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 tha t increasing pressure, or reducing the fatty acid composition will reduce t he desire far interfacial curvature towards the water and so may favour the formation of inverse bicontinuous cubic phases. We have recorded the exces s water, pressure-temperature phase diagrams of 2:1 FA/PC mixtures of chain length C-12, C-14, C-16 and C-18, over the range 1-1000 bar and approximat ely 0-100 degrees C. There is no evidence in the data of the appearance of new bicontinuous cubic phases under the application of pressure. However, w e 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 b ased on the P minimal surface, in the C-16 system, In the C-18 system we in duce a Pn3m cubic phase, which we believe corresponds to the D minimal surf ace, which co-exists with an inverse hexagonal phase.