Orientational Order in Liquids upon Condensation in Nanochannels: An Optical Birefringence Study on Rodlike and Disclike Molecules in Monolithic Mesoporous Silica
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.