Second harmonic generation and atomic force microscopy force-distance measurements of a corundum-water interface: Point of zero charge and water structure near a charged interface

We have used two separate techniques, second harmonic generation (SHG) and Atomic Force Microscopy (AFM) force distance measurements to measure the p.z.c. of a corundum-water interface as a function of pH, ionic strength and electrolyte composition. Both techniques indicate a p.z.c. for this interface in the pH range 4--6. The established model for the SHG experiments was found to be of limited use with regard to simultaneously modeling both the ionic strength and pH dependence observed in the SH response. This inadequacy is probably related to the assumption that there is a linear increase in the number of oriented water molecules near the charged oxide-water interface with increasing potential. By using a "damping factor" which increases at higher potentials, the simultaneous ionic strength and pH dependence was modeled more successfully (as determined by a chi-squared statistic). The model is consistent with present knowledge of water structure near a charged oxide-water interface, but only an approximation was used since there is still a large amount of uncertainty about water structure.