Sidestepping Intermolecular Hydrogen Bonds: How Single Water Molecules Adsorb and Assemble on the Calcite(104)-(2 × 1) Surface.

The adsorption of water on mineral surfaces has a decisive impact on processes in the geological, geochemical, biological, and technological contexts. In this work, we investigate the water/calcite(104)-(2 × 1) interface at the single-molecule level by direct imaging with CO-tip-assisted noncontact atomic force microscopy (NC-AFM) and by density functional theory (DFT) calculations combined with NC-AFM image simulations. For single water molecules, the adsorption geometries within the (2 × 1) calcite unit cell are consistently identified by experiments and simulations. The energetic difference between the energetically most favorable adsorption position next to a bulk-like carbonate row (QS water) and the less favorable adsorption geometry next to a reconstructed carbonate row (PR water) can be explained by the local relaxation of the calcite(104) surface nearby a PR water molecule that locally restores the unreconstructed (1 × 1) surface structure. Different combinations of QS and PR water molecules yield a variety of water dimer configurations. We find that the dimer adsorption energy is mostly identical to the sum of the individual molecule energies. From successively raising the sample temperature up to 170 K at half monolayer coverage, we observe that all water molecules move to the most favorable QS position. Overall, the first layer of water molecules on the calcite(104) surface is strongly bound to the substrate in the absence of intermolecular hydrogen bonds.