Department of Physics, Technical University Munich, 85748 Garching, Germany.
J Chem Phys. 2010 Oct 28;133(16):164511. doi: 10.1063/1.3490666.
We study the liquid structure and solvation forces of dense monovalent electrolytes (LiCl, NaCl, CsCl, and NaI) in a nanometer slab-confinement by explicit-water molecular dynamics (MD) simulations, implicit-water Monte Carlo (MC) simulations, and modified Poisson-Boltzmann (PB) theories. In order to consistently coarse-grain and to account for specific hydration effects in the implicit methods, realistic ion-ion and ion-surface pair potentials have been derived from infinite-dilution MD simulations. The electrolyte structure calculated from MC simulations is in good agreement with the corresponding MD simulations, thereby validating the coarse-graining approach. The agreement improves if a realistic, MD-derived dielectric constant is employed, which partially corrects for (water-mediated) many-body effects. Further analysis of the ionic structure and solvation pressure demonstrates that nonlocal extensions to PB (NPB) perform well for a wide parameter range when compared to MC simulations, whereas all local extensions mostly fail. A Barker-Henderson mapping of the ions onto a charged, asymmetric, and nonadditive binary hard-sphere mixture shows that the strength of structural correlations is strongly related to the magnitude and sign of the salt-specific nonadditivity. Furthermore, a grand canonical NPB analysis shows that the Donnan effect is dominated by steric correlations, whereas solvation forces and overcharging effects are mainly governed by ion-surface interactions. However, steric corrections to solvation forces are strongly repulsive for high concentrations and low surface charges, while overcharging can also be triggered by steric interactions in strongly correlated systems. Generally, we find that ion-surface and ion-ion correlations are strongly coupled and that coarse-grained methods should include both, the latter nonlocally and nonadditive (as given by our specific ionic diameters), when studying electrolytes in highly inhomogeneous situations.
我们通过显式水分子动力学 (MD) 模拟、隐式水分子蒙特卡罗 (MC) 模拟和修正的泊松-玻尔兹曼 (PB) 理论研究了纳米片层限制下单价电解质 (LiCl、NaCl、CsCl 和 NaI) 的液体结构和溶剂化力。为了在隐式方法中进行一致的粗粒化并考虑特定的水合作用效应，我们从无限稀释 MD 模拟中推导出了真实的离子-离子和离子-表面对势。从 MC 模拟中计算出的电解质结构与相应的 MD 模拟吻合得很好，从而验证了粗粒化方法。如果采用真实的、由 MD 推导得到的介电常数，这种吻合度会进一步提高，这部分修正了 (由水介导的) 多体效应。进一步分析离子结构和溶剂化压力表明，与 MC 模拟相比，当 PB (NPB) 的非局部扩展在较宽的参数范围内表现良好，而所有局部扩展则大多失败。将离子映射到带电、不对称且不可加的二元硬球混合物的 Barker-Henderson 映射表明，结构相关性的强度与盐特异性不可加性的大小和符号密切相关。此外，全离子 NPB 分析表明，Donnan 效应主要由位阻相关决定，而溶剂化力和过充电效应主要由离子-表面相互作用决定。然而，对于高浓度和低表面电荷，位阻对溶剂化力的修正具有强烈的排斥性，而过充电也可以在强相关体系中由位阻相互作用引发。一般来说，我们发现离子-表面和离子-离子相关性是强烈耦合的，当研究高度不均匀情况下的电解质时，粗粒化方法应同时包含后者，即非局部和不可加性 (如我们特定的离子直径所示)。
