EFFECT OF ADSORBED WATER ON WETTING PROPERTIES OF BOROSILICATE GLASS QUARTZ AND SAPPHIRE

Contact angles of a variety of hydrophobic liquids were measured on clean surfaces of borosilicate glass, quartz, and sapphire at 20°C with the RH at 0.6% and 95%. The adsorption of water molecules at 0.6% RH resulted in the formation of only a fraction of a monolayer; yet it converted each high-energy solid surface to one having a low critical surface tension of wetting, γc. The increased amount of water adsorbed at 95% RH to form a film comprising little more than a condensed monolayer further lowered γc to a value somewhat above that of bulk water. At each relative humidity investigated the value of γc was the same for each of these three solids; hence γc was dependent upon the surface concentration of adsorbed water but was independent of the chemical nature of the solid substrate. The somewhat different values of γc obtained on using several different homologous series of hydrophobic liquids are explained in terms of differences in the contribution from nondispersion interfacial forces acting between the water film and the contacting organic liquid molecules. These results reveal clearly the important influence of only a monolayer of adsorbed water upon the spreading, adhesion, and contact angles of organic liquids resting upon high-energy, hydrophilic, solid surfaces. © 1969.