Abstract
The physisorption of the vapor of pure nonpolar liquids (alkanes and cycloalkanes) and water onto a thermally oxidized silicon wafer was studied by means of a phase modulation ellipsometer. The resulting film thickness isotherms show complete wetting for all studied nonpolar liquids (n-pentane, n-hexane, cyclopentane, and cyclohexane) and incomplete wetting for water on the SiO2(27 nm)/Si-wafer. The film thickness isotherms of the wetting systems were analysed in terms of the Frenkel-Halsey-Hill (FHH) equation, In (p/p0) = –(α/kT)·d−n, where n is expected to have a value of 3 for non-retarded dispersion force interactions. The present systems, however, exhibit a much smaller value, n = 1.5±0.2. It appears that for small film thickness (a few monolayers) the exponent n depends on the nature of the adsorbent surface and increases with the strength of the adsorbate-adsorbent interaction; in this regime the FHH equation can be taken merely as an empirical relation.
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