  We present high-resolution optical birefringence measurements upon sequential
filling of an array of parallel-aligned nanochannels (14~nm mean diameter) with
rod-like (acetonitrile) and disc-like (hexafluorobenzene) molecules. We will
demonstrate that such birefringence isotherms, when performed simultaneously
with optically isotropic and index-matched counterparts (neopentane and
hexafluoromethane), allow one to characterize the orientational state of the
confined liquids with a high accuracy as a function of pore filling. The pore
condensates are almost bulk-like, optically isotropic liquids. For both
anisotropic species we find, however, a weak orientational order (of a few
percent at maximum) upon film-condensation in the monolithic mesoporous
membrane. It occurs upon formation of the second and third adsorbed layer,
only, and vanishes gradually upon onset of capillary condensation. Presumably,
it originates in the breaking of the full rotational symmetry of the
interaction potential at the cylindrical, free liquid-vapor interface in the
film-condensed state rather than at the silica-liquid interface. This
conclusion is corroborated by comparisons of our experimental results with
molecular dynamics simulations reported in the literature.
