In general, in nano-porous materials with small pore sizes and windows, the substrate/solvent will immediately contact with the surface area of the material rather than inside pores and channels. Here, we report the results of our comprehensive atomistic molecular dynamics simulations on deciphering the intermolecular hydrogen bond network of water on the exterior surface of a nanoparticle model of ZIF-8 vs. the inner surface of its pristine crystalline bulk model. We modeled a nanoparticle which has a realistic finite size of ~5.1 nm with edges containing under-coordinated Zn<sup>2+</sup>metal sites. To model water, we employed the flexible and anharmonic four-site q-TIP4P/F water potential model as implemented in our in-house developed general purpose software package DL_POLY Quantum v1.0 derived from DL_POLY Classic. We show that water exposed to the surface of the nanoparticle exhibits both interfacial and bulk-like characters. Moreover, we found that larger droplets are formed as water content increase and then started to diffuse into nanopores. As the confined water molecules are pushed to the vacant pores due to hydrophobic inner surface, the outer surface water molecules form coordinative bonds with under-coordinated Zn<sup>2+ </sup>metal centers which act as nucleation sites for the water droplets to form and hence making the surface hydrophilic.

Characterizing water local structure and dynamics on the surfaces of zeolitic-imidazolate framework nanoparticles

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ACS Fall 2022

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