Molecular structure of a mineral/water interface:: Effects of surface NanoRoughness of α-Al2O3 (0001)

Surface disorder on a nanometer scale is shown to have fundamental influence on the molecular structure of alpha-Al2O3(0001)/water interfaces which have been studied by sum-frequency vibrational spectroscopy. On rough surfaces, we observe isolated surface hydroxyl groups which do not couple to the network of hydrogen-bonded water molecules of the interface and which we assign to aluminol groups in hydrophobic nanopores of the surface. The pH dependence of O-H stretching vibrations of water molecules at the rough interface is distinctly different from that of an atomically smooth interface and points to different pK values for deprotonation of surface hydroxyl groups at defect sites and regular terrace sites.