Inverse bicontinuous cubic phases in fatty acid/phosphatidylcholine mixtures: the effects of pressure and lipid composition

We report on the effect of hydrostatic pressure on the structure and phase behaviour of symmetric 2:1 (mol/mol) fatty acid/phosphatidylcholine (FA/PC) mixtures. In addition, the effect of changing the fatty acid/phospholipid composition has been explored. The working hypothesis we have used, was that increasing pressure, or reducing the fatty acid composition will reduce the desire for interfacial curvature towards the water and so may favour the formation of inverse bicontinuous cubic phases. We have recorded the excess water, pressure–temperature phase diagrams of 2:1 FA/PC mixtures of chain length C12, C14, C16 and C18, over the range 1–1000 bar and approximately 0–100°C. There is no evidence in the data of the appearance of new bicontinuous cubic phases under the application of pressure. However, we have been able to induce cubic phases by reducing the FA/PC ratio from 2:1 to 1:1. We then find that we induce an Im3m cubic phase, believed to be based on the P minimal surface, in the C16 system. In the C18 system we induce a Pn3m cubic phase, which we believe corresponds to the D minimal surface, which co-exists with an inverse hexagonal phase