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

The pH and ionic strength dependence of light generated at a corundum-solution interface by the nonlinear optical process of second harmonic generation (SHG) is reported. A point of zero salt effect occurs in the pH range 5 to 6. The pH and ionic strength dependence of the SHG is qualitatively consistent with a model describing SHG from a charged mineral/water interface from Ong et al. (1992) and Zhao et al. (1993a, 1993b), but certain aspects of the model appear inadequate to describe the full range of our data. Atomic force microscopy (AFM) force-distance measurements, though imprecise, were consistent with a point of zero charge (p.z.c.) for the interface also in the pH range 5 to 6. The SHG (and AFM) results are different from expectation; the observed p.z.s.e. (and presumably also the p.z.c.) is considerably lower than the accepted point of zero charge of clean alumina powders (pH 8-9.4; Parks, 1965; Sverjensksy and Sahai, 1996). Although the reasons for this are unclear, SHG holds promise as a probe of oxide-water interfaces that is independent of interpretation of acid-base titration stoichiometry.