Observation of Molecular Layering in Thin Liquid Films Using X-Ray Reflectivity

We report the direct observation of internal layering in thin ( $\sim 45–90Å$) liquid films of nearly spherical, nonpolar molecules, tetrakis(2-ethylhexoxy)silane, using synchrotron x-ray reflectivity. The Patterson functions have secondary maxima indicating layer formation, and model-independent fitting to the reflectivity data shows that there are three electron density oscillations near the solid-liquid interface, with a period of $\sim 10Å$ (consistent with the molecular dimensions). The oscillation amplitude has a strong inverse dependence on the substrate surface roughness.