Observation of molecular layering in thin liquid films using X-Ray reflectivity

We report the direct observation of internal layering in thin (˜45–9Å) 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 ˜10Å (consistent with the molecular dimensions). The oscillation amplitude has a strong inverse dependence on the substrate surface roughness.