Mamontov E
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA.
J Chem Phys. 2004 Nov 8;121(18):9087-97. doi: 10.1063/1.1804152.
A quasielastic neutron scattering experiment has revealed the dynamics of surface water in a high surface area zirconium oxide in the temperature range of 300-360 K. The characteristic times of the rotational (picoseconds) and translational (tens of picoseconds) components of diffusion motion are well separated. The rotational correlation time shows an Arrhenius-type behavior with an activation energy of 4.48 kJ/mol, which is lower compared to bulk water. The rotational diffusion at room temperature is slower by about a factor of 2 compared to bulk water, whereas the translational diffusion slows down by a factor of 40. In contrast to bulk water, the translational correlation time exhibits an Arrhenius-type temperature dependence with an activation energy of 11.38 kJ/mol. Comparison of different models for jump diffusion processes suggests that water molecules perform two-dimensional jumps at a well-defined, almost temperature-independent distance of 4.21-4.32 A. Such a large jump distance indicates a low molecular density of the layer of diffusing molecules. We argue that undissociated water molecules on an average form two hydrations layers on top of the surface layer of hydroxyl groups, and all the layers have similar molecular density. Quasielastic neutron scattering experiment assesses the dynamics of the outermost hydration layer, whereas slower motion of the water molecules in the inner hydration layer contributes to the elastic signal.
准弹性中子散射实验揭示了高比表面积氧化锆表面水在300 - 360 K温度范围内的动力学。扩散运动的旋转（皮秒级）和平动（几十皮秒级）分量的特征时间有明显区分。旋转关联时间呈现阿仑尼乌斯型行为，活化能为4.48 kJ/mol，与体相水相比更低。室温下的旋转扩散比体相水慢约2倍，而平动扩散减慢40倍。与体相水不同，平动关联时间呈现阿仑尼乌斯型温度依赖性，活化能为11.38 kJ/mol。对不同跳跃扩散过程模型的比较表明，水分子在4.21 - 4.32 Å这一明确的、几乎与温度无关的距离上进行二维跳跃。如此大的跳跃距离表明扩散分子层的分子密度较低。我们认为，平均而言，未离解的水分子在羟基表面层之上形成两个水合层，且所有层具有相似的分子密度。准弹性中子散射实验评估最外层水合层的动力学，而内层水合层中水分子较慢的运动对弹性信号有贡献。
