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.