Effect of Adsorbed Water on the Critical Surface Tension of Wetting on Metal Surfaces

摘要

Contact angles of a variety of pure polar and nonpolar, nonhydrophilic liquids, covering a wide range of surface tensions, were measured at 20C on clean, smooth surfaces of 14 metals and one metal oxide. At low RH of only 0.6% and high RH of 95%, each of these high-energy, solid surfaces was converted to one of much lower critical surface tension of wetting g by the physical adsorption of a thin film of water. The formation of only a fraction of a monolayer at 0.6% RH decreased g to about 45 dynes/cm for each metal surface; additional water adsorption at 95% RH to form a more condensed film further lowered g to a common value of 38 dynes/cm. Since g 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 on the surface concentration of the adsorbed water. These results were the same as previously obtained for glass surfaces. It is therefore proposed that the surface energy of any clean, smooth, high-energy, hydrophilic surface, whether glass, metal or metal oxide, after exposure to a humid atmosphere depends on the surface concentration of water adsorbed on the surface and that the chemical nature of the underlying hydrophilic substrate has little other effect on wetting and spreading properties. (Author)