WETTING FILMS ON SILICA

Measurements are reported of the thickness of equilibrium wetting films of dilute solutions of KCl, LiCl, BaCl2, and LaCl3 on polished vitreous silica. Under constant hydrostatic pressure, film thickness decreases with increase of salt concentration and with increase of valency of the cation. The results are in approximate agreement with Langmuir's theory of wetting films, confirming that the fundamental origin of the phenomenon is the diffuse electrical double layer. However, the experimental film thicknesses with KCl and BaCl2 appear to be significantly greater than values calculated on the assumption that no charge exists at the air/water interface. The discrepancies could be accounted for by the presence of a relatively weak negative electrical double layer, itself depending on the nature and concentration of electrolyte in the solution. Reduction of the zeta-potential to approximately zero by addition of LaCl3 led to a very thin residual film; such films are particularly susceptible to contamination by traces of hydrophobic impurities. © 1969.