Elucidating and manipulating pressure-induced water intrusion-extrusion in tunable hydrophobic Co/Zn bimetallic ZIFs: Roles of pore size and hydrogen bond

Porous heterogeneous lyophobic systems (HLSs) find potential applications in energy restoring, dissipating, and absorbing. However, the development of controllable HLSs still lacks rational structure design of nanoporous materials matching the molecular sizes of adopted liquids. Besides that, thoroughly understanding the underlying transportation mechanism in the confined nano channels is greatly challenging. In this work, a series of Co/Zn bimetallic zeolitic imidazolate frameworks (ZIFs) with tunable structures were synthesized via regulating the Co to Zn ratios and employed to investigate the intrusion-extrusion of liquid water in confined nanopores. Structural characterizations confirm the heterometallic coordination in the Co/Zn-doped frameworks. Water intrusion-extrusion experiments unlock the relationship between the intrusion pressure and the nanopore size and realize the evolution of the HLSs between molecular spring and shock-absorber. In addition, cycling tests indicate the reversible structure change of Co/Zn ZIFs encountering pressure-induced water intrusion. In combination with molecular dynamics simulations, we present that the water multimers intrude into nanopores of ZIFs in chain-like forms along with dissociation of hydrogen bonds (HBs). Water molecules in the pre-intrusion state exhibit reduced HBs in response to the increase of pressure and linear structure with 1.6-3.0 HBs on average. After transition to the post-intrusion situation, the associative configuration of water tends to exhibit the tetrahedral structure. Herein, we highlight the roles of pore size and HB in synergically dominating the pressure-induced intrusion-extrusion of liquid water in hydrophobic nanopores. Furthermore, the present work can also guide the development of functional guest-host systems based on porous architectures.