Aqueous wetting films on fused quartz

Using an image analyzing interferometer, IAI, the interfacial characteristi cs of an isothermal constrained vapor bubble, CVB, in a quartz cuvette were studied as a precursor to heat transfer research. The effects of pH and el ectrolyte concentration on the meniscus properties (curvature and adsorbed film thickness) and the stability of the aqueous wetting films were evaluat ed. The surface potential in the electric double layer was a function of th e cleaning and hydroxylation of the quartz surface. The disjoining pressure isotherm for pure water was very close to that predicted by the Langmuir e quation. For aqueous solutions of moderate electrolyte concentration, the G ouy-Chapman theory provided a good representation of the electrostatic effe cts in the film. The effect of temperature on the film properties of aqueou s solutions and pure water was also evaluated: The meniscus curvature decre ased with increasing temperature, while Marangoni effects, intermolecular f orces, and local evaporation and condensation enhanced waves on the adsorbe d film layer. Pure water wetting films were mechanically metastable, breaki ng into droplets and very thin films (less than 10 nm) after a few hours. A queous wetting films with pH 12.4 proved to be stable during a test of seve ral months, even when subjected to temperature and mechanical perturbations . The mechanical stability of wetting films can explain the reported differ ences between the critical heat fluxes of pure water and aqueous solutions. The IAI-CVB technique is a simple and versatile experimental technique for studying the characteristics of interfacial systems. (C) 1999 Academic Pre ss.