Forced intrusion of water and aqueous solutions in microporous materials: from fundamental thermodynamics to energy storage devices

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.

Influence of cation nature on high pressure intrusion of aqueous salt solutions in pure silica MFI-type zeolite

High pressure intrusion-extrusion of water and aqueous salt solutions in hydrophobic porous solids such as pure silica zeolites (zeosils) is a promising way to absorb and store mechanical energy. The intrusion pressure has already been shown to be increased with salt concentration, but the influence of cation nature is not yet fully understood. The intrusion-extrusion experiments of aqueous chloride solutions of alkali, alkaline-earth and transition metals (MCln, where Mn+ = Li+, Na+, K+, Rb+, Cs+, Ca2+, Mg2+, Sr2+, Al3+, Mn2+, Ni2+ Co2+, Cu2+, Zn2+, Cd2+) in MFI-type zeosil (silicalite-1) have been performed to study this effect. In contrast to the anions, the reported results indicate that the cation nature does not have a significant influence on intrusion behavior. All the systems demonstrate a spring behavior with fully reversible intrusion except the ones with highly concentrated ZnCl2 and CsCl solutions, where a small part of the liquid remains trapped in the pores in the first intrusion-extrusion cycle. At fixed H2O/salt molar ratio, a strong influence of cation nature on intrusion pressure is observed. For alkali, alkaline-earth metal and aluminium cations, the intrusion pressure rises with cation charge, whereas the increase of cation size leads to a pressure decrease. These trends are also in agreement with cation hydration enthalpy value for alkali and alkaline-earth metal cations, but no correlation is observed for the most part of the transition metal ones except Zn and Cd.

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.

Thermodynamics of Polyethylene Glycol Intrusion in Microporous Water

Polymers can be used to augment the properties of microporous materials, affording enhanced processability, stability, and compatibility. Manipulating polymers to target specific properties, however, requires detailed knowledge of how different polymers and microporous materials interact. Here, we report a study of the thermodynamics of polyethylene glycol (PEG) intrusion into a representative hydrophobic zeolite (silicalite-1) and metal-organic framework [ZIF-67; Co(2-methylimidazolate)2] in water, both of which can be formed into colloidally stable aqueous dispersions─termed "microporous water"─with dry, guest-accessible pore networks. Through a combination of O2 capacity measurements and isothermal titration calorimetry (ITC), we establish relationships between PEG intrusion behavior, polymer length, polymer end groups, and the structure of the microporous framework. In particular, we find that PEG intrusion is exothermic for silicalite-1 but endothermic for ZIF-67. Our results provide fundamental insights into polymer intrusion in microporous materials that should inform efforts to design composite solids and fluids with enhanced functionality.

Effects of Nanoconfinement on Dynamics in Concentrated Aqueous Magnesium Chloride Solutions

Water behavior in various natural and manufactured settings is influenced by confinement in organic or inorganic frameworks and the presence of solutes. Here, the effects on dynamics from both confinement and the addition of solutes are examined. Specifically, water and ion dynamics in concentrated (2.5-4.2 m) aqueous magnesium chloride solutions confined in mesoporous silica (2.8 nm pore diameter) were investigated using polarization selective pump-probe and 2D infrared spectroscopies. Fitting the rotational and spectral diffusion dynamics measured by the vibrational probe, selenocyanate, with a previously developed two-state model revealed distinct behaviors at the interior of the silica pores (core state) and near the wall of the confining framework (shell state). The shell dynamics are noticeably slower than the bulk, or core, dynamics. The concentration-dependent slowing of the dynamics aligns with behavior in the bulk solutions, but the spectrally separated water-associated and Mg2+-associated forms of the selenocyanate probe exhibit different responses to confinement. The disparity in the complete reorientation times is larger upon confinement, but the spectral diffusion dynamics become more similar near the silica surface. The length scales that characterize the transition from surface-influenced to bulk-like behavior for the salt solutions in the pores are discussed and compared to those of pure water and an organic solvent confined in the same pores. These comparisons offer insights into how confinement modulates the properties of different liquids.

Calorimetric Heats of Intrusion of LiCl Aqueous Solutions in Hydrophobic MFI-Type Zeosil: Influence of the Concentration

For the first time, we report calorimetric measurements of intrusion of aqueous LiCl solutions in a hydrophobic pure siliceous MFI zeolite (silicalite-1) under high pressure. Our results show that the intrusion heats are strongly dependent on the LiCl concentration. The intrusion process is endothermic for diluted solutions (molar H2O/LiCl = 12) as well as for water, but it becomes exothermic for a concentration close to saturation (molar H2O/LiCl = 4). Analysis of the data in the framework of wetting thermodynamics shows that besides surface wetting, other phenomena occur during intrusion, such as hydrogen-bond weakening and composition change. In all cases, water is preferentially intruded so that the intruded phase becomes more diluted than the bulk solution. In the case of the most diluted solution, only water molecules seemed to be intruded. Furthermore, silicalite-1 is shown to be very stable in the presence of LiCl solution, with no noticeable structural and textural modifications observed after intrusion.

