Liquid Order at the Interface of KDP Crystals with Water: Evidence for Icelike Layers

Abstract

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.

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

“…9 Recently, a single icelike layer was found at the RuO 2 -water interface, 10 and we derived the complete ordering components of a potassium dihydrogen phosphate (KDP) {101} surface in contact with water. 11 From these studies the picture emerges that a liquid in contact with a crystal surface has both solid and liquid properties. The transition to bulk liquid behavior appears to depend strongly on the particular system.…”

“…11 Water in contact with the positively charged KDP-͕010͖ surface shows icelike ordering in the first two water layers and weak but clear in-plane ordering in the third water layer. For the brushite system, which contains water layers in the crystal structure, less ordering is found.…”

“…15 In order to fully determine the liquid ordering, it is crucial to measure rods with different in-plane momentum transfer. 11 The specular rod is only sensitive to the electron density perpendicular to the surface and ignores in-plane order. The other rods probe the lateral order as well.…”

“…16 In order to obtain data with the required accuracy, the water layer has to be very thin, because otherwise the background signal becomes too high. 11 We therefore exploited the fact that in a humid environment the crystal is covered by a thin water layer. The experimental setup consists of a sample environmental chamber 17 that was coupled to a z-axis diffractometer and that can operate in different modes from dry to 100% humidity.…”

“…The in-plane order of the water can be taken into account using an additional term in the density distribution depending on the in-plane momentum transfer and in-plane DW parameters. 11 The electron density distribution for the specular rod is the genuine distribution, which includes the electron density of the water layers without in-plane order. The profiles for a nonspecular rod show the part of the electron density that corresponds to that Fourier component in the liquid.…”