X-RAY STUDY OF PHASE TRANSITIONS IN AMPHIPHILIC SYSTEMS

Abstract
Amphiphilic molecules in solution in water can build disordered micelles or periodically organized fluid films when the concentration of amphiphilic molecules is large enough. The geometry of the different phases observed can be reduced to the one of the surface (not necessarily connex) that separates the water medium from the amphiphilic one. Therefore transition between two phases corresponds to a change of the topology of this surface. With an accurate structural characterization by X-ray diffraction, it is possible to study how the connectivity of the two media does change. Such a study, using C₁₂EO₆ as amphiphilic molecule, for the transition between an hexagonal and a cubic phase -space group Ia3d- reveals clear epitaxial relations. These relations are useful to understand how the two fascinating interwoven labyrinths of the cubic phase can grow from the 2D network of infinite amphiphilic rods of the hexagonal one. We propose a geometrical model for possible fluctuations in the hexagonal phase, which are playing the role of precursors for the transition. From a less amphiphilic concentrated hexagonal phase, transition towards a micellar phase leads to a much stronger disorganization without epitaxial relations. But a local hexagonal order seems to persist in the micellar phase near the transition line.