Fundamentals and Applications of Surface Wetting

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.

Spontaneous Dipole Reorientation in Confined Water and Its Effect on Wetting/Dewetting of Hydrophobic Nanopores

The properties of nanoconfined fluids are important for a broad range of natural and engineering systems. In particular, wetting/dewetting of hydrophobic nanoporous materials is crucial due to their broad applicability for molecular separation and liquid purification; energy storage, conversion, recuperation, and dissipation; for catalysis, chromatography, and so on. In this work, a rapid, orchestrated, and spontaneous dipole reorientation was observed in hydrophobic nanotubes of various pore sizes d (7.9-16.5 Å) via simulations. This phenomenon leads to the fragmentation of water clusters in the narrow nanopores (d = 7.9, 10 Å) and strongly affects dewetting through cluster repulsion. The cavitation in these pores has an electrostatic origin. The dependence of hydrogen-bonded network properties on the tube aperture is obtained and is used to explain wetting (intrusion)-dewetting (extrusion) hysteresis. Computer simulations and experimental data demonstrate that d equals ca. 12.5 Å is a threshold between a nonhysteretic (spring) behavior, where intrusion-extrusion is reversible, and a hysteretic one (shock absorber), where hysteresis is prominent. This work suggests that water clustering and the electrostatic nature of cavitation are important factors that can be effectively exploited for controlling the wetting/dewetting of nanoporous materials.

Tuning Wetting-Dewetting Thermomechanical Energy for Hydrophobic Nanopores via Preferential Intrusion

Heat and the work of compression/decompression are among the basic properties of thermodynamic systems. Being relevant to many industrial and natural processes, this thermomechanical energy is challenging to tune due to fundamental boundaries for simple fluids. Here via direct experimental and atomistic observations, we demonstrate, for fluids consisting of nanoporous material and a liquid, one can overcome these limitations and noticeably affect both thermal and mechanical energies of compression/decompression exploiting preferential intrusion of water from aqueous solutions into subnanometer pores. We hypothesize that this effect is due to the enthalpy of dilution manifesting itself as the aqueous solution concentrates upon the preferential intrusion of pure water into pores. We suggest this genuinely subnanoscale phenomenon can be potentially a strategy for controlling the thermomechanical energy of microporous liquids and tuning the wetting/dewetting heat of nanopores relevant to a variety of natural and technological processes spanning from biomedical applications to oil-extraction and renewable energy.

Reversible Surface Energy Storage in Molecular-Scale Porous Materials

Forcible wetting of hydrophobic pores represents a viable method for energy storage in the form of interfacial energy. The energy used to fill the pores can be recovered as pressure-volume work upon decompression. For efficient recovery, the expulsion pressure should not be significantly lower than the pressure required for infiltration. Hysteresis of the wetting/drying cycle associated with the kinetic barrier to liquid expulsion results in energy dissipation and reduced storage efficiency. In the present work, we use open ensemble (Grand Canonical) Monte Carlo simulations to study the improvement of energy recovery with decreasing diameters of planar pores. Near-complete reversibility is achieved at pore widths barely accommodating a monolayer of the liquid, thus minimizing the area of the liquid/gas interface during the cavitation process. At the same time, these conditions lead to a steep increase in the infiltration pressure required to overcome steric wall/water repulsion in a tight confinement and a considerable reduction in the translational entropy of confined molecules. In principle, similar effects can be expected when increasing the size of the liquid particles without altering the absorbent porosity. While the latter approach is easier to follow in laboratory work, we discuss the advantages of reducing the pore diameter, which reduces the cycling hysteresis while simultaneously improving the stored-energy density in the material.

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

Wetting of a solid by a liquid is relevant for a broad range of natural and technological processes. This process is complex and involves the generation of heat, which is still poorly understood especially in nanoconfined systems. In this article, scanning transitiometry was used to measure and evaluate the pressure-driven heat of intrusion of water into solid ZIF-8 powder within the temperature range of 278.15-343.15 K. The conditions examined included the presence and absence of atmospheric gases, basic pH conditions, solid sample origins, and temperature. Simultaneously with these experiments, molecular dynamics simulations were conducted to elucidate the changing behavior of water as it enters into ZIF-8. The results are rationalized within a temperature-dependent thermodynamic cycle. This cycle describes the temperature-dependent process of ZIF-8 filling, heating, emptying, and cooling with respect to the change of internal energy of the cycle from the calculated change in the specific heat capacity of the system. At 298 K the experimental heat of intrusion per gram of ZIF-8 was found to be -10.8 ± 0.8 J·g-1. It increased by 19.2 J·g-1 with rising temperature to 343 K which is in a reasonable match with molecular dynamic simulations that predicted 16.1 J·g-1 rise. From these combined experiments, the role of confined water in heat of intrusion of ZIF-8 is further clarified.

