Moira K. Ridley

Surface protonation at the rutile (110) interface: explicit incorporation of solvation structure within the refined MUSIC model framework.

The detailed solvation structure at the (110) surface of rutile (alpha-TiO2) in contact with bulk liquid water has been obtained primarily from experimentally verified classical molecular dynamics (CMD) simulations of the ab initio-optimized surface in contact with SPC/E water. The results are used to explicitly quantify H-bonding interactions, which are then used within the refined MUSIC model framework to predict surface oxygen protonation constants. Quantum mechanical molecular dynamics (QMD) simulations in the presence of freely dissociable water molecules produced H-bond distributions around deprotonated surface oxygens very similar to those obtained by CMD with nondissociable SPC/E water, thereby confirming that the less computationally intensive CMD simulations provide accurate H-bond information. Utilizing this H-bond information within the refined MUSIC model, along with manually adjusted Ti-O surface bond lengths that are nonetheless within 0.05 A of those obtained from static density functional theory (DFT) calculations and measured in X-ray reflectivity experiments (as well as bulk crystal values), give surface protonation constants that result in a calculated zero net proton charge pH value (pHznpc) at 25 degrees C that agrees quantitatively with the experimentally determined value (5.4+/-0.2) for a specific rutile powder dominated by the (110) crystal face. Moreover, the predicted pHznpc values agree to within 0.1 pH unit with those measured at all temperatures between 10 and 250 degrees C. A slightly smaller manual adjustment of the DFT-derived Ti-O surface bond lengths was sufficient to bring the predicted pHznpcvalue of the rutile (110) surface at 25 degrees C into quantitative agreement with the experimental value (4.8+/-0.3) obtained from a polished and annealed rutile (110) single crystal surface in contact with dilute sodium nitrate solutions using second harmonic generation (SHG) intensity measurements as a function of ionic strength. Additionally, the H-bond interactions between protolyzable surface oxygen groups and water were found to be stronger than those between bulk water molecules at all temperatures investigated in our CMD simulations (25, 150 and 250 degrees C). Comparison with the protonation scheme previously determined for the (110) surface of isostructural cassiterite (alpha-SnO2) reveals that the greater extent of H-bonding on the latter surface, and in particular between water and the terminal hydroxyl group (Sn-OH) results in the predicted protonation constant for that group being lower than for the bridged oxygen (Sn-O-Sn), while the reverse is true for the rutile (110) surface. These results demonstrate the importance of H-bond structure in dictating surface protonation behavior, and that explicit use of this solvation structure within the refined MUSIC model framework results in predicted surface protonation constants that are also consistent with a variety of other experimental and computational data.

Calcium adsorption at the rutile-water interface: A potentiometric study in NaCl media to 250 C

Abstract Calcium adsorption by rutile was studied potentiometrically from 25 to 250°C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca2+ was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca2+ adsorption directly. These experiments revealed that Ca2+adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pHznpc(NaCl) – pH). That is, as temperature increased, Ca2+ adsorption commenced at progressively more positive pHznpc(NaCl) – pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca2+ adsorption at all temperatures as a result of either increased competition from Na+ or greater complexation of Ca2+ by Cl−. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H+ released / moles Ca2+ adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca2+ adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca2+ waters of hydration, are believed to be primarily responsible for the increase in adsorption with temperature.

Modeling the surface complexation of calcium at the rutile-water interface to 250°C

Abstract The adsorption behavior of metal-(hydr)oxide surfaces can be described and rationalized using a variety of surface complexation models. However, these models do not uniquely describe experimental data unless some additional insight into actual binding mechanisms for a given system is available. This paper presents the results of applying the MUlti SIte Complexation or MUSIC model, coupled with a Stern-based three layer description of the electric double layer, to Ca2+ adsorption data on rutile surfaces from 25 to 250°C in 0.03 and 0.30 m NaCl background electrolyte. Model results reveal that the tetradentate adsorption configuration found for Sr2+ adsorbed on the rutile (110) surface in the in situ X-ray standing wave experiments of Fenter et al. (2000) provides a good fit to all Ca2+ adsorption data. Furthermore, it is also shown that equally good fits result from other plausible adsorption complexes, including various monodentate and bidentate adsorption configurations. These results amply demonstrate the utility of in situ spectroscopic data to constrain surface complexation modeling, and the ability of the MUSIC model approach to accommodate this spectroscopic information. Moreover, this is the first use of any surface complexation model to describe multivalent ion adsorption systematically into the hydrothermal regime.

