X-ray reflectivity of classical smectic-A liquid crystals

Using X-ray reflectivity smectic layering is investigated on both sides of a classical monolayer smectic-A to nematic phase transition in a polar NCS substituted compound. At the surface, only monolayering is observed ; there is no breaking of up-down symmetry. This contrasts with the situation of a smectic-A1 phase in a polar CN substituted compound, where bilayering occurs at the surface. Consequences for molecular models of smectic-A phases are discussed.