Stable Compressible Liquids Made of Hierarchical MOF Nanocrystals.

Compressible liquids can be produced by dispersing nanoparticles containing hydrophobic pores as colloidal suspensions in water. Due to the water intrusion into the hydrophobic nanopores under pressure, these compressible liquids exhibit significantly greater compressibility than traditional liquids, lending them to energy storage and absorption applications. Metal-organic frameworks (MOFs) such as ZIF-8 have been proposed for this application due to their large porosity, but their physical and chemical stability in aqueous environments presents challenges, prone to hydrolysis or separation from the liquid phase. In this work, the stability concerns of ZIF-8 used for compressible liquids have been circumvented by producing nanoparticles of mesoporous ZIF-8 by a template-directed synthesis. The stability, compressibility, and intrusion kinetics were compared between ZIF-8 with and without mesopores. The mesoporous ZIF-8, uniquely containing hydrophobic micropores and hydrophilic mesopores, presents compressibility comparable to that of conventional ZIF-8 due to the hydrophobic micropores but has the added benefit of significantly increased physical and chemical stability due to the hydrophilic mesopores. The presence of mesopores slightly reduces the water intrusion pressure and accelerates the kinetics that can benefit the cyclic compressibility for vibrations or repeated impact applications as water molecules reversibly intrude and extrude the micropores. This work can inspire future endeavors on understanding and developing compressible and porous liquids with sufficient stability for practical uses.