Vapor Adsorption on Mica and Silicon: Entropy Effects, Layering, and Surface Forces

We have measured the adsorption isotherms of n-pentane, cyclohexane, tetrachloromethane, octamethylcyclotetrasiloxane (OMCTS), and water on muscovite mica and silicon. An angle-averaging, refractive-index-matching ellipsometric technique has been used to avoid problems due to anisotropy and back-reflection in the case of mica. With OMCTS on mica layering effects like those found with simple gases on graphite and single crystals at low temperatures are observed. OMCTS on silicon shows no layering, but the isotherm is qualitatively different from that of the other nonpolar liquids. The water isotherm on the slightly hydrophobic silicon surface is convex upward but smooth and shows only little adsorption (51 nm) even at saturation. Up to thicknesses equivalent to about half a statistical monolayer, all other isotherms are smooth and linear, showing the importance of entropy effects at low coverages. The isotherms of the nonpolar liquids except OMCTS on both mica and silicon are very similar when normalized by the molecular diameter. Close to saturation differences in wetting are seen. Cyclohexane and tetrachloromethane wet silicon, and n-pentane wets both mica and silicon. None of the isotherms agrees with the Lifshitz theory of van der Waals forces at separations below 2 nm. The results suggest that structural effects are present in many liquid adsorbed films at room temperature, provided that the substrates are smooth and homogeneous. The absence of layering does not imply agreement with the Lifshitz theory of van der Waals forces.