Influence of Surface Wettability on Adsorption Isotherms of Water Vapor

Abstract

Using an exponential form of an isotherm of adsorption the excess surface free energy of a solid surface covered with an adsorption or wetting film Δγsvwas calculated on the basis of Gibbs equation starting from the state of an infinitely thick film. Isotherms of adsorption and of excess surface free energy were calculated for different values of two parameters that characterize the range of action of surface forces and corresponding decay length. In the case of partial wetting of a solid surface, isotherms consisting of two exponential terms were used, as was proposed earlier by A. Adamson. Equilibrium contact angles that are formed with droplets of bulk liquid were calculated using the Frumkin–Derjaguin approach in dependence on the parameters of isotherms. Calculations of contact angles that are formed between convex capillary menisci in thin pores, at relative vapor pressure lower than 1, have shown that a transition from partial wetting of a lone solid surface (atp/ps= 1) to complete wetting in thin pores (atp/ps< 1) may occur. Corresponding threshold values of relative vapor pressure and pore radii were calculated. In the case of complete wetting, corrections for capillary pressures that arise due to formation of a transition zone between wetting films and curved meniscus surfaces were estimated. The calculations are limited by the values of relative vapor pressure near saturation when thick enough wetting films are formed on pore surfaces. Due to the application of model isotherms with some adopted parameters, the results obtained are qualitative in nature. However, the methods of calculation developed for the excess free energy of solid surfaces covered with thin adsorption of thicker wetting films and of contact angles outside and inside thin pores may be of general interest.