From Cylinders to Bilayers: A Structural Study of Phase Transformations in a Lyotropic Liquid Crystal

The structural relations of the four intermediate mesophases between the hexagonal phase and the lamellar phase of the sodium dodecyl sulfate-water system are examined. The system manages to cross from a situation with homogeneously curved interfaces (cylinders) to another one with zero curvature (bilayers) in a nearly continuous way. The first step is the transformation of the cylinders into ribbons; then the ribbons become connected to each other at regular intervals to form a three-dimensional array of rods. Subsequently this array is distorted to give a layered structure where the layers are spaced as in a lamellar phase but retain the correlations of the cubic phase in the other directions. The final step is the transition to a system of flat layers. The only physical parameter which appears to change in consistent way through this series of structures is the mean curvature of the interfaces. Comparison with other systems which adopt different solutions to transform a cylinder into a bilayer is discussed.