Physical Science Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
Department of Chemical Engineering, University of Washington, Seattle, Washington 98185, USA.
J Chem Phys. 2017 Oct 28;147(16):161716. doi: 10.1063/1.4994912.
Determining the solvation free energies of single ions in water is one of the most fundamental problems in physical chemistry and yet many unresolved questions remain. In particular, the ability to decompose the solvation free energy into simple and intuitive contributions will have important implications for models of electrolyte solution. Here, we provide definitions of the various types of single ion solvation free energies based on different simulation protocols. We calculate solvation free energies of charged hard spheres using density functional theory interaction potentials with molecular dynamics simulation and isolate the effects of charge and cavitation, comparing to the Born (linear response) model. We show that using uncorrected Ewald summation leads to unphysical values for the single ion solvation free energy and that charging free energies for cations are approximately linear as a function of charge but that there is a small non-linearity for small anions. The charge hydration asymmetry for hard spheres, determined with quantum mechanics, is much larger than for the analogous real ions. This suggests that real ions, particularly anions, are significantly more complex than simple charged hard spheres, a commonly employed representation.
确定单个离子在水中的溶剂化自由能是物理化学中最基本的问题之一，但仍有许多悬而未决的问题。特别是，将溶剂化自由能分解为简单直观的贡献的能力将对电解质溶液模型产生重要影响。在这里，我们根据不同的模拟方案，给出了各种类型的单离子溶剂化自由能的定义。我们使用密度泛函理论相互作用势能和分子动力学模拟计算了带电硬球的溶剂化自由能，并将其与 Born（线性响应）模型进行了比较，以分离电荷和空化的影响。我们表明，使用未经修正的 Ewald 求和会导致单离子溶剂化自由能出现不合理的值，并且阳离子的充电自由能与电荷呈近似线性关系，但对于小阴离子则存在较小的非线性。用量子力学确定的硬球的电荷水合不对称性远大于类似的真实离子。这表明，真实离子，特别是阴离子，比常用的简单带电硬球复杂得多。
