Fluctuations and phase transformations in a lyotropic liquid crystal

We are currently studying lamellar/cubic and cubic/hexagonal phase transformations in the lyotropic system hexaethylene glycol mono-n-dodecyl ether (C12EO6)/water. These transformations imply very dramatic changes of the topologies of the structures adopted by amphiphilic molecules. In a previous paper we presented the observation of epitaxial relations involving reticular planes ((001) in the lamellar phase, {211} in the cubic phase, and {10} in the hexagonal phase) and directions (〈111〉 in the cubic phase and cylinder axes in the hexagonal phase) when one structure grows within another. In this paper we present the observation of structural fluctuations appearing in the vicinity of the phase transformations and discuss the nature of the processes taking place at such transformations. This information was obtained by looking for additional scattering, most often diffuse, around and away from the Bragg spots obtained from monocrystals of each structure. In the particular case of the lamellar phase, in the vicinity of its transformation into the cubic phase, it reveals the existence of a fragmentation of the amphiphilic bilayers into cylinders arranged on a local hexagonal lattice. This unexpected fact suggests that similar processes are involved in lamellar/cubic and hexagonal/cubic transformations: the cubic phase grows from cylinders of either hexagonal or lamellar phases with the setting up of correlations between the cylinders which become the 〈111〉 directions of the cubic phase; conversely, hexagonal and lamellar phases grow from the 〈111〉 axes of the cubic phase with a decrease of the correlations between the 〈111〉 directions, which become the cylinders of the hexagonal or lamellar phases. This process, which stresses the role of cylindrical structures and fluctuations, is in quite good agreement with the observation of linear displacements along the 〈111〉 axes in the cubic phase.