Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, United States.
J Phys Chem B. 2012 Aug 16;116(32):9776-83. doi: 10.1021/jp305226j. Epub 2012 Aug 7.
Charge hydration asymmetry (CHA) manifests itself in the experimentally observed strong dependence of free energy of ion hydration on the sign of the ion charge. This asymmetry is not consistently accounted for by popular models of solvation; its magnitude varies greatly between the models. While it is clear that CHA is somehow related to charge distribution within a water molecule, the exact nature of this relationship is unknown. We propose a simple, yet general and rigorous criterion that relates rotational and charge inversion properties of a water molecule's charge distribution with its ability to cause CHA. We show which electric multipole components of a water molecule are key to explain its ability for asymmetric charge hydration. We then test several popular water models and explain why specific models show none, little, or strong CHA in simulations. We use the gained insight to derive an analogue of the Born equation that includes the missing physics necessary to account for CHA and does not rely on redefining the continuum dielectric boundary. The proposed formula is as simple as the original, does not contain any fitting parameters, and predicts hydration free energies and entropies of spherical cations and anions within experimental uncertainty. Our findings suggest that the gap between the practical continuum electrostatics framework and the more fundamental explicit solvent treatment may be reduced considerably by explicitly introducing CHA into the existing continuum framework.
电荷水合不对称性 (CHA) 表现在实验观察到的离子水合自由能强烈依赖于离子电荷的符号上。这种不对称性在流行的溶剂化模型中并没有得到一致的解释；其大小在不同模型之间差异很大。虽然很明显，CHA 与水分子内的电荷分布有关，但这种关系的确切性质尚不清楚。我们提出了一个简单但通用且严格的准则，将水分子的电荷分布的旋转和电荷反转性质与其引起 CHA 的能力联系起来。我们展示了水分子的哪些电多极子成分是解释其不对称电荷水合能力的关键。然后，我们测试了几种流行的水分子模型，并解释了为什么特定的模型在模拟中显示出没有、很少或强烈的 CHA。我们利用获得的洞察力推导出一个包含解释 CHA 所需的缺失物理但不依赖于重新定义连续介电边界的 Born 方程的类似物。所提出的公式与原始公式一样简单，不包含任何拟合参数，并在实验不确定度内预测球形阳离子和阴离子的水合自由能和熵。我们的研究结果表明，通过将 CHA 明确引入现有的连续体框架中，实际连续体静电学框架和更基本的显式溶剂处理之间的差距可能会大大缩小。
