Structure and Dynamics of Water at Step Edges on the Calcite {101̅4} Surface

Abstract

The behavior of liquid water around obtuse and acute steps parallel to <4 ത 41> on the {1014} cleavage surface of calcite has been investigated by means of classical molecular dynamics simulations performed with a force-field fitted against thermodynamic properties. Water density maps, radial distribution functions and water average residence times have been investigated. The structure and dynamics of the first two ordered hydration layers, which have been previously observed for the basal surface of calcite, are found to be disrupted by the presence of the steps over a range of five molecular rows either side of the step edge. Calcium sites along the step top edge can coordinate up to three water molecules, as compared with just the single water that can be adsorbed per calcium ion on the flat surface. Water residence times at calcium sites in the vicinity of the step span greater than 2 orders of magnitude, from tenths to several tens of ns, as compared to 2 ns and 0.2 ns for the flat surface and a calcium ion in aqueous solution, respectively. The occurrence of waters with long residence times at the step corners points towards the possible role of step dehydration as a rate-limiting factor in calcite crystal growth. Indeed, the different distributions of slow and fast waters along the obtuse and acute steps appear to correlate with the different rates of growth observed for the two types of steps.

Water Structure, Dynamics and Ion Adsorption at the Aqueous {010} Brushite Surface

“…Analysis of the residence times of water around the surface calcium sites of the flat surface indicates that they are particularly long, with no water exchange observed over the course of the 50 ns simulation. To place this in context, the exchange is much slower than for an isolated calcium ion in pure water or for calcium at the basal surface of calcite, which have been previously reported using the same force field as ≈0.2 and ≈2 ns, respectively [29].…”

“…This approximation, if anything, should lead to the simulation overestimating the sharpness of peaks in the electron density distribution, though in practice this is not observed to be the case. The method of fitting the residence times has been previously described by De La Pierre et al [29].…”

“…For water, the isosurfaces shown are in transparent colours where the darker shade is used for the higher density (blue: 2 atoms/Å 3 and grey 0.05 atoms/Å 3 ). Labels indicate the atomic row position (Upper or Lower) and its proximity to the step (1, 2, 3, 4 or 5) using the same notation established for calcite [29].…”

Crystallization pathways in the Great Artesian Basin (Australia) spring mound carbonates: Implications for life signatures on Earth and beyond

“…Crucially, all stages of nucleation and growth documented by nanoscale investigation indicate that nucleation and growth occurred directly from the solution, without the intervention of organic templates (Frisia et al, 2018). The subsequent growth of calcium carbonate into micrometre-scale to millimetre-scale crystals likely takes place by propagation of lattice steps at spiral dislocations (De La Pierre et al, 2016). At the growth stage, the rate limiting process becomes the flux of ions, ion pairs or even nanoparticles to growth sites.…”

Ion dissolution mechanism and kinetics at kink sites on NaCl surfaces

“…Desolvation is widely accepted as the rate-limiting process for growth of ionic crystals and biominerals; although a number of desolvation hypotheses have been proposed (1,3,4,(6)(7)(8)(9)(10)(11)(12)(13), our actual understanding of the desolvation process remains largely incomplete. Only a few computational studies focus on attachment/detachment of a solute at a crystal surface (12,(14)(15)(16)(17)(18)(19)(20)(21), and only recently have kink sites been examined (22,23).…”