Heterogeneous Microscopic Dynamics of Intruded Water in a Superhydrophobic Nanoconfinement: Neutron Scattering and Molecular Modeling

With their strong confining porosity and versatile surface 2 chemistry, Zeolitic Imidazolate Frameworks-including the 3 prototypical ZIF-8-display exceptional properties for vari-4 ous applications. In particular, the forced intrusion of water 5 at high pressure (∼ 25 MPa) into ZIF-8 nanopores is of in-6 terest for energy storage. Such a system reveals also ideal to 7 study experimentally water dynamics and thermodynamics 8 in ultra-hydrophobic confinement. Here, we report on neu-9 tron scattering experiments to probe the molecular dynam-10 ics of water within ZIF-8 nanopores under high pressure up 11 to 38 MPa. In addition to an overall confinement-induced 12 slowing down, we provide evidence for strong dynamical 13 heterogeneities with different underlying molecular dynam-14 ics. Using complementary molecular simulations, these het-15 erogeneities are found to correspond to different microscopic 16 mechanisms inherent to vicinal molecules located in strongly 17 adsorbing sites (ligands) and other molecules nanoconfined 18 in the cavity center. These findings unveil a complex micro-19 scopic dynamics, which results from the combination of sur-20 face residence times and exchanges between the cavity sur-21 face and center.