NSF-DFG Confine: Diffusion of Water Confined in Patterned Hydrophilic-Hydrophobic Nanopores

水对生命是必不可少的，了解水分子如何在密闭空间中运动是非常重要的。在这里，我们关注金属有机骨架(MOF)，它在整个内部孔隙中具有交替的亲水和疏水网格状瓷砖区域的明确图案，成为从沙漠空气中收集水的重要材料。正是水分子在这个有图案的空间内的扩散以及亲水和疏水区域之间的相互作用是独特的，也是我们建议研究的。我们的计算研究计划与实验密切相关。计算将使用水跳跃的过渡态理论来研究晶内扩散。固体核磁共振和准弹性中子散射(QENS)的数据将被用来验证预测。为了连接宏观扩散数据，例如，通过摆动频率响应测量获得的数据，需要宏观动力学模型，该模型不仅考虑纳米孔内的扩散，而且还考虑MOF颗粒之间的大孔中的扩散。我们的目标是(I)从从头算自由能计算水分子在吸附中心晶格上的化学精确吸附常数；(Ii)用从头算过渡态理论计算水分子在吸附中心之间跳跃的扩散速率常数；(Iii)分别用大正则蒙特卡罗(GCMC)和动力学蒙特卡罗(KMC)模拟预测吸附等温线和扩散系数作为负载的函数；我们的首要目标是研究水在MOF中的扩散，以促进对具有亲水和疏水区域模式的受限空间中水分子的吸收和释放的认识。这将能够预测结构变化和控制水在孔隙中的扩散率，并最终控制水的吸附/解吸动力学。有了这一新的基本认识，我们将能够提高财政部的集水性能和水生产率，最终造福社会。总体而言，我们的工作将导致集水材料开发的范式转变，并将其从一种反复试验的活动转变为对孔隙中水行为的精确分子设计。随着对MOF中水分扩散的了解，我们将加快发展集水器，为不同气候的数十亿人提供清洁的水。

Water is essential to life and understanding how water molecules move in confined spaces is of fundamental interest. Here, we focus on metal-organic frameworks (MOFs) with well-defined patterns of alternating hydrophilic and hydrophobic grid-like tiled regions throughout the internal pores which became prominent materials for water harvesting from desert air. It is the diffusion of water molecules within this patterned space and the interplay between the hydrophilic and hydrophobic regions that is unique, and which we propose to study.Our computational research program is closely connected with experiments. Computation will study the intracrystalline diffusion using transition state theory for the water jumps. Data from solid-state NMR, and quasi-elastic neutron scattering (QENS) will be used to verify the predictions. To connect with macroscopic diffusion data, e.g., obtained with swing frequency response measurements, macrokinetic models are needed which consider not only diffusion within the nanopores, but also in the macropores between the MOF pellets. We aim to (i) compute chemically accurate adsorption constants for water molecules on a lattice of adsorption sites from ab initio free energies; (ii) compute chemically accurate diffusion rate constants for jumps of water molecules between adsorption sites using ab initio transition state theory; (iii) predict adsorption isotherms and diffusivities as a function of loading using Grand Canonical Monte Carlo (GCMC) and kinetic Monte Carlo (kMC) simulation, respectively, on a lattice of sites; (iv) suggest new MOFs with improved water harvesting properties.Our overarching objective is to study water diffusion in MOFs to advance knowledge about uptake and release of water molecules in confined space with patterns of hydrophilic and hydrophobic regions. This will enable predictions for structural changes and controlling the diffusivity of water in the pores and ultimately the water adsorption/desorption kinetics. With this new fundamental understanding, we will be able to improve the water-harvesting properties of the MOF and its water productivity, ultimately benefiting society. Overall, we envision that our work will lead to a paradigm shift in the development of water-harvesting materials, and transform it from a trial-and-error activity to precision molecular design of the water behavior in the pores. With a better understanding of water diffusion in MOFs, we will accelerate the development of water harvesters to provide clean water to billions of people across varying climates.