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David J. Wesolowski,
Jorge O. Sofo,
Andrei V. Bandura,
Zhan Zhang,
Eugene Mamontov,
Milan Předota,
Nitin Kumar,
James D. Kubicki,
Paul R. C. Kent,
Lukas Vlcek,
Michael L. Machesky,
Paul A. Fenter,
Peter T. Cummings,
Lawrence M. Anovitz,
Adam A. Skelton, Jörgen Rosenqvist
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ABSTRACT: Liu and co-workers [ Phys. Rev. B 82 161415 (2010)] discussed the long-standing debate regarding whether H2O molecules on the defect-free (110) surface of rutile (α-TiO2) sorb associatively, or there is dissociation of some or all first-layer water to produce hydroxyl surface sites. They conducted static density functional theory (DFT) and DFT molecular dynamics (DFT-MD) investigations using a range of cell configurations and functionals. We have reproduced their static DFT calculations of the influence of crystal slab thickness on water sorption energies. However, we disagree with several assertions made by these authors: (a) that second-layer water structuring and hydrogen bonding to surface oxygens and adsorbed water molecules are ‘‘weak’’; (b) that translational diffusion of water molecules in direct contact with the surface approaches that of bulk liquid water; and (c) that there is no dissociation of adsorbed water at this surface in contact with liquid water. These assertions directly contradict our published work, which compared synchrotron x-ray crystal truncation rod, second harmonic generation, quasielastic neutron scattering, surface charge titration, and classical MD simulations of rutile (110) single-crystal surfaces and (110)-dominated powders in contact with bulk water, and (110)-dominated rutile nanoparticles with several monolayers of adsorbed water.
Phys. Rev. B. 04/2012; 85(16).
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Michael Machesky,
David Wesolowski, Jörgen Rosenqvist,
Milan Předota,
Lukas Vlcek,
Moira Ridley,
Vaibhav Kohli,
Zhan Zhang,
Paul Fenter,
Peter Cummings,
Serguei Lvov,
Mark Fedkin,
Victor Rodriguez-Santiago,
James Kubicki,
Andrei Bandura
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ABSTRACT: 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.
Langmuir 03/2011; 27(8):4585-93. · 4.19 Impact Factor
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ABSTRACT: The acid-base properties of cassiterite (alpha-SnO2) surfaces at 10-50 degrees C were studied using potentiometric titrations of powder suspensions in aqueous NaCl and RbCl media. The proton sorption isotherms exhibited common intersection points in the pH range of 4.0-4.5 under all conditions, and the magnitude of charging was similar but not identical in NaCl and RbCl. The hydrogen bonding configuration at the oxide-water interface, obtained from classical molecular dynamics (MD) simulations, was analyzed in detail, and the results were explicitly incorporated in calculations of protonation constants for the reactive surface sites using the revised MUSIC model. The calculations indicated that the terminal SnOH2 group is more acidic than the bridging Sn2OH group, with protonation constants (log KH) of 3.60 and 5.13 at 25 degrees C, respectively. This is contrary to the situation on the isostructural alpha-TiO2 (rutile), apparently because of the difference in electronegativity between Ti and Sn. MD simulations and speciation calculations indicated considerable differences in the speciation of Na+ and Rb+, despite the similarities in overall charging. Adsorbed sodium ions are almost exclusively found in bidentate surface complexes, whereas adsorbed rubidium ions form comparable numbers of bidentate and tetradentate complexes. Also, the distribution of adsorbed Na+ between the different complexes shows a considerable dependence on the surface charge density (pH), whereas the distribution of adsorbed Rb+ is almost independent of pH. A surface complexation model (SCM) capable of accurately describing both the measured surface charge and the MD-predicted speciation of adsorbed Na+/Rb+ was formulated. According to the SCM, the deprotonated terminal group (SnOH(-0.40)) and the protonated bridging group (Sn2OH+0.36) dominate the surface speciation over the entire pH range of this study (2.7-10). The complexation of medium cations increases significantly with increasing negative surface charge, and at pH 10, roughly 40% of the terminal sites are predicted to form cation complexes, whereas anion complexation is minor throughout the studied pH range.
Langmuir 09/2009; 25(18):10852-62. · 4.19 Impact Factor
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ABSTRACT: An electrophoresis cell developed in our laboratory was utilized to determine the zeta potential at the SnO(2) (cassiterite)/aqueous solution (10(-3) mol kg(-1) NaCl) interface over the temperature range from 25 to 260 degrees C. Experimental techniques and methods for the calculation of zeta potential at elevated temperature are described. From the obtained zeta potential data as a function of pH, the isoelectric points (IEPs) of SnO(2) were obtained for the first time. From these IEP values, the standard thermodynamic functions were calculated for the protonation-deprotonation equilibrium at the SnO(2) surface, using the 1-pK surface complexation model. It was found that the IEP values for SnO(2) decrease with increasing temperature, and this behavior is compared to the predicted values by the multisite complexation (MUSIC) model and other semitheoretical treatments, and were found to be in excellent agreement.
Langmuir 08/2009; 25(14):8101-10. · 4.19 Impact Factor
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ABSTRACT: Our quasielastic neutron-scattering experiments and molecular-dynamics simulations probing surface water on rutile (TiO2) have demonstrated that a sufficiently high hydration level is a prerequisite for the temperature-dependent crossover in the nanosecond dynamics of hydration water. Below the monolayer coverage of mobile surface water, a weak temperature dependence of the relaxation times with no apparent crossover is observed. We associate the dynamic crossover with interlayer jumps of the mobile water molecules, which become possible only at a sufficiently high hydration level.
Physical Review E 06/2009; 79(5 Pt 1):051504. · 2.26 Impact Factor
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Michael L Machesky,
Milan Predota,
David J Wesolowski,
Lukas Vlcek,
Peter T Cummings, Jörgen Rosenqvist,
Moira K Ridley,
James D Kubicki,
Andrei V Bandura,
Nitin Kumar,
Jorge O Sofo
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ABSTRACT: 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.
Langmuir 11/2008; 24(21):12331-9. · 4.19 Impact Factor
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ABSTRACT: The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH)3 solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol−1 of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol−1 are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q1 or Q2 silicon sites, respectively. The dissolution mechanism varies with ΔG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q1 sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q1 reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q2 rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q3 sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
Geochimica et Cosmochimica Acta.
