Computational Design of Ultraselective Desalination Membranes using Molecular Simulations and Path Sampling Techniques

半透膜选择性地阻止不需要的分子和离子在流体中通过，同时允许需要的分子通过膜。这种类型的技术是许多应用程序以及自然过程的关键。例如，在海水淡化中使用的膜只具有透水性，并排斥大多数其他离子和分子。类似地，生物细胞的特征是能够调节小分子和离子进出细胞的膜。在分子水平上，膜的选择性是由其纳米结构决定的，即其组成纳米孔的几何形状、拓扑结构和化学性质。然而，关于膜的结构是如何从根本上与它对某些离子和分子的选择性相关的，还有很多东西有待了解。一方面，现有的实验技术缺乏必要的时空分辨率来表征膜结构和探测孤立的溶质（分子或离子）通过事件。另一方面，传统的分子模拟技术可以在正确的长度尺度上提供有关事件的信息，但是观察不需要的溶质通过高选择性膜所需的平均时间超出了标准分子模拟方法的范围。这些因素限制了我们理解重要自然过程（如生物膜传输和多孔介质中的溶质传输）的能力，并阻碍了合理设计用于海水淡化、气体和化学分离应用的超选择性膜的努力。本提案的目的是利用分子动力学模拟和先进的路径采样技术对纳米孔膜的结构-选择性关系进行系统的研究。研究人员最近开发了一种新的路径采样算法，可以在实际操作条件下准确有效地估计任意长的溶质输运时间尺度。该算法还能够重建溶质输运机制的准确和具有统计代表性的图像。这种方法将用于探索压力驱动的溶质通过具有明确几何和化学性质的纳米孔膜的动力学和分子机制。该项目侧重于用于海水淡化应用的膜，但开发的计算工具可以普遍应用于其他基于膜的分离过程。该研究计划首先着手开发新的计算工具和方法来研究溶质通过膜的运输。随后，研究者将进行一系列基于假设的计算，以解决有关膜中结构-选择性关系和纳米尺度限制下阻碍运输的重要基本问题。为此，将研究具有明确结构和化学性质的简单模型碳基膜和合成膜，如沸石和金属有机框架。与耶鲁大学实验组的合作补充了建模工作，因为将合成具有明确结构的膜，并评估膜基脱盐的性能。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Semipermeable membranes selectively impede the passage of undesirable molecules and ions in a fluid, while allowing the desirable molecules to pass through the membrane. This type of technology is key to many applications as well as natural processes. For instance, membranes that are only water permeable and reject most other ions and molecules are used in water desalination. Similarly, biological cells feature membranes capable of modulating the passage of small molecules and ions into and out of the cell. On a molecular level, a membrane’s selectivity is dictated by its nanostructure, i.e., the geometry, topology, and chemistry of its constituent nanopores. However, there is much yet to learn about how a membrane's structure is fundamentally related to its selectivity for certain ions and molecules. On one hand, existing experimental techniques lack the necessary spatiotemporal resolution to characterize membrane structure and to probe isolated solute (the molecule or ion) passage events. On the other hand, conventional molecular simulation techniques can provide information about events at the correct length-scale, but the average time it takes to observe an undesirable solute passing through a highly selective membrane is beyond the reach of standard molecular simulation methods. These factors limit our ability to understand important natural processes (such as biological membrane transport and solute transport in porous media) and hamper efforts to rationally design ultraselective membranes for desalination and gas and chemical separation applications.The goal of this proposal is to conduct a systematic investigation of the structure-selectivity relationship in nanoporous membranes using molecular dynamics simulations and advanced path sampling techniques. The investigator recently developed a novel path sampling algorithm that makes it possible to accurately and efficiently estimate arbitrarily long solute transport timescales under operationally realistic conditions. The algorithm is also capable of reconstructing an accurate and statistically representative picture of the solute transport mechanism. This approach will be used to probe the kinetics and molecular mechanisms of pressure-driven solute transport through nanoporous membranes with well-defined geometries and chemistries. The project focuses on membranes used in desalination applications, but the developed computational tools can be universally applied to other membrane-based separation processes. The research plan sets out first to develop new computational tools and methods for studying solute transport through membranes. Subsequently, the investigator will conduct a set of hypothesis-based calculations that address important fundamental questions about the structure-selectivity relationship in membranes and hindered transport under nanoscale confinement. To this end, both simple model carbon-based membranes with well-defined structures and chemistries and synthetic membranes such as zeolites and metal-organic frameworks will be examined. Collaboration with experimental groups at Yale complements the modeling efforts, as membranes with well-defined structures will be synthesized and performance in membrane-based desalination will be assessed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.