温度对液态水结构的影响:中子及X射线散射研究

Structure of Liquid Water Studied by Neutron and X-Ray Scattering: Temperature Effects

采用中子散射和X射线散射研究了液态水在298~373 K温度范围内的结构,通过偏径向分布函数(PDF)、配位数分布(CN)、角分布(ADF)及空间密度分布(SDF)等讨论了温度对液态水结构的影响。整体来看,液态水具有"不规则四面体"氢键网络的短程有序结构,该有序度可延续到第三水合层。液态水分子的第一水合层中,围绕中心水分子约有4.8个水分子,然而其中仅有约3.3个水分子与中心水分子通过氢键相键合,约1/3进入到第一水合层的水分子并未与中心水分子直接键合,也正是这些间隙水分子的存在加剧了液态水结构的复杂性。温度对液态水的有序度存在一定的影响,在298~373 K的有限温度变化范围内,温度对液态水中氢键的键长、键角分布及第一水合层SDF的影响不大。从298K升温到373K,O(W)-O(W)距离仅增加0.03?,氢键数目也仅有微小减少,温度对第二和第三水合层的影响则要显著很多。

Structure of liquid water in the temperature range of 298~373 K was studied by neutron and X-ray scattering. The influences of temperature on the structure of liquid water were discussed in terms of partial radial distribution function(PDF), coordination number distribution(CN), angular distribution function(ADF) and spatial density function(SDF). Liquid water possesses a short-range ordered, "Irregular tetrahedron" hydrogen-bonded network structure, which extends to the third sphere. Around 4.8 water molecules run into the first sphere of liquid water, but only about 3.3 of them bond with the central water molecule. About 1/3 of the water molecules act as the interstitial water, which enters the first layer but does not directly bond with the central water molecule. It is the presence of these interstitial water molecules that aggravates the complexity of the structure of liquid water. High temperature disturbs the structure of liquid water to a certain extent. From 298 K to 373 K, the O(W)…O(W) distance increases by 0.03 ?, the number of hydrogen bonds decreases by 0.1, and the ∠O(W)…O(W)…O(W) angle distribution widens. In the limited temperature range of 298~373 K in the present work, the temperature effects are slight in the SDF for the first sphere, while much more significant effect is observed for the second and the third spheres.