Calcite (CaCO3) is an important rock forming mineral that is widely found in nature, both in rocks, such as limestone, and in biominerals, such as sea shells. The growth of calcite from aqueous solution has long been a topic of research, with the aim of both controlling crystal growth, to emulate nature, and inhibiting it, to prevent limescale deposition in pipes and appliances. The absorption and desorption of water on calcite surfaces is a fundamental part of the growth and dissolution process, but there has been some debate about what happens to water molecules when they adsorb on to the calcite surface. Results from experiments appeared to suggest the water dissociates to H+ and OH- ions but calculations found this dissociation to be energetically unfavourable.
In a recent paper in the Journal of Physical Chemistry, researchers from the CMMP group and the London Centre for Nanotechnology at UCL have calculated the interaction between water molecules and defects, such as missing ions and steps, on the surface of calcite. They found that there was a strong interaction between the water molecules and the surface steps but the molecules remained associated. Near carbonate (CO32-) vacancies, however, the water molecule dissociated, with the H+ ion combining with an anion to create a bicarbonate (HCO3-) ion and the OH- ion sitting in the hole left by the vacancy. These results could explain the earlier discrepancy between theory and experiment.
This work has been published in the Journal of Physical Chemistry (J.S. Lardge, D.M. Duffy, M.J. Gillan and M. Watkins, J. Phys. Chem. C 114, 2664 (2010))
Journal link: http://pubs.acs.org/doi/pdfplus/10.1021/jp909593p
Figure: Dissociated water molecule on a calcite surface.