Porous systems are relevant in many scientific fields, from chemistry to physics to engineering. They are used as catalysts, for separation of liquids and gasses, chromatography, for trapping polluting chemicals and for storing gasses, including CO2 sequestration, just to mention few examples. Hydrophobic porous materials were considered for energy applications, energy storage and dissipation. Upon intrusion/extrusion the process stores/returns an energy proportional to the intrusion/extrusion pressure, Pint/Pext, times the intruded/extruded liquid volume. If Pint ~ Pext the systems act as “mechanical batteries”, it Pint &gt;&gt; Pext it behave as shock absorbers/dampers. In the latter case, it was shown that the energy dissipated in intrusion/extrusion cycles can be partially/totally converted into electric current. Contrary to conventional systems, selected {porous media + liquid} systems absorb heat from the environment when the process is accompanied by intrusion. One can, hence, develop an intrusion/extrusion energy conversion device with a nominal efficiency, electric energy produced over the amount of mechanical energy invested, higher than 100%. This is, indeed, the objective of our Electro-Intrusion project. To develop or optimize the applications mentioned above one needs to lay down the theoretical foundations of intrusion/extrusion in nanoporous systems, which is still lacking. The Laplace-Washburn law allows predicting intrusion pressure in large-pore materials, but it is inadequate for nanopores, like zeolites and metal-organic frameworks (MOFs). Moreover, no law exists to predict extrusion pressure, making difficult to interpret experiments and design materials of desired characteristics, e.g., MOFs with either large or negligible hysteresis. For example, no explanation exists why ZIF-8 and Cu2(tebpz) (tebpz=3,3',5,5'-tetraethyl-4,4'-bipyrazolate), two MOFs with porosities of comparable size, have a very different hysteretic behavior. In this talk, I present our recent works unraveling some of these puzzles. We used advanced sampling techniques for classical and ab initio molecular dynamics to perform ad hoc in silico experiments. The analysis of our simulations allowed us to identify the principles governing the wetting and drying of nanoporous systems and determining their hysteresis. The results of detailed comparisons with experimental data will also be discussed.

Wetting and drying of nanoporous systems: From theoretical modeling to design principles of novel energy materials.

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ACS Fall 2022

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