Underlying Adsorption Mechanisms of Water in Hydrophobic and Hydrophilic Zeolite Imidazolate Frameworks: ZIF-71 and ZIF-90

The hydration of porous materials is relevant for separation, transport, and catalysis purposes, among others. In this respect, zeolitic imidazolate frameworks (ZIFs) have received considerable attention since they have shown remarkable resistance to water as well as to other organic solvents. Studies on water adsorption in ZIFs are however relatively scarce and primarily focused on the effect of the host composition and porosity on their hydrophobic or hydrophilic nature. In this work, we explore the underlying adsorption mechanisms of water in ZIF-71 and ZIF-90, which are experimentally known structures. These ZIFs have been reported hydrophobic and hydrophilic, respectively. We conducted Monte Carlo simulations using previously validated models and force fields to compute the adsorption isotherms of water in both ZIFs at room temperature. Although the polar functional group in ZIF-90 leads to adsorption in the gas phase, a following rapid cage filling occurs as in ZIF-71. A consistent description of this phenomenon is provided in terms of hydrogen bonding formation between water molecules. In the low-coverage regime, the preferential adsorption sites are identified and interactions with water comprehensively characterized.