Theoretical Insights into the Hydrated (10.4) Calcite Surface: Structure, Energetics, and Bonding Relationships

Abstract

Roothaan-Hartree-Fock molecular orbital methods were applied to investigate the ground-state structural, energetic properties, and bonding relationships of the hydrated (10.4) calcite surface. The adsorption of water molecules was modeled at the 6-31G(d,p) level of theory using Ca(n)(CO(3))(n) slab cluster models (4 <or= n <or= 18) with a varying number of H(2)O monomers (2 <or= (H(2)O)(n) <or= 6) interacting with the surface. Modeling results add fresh insights into the detailed 3D structural registry of the first and second hydration layers and the reconstructed (10.4) calcite surface, complementary to the information acquired from earlier atomistic, density functional, X-ray scattering, and grazing incidence X-ray diffraction studies. Both the modeled energies and geometries agree best with results of earlier density functional calculations, supporting the associative character of adsorbed water molecules. Two adsorption configurations are postulated: (i) H(2)O molecules interacting with surface Ca through ionic bonding and by hydrogen bonding to a surface O with their dipole slightly oblique above the surface (1st hydration layer), and (ii) H(2)O molecules that hydrogen bond to surface O and to H(2)O molecules in the first hydration layer with their dipole nearly parallel to the surface (2nd hydration layer). These interactions are consistent with the "chemisorption" and "physisorption" of H(2)O on calcite surfaces, proposed on the basis of previous thermogravimetric and Fourier-transformed infrared studies. Most significant is the distortion of the surface Ca-O octahedra caused by the relaxation (and possibly rupture) of some Ca-O bonds upon hydration, weakening the topmost atomic layer. These findings are consistent with interpretations of X-ray photoelectron spectroscopy, density functional theory, and electrokinetic studies that suggest the preferential release of surface Ca atoms over surface CO(3) groups upon hydration of the cleavage surface. These insights will help to elucidate mechanisms of carbonate mineral dissolution, the rearrangement of surface layers, ion replacement, charge development, and solute transport through subsurface lattice layers.

Nucleation of metastable aragonite CaCO 3 in seawater

“…Surface energies are obtained from DFT computations of hydrated calcium carbonate surface slabs with partial Mg 2+ substitution on calcium sites (SI Appendix, section S.I.6). The solvated surface is modeled by adsorption of explicit water molecules on the cation sites, which physically captures the thermodynamic effect of water hydration (34). The dominant facets of the aragonite nucleus are predicted to be the (001), (011), (010), and (110) forms, and for calcite the ð1014Þ and ð1010Þ forms, consistent with experimental morphologies and other calculations in the literature (35,36).…”

“…27. Hydrated surface energies were calculated with explicit H 2 O molecules, placed 2.4 Å above each calcium ion (34), oriented with lone-pair orbitals facing the nearest Ca 2+ or Mg 2+ ion and a hydrogen pointed toward the nearest oxygen atom, then relaxed by simulated annealing. The use of H 2 O adsorbates is a reasonable approximation for hydrated surface energies under pH values corresponding to neutral and ocean water (pH = 8.1).…”

High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy

“…4 doubling the surface unit cell in the b dimension) that has been ascribed to a slight rotation of some surface carbonate groups. While the (2 xl) superstructure on calcite (104) face matches with low energy electron diffraction 122] and computer modelling studies [23,24], it has not been observed by Rode et al [20] using frequency modulation AFM.…”

Influence of surface conductivity on the apparent zeta potential of calcite

“…Calcite surface functional groups of the dominating crystallographic plane (calcite (1 0 4) surface) behave differently than those of oxides, because they are assumed to be controlled by two different types of sites resulting from the hydrolysis of surface water molecules: a hydroxylated calcium cation, >CaOH, and a protonated carbonate anion, >CO 3 H surface site [8,11]. This assumption of hydrolysis of surface water molecules at the calcite-water interface was recently questioned by a theoretical study showing that surface water molecules may not be dissociated and that >CaOH 2 and >CO 3 surface sites may thus control the calcite surface speciation [58]. Following the recent surface complexation model of Heberling et al [10], we will also consider the presence of >CaOH 2 and >CO 3 surface sites at the calcite surface.…”