Electric double layer at the rutile (110) surface. 2. Adsorption of ions from molecular dynamics and X-ray experiments

Molecular dynamics (MD) simulations were conducted to characterize the microstructure of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) for hydroxylated and nonhydroxylated surfaces, each either neutral or negatively charged. The fully atomistic description of the rutile surface and its interactions with the fluid phase was based on ab initio calculations, while the aqueous phase was described by the SPC/E model and existing parametrizations for Rb+, Na+, Sr2+, Zn2+, Ca2+, and Cl- ions. Formation of inner-sphere complexes of cations with surface oxygens was identified for all cations studied. On negatively charged surfaces, Zn2+ is shown to sorb at two bidentate sites, between a bridging and terminal oxygen, and between two terminal oxygens (hydroxylated surface only), while all other cations occupy a tetradentate site, in contact with two terminal and two bridging oxygens in adjacent rows on the crystal surface, and directly above an additional triply coordinated oxygen in the Ti-O surface plane. These differences in inner-sphere binding configuration appear to be related to the bare ionic radii of the cations. Simulation results agree very well with X-ray standing wave and crystal truncation rod studies of the inner-sphere adsorption sites of the cations Rb+ and Sr2+. MD and X-ray results for Zn2+ adsorption are qualitatively consistent, but important differences in adsorption heights are discussed. Both MD simulations and X-ray studies indicate that, on rutile (110), interaction of Cl- with neutral and negatively charged surfaces and with sorbed, multivalent cations is minimal. The hydroxylated surface gives better agreement with experiments than the nonhydroxylated surface and is therefore inferred to be the dominant surface in contact with aqueous solutions at ambient conditions. At the negative, hydroxylated surface, the MD results indicate that Sr2+ and Ca2+ also form outer-sphere species that are laterally ordered with respect to the crystal surface structure, though these are much less abundant than the inner-sphere species. At positively charged hydroxylated surfaces, MD simulations indicate Cl- adsorption in the tetradentate site 4.3 Angstrom above the surface, with longer-range ordering of ions and water molecules than was observed on neutral or negatively charged surfaces. The adsorption geometries of ions are not sensitive to an increase of temperature to 448 K.