Point of zero charge of a corundum-water interface probed with optical second harmonic generation (SHG) and atomic force microscopy (AFM): New approaches to oxide surface charge

Structures and Charging of α-Alumina (0001)/Water Interfaces Studied by Sum-Frequency Vibrational Spectroscopy

“…3 and 6 that at pH 3.3 and 9.3, the alumina surface is already significantly protonated and deprotonated, respectively. From the pK values of reactions (8) and (11) In previous interfacial studies, the pH value at which dissolution of salt has no effect on the interfacial structure, known as the point of zero salt effect (p.z.s.e), was often used to find p.z.c., 21,22,53 because in the absence of surface charges, salt ions are not attracted to the interface. In our SFVS study, this means that at the p.z.c., addition of salt into the aqueous solution should not change the spectrum.…”

Bond-valence methods for pKa prediction: critical reanalysis and a new approach

“…If only doubly coordinated groups are present, force and streaming potential measurements on this surface have consistently supported the idea that doubly coordinated aluminol groups contribute to surface charge under pH conditions not predicted by the MUSIC method. Stack et al (2001) performed AFM force titrations and second harmonic generation experiments to determine the point of zero charge for the corundum basal surface, although the surface was miscut by ϳ5°. They observed a PZC of pH 5 to 6 for this surface, in contrast to the generally accepted figure of pH ϳ 9 for alumina samples.…”

“…They observed a PZC of pH 5 to 6 for this surface, in contrast to the generally accepted figure of pH ϳ 9 for alumina samples. Kosmulski (2003) objected that this low PZC likely resulted from surface contamination, but Stack et al (2003) responded that Auger Electron Spectroscopy characterization of the surface in question did not reveal the presence of any contaminants other than adventitious C. In addition, it was pointed out that these results only applied to a single surface on corundum, rather than the entire surface, for which the accepted PZC of pH ϳ 9 might still apply. Franks and Meagher (2003) used streaming potential measurements and AFM force titrations to obtain isoelectric points (IEP) of pH 5 to 6 for the (001) surface of sapphire.…”

The pH-Dependent Surface Charging and the Points of Zero Charge

“…Certainly, the significance of these correlations is limited by the quality of the PZC data. The well-established PZC of aluminum oxide at a pH of about 9 was recently challenged (8), and a new method to determine the PZC from second-harmonic generation was proposed, which led to an apparent PZC at pH 5-6. While new or improved spectroscopic methods brought about substantial progress in the studies of specific adsorption of ions, the new methods proposed to study the primary surface charging were less successful.…”