Modeling the Phase Behavior of the Inverse Hexagonal and Inverse Bicontinuous Cubic Phases in 2:1 Fatty Acid/Phosphatidylcholine Mixtures

In this paper we present simplified models of the energetics of the inverse hexagonal (H II ) and inverse bicontinuous cubic phases (,, and ) based upon the competition between the energetics of monolayer bending and the packing of amphiphiles within each mesophase. These are used to interpret our experimental observations of the relative stability of these mesophases in 2:1 (mol:mol) fatty acid/phosphatidylcholine mixtures (Templer, R.H.; Seddon, J.M.; Warrender, N.A.; Syrykh, A.; Huang, Z.; Winter, R.; Erbes, J. J. Phys. Chem., submitted for publication). We have used our models to rationalize the following observations made at atmospheric pressure: the phase sequences with water composition, the destabilization of the inverse bicontinuous cubics with increasing chain lengths and temperature, and the phase transition from to with increasing temperature. We show, for the first time, that phase transitions between the inverse bicontinuous cubic phases may be explained in terms of a change in their interfacial geometry. Data from the C 12 mixtures indicates that the preferred radius of curvature of the system decreases by 0.09 Å °C -1 and that the ratio of the rigidity for coupled mode bending to the bare bending rigidity is 0.14. The latter lies in the range predicted by theory (Templer, R. H.; Khoo, B. J; Seddon, J. M. Langmuir, in preparation).