Inverse Bicontinuous Cubic Phases in 2:1 Fatty Acid/Phosphatidylcholine Mixtures. The Effects of Chain Length, Hydration, and Temperature

Using small-angle X-ray diffraction and polarizing microscopy, we have investigated the phase and structural behavior of inverse bicontinuous cubic phases in a homologous series of 2:1 (mol:mol) saturated fatty acid/phosphatidylcholine mixtures in water. For the C 12 fatty acid/phosphatidylcholine mixtures, we find three inverse bicontinuous cubic phases, which our analyses indicate are based upon the P, D, and G minimal surfaces. Between 35 and 49 °C these phases are observed to occur in the sequence G → D → P as the water content is increased. The excess water P cubic disappears above 49 °C when it is replaced by the D cubic in coexistence with the inverse hexagonal phase. For C 14 fatty acid/phosphatidylcholine mixtures, we find only the P cubic in coexistence with the inverse hexagonal phase. For longer chain length fatty acid/phosphatidylcholine mixtures, the inverse bicontinuous cubics are entirely absent, their place being taken by the inverse hexagonal phase. Most of our evidence for the structure of the cubic phases being based on the P, D, and G minimal surfaces comes from our modeling of the isothermal swelling data in the C 12 system. This modeling has also been used to fit the swelling behavior in the inverse hexagonal phase and has allowed us to determine the location of the pivotal surface for the 2:1 fatty acid/phosphatidylcholine mixtures for chain lengths between C 12 and C 18 . With little variation as a function of chain length or hydration, the pivotal surface appears to be located between the second and third CH 2 groups of the hydrocarbon chains with an average molecular area per 2:1 (mol:mol) saturated fatty acid/phosphatidylcholine complex of 109.5 ± 1.5 Å 2 . As far as we are aware this is the first self-consistent determination of pivotal surface geometry spanning four phases and four different chain lengths.