Effect of adsorbed water on the critical surface tension of wetting on metal surfaces

Abstract

Abstract Contract angles of a variety of pure nonhydrophilic liquid polar and nonpolar compounds, covering a wide range of surface tensions, were measured on clean smooth surfaces of fourteen metals and one metal oxide. At 0.6% relative humidity (RH) and 95% RH, each of these high-energy, solid surfaces was converted at 20°C to one of much lower critical surface tension of wetting (γc) by the physical adsorption of a thin film of water. The formation of only a fraction of a monolayer at 0.6% RH decreased γc to about 45 dynes/cm for each metal surface; additional water adsorption at 95% RH to form a more condensed film further lower γc to a common value of 38 dynes/cm. Since γc values were nearly identical for each surface at each extreme of the RH, the surface energy of these hydrophilic solids must be independent of the constitution of the underlying solid substrate and dependent only upon the surface concentration of the adsorbed water.