Ion Adsorption at the Rutile−Water Interface:  Linking Molecular and Macroscopic Properties

Abstract

A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na+, Rb+, Ca2+, Sr2+, Zn2+, Y3+, Nd3+) with macroscopic observables (H+ release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.

Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides

“…As mentioned in Section 3.1.2, it has been shown recently (Zhang et al, 2004) that a number of mono-, diand trivalent metal ions may form quattro-dentate complexes at the 1 1 0 face of rutile (Fig. 2).…”

“…The difference was attributed to the presence of a difference in hydration. However recently, it has been shown by spectroscopy that at the 1 1 0 face of rutile (Zhang et al, 2004), cations may coordinate to surface groups forming quattrodentate complexes. This is not only observed for a variety of divalent and trivalent ions, but it was also found for Rb + at very high pH and concentration.…”

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

“…Both anatase and rutile (001) are reactive with OH-groups accumulating on the surface over the course of the simulations. We emphasize that the varieties in water adsorption behavior demonstrated here must be accounted for in classical modeling of, e.g., ion adsorption 107 and biomolecular functionalization on wet TiO 2 surfaces. 108 The strong TiO 2 surface-water interactions are also confirmed by the distribution of the hydrogen bond angle θ as a function of the SSD.…”

Inner-sphere complexation of cations at the rutile–water interface: A concise surface structural interpretation with the CD and MUSIC model

“…Measurements with these techniques reveal precise structural information on the coordination geometry of specifically adsorbed ions. Similarly, crystal truncation rod analysis provides information on the height of adsorbed ions above crystal surface planes (Fenter et al, 2002;Zhang et al, 2004;Zhang et al, 2006, and references therein). However, X-ray techniques also have limitations.…”