Micellar hexagonal phases in lyotropic liquid crystals

The hexagonal cell parameter a of the system sodium dodecyl lauryl sulfate and water as a function of volume concentration cᵥ in phase H_α shows the functional behavior expected for micelles of finite length: a{∝}cᵥ^-1/3. The interpretation of x-ray data based on finite micelles leads to an alternative description of the hexagonal phase H_α: spherocylindrical micelles of constant radius with length that may grow along the range of the H_α phase. Results are compared with recent statistical-mechanical calculations for the isotropic I-H_α transition. The absence of diffraction in the direction perpendicular to the hexagonal plane is ascribed to polydispersity of micellar length, which also is a necessary condition for the occurrence of direct I-H_α transitions.