DOI: 10.1051/jp2:1994206
J. Phys. II France 4 (1994) 1393-1416

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

Paolo Mariani¹, Lia Q. Amaral², Letizia Saturni¹ and Hervé Delacroix<sup>3

1</sup>  Istituto di Scienze Fisiche, Facoltà di Medicina e Chirurgia, Università di Ancona, via Ranieri, 60131 Ancona, Italy.
²  Instituto de Fisica, Universidade de Sao Paulo, Caixa Postal 20516, CEP 01498-970, Sao Paulo, SP, Brazil.
³  Centre de Génétique Moléculaire, Laboratoire Propre du CNRS, 91198 Gif-sur-Yvette France

(Received 22 December 1993, revised 4 March 1994, accepted 26 April 1994)

Abstract
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 Q ²³⁰ (Ia3d) and/or cubic micellar Q ²²³ (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 Q ²³⁰ cubic phase and from the globular micelles packed in the Q ²²³ 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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