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

Abstract

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 C12 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 C14 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 o...