence, simple EtAc-addition has an effect similar to dehydration. For EtAc/water ratios above 2/3 the liquid crystalline phases dissolve. The phase behavior is here discussed in view of related phase behaviors for water-miscible solvent/MO/water systems. For instance, an interpretation of the swelling behavior of the sponge phase (L₃), present in the water-miscible solvent(s)/MO/water systems, shows that solvents partitioned to the polar domains strongly increases the spontaneous curvature of the MO-films. The reason is probably weaker hydrophobic interactions in interfacial regions. As expected, in case of water-miscible solvents, the ternary phase behaviors can be understood by consider water and water-miscible solvent as one “mixed solvent”.