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.

Effects of Size and Porosity on the Hydrophobicity of Hierarchical Nanoparticles

Hierarchical nanoporous particles combine properties of microporous and mesoporous materials that are widely exploited for energy storage and conversion, separation of gases and liquids, water purification and desalination, fabrication of nanodevices, etc. Hierarchical meso/microporous level-2 and level-3 Menger sponge particles immersed in water were investigated using computer simulation methods to demonstrate a synergetic effect of additional porosity on the wettability of materials. The Menger sponge is an object with a fractal dimension. At each level, the particles are composed of the same structural blocks. The hydrophobicity of the blocks was shown to depend on their size and position in the nanoparticles. The additional porosity decreases the hydrophobicity of the particles due to the partial breaking of hydrogen bonds between water molecules in the pores. This effect can be used to tune and modify the hydrophobicity and wettability of bulky porous materials, nanoparticles, and nanostructured surfaces.

Molecular Simulation of the Impact of Defects on Electrolyte Intrusion in Zeolites

We have investigated through molecular simulation the intrusion of electrolytes in two representative pure-silica zeolites, silicalite-1 and chabazite, in which point defects were introduced in varying amounts. We distinguish between two types of defects, considering either "weak" or "strong" silanol nest defects, resulting in different hydration behaviors. In the presence of weak defects, the hydration process occurs through a homogeneous nucleation process, while with strong defects, we observe an initial adsorption followed by a filling of the nanoporous volume at a higher pressure. However, we show that electrolytes do not penetrate the zeolites, and these defects appear to have only marginal influence on the thermodynamics of electrolyte intrusion. While replacing pure water by the electrolyte solution shifts the intrusion pressure toward higher values because of the drop of water saturation vapor pressure, an increase in hydrophilicity of the framework due to point defects has the opposite effect, showing that controlling the amount of defects in zeolites is crucial for storage energy applications.

Alchemical Osmostat for Monte Carlo Simulation: Sampling Aqueous Electrolyte Solution in Open Systems

Molecular simulations involving electrolytes are usually performed at a fixed amount of salt ions in the simulation box, reproducing macroscopic concentration. Although this statement is valid in the bulk, the concentration of an electrolyte confined in nanoporous materials such as MOFs or zeolites is greatly affected and remains a priori unknown. The nanoporous material in equilibrium with the bulk electrolyte exchange water and ions at a given chemical potential Δμ in the semi-grand-canonical ensemble, that must be calibrated in order to determine the concentration in the nanoporous material. In this work, we propose an algorithm based on nonequilibrium candidate Monte Carlo (NCMC) moves to ultimately perform MC simulations in contact with a saline reservoir. First, we adapt the Widom insertion technique to calibrate the chemical potential by alchemically transmuting water molecules into ions by using NCMC moves. The chemical potential defines a Monte Carlo osmostat in the semi-grand-constant volume and temperature ensemble (Δμ, N, V, T) to be added in a Monte Carlo simulation where the number of ions fluctuates. In order to validate the method, we adapted the NCMC move to determine the free energy of water solvation and subsequently explore thermodynamics of electrolyte solvation at infinite dilution in water. Finally, we implemented the osmostat in MC simulations initialized with bulk water that are driven toward electrolyte solutions of similar concentration as the saline reservoir. Our results demonstrate that alchemical osmostat for MC simulation is a promising tool for use to sample electrolyte insertion in nanoporous materials.

Energetic Performance of Pure Silica Zeolites under High-Pressure Intrusion of LiCl Aqueous Solutions: An Overview

An overview of all the studies on high-pressure intrusion-extrusion of LiCl aqueous solutions in hydrophobic pure silica zeolites (zeosils) for absorption and storage of mechanical energy is presented. Operational principles of heterogeneous lyophobic systems and their possible applications in the domains of mechanical energy storage, absorption, and generation are described. The intrusion of LiCl aqueous solutions instead of water allows to considerably increase energetic performance of zeosil-based systems by a strong rise of intrusion pressure. The intrusion pressure increases with the salt concentration and depends considerably on zeosil framework. In the case of channel-type zeosils, it rises with the decrease of pore opening diameter, whereas for cage-type ones, no clear trend is observed. A relative increase of intrusion pressure in comparison with water is particularly strong for the zeosils with narrow pore openings. The use of highly concentrated LiCl aqueous solutions instead of water can lead to a change of system behavior. This effect seems to be related to a lower formation of silanol defects under intrusion of solvated ions and a weaker interaction of the ions with silanol groups of zeosil framework. The influence of zeosil nanostructure on LiCl aqueous solutions intrusion-extrusion is also discussed.