Mechanism of Water Intrusion into Flexible ZIF-8: Liquid Is Not Vapor

Abstract

Zeolitic Imidazolate Frameworks (ZIF) find application in storage and dissipation of mechanical energy. Their distinctive properties linked to their (sub)nanometer size and hydrophobicity allow for water intrusion only under high hydrostatic pressure. Here we focus on the popular ZIF-8 material investigating the intrusion mechanism in its nanoscale cages, which is the key to its rational exploitation in target applications. In this work, we used a joint experimental/theoretical approach combining in operando synchrotron experiments during highpressure intrusion experiments, molecular dynamics simulations, and stochastic models to reveal that water intrusion into ZIF-8 occurs by a cascade filling of connected cages rather than a condensation process as previously assumed. The reported results allowed us to establish structure/function relations in this prototypical microporous material, representing an important step to devise design rules to synthesize porous media.

Effect of Crystallite Size on the Flexibility and Negative Compressibility of Hydrophobic Metal–Organic Frameworks

“…Following from this, one would expect that a higher intrusion pressure ( P int ) would be associated with a smaller crystallite size. However, the opposite trend is observed: P int grows with increasing crystallite size, with a much greater dependence for crystallites of less than 100 nm. , In addition, the third sample, macroZIF-8, extends this trend of crystallite size versus flexibility (Figure ), which parallels the trend of crystallite size versus P int already reported.…”

“…However, the size of the crystallites considered experimentally (approximately 35, 80, and 270 nm for nano1-, nano2-, and macroZIF-8 respectively) are beyond the scope of atomistic simulations. Therefore, a different approach was taken, in which two different computational samples were considered: the first consists of a slab immersed in water, which has been used successfully in previous works; − the second is a triperiodic simulation box obtained by duplicating the unit cell in each direction . We started our investigation exploring two aspects via the slab sample: (i) whether intruded (potentially metastable) states existed at the experimental P int of nano1- and nano2ZIF-8, and (ii) whether the characteristics of the intruded water were different at the different crystallite sizes.…”

Exploring the Heat of Water Intrusion into a Metal–Organic Framework by Experiment and Simulation

“…Once established that the change of the specific heat capacity between intruded and extruded states is responsible for the trend of heat of intrusion on the temperature, the authors focused on the possible origin of the sign and absolute value of Δ c p . From previous simulations and experiments, it is estimated that the density of water confined within ZIF-8 cavities is approximated to be 0.7 kg·dm –3 , which is much lower than the value of the bulk liquid. ,, It is also noticed that the specific heat of bulk liquid water decreases with increasing density . Thus, the authors expect that the reduction of the density upon intrusion, from the bulk to the liquid-like confined value, is associated with an increase of heat capacity of bulk water (a positive Δ c p ), which is consistent with simulation results.…”

“…The complete extrusion of water from ZIF-8 shows that it can function as a spring or as a shock absorber depending on the dynamic conditions under multiple cycles. This has been demonstrated in the past in various articles. ,,− Additionally, when gas is present in the sample, the intrusions and extrusion isotherms do not show the level of detail with each process. The intrusion volumes are also slightly diminished by 0.05 cm 3 ·g –1 with the two-step extrusion still partially visible.…”

Energy absorption and storage of nanofluidic solid–liquid composite material under high strain rates

“…having high energy-absorbing ratio with low working pressure [13]. Numerous studies have focused on the effects of nanoporous particle composition, size, hydrophobic properties and microstructure on infiltration pressure and solid-liquid interactions [21][22][23][24][25]. For the scale effect of nanopores (from molecular scale to macroporous scale), the critical infiltration pressure (0.3-100 MPa) increases with decreasing pore size [26][27][28].…”