Structural phase transformations induced by the addition of decanol to the hexagonal phase of the binary sodium decyl sulphate/water system

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Abstract
The hexagonal phase of the sodium decyl sulphate/water system transforms into a lamellar phase on the introduction of decanol. This transformation occurs in several steps. We present here a study of the sequence of the corresponding phase transformations. The various phases are identified according to their textures by optical microscopy. It can be seen that, as the decanol/soap ratio increases the two dimensional hexagonal phase is followed by two dimensional rectangular phases before the one dimensional lamellar phase is reached. The symmetries of the structures of the phases and the shapes of their aggregates of amphiphilic molecules were determined by small angle X-ray and neutron scattering studies (SAXS and SANS). Two rectangular phases with cmm and pgg symmetries show up successively between the hexagonal and lamellar phases. The shape of the aggregates evolves along the sequence in an unexpected manner. In the two dimensional hexagonal phase, the aggregates are cylinders with an isotropic circular section at low decanol/soap ratio, which become anisotropic as this ratio increases, i.e. the aggregates become ribbon-like aggregates. The aggregates keep this shape in the rectangular phases, with changes of size, and also, most probably, in the lamellar phase near the two dimensional rectangular phases where it can be seen that the lamellae are fragmented. The local organization of decanol and sodium decyl sulphate molecules within the ribbon-like aggregates was also investigated by SANS and deuteron magnetic resonance (DMR). The SANS studies show that the two molecules are not distributed uniformly within the aggregates, while the DMR measurements show that both amphiphilic molecules stay anchored at the amphiphile/water interface by their polar heads. It can then be inferred that the decanol molecules are preferably in the regions of lowest interfacial curvature and the sodium decyl sulphate molecules are in the regions of highest interfacial curvature. This study shows that addition of decanol in the hexagonal phase induces deformation of the cylinders into ribbons by a local flattening of the interface. This deformation, which starts already deep within the hexagonal phase, is not associated with a change of symmetry of the structure. It also appears that a further deformation of the ribbons into lamellae is not needed for the rectangular phase to change into the lamellar phase when the decanol content increases. Thus, in this sequence of phases, the symmetry of the aggregates and those of their organization are not necessarily related.