Wettability: thermodynamic relationships between vapour adsorption and wetting

Starting from the Gibbs adsorption equation the thermodynamic relationships between vapour adsorption and wetting are derived. The surface pressure π SV of a film on a solid formed by vapour adsorption is calculated from the vapour adsorption isotherm. The surface pressure is related to the spreading tension and to the contact angle in the vapour—liquid—solid system. The surface pressure at the saturated vapour pressure determines, together with the interfacial tension of the liquid, the difference between the interfacial tension of a solid and a solid—liquid interface. Knowledge of the value of π SV for two different immiscible liquids allows the prediction of the wettability in a liquid—liquid—solid system. The procedure is illustrated for the water—octane—silica system. The value of the spreading tension obtained from the individual adsorption isotherms indicates that the silica surface has a strong preference to be water wet.