Department of Physical Chemistry II, Ruhr-University Bochum, Bochum, Germany.
Department of Theory & Bio-systems, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany.
Phys Chem Chem Phys. 2020 Jun 4;22(21):12140-12153. doi: 10.1039/c9cp06845g.
We report a terahertz absorption spectroscopy study of MgSO4 aqueous solutions in the concentration range 0.1 mol dm-3 to 2.4 mol dm-3. Accompanying classical MD simulations were carried out that use a polarizable force field parameterized to reproduce the solution thermodynamics. Contrary to prior reports, we find no evidence of contact ion pairs, even close to the solubility limit. Only solvent separated and different types of solvent shared ion pairs are found, being abundant even at the lowest concentration investigated here. The structure of the solution is concentration-dependent: the number of both types of ion pairs grows with increasing salt concentration. The combined theoretical and experimental analysis of the spectra in the frequency region 50-640 cm-1 suggests that the dynamics of water directly between two ions in solvent shared configuration is very strongly perturbed, via a cooperative, supra-additive, effect arising from the two ions. At high concentrations, the results support a scenario, where the perturbations in the water dynamics extend up to the third hydration layer via a cooperative, but additive, effect involving multiple ions. The SO42- and its hydration shell are much more strongly perturbed by the presence of the counterions than the first hydration shell of Mg2+. It is further shown that our simulations and observations are in agreement with thermodynamic properties of aqueous MgSO4 solutions derived by other methods.
我们报告了在 0.1 mol dm-3 至 2.4 mol dm-3 浓度范围内硫酸镁水溶液的太赫兹吸收光谱研究。同时进行了伴随的经典 MD 模拟，该模拟使用了经过参数化以再现溶液热力学的极化力场。与先前的报告相反，我们甚至在接近溶解度极限的情况下也没有发现接触离子对的证据。仅发现溶剂分离和不同类型的溶剂共享离子对，即使在研究的最低浓度下也很丰富。溶液的结构随浓度而变化：两种类型的离子对的数量随着盐浓度的增加而增加。对光谱在 50-640 cm-1 频率范围内的理论和实验联合分析表明，通过来自两个离子的协同、超加和效应，溶剂共享构型中两个离子之间的水直接动力学受到非常强烈的干扰。在高浓度下，结果支持这样一种情景，即水动力学的干扰通过涉及多个离子的协同但加和效应扩展到第三水合层。与 Mg2+的第一水合壳层相比，SO42-及其水合壳层受到反离子的强烈干扰。进一步表明，我们的模拟和观察结果与其他方法得出的硫酸镁水溶液热力学性质一致。
