We present the equilibrium thermodynamics of a layer of water adsorbed on a wettable solid. It seems reasonable to express the adsorption potential with the integration of the van der Waals potential of the power -6 over a given domain of the solid material. First we develop the theory with regard to a layer of water adsorbed on an ice sphere. The theory is then extended to a layer of water adsorbed on a wettable solid of any shape. The chemical potential inside the adsorption layer includes the pressure of water, i.e., the disjoining pressure introduced by Derjaguin. The vapor/water interface, if it exists, can be located by solving the differential equation that the formula of the chemical potential valid on the interface produces when the water pressure is equated to the capillary pressure expressed in terms of the curvature of the surface. The constant that defines the van der Waals potential can be expressed with the material constants if the water is overlaid by the vapor. Importance of the disjoining pressure for causing a flow of adsorbed water is recognized. The theory applies to any liquid that wets a solid surface. © 1992.
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