Between cylinders and bilayers : structures of intermediate mesophases of the SDS/water system

The classical geometries for the packing of amphiphile molecules in the ordered phases of lyotropic systems are the lamellar phase where bilayers of amphiphilic molecules are separated by water layers, and the hexagonal phase where cylinders of amphiphile are arranged on a two-dimensional hexagonal lattice. These phases only exist for definite compositions of the water/amphiphile system ; in between them, there usually lies a vast concentration region which allows the system to cross from zero curvature (bilayers) to finite curvature (cylinders). The problem with such intermediate regions is that they may contain a large number of mesophases, each limited to a very narrow range of compositions (∼ 1 %). To solve this problem we deliberately create a spatial gradient in the concentration of water/amphiphile of each sample ; such samples are scanned by the X-ray beam of a synchrotron source, allowing diffraction patterns to be recorded at concentration intervals ∼ 0.05 %. For the system SDS/water we have identified 4 intermediate mesophases : a tetragonal phase, a cubic phase, a rhomboedral phase and a two-dimensional monoclinic phase. These structures show that the system manages to cross from a planar geometry to a cylindrical geometry in a nearly continuous way.