Quality-dependent performance of hydrophobic ZIF-67 upon high-pressure water intrusion-extrusion process

Zeolitic imidazolate framework (ZIF) microporous materials have already been employed in many fields of energetic and environmental interest since the last decade. The commercial scale production of some of these materials makes them more accessible for their implementation in industrial processes; however, their massive synthesis may entail modifications to the preparation protocols, which may result in a loss in the optimization of this process and a drop in the material's quality. This fact may have implications for the performance of these materials during their lifetime, especially when they are used in applications such as energy dissipation, in which they are subjected to several operating cycles under high pressures. This study focuses on ZIF-67, a material that has demonstrated in the past its ability to dissipate energy through the water intrusion-extrusion process under high pressure. Two ZIF-67 samples were synthesized using different protocols, and 2 batches of different qualities (labelled as high quality (HQ) and low quality (LQ)) were obtained and analysed by water porosimetry to study their performance in the intrusion-extrusion process. Unexpectedly, minor structural differences, which are typically neglected especially under production conditions, had a dramatic effect on their performance. The results presented in this study reiterate the importance of quality control with respect to reproducibility of experimental results. In a broader perspective, they are critical to the technology transfer from academia to industry.

What keeps nanopores boiling

The liquid-to-vapor transition can occur under unexpected conditions in nanopores, opening the door to fundamental questions and new technologies. The physics of boiling in confinement is progressively introduced, starting from classical nucleation theory, passing through nanoscale effects, and terminating with the material and external parameters that affect the boiling conditions. The relevance of boiling in specific nanoconfined systems is discussed, focusing on heterogeneous lyophobic systems, chromatographic columns, and ion channels. The current level of control of boiling in nanopores enabled by microporous materials such as metal organic frameworks and biological nanopores paves the way to thrilling theoretical challenges and to new technological opportunities in the fields of energy, neuromorphic computing, and sensing.

Hydrophilic interaction liquid chromatography with methanol-water eluent on a zeolite

BACKGROUND

Hydrophilic interaction chromatography (HILIC) works with organic solvent-water mixtures as eluent and is based on the formation of a water enriched liquid phase on the surface of a hydrophilic stationary phase. Hydrophilic solutes are retained on that stagnant water-rich film depending on the difference of solvation compared to the mobile phase composition. However, the enhancement of selectivity by increasing the fraction of organic cosolvent is coupled with a limitation the analyte solubility, and the improvement of the HILIC principle by new hydrophilic stationary phases is the remaining option.

RESULTS

Y-zeolite (faujasite, FAU type) in the Na⁺-form with an average particle diameter of 5 μm was used as packing material in a 125 mm long HPLC column. The chromatographic response of the column was tested in methanol-water mixtures as eluent after injection of several aliphatic alcohols, polyols and monosaccharides with eluent conditions where no separation occurs on diol functionalized silica. On the zeolite the retention time increases according to ethylene glycol < glycerol < erythritol < sorbitol < inositol. The separation principle is explained to be superposed by two effects: firstly, a partition equilibrium between the water-rich phase in the zeolite micropores exists, and secondly, selective interactions with the inner crystalline pore surface and fixed-position Na⁺ ions, both serving to enhance the selectivity. Furthermore, arabinose and fructose monosaccharides could be separated into their tautomeric forms. Only upon increasing the temperature from 20 to 60 °C the tautomeric pattern merges into a single peak.

SIGNIFICANCE AND NOVELTY

Instead of the stagnant water rich surface layer, zeolite micropores now take over that function. As a result, the selectivity among polyols and between α/β-arabinopyranose and β-fructopyranose/β-fructofuranose tautomers is extraordinary superior towards conventional hydrophilic interaction liquid chromatography (HILIC).

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

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 high-pressure 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.

Microporous water with high gas solubilities

Liquids with permanent microporosity can absorb larger quantities of gas molecules than conventional solvents¹, providing new opportunities for liquid-phase gas storage, transport and reactivity. Current approaches to designing porous liquids rely on sterically bulky solvent molecules or surface ligands and, thus, are not amenable to many important solvents, including water^2-4. Here we report a generalizable thermodynamic strategy to preserve permanent microporosity and impart high gas solubilities to liquid water. Specifically, we show how the external and internal surface chemistry of microporous zeolite and metal-organic framework (MOF) nanocrystals can be tailored to promote the formation of stable dispersions in water while maintaining dry networks of micropores that are accessible to gas molecules. As a result of their permanent microporosity, these aqueous fluids can concentrate gases, including oxygen (O₂) and carbon dioxide (CO₂), to much higher densities than are found in typical aqueous environments. When these fluids are oxygenated