Effect of water loading on the stability of pristine and defective UiO-66

Materials used for water treatment purposes need to be stable for easy handling and cost-effectiveness. UiO-66 has been identified as a promising option. In this work, we investigate the impact of water loading on the structural and mechanical properties of pristine and defective UiO-66 using classical molecular simulations. We employ and compare two approaches for modeling non-bonded interactions between the framework and water molecules: direct Lorentz-Berthelot (L-B) mixing and hybrid mixing. We conducted molecular dynamics simulations to examine the spatial arrangement of water molecules within the framework, water affinity for specific framework interaction sites, and their impact on the framework's structural parameters under atmospheric conditions, high hydrostatic pressures, and increased water loading. Our results indicate that both methods predict water affinity near zirconium clusters, but differ in identifying principal interaction sites and interaction strength. L-B mixing predicts strong binding to linker oxygen atoms, restricting water movement, while hybrid mixing indicated dynamic water behavior, with site-to-site hopping and pore-to-pore movement observed at moderate and high loadings. Structural analysis at increased water loadings showed adsorption-induced expansion using L-B mixing due to linker-cluster bond stretching, contrasting with slight system contraction predicted by hybrid mixing. High-pressure NPT simulations evidence that water loading reduces amorphization pressure, although values obtained using both approaches differ significantly at moderate and high loadings.