Monitoring Hydrous Metal Oxide Surface Charge and Adsorption by STIRS

The first in situ infrared spectroscopic measurements of surface excess solvated ion concentrations in the electrical double layer resulting from metal oxide surface charge are reported. Enhanced concentrations of perchlorate and tetramethylammonium ions have been detected at the surface of a TiO2 gel film. A new technique, surface titration by internal reflection spectroscopy (STIRS), has been used in which spectra are recorded after flow-induced, stepwise pH changes. A measured isoelectric point for TiO2 of 5 concurs with those from other methods. The perturbing effect of specific adsorption on the TiO2 surface charge is clearly indicated by the STIRS spectra of the system containing oxalate ion.