Molecular layering in a liquid on a solid substrate: an X-ray reflectivity study

We report the observation of molecular layering in a liquid at the solid-liquid interface, using X-ray reflectivity. The liquid was tetrakis (2-ethylhexoxy) silane (TEHOS), which is nonpolar and insulating, with spherical molecules of about 10 Angstrom diameter. We studied both thin (45-90 Angstrom) and relatively thick (similar to 5000 Angstrom) films, which are prepared by dipping in a dilute solution and by pouring and draining the pure material, respectively. For 45-90 Angstrom thick films, three layers of density oscillations are seen near the solid-liquid interface, with a spacing comparable to the molecular size. There is an inverse dependence on the substrate surface roughness. For similar to 5000 Angstrom thick films, we observed a diffraction peak with a correlation length of similar to 32 Angstrom. (C) 2000 Elsevier Science B.V. All rights reserved.