Prediction of the wetting condition from the Zeta adsorption isotherm
Abstract
We use the Zeta adsorption isotherm and propose a method for determining the conditions at which an adsorbed vapour becomes an adsorbed liquid. This isotherm does not have a singularity when vapour phase pressure, PV, is equal to the saturation-vapour pressure, Ps, and is empirically supported by earlier studies for PV < Ps. We illustrate the method using water and three hydrocarbon vapours adsorbing on silica. When the Zeta isotherm is combined with Gibbsian thermodynamics, an expression for γSV, the surface tension of the solid–vapour interface as a function of xV(≡PV/Ps) is obtained, and it is predicted that adsorption lowers γSV from the surface tension of the substrate in the absence of adsorption, γS0, to that at the wetting condition. The wetting hypothesis indicates that γSV at wetting, xVw, is equal γLV, the surface tension of the liquid–vapour interface. For water vapour adsorbing on silica, adsorption lowers γSV to γLV at xVw equal unity, but for the hydrocarbons heptane, octane and toluene adsorbing on silica xVw is found to be 1.40, 1.30 and 1.32 respectively.