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.