Room temperature adsorption of water by aluminum thin films

Thin films of aluminum were prepared under ultra-high vacuum conditions in order to investigate the low temperature, low pressure adsorption of water vapor by the aluminum. The kinetics of the water vapor-aluminium reaction have been found to be essentially different than the oxygen-aluminum reaction previously reported. In contrast to the “dry” oxygen uptake kinetics, a plot of the sticking coefficient of H₂O versus the total weight gain of the film indicates that the sticking coefficient of H₂O passes through a maximum. As a result of the present mass adsorption measurements of water by fresh aluminum surfaces and Huber and Kirk's previous contact potential studies of an oxidized aluminum surface upon exposure to water vapor, a model is suggested, based on the simultaneous lateral growth of oxide nuclei and first order adsorption of water dipoles on the growing oxide nuclei surfaces. The model quantitatively describes the kinetics of the mass adsorption of water and also predicts the contact potential behavior of a fresh aluminum surface upon exposure to water vapor. A sticking coefficient of approximately 0.05 is indicated for H₂O on bare aluminum while 0.11 corresponds to the sticking coefficient of water dipoles on the oxide nuclei surfaces.