Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, and The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi 444-8585, Japan.
J Phys Chem A. 2010 Dec 9;114(48):12573-84. doi: 10.1021/jp1084795. Epub 2010 Nov 15.
The solvation of halide ions at the water/vapor interface is investigated by using molecular dynamics simulations with nonpolarizable molecular mechanical (MM), polarizable MM, and quantum mechanical (QM)/MM methods. The free energy profile of the ion solvation is decomposed into the energy and the entropic contributions along the ion displacement from inside to the surface of water. It is found that the surface affinity of the ion, relative to the bulk value, is determined by a subtle balance between the energetic destabilization and the entropic stabilization with the ion displacement. The amount of energetic destabilization is found to be reduced when nonadditive interactions are included, as in the polarizable MM and QM/MM models. The structure of water around the ion at the interface is also largely modified when the higher order effects are considered. For example, the induced dipole effect enhances the solvation structure around the ion at the interface significantly and thus reduces the amount of entropic stabilization at the interface, relative to in the bulk. It is found that this induced dipole effect causes the slowing in the ion-water hydrogen bond dynamics at the interface. On the other hand, the higher order induced multipole effects in the QM/MM method suppress both the excessive enhancement of the solvation structure and the slowing of the ion-water hydrogen bond dynamics at the interface. The present study demonstrates that not only the induced dipole moment but also the higher order induced multipole moments, which are neglected in standard empirical models, are essential for the correct description of the ion solvation at the water/vapor interface.
通过使用非极化分子力学 (MM)、极化 MM 和量子力学 (QM)/MM 方法的分子动力学模拟，研究了卤化物离子在水/蒸汽界面的溶剂化作用。离子溶剂化的自由能曲线沿着离子从水内部向表面的位移被分解为能量和熵贡献。结果表明，相对于体相值，离子在表面的亲和力由离子位移时的能量去稳定化和熵稳定化之间的微妙平衡决定。发现当包括非加和相互作用时，如在极化 MM 和 QM/MM 模型中，能量去稳定化的程度会降低。当考虑更高阶效应时，离子周围水的结构在界面上也会发生很大的变化。例如，诱导偶极子效应显著增强了界面处离子周围的溶剂化结构，从而相对于体相减少了界面处的熵稳定化程度。结果表明，这种诱导偶极子效应导致离子-水氢键在界面处的动力学减慢。另一方面，QM/MM 方法中的更高阶诱导多极效应抑制了界面处溶剂化结构的过度增强和离子-水氢键动力学的减慢。本研究表明，不仅诱导偶极矩，而且在标准经验模型中被忽略的更高阶诱导多极矩，对于正确描述水/蒸汽界面处的离子溶剂化作用也是必不可少的。
