Abal João P K, Bordin José Rafael, Barbosa Marcia C
Institute of Physics, Federal University of Rio Grande do Sul, 91501-970, Porto Alegre, Brazil.
Phys Chem Chem Phys. 2020 May 21;22(19):11053-11061. doi: 10.1039/d0cp00484g. Epub 2020 May 6.
Water scarcity is a reality in our world, and scenarios predicted by leading scientists in this area indicate that it will worsen in the next decades. However, new technologies based on low-cost seawater desalination can prevent the worst scenarios, providing fresh water for humanity. With this goal, membranes based on nanoporous materials have been suggested in recent years. One of the materials suggested is MoS, and classical Molecular Dynamics (MD) simulation is one of the most powerful tools to explore these nanomaterials. However, distinct force fields employed in MD simulations are parameterized based on distinct experimental quantities. In this paper, we compare two models of salt that were built based on distinct properties of water-salt mixtures. One model fits the hydration free energy and lattice properties, and the second fits the crystal density and the density and the dielectric constant of water and salt mixtures. To compare the models, MD simulations for salty water flow through nanopores of two sizes were used - one pore big enough to accommodate hydrated ions, and one smaller in which the ion has to dehydrate to enter - and two rigid water models from the TIP4P family - TIP4P/2005 and TIP4P/ε. Our results indicate that the water permeability and salt rejection by the membrane are more influenced by the salt model than by the water model, especially for the narrow pore. In fact, completely distinct mechanisms were observed, and they are related to the characteristics employed in the ion model parameterization. The results show that not only can the water model influence the outcomes, but the ion model plays a crucial role when the pore is small enough.
水资源短缺是当今世界的一个现实情况，该领域顶尖科学家预测的情况表明，在未来几十年里这一情况将恶化。然而，基于低成本海水淡化的新技术可以防止最糟糕的情况发生，为人类提供淡水。为了实现这一目标，近年来有人提出了基于纳米多孔材料的膜。其中一种被提出的材料是MoS，经典分子动力学（MD）模拟是探索这些纳米材料最强大的工具之一。然而，MD模拟中使用的不同力场是根据不同的实验量进行参数化的。在本文中，我们比较了基于水盐混合物不同性质构建的两种盐模型。一种模型拟合水化自由能和晶格性质，另一种模型拟合晶体密度以及水和盐混合物的密度与介电常数。为了比较这些模型，我们使用了MD模拟来研究盐水通过两种尺寸纳米孔的流动——一种孔足够大以容纳水合离子，另一种较小，离子必须脱水才能进入——以及TIP4P家族的两种刚性水模型——TIP4P/2005和TIP4P/ε。我们的结果表明，膜的水渗透性和盐截留率受盐模型的影响比水模型更大，尤其是对于窄孔而言。事实上，观察到了完全不同的机制，它们与离子模型参数化中所采用的特性有关。结果表明，不仅水模型会影响结果，而且当孔足够小时，离子模型也起着至关重要的作用。
