PHASE CHANGE IN AN ADSORBED VAPOR LEADING TO THE WETTING TRANSITIONS
We conducted two sets of exmperiments to investigate the adsorption behaviors at the solid-vapor interface and to examine the Zeta adsorption isotherm model. When the pressure is less than the saturation-vapor pressure, Ps, the gravimetric measurements of vapor adsorption on silicon powder are conducted. Then the interferometer spectra analysis is used to measure the adsorbed thickness when the pressure is greater than Ps. Reuslts show that for heptane adsorbing on silicon, the amount adsorbed is positive when pressure is greater than Ps and the isotherms predicted by Zeta adsoprtion isotherm model agree well the experimental measurements. As the pressure increases, the amount of adsorption undergoes a sharp increase and then tends to a constant. The adsorbed vapor can be approximated as a collection of molecular clusters. When the pressure is near zero, the adsorbate consists primarily of single molecules, and the number of occupied sites is small. As the pressure progressively increases, the clusters with a larger number of molecules appear in the adsorbate and the number of unoccupied sites gradually decreases to zero, the clusters merge to initiate the liquid phase; Zeta adsorption isotherm predicts a phase transition to occur at a pressure that is greater than Ps, and at that pressure, the entropy is continuous, but its first-order partial differential with respect to pressure is not, indicating an adsorbed liquid phase starts to form, and then it wets the surface at a larger pressure.