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 <4̅41> on the {101̅4} 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 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 toward 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.