We present a surface x-ray diffraction study on the KDP-water interface in which the structure of both the crystalline and liquid part of the interface has been measured. We have been able to determine the ordering components in the liquid in both the perpendicular and parallel directions. We find interface-induced ordering in the first four layers of water molecules. The first two layers behave icelike and are strongly bound to the surface. The next two layers are more diffuse and show only minor lateral and perpendicular ordering. Subsequent layers are found to behave similar to a bulk liquid.

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We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.

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.

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