Evidence for density anomalies of liquids at the solid/liquid interface

We show that the density of liquids is altered significantly next to a hard wall. Thin films of van der Waals liquids on silicon substrates have been prepared and investigated by means of X-ray reflectivity. This probe yields density profiles on an angstrom scale. The region next to the substrates is highly sensitive to the shape of the molecules of the liquid. Our results are in agreement with published density functional calculations. Small, almost spherical molecules exhibit a one-dimensional ordering perpendicular to the surface while more complex molecules lead to a low-density region in the vicinity of the substrate.