Rapid Water Diffusion Through Extended Networks in MOFs.

The performance of metal-organic frameworks (MOFs) is profoundly shaped by the distribution and diffusion of confined water, yet condensable molecules such as H2O remain less explored than noncondensable gases. Here, we combine molecular dynamics and Monte Carlo simulations with infrared spectroscopy and water adsorption experiments to reveal nonmonotonic, loading-dependent water diffusion behaviors in four representative MOFs: ZIF-90, ZIF-8, ZIF-67, and UiO-66. At low to medium loadings, water sequentially occupies cages to form clusters, with diffusivity decreasing as water loading increases. At high loadings, they organize into an extended network, leading to rapid water diffusion. Such counterintuitive transitions contrast sharply with the behavior of noncondensable molecules and strongly correlate with the enthalpy of vaporization across various water models, emphasizing the dominant role of water-water interactions in governing diffusion in MOFs. Our findings clarify the unique loading-dependent diffusion behavior of condensable molecules in MOFs and guide the rational design of high-performance MOF-based materials.