Liquid ordering at the Brushite-{010}–water interface

Abstract

Using surface x-ray diffraction, we have determined the atomic structure of the ͕010͖ interface of brushite, CaHPO 4 ·2͑H 2 O͒, with water. Since this biomineral contains water layers as part of its crystal structure, special ordering properties at the interface are expected. We found that this interface consists of two water bilayers with different ordering properties. The first water bilayer is highly ordered and can be considered as part of the brushite crystal structure. Surprisingly, the second water bilayer exhibits no in-plane order, but shows only layering in the perpendicular direction. We propose that the low level of water ordering at the interface is correlated with the low solubility of brushite in water.

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

“…Arsic et al estimated in [16] that there was a coverage of approximately 2-3 layers of water during the experiments as opposed to bulk water for the simulated interface, which could cause a discrepancy between simulated and measured density profiles. However, a comparison to results using other methods suggests this was an underestimate.…”

“…In the as-synthesised morphology, the {010} face is most commonly exposed [14,15]. When imaged via atomic force microscopy (AFM) in water the {010} brushite surface typically exhibits steps of height 7.6 Å [15,16], which correspond to one bilayer of calcium phosphate and one bilayer of water molecules. The dissolution of brushite is then observed to proceed via the formation of triangular etch pits with [001], [201] and [101] steps [17,18] (see Figure 1).…”

“…Although there have been studies of the brushite-water interface, to date there has been no precise determination of the water ordering. Jiang et al [20] studied the adsorption of aspartic acid on brushite and as a part of their study attempted to reproduce the SXRD spectrum from Arsic et al [16]. The force field employed was based upon existing interatomic potentials (OPLS-AA [21], TIP3P water model [22]) with modifications for brushite based on Hauptmann's model [23] with partial charges from density functional theory (DFT).…”

“…Furthermore, it might be supposed that the water ordering of the second layer of water should also be particularly strong and follow that which occurs in the bulk. With this as a hypothesis, surface X-ray diffraction (SXRD) was used by Arsic et al [16] to determine the electron density distribution of the water-brushite interface. However, less in-plane ordering was found than expected.…”

The Link between Brushite and Gypsum: Miscibility, Dehydration, and Crystallochemical Behavior in the CaHPO4·2H2O–CaSO4·2H2O System

“…Another calcium phosphate, brushite CaHPO 4 ·2H 2 O, has received increasing attention due to its significance in technological and environmental issues [4][5][6] . Brushite is a major component of kidney and bladder stones and is widely used as a coating for bone implants 7 .…”