X-ray diffraction evidence of ordering in a normal liquid near the solid-liquid interface

Department of Physics and Astronomy, Northwestern University Evanston, IL 60208-3112, USA

Received: 25 November 1999
Accepted: 9 March 2000

Abstract

We have seen X-ray diffraction peaks establishing that the structure of normal liquids at solid-liquid interfaces is significantly different from the bulk structures. Wetting film thicknesses of ~5000 Å were formed on silicon (111) surfaces with native oxide by pouring and draining the pure liquids; such films are thin enough for X-rays to penetrate easily, but thick enough to eliminate undesired fringes corresponding to the film thickness. The liquids studied were tetrakis(2-ethylhexoxy)silane (TEHOS) and tetrakis(trimethylsiloxy)silane (TTMSS). The observed diffraction peaks are in the specular direction, showing that the liquid molecules form layers parallel to the interface. The layer spacings are comparable to the molecular dimensions, and the peak widths indicate that there are 3–6 layers.