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

Using surface x-ray diffraction, we have determined the atomic structure of the {010} interface of brushite, CaHPO4.2(H2O), 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.