State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences , Dalian, China.
J Phys Chem B. 2013 Dec 12;117(49):15395-406. doi: 10.1021/jp404923y. Epub 2013 Aug 13.
The optical Kerr effect (OKE) and dielectric relaxation spectroscopy (DRS) signals of MgCl2 aqueous solutions were modeled based on the same group of molecular dynamics simulation trajectories. Plausible agreement between the simulations and the experiments allows us to analyze the microscopic origin of the different concentration dependences of relaxation times probed by these two techniques. Our simulations suggest that a significant amount of cations and anions associate in the solutions due to the ion pairing effect. These ion assemblies as well as the free cations form suspending clusters together with the water molecules in their cation hydration shells. The dynamics of water molecules in these clusters is significantly hindered, while that of other water molecules remains largely unaffected. The relaxation times measured by OKE and DRS have different concentration dependences because DRS probes only the rotational dynamics of water molecules outside of the cluster while OKE contains the information of dynamics of all of the water molecules in the solutions. Such findings provide us a microscopic picture on how the ion hydration affects the water dynamics in certain ionic solutions in which ion pairing plays an important role.
基于同一组分子动力学模拟轨迹，对 MgCl2 水溶液的光克尔效应（OKE）和介电弛豫光谱（DRS）信号进行了建模。模拟与实验之间的合理一致性使我们能够分析这两种技术探测到的弛豫时间的不同浓度依赖性的微观起源。我们的模拟表明，由于离子配对效应，大量的阳离子和阴离子在溶液中缔合。这些离子缔合体以及游离的阳离子与它们的阳离子水合壳中的水分子一起形成悬浮簇。这些簇中水分子的动力学受到显著阻碍，而其他水分子的动力学则基本不受影响。OKE 和 DRS 测量的弛豫时间具有不同的浓度依赖性，因为 DRS 仅探测簇外水分子的旋转动力学，而 OKE 包含溶液中所有水分子动力学的信息。这些发现为我们提供了一个微观图景，说明离子水合如何影响某些离子溶液中的水动力学，其中离子配对起着重要作用。
