Hexagonal-cubic phase transitions in lipid containing systems : epitaxial relationships and cylinder growth

In order to approach the nature of the phase transitions, three lipid-water systems (namely PaLPC, OLPC and DTAC), which exhibit a sequence of phases of type I, including hexagonal (which exists in the central region of the three phase diagrams) as well as cubic bicontinuous Q230 (Ia3d) and/or cubic micellar Q223 (Pm3n) phases, have been considered. On the basis of a recently proposed alternative description of the hexagonal phase in terms of finite micelles, the structural parameters as a function of the lipid concentration are analysed in terms of micellar growth inside the hexagonal domain. Moreover, considering the epitaxial relationships occurring at the hexagonal to cubic phase transition, the growth of the hexagonal phase from the 3D networks of rods of the Q230 cubic phase and from the globular micelles packed in the Q223 cubic symmetry has been followed. As a result, a different behaviour has been observed in the three lipid systems. These differences are discussed in relation to the phase sequence : in particular, from a simple geometrical approach, based on the analysis of the projection of the electron density distribution on the cubic phase in the direction of the cylinder growth, a mechanism for the process of transformation of the structure elements is proposed.

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