Department of Chemistry, University of Cincinnati , Cincinnati, Ohio 45221, United States.
J Phys Chem B. 2017 Mar 9;121(9):2189-2201. doi: 10.1021/acs.jpcb.7b00104. Epub 2017 Feb 23.
The law of matching water affinities (LMWA) is explored in classical molecular dynamics simulations of several alkali halide ion pairs, spanning the size range from small kosmotropes to large chaotropes. The ion-ion potentials of mean force (PMFs) are computed using three methods: the local molecular field theory (LMFT), the weighted histogram analysis method (WHAM), and integration of the average force. All three methods produce the same total PMF for a given ion pair. In addition, LMFT-based partitioning into van der Waals and local and far-field electrostatic free energies and assessment of the enthalpic, entropic, and ion-water components yield insights into the origins of the observed free energy profiles in water. The results highlight the importance of local electrostatic interactions in determining the shape of the PMFs, while longer-ranged interactions enhance the overall ion-ion attraction, as expected in a dielectric continuum model. The association equilibrium constants are estimated from the smooth WHAM curves and compared to available experimental conductance data. By examining the variations in the average hydration numbers of ions with ion-ion distance, a correlation of the water structure in the hydration shells with the free energy features is found.
本文采用经典分子动力学模拟方法，研究了几种碱金属卤化物离子对的水合亲和力规律（LMWA），涵盖了从小尺寸的向列相到大尺寸的胆甾相的范围。使用三种方法计算了离子-离子平均力势（PMF）：局部分子场理论（LMFT）、加权直方图分析方法（WHAM）和平均力积分。对于给定的离子对，这三种方法都产生了相同的总 PMF。此外，基于 LMFT 的范德华、局部和远场静电自由能划分，以及焓、熵和离子-水成分的评估，深入了解了在水中观察到的自由能分布的起源。结果强调了局部静电相互作用在确定 PMF 形状方面的重要性，而长程相互作用则如介电连续体模型所预期的那样增强了离子-离子的整体吸引力。通过平滑的 WHAM 曲线估计了缔合平衡常数，并将其与可用的实验电导数据进行了比较。通过考察离子-离子距离与离子平均水合数的变化，发现水合壳层中的水结构与自由能特征之间存在相关性。
