Water Orientation at the Anatase TiO₂ Nanoparticle Interface: A Probe of Surface pK_a Values

The acid-base properties of surfaces significantly influence catalytic and (photo)electrochemical processes. Estimation of acid dissociation constants (pK_a values) for colloids is commonly performed through electroanalytical techniques or spectroscopic methods employing label molecules. Here, we show that polarimetric angle-resolved second harmonic scattering (AR-SHS) can be used as an all-optical, label-free probe of colloid surface pK_a values. We apply AR-SHS to dispersions of 100 nm anatase TiO₂ particles to extract surface potential and surface susceptibility, a measure of interfacial water orientation, as a function of pH. The surface potential follows changes in surface charge density, while the interfacial water orientation inverts at pH ∼4.8, ∼6, and ∼7.6. As the variation in bulk pH modifies the populations of Ti-OH₂⁺, Ti-OH, and Ti-O^- interfacial groups, a change in water orientation reports on the ratio of protonated/deprotonated species. Such observation allows for pK_a evaluation from plots of surface susceptibility versus pH. A Nerstian trend in the surface potential is additionally demonstrated.