Abstract
Understanding the growth processes of calcium phosphate minerals in aqueous environments has implications for both health and geology. Brushite, in particular, is a component of certain kidney stones and is used as a bone implant coating. Understanding the water–brushite interface at the molecular scale will help inform the control of its growth. Liquid-ordering and the rates of water exchange at the brushite–solution interface have been examined through the use of molecular dynamics simulation and the results compared to surface X-ray diffraction data. This comparison highlights discrepancies between the two sets of results, regardless of whether force field or first principles methods are used in the simulations, or the extent of water coverage. In order to probe other possible reasons for this difference, the free energies for the adsorption of several ions on brushite were computed. Given the exothermic nature found in some cases, it is possible that the discrepancy in the surface electron density may be caused by adsorption of excess ions.
Highlights
Calcium phosphate phases are ubiquitous both as biominerals and geologically
There is some discrepancy in z positions and the classical force fields do not quite reproduce the peaks for the brushite Ca atoms, but overall the electron density profiles have similar heights, widths and z positions of peaks
surface X-ray diffraction (SXRD) and those obtained from the present simulations
Summary
Calcium phosphate phases are ubiquitous both as biominerals and geologically. The mineral apatite (Ca5 (PO4 ) X, where X is an anion such as F− , Cl− or OH− ) is the most stable of the calcium phosphate phases over a large pH range. Brushite forms a aggressive kidney stone variant [2], and while a correlation between pH and calcium content has been identified there is not yet a full understanding of this undesirable calcification [3]. Brushite is a favoured choice because of its high solubility, but there is a limited understanding in regard to its successful resorption into the body [5,6]. Understanding these growth and dissolution processes, in a biological, geological [7] and industrial [8,9] context relies on understanding brushite’s structure and especially its interaction with water at the morphologically important surfaces
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