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

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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.