Oxygen adsorption isotherms at the surfaces of liquid Cu and Au-Cu alloys and their interfaces with Al2O3 detected by wetting experiments

The wetting of Cu-O and Au-Cu-O alloys on alumina single-crystals and the surface free energy of these liquids have been determined as a function of oxygen activity. The explored oxygen activity range is from 10(-10) to the value corresponding to the formation of the mixed oxide AlCuO2. The oxygen adsorption at the surface of a metallic liquid and its interface with an oxide are shown to mainly depend on the external phases in contact with the liquid (vapour or oxide respectively). The free energies of the surfaces show monotonic decreases with log a(O) above a critical value of a(O). The free energies of the metal-oxide interfaces also show monotonic decreases and stepped decreases at low oxygen activity. These are interpreted as oxygen adsorption transitions corresponding to the formation of a 2D-interfacial phase. The independence of surface and interfacial oxygen adsorptions suggests that the wetting and adhesion of a liquid metal on an oxide are not systematically improved with increasing oxygen activity.