Molecular Structure of Canonical Liquid Crystal Interfaces

Numerous applications of liquid crystals rely on control of molecular orientation at an interface. However, little is known about the precise molecular structure of such interfaces. In this work, synchrotron X-ray reflectivity measurements, accompanied by large-scale atomistic molecular dynamics simulations, are used for the first time to reconstruct the air-liquid crystal interface of a nematic material, namely, 4-pentyl-4'-cyanobiphenyl (5CB). The results are compared to those for 4-octyl-4'-cyanobiphenyl (8CB) which, in addition to adopting isotropic and nematic states, can also form a smectic phase. Our findings indicate that the air interface imprints a highly ordered structure into the material; such a local structure then propagates well into the bulk of the liquid crystal, particularly for nematic and smectic phases.