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 in verse hexagonal (Hn) and inverse bicontinuous cubic phases (Q(II)(P), Q(II)(D), Md Q(II)(G)) based upon the competition between the energeti cs of monolayer bending and the packing of amphiphiles within each mes ophase. These are used to interpret our experimental observations of t he 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., sub mitted for publication): We have used our models to rationalize the fo llowing observations made at atmospheric pressure: the phase sequences with water composition, the destabilization of the inverse bicontinuo us cubics with increasing chain lengths and temperature, and the phase transition from Q(II)(P) to Q(II)(D) with increasing temperature. We show, for the first time, that phase transitions between the inverse b icontinuous cubic phases may be explained in terms of a change in thei r interfacial geometry. Data from the C-12 mixtures indicates that the preferred radius of curvature of the system decreases by 0.09 Angstro m degrees C-1 and that the ratio of the rigidity for coupled mode bend ing to the bare bending rigidity is 0.14. The latter lies in the range predicted by theory (Templer, R. Il.; Khoo, B, J; Seddon, J. M. Langm uir, in preparation).