Structure of calcite–aqueous NaCl solution interfaces from ambient to elevated temperatures

抄録

The presence of adsorbed ions on calcite surfaces can significantly affect the adsorption and desorption of organic molecules, which is critical for oil recovery and biomineralization. In this study, the structure of calcite–artificial seawater interfaces from 25 to 80 °C was experimentally and theoretically investigated by surface X–ray scattering and molecular dynamics simulations, respectively. The small difference in the CTR scattering profiles at different temperatures could be attributed to the relaxed outermost calcite surface. The electron density profile of the NaCl solution (0.5 mol/kg) exhibits peaks near the calcite surfaces. The two peaks closest to the surface can be interpreted as adsorbed water molecules, inner–sphere Na⁺ complexes, and inner– and outer–sphere Cl⁻ complexes. Thus, the adsorbed Cl⁻ formed two peaks near the calcite’s surface, while Na⁺ formed a single peak as an inner–sphere complex. It should be noted that there was no strong covalent bond between these inner–sphere complexes and the calcite surface. These structural differences between adsorbed cations and anions could be explained by the balance of the interactions between the surface Ca²⁺ and CO₃²⁻, adsorbed ions, and the surrounding water molecules. The presence of inner–sphere Cl⁻ complexes destabilizes surface Ca²⁺, whereas Na⁺ has an insignificant effect on the structure of surface CO₃²⁻. Adding a small amount (0.045 mol/kg) of Mg²⁺ and SO₄²⁻ appears to enhance the relaxation of the interfacial structure.