Sequence of Phase Transformations in the Ternary Lyotropic System Sodium Decyl Sulfate/2-Octanol/Water

Overview & Synopsis

An oriented hexagonal phase of the ternary lyotropic system sodium decyl sulfate/2-octanol/water transforms into a lamellar phase. The transformation occurs in several steps due to a decrease of the water content by slow evaporation in the course of time while the 2-octanol/sodium decyl sulfate molar ratio remains constant. The following sequence is identified by small angle X-ray scattering studies: hexagonal phase ↦ hexagonal phase coexisting with a rectangular phase ↦ cubic phase ↦ cubic phase coexisting with a lamellar phase ↦ lamellar phase. Epitaxial relations between the different phases are pointed out. The diffraction pattern of the lamellar phase exhibits in addition to the characteristic reflections of the latter, diffuse scattering which corresponds to density fluctuations within the amphiphilic bilayers. According to the Bragg reflections the cubic phase is a centred one and we propose a new model for this phase. The structure reminds us of that of the cubic phase which space group is Im3m, but the rods of both sub-lattices are no longer cylinders as in Im3m but ribbons and therefore the space group is Im3.