RbCl和CsCl水溶液结构的X射线散射及经验势结构精修模拟

采用X射线散射法研究了RbCl和CsCl水溶液的结构，利用基于经验势的结构精修(EPSR)方法获得了溶液中的水合Cl^-、Rb⁺、Cs⁺、离子缔合及本体水的对径向分布函数、配位数分布及空间密度分布(3D结构)等结构信息。在水溶液中，Cl^-具有相对稳定的6水合结构，其水合距离为0.321 nm，外加阳离子对其水合作用的影响不明显。7.3 ± 1.4个水分子与Rb⁺水合，其特征水合距离为0.297 nm，8.4 ± 1.6个水分子与Cs⁺水合其水合距离为0.312 nm。Cs⁺不具有第二水合层，而Rb⁺表现出了更强的水合能力，具有较明显的第二水合层。Cl^-、Rb⁺及Cs⁺常被认为是“结构破坏”型离子。从微观角度来看这种所谓的“结构破坏”主要体现在破坏了本体水分子的第二水合层保持四面体构型的趋势。RbCl和CsCl水溶液中部分存在着Rb-Cl和Cs-Cl直接接触离子对，在1.0 mol·dm^-3的溶液中Rb-Cl及Cs-Cl的特征距离分别为0.324和0.336 nm，溶剂分割离子对的距离则都在0.6 nm左右。相对于Cs⁺，Rb⁺与Cl^-离子之间表现出了更强的缔合能力。

Structure of Aqueous RbCl and CsCl Solutions Using X-Ray Scattering and Empirical Potential Structure Refinement Modelling

X-ray scattering measurements were performed on 1.0 mol·dm^-3 RbCl and CsCl aqueous solutions. The X-ray structure factors were subjected to empirical potential structure refinement to extract detailed structural information on hydrated Cl^-, Rb⁺, Cs⁺, and ion association, as well as bulk water, in terms of the individual site-site pair correlation functions, coordination number distributions, and spatial density functions (three-dimensional structure). Cl^- is found to have a relatively stable six-fold coordination of water molecules with a Cl^--H₂O distance of 0.321 nm, and without a significant cation effect on its local structure. Rb⁺ is surrounded on an average by 7.3 ± 1.4 water molecules with a Rb⁺-H₂O distance of 0.297 nm, whereas 8.4 ± 1.6 water molecules hydrate Cs⁺ at a Cs⁺-H₂O distance of 0.312 nm. It is likely that Rb⁺ has a stronger hydration shell than Cs⁺, as evidenced by the presence of the second hydration shell of the former. Contact ion-pairs are partially formed in both solutions and characterized by the Rb⁺-Cl^- and Cs⁺-Cl^- distances of 0.324 nm and 0.336 nm. The solvent-separated ion pairs for both ions are discernible at around 0.6 nm. Rb⁺ has a stronger electrostatic interaction and hence a relatively stronger ion association with Cl^- than Cs⁺.