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 i s highly sensitive to the shape of the molecules of the liquid. Our results are in agreement with published density functional calculations. Small, al most spherical molecules exhibit a one-dimensional ordering perpendicular t o the surface while more complex molecules lead to a low-density region in the vicinity of the substrate.