Comparison of cation adsorption by isostructural rutile and cassiterite.

Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.

Anatase nanoparticle surface reactivity in NaCl media: A CD-MUSIC model interpretation of combined experimental and density functional theory studies

The effect of particle size on the primary charging behavior of a suite of monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been quantitatively examined with macroscopic experimental studies. The experimental results were evaluated using surface complexation modeling, which explicitly incorporated corresponding molecular-scale information from density functional theory (DFT) simulation studies. Potentiometric titrations were completed in NaCl media, at five ionic strengths (from 0.005 to 0.3 m), and over a wide pH range (3-11), at a temperature of 25 °C. From the experimental results, the pH of zero net proton charge (pHznpc) for the 4 and 20 nm diameter samples was 6.42, whereas the pHznpc was 6.22 for the 40 nm sample. The slopes of the net proton charge curves increased with an increase in particle size. Multisite surface complexation and charge distribution (CD) models, with a Basic Stern layer description of the electric double layer, were developed to describe all experimental data. Fits to the experimental data included an inner-sphere Na-bidentate species, an outer-sphere Na-monodentate species, and outer-sphere Cl-monodentate species. DFT simulations found the Na-bidentate species to be the most stable species on the (101) anatase surface (the predominant crystal face). The CD value for the Na-bidentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The Stern layer capacitance value varied systematically with particle size. The collective experimental and modeling studies show that subtle differences exist in the interface reactivity of nanometer diameter anatase samples. These results should help to further elucidate an understanding of the solid-aqueous solution interface reactivity of nanosized particles.

The Protonation Behavior of Metal Oxide Surfaces to Hydrothermal Conditions

Metal oxide surface protonation under hydrothermal conditions is summarized. Important concepts and definitions are introduced first, followed by a brief overview of experimental methods and presentation of representative results. Finally, the modeling methods that are most useful in predicting surface protonation behavior between 0 and 300oC are presented and compared.

Experimental Study of Strontium Adsorption on Anatase Nanoparticles as a Function of Size with a Density Functional Theory and CD Model Interpretation

The effect of particle size on the adsorption of Sr(2+) onto monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been evaluated quantitatively with macroscopic experimental studies. The adsorption of Sr(2+) onto the anatase particles was evaluated by potentiometric titrations in NaCl media, at two ionic strengths (0.03 and 0.3 m), and over a wide range of pH (3-11) and surface loadings, at a temperature of 25 °C. Adsorption of Sr(2+) to the surface of the 20 and 40 nm diameter samples was similar, whereas the Sr(2+) adsorption titration curves were shallower for the 4 nm diameter samples. At high pH, the smallest particles adsorbed slightly less Sr(2+) than was adsorbed by the larger particles. At the molecular scale, density functional theory (DFT) calculations were used to evaluate the most stable Sr(2+) surface species on the (101) anatase surface (the predominant crystal face). An inner-sphere Sr-tridentate surface species was found to be the most stable. The experimental data were described with a charge distribution (CD) and multisite complexation (MUSIC) model, with a Basic Stern layer description of the electric double layer. The resulting surface complexation model explicitly incorporated the molecular-scale information from the DFT simulation results. For 20 and 40 nm diameter anatase, the CD value for the Sr-tridentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The CD value for the 4 nm sample was smaller than that for the 20 and 40 nm samples, reflecting the shallower Sr(2+) adsorption titration curves. The adsorption differences between the smallest and larger anatase particles can be rationalized by water being more highly structured near the 4 nm anatase sample and/or the Sr-tridentate surface species may require more well-developed surface terraces than are present on the 4 nm particles.