Effects of Defects and Presence of Open-Metal Sites on the Structure and Dynamics of Water in Hydrophobic Zeolitic Imidazolate Frameworks.

Most of the chemistry in nanoporous materials with small pore sizes and windows takes place on the outer surface, which is in direct contact with the substrate/solvent, rather than within the pores and channels. Here, we report the results of our comprehensive atomistic molecular dynamics (MD) simulations to decipher the interaction of water with a realistic finite ∼5.1 nm nanoparticle (NP) model of ZIF-8, with edges containing undercoordinated Zn metal sites, vs a conventionally employed pristine crystalline bulk (CB) model. The hydrophobic interior surface of the CB model imparts significant dynamical behavior on water molecules with (i) increasing diffusivity from the surface toward the center of the pores and (ii) confined water, at low concentration, showing similar diffusivity to that of the bulk water. On the other hand, water molecules adsorbed on the surface of the NP model exhibit a range of characteristics, including "coordinated", "confined", and "bulk-like" behavior. Some of the water molecules form coordinative bonds with the undercoordinated Zn metal centers and act as nucleation sites for the water droplets to form, facilitating diffusion into the pores. However, diffusion of water molecules is limited to the areas near the surface and not all the way to the core of the NP model. Our atomistic MD simulations provide insights into the stability of ZIFs in aqueous solutions despite hydrolysis of their outer surface. Such insights are helpful in designing more robust nanoporous materials for applications in humid environments.