Acid-base properties of a goethite surface model: A theoretical view     

Adelia J.A. Aquino  Daniel Tunega  Georg Haberhauer  Hans Lischka 《Geochimica et cosmochimica acta》2008,72(15):3587-3602

Density functional theory is used to compute the effect of protonation, deprotonation, and dehydroxylation of different reactive sites of a goethite surface modeled as a cluster containing six iron atoms constructed from a slab model of the (1 1 0) goethite surface. Solvent effects were treated at two different levels: (i) by inclusion of up to six water molecules explicitly into the quantum chemical calculation and (ii) by using additionally a continuum solvation model for the long-range interactions. Systematic studies were made in order to test the limit of the fully hydrated cluster surfaces by a monomolecular water layer. The main finding is that from the three different types of surface hydroxyl groups (hydroxo, μ-hydroxo, and μ₃-hydroxo), the hydroxo group is most active for protonation whereas μ- and μ₃-hydroxo sites undergo deprotonation more easily. Proton affinity constants (pK_a values) were computed from appropriate protonation/deprotonation reactions for all sites investigated and compared to results obtained from the multisite complexation model (MUSIC). The approach used was validated for the consecutive deprotonation reactions of the [Fe(H₂O)₆]³⁺ complex in solution and good agreement between calculated and experimental pK_a values was found. The computed pK_a for all sites of the modeled goethite surface were used in the prediction of the pristine point of zero charge, pH_PPZN. The obtained value of 9.1 fits well with published experimental values of 7.0-9.5.  相似文献   

Effect of the Si/Al ordering on structural parameters and the energetic stabilization of vermiculites – a theoretical study     

Tunega  D.  Lischka  H. 《Physics and Chemistry of Minerals》2003,30(9):517-522

The effect of the Si/Al distribution in the tetrahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the structural formula (Mg₄)(Mg₁₂)(Si₈Al₈)O₄₀(OH)₈·24(H₂O) per unit cell were fully optimized. The models differ by the T···Mg²⁺···T coordination of the interlayer Mg²⁺ cations by two central cations from the adjacent tetrahedral sheets of the 2:1 vermiculite layers (T,T=Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg²⁺ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the vermiculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg²⁺ cations.  相似文献