THIN HYDROCARBON AND WATER FILMS ON BARE AND METHYLATED SILICA - VAPOR ADSORPTION, WETTABILITY, ADHESION, AND SURFACE FORCES

The vapor adsorption of different n-alkanes, cyclohexane, toluene, and water on bare and methylated pyrogenic silica (Aerosil OX-50) has bee n studied gravimetrically. Linear adsorption isotherms of the n-alkane s and of cyclohexane on both substrates are found up to high relative vapor pressures; the same holds true for toluene on methylated silica. The linearity of the isotherms indicates relatively weak lateral inte ractions. On bare silica, the adsorption, expressed in mol/m(2), of th e n-alkanes studied (C7-C9) is independent of chain length. The adsorp tion strongly increases after the coverage, corresponding to a monolay er of alkanes oriented perpendicular to the surface has been reached. Methylation of the silica is found to decrease the adsorption of all t he adsorptives studied. On methylated silica the octane isotherm lies below the coverage of a monolayer parallel to the surface until just b efore saturation. From the adsorption data, surface pressure isotherms are constructed, and the work of adhesion is obtained. The work of ad hesion reveals that the Lifshits-van der Waals part of the silica surf ace tension is reduced from 44 mJ/m(2) for bare (pyrogenic) silica to 30 mJ/m(2) for methylated silica. The adsorption data are also convert ed to disjoining pressure isotherms. At small film thicknesses, the di sjoining pressure can be described by an exponential short-range inter action.