Physical Chemistry II, Ruhr-University Bochum, Germany.
Phys Chem Chem Phys. 2013 Jun 7;15(21):8383-91. doi: 10.1039/c3cp50865j. Epub 2013 Apr 29.
Ion radius and charge density are important parameters that determine the solvation behavior in aqueous electrolyte solutions. Here, we report on high precision THz absorption measurements of solvated LaCl3 and LaBr3 using narrow-band (75-90 cm(-1)) p-Ge laser and wideband (30-350 cm(-1)) Fourier transform spectroscopy. The concentration dependent absorption up to 3.3 M shows a prominent nonlinearity indicating ion pair formation with increasing electrolyte concentration. A more detailed analysis in terms of a chemical equilibrium model allowed us to separate the ion and ion pair contributions from bulk and solvation water. Thus we were able to characterize anion and cation solvation independently. The center frequencies of the Cl(-) and Br(-) rattling modes are in agreement with those found in aqueous alkali and earth alkali halide solutions. The coupling between anion and cation hydration is found to be small. Based upon our detailed analysis we propose increasing formation of solvent shared ion pairs with increasing solute concentration. The well defined ion resonances imply that in spite of its high charge density La(3+) acts locally on the water structure. Terahertz absorption spectroscopy is found here to be an experimental tool which allows us to directly observe solute hydration shells as well as ion pair formation.
离子半径和电荷密度是决定水相电解质溶液中溶剂化行为的重要参数。在这里，我们报告了使用窄带（75-90 cm(-1)) p-Ge 激光和宽带（30-350 cm(-1)) 傅里叶变换光谱对溶剂化 LaCl3 和 LaBr3 的高精度太赫兹吸收测量。高达 3.3 M 的浓度依赖性吸收显示出明显的非线性，表明随着电解质浓度的增加形成离子对。根据化学平衡模型的更详细分析，我们能够将离子和离子对的贡献与本体和溶剂水分离。因此，我们能够独立地对阴离子和阳离子的溶剂化进行表征。Cl(-) 和 Br(-) rattling 模式的中心频率与在水相碱金属和碱土金属卤化物溶液中发现的频率一致。阴离子和阳离子水合的耦合被发现很小。基于我们的详细分析，我们提出随着溶质浓度的增加，形成溶剂共享离子对的比例增加。明确的离子共振意味着，尽管 La(3+) 的电荷密度很高，但它在局部作用于水结构。太赫兹吸收光谱被发现是一种实验工具，它允许我们直接观察溶质水合壳以及离子对的形成。
