SURFACE REACTIVITY OF NANOCRYSTALLINE ANATASE

The chemical and electrostatic interactions at mineral-water interfaces is of fundamental importance in many geochemical processes, which has lead to considerable interest in characterizing this interface region by theoretical and experimental means. Specifically, the development of surface charge has been studied frequently as a function of pH, at the macroscopic scale. Data predicting the surface behavior of macroscopic crystals may not apply directly to natural, low temperature systems, where nano-sized crystals represent a major fraction of available reactive surfaces. Therefore, to accurately model and predict the role of natural nano-sized particles in the environment it is essential to understand the size-dependencies of mineral-water interface properties. This presentation will summarize our experimental and modeling efforts investigating the size-dependence of surface protonation, and pHznpc and IEP values of nanometer anatase (TiO₂). A suite of anatase samples ranging in particle size from <5nm to 200nm were studied. Potentiometric titrations and electrophoretic mobility studies were completed, with the two experimental techniques matching as closely as the different procedures permitted. Titrations were performed in NaCl media at ionic strengths from 0.005 to 0.3 molality, at 25ºC. The surface charge of the anatase was enhanced with increasing ionic strength. Moreover, the experimental data suggests that the pHznpc values increase with decreasing particle size. The experimental results were rationalized using the 1-pK and MUSIC surface complexation models, in combination with a basic Stern-layer representation of electrical double layer (EDL) structure. MUSIC model fits were constrained by complementary molecular dynamics modeling results, which provided Ti-O bond lengths. To adequately model and describe the experimental data of the smallest particles at the lower ionic strengths, the spherical symmetry of the diffuse portion of the EDL must be accounted for. Research sponsored by: NSR–NIRT initiative EAR–0124001.