WETTABILITY - THERMODYNAMIC RELATIONSHIPS BETWEEN VAPOR ADSORPTION AND WETTING

Starting from the Gibbs adsorption equation the thermodynamic relation ships between vapour adsorption and wetting are derived. The surface p ressure pi(SV) of a film on a solid formed by vapour adsorption is cal culated from the vapour adsorption isotherm. The surface pressure is r elated to the spreading tension and to the contact angle in the vapour -liquid-solid system. The surface pressure at the saturated vapour pre ssure 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 pi(SV) for two different immiscible liquids allows the prediction of the wettability in a liqui d-liquid-solid system. The procedure is illustrated for the water-octa ne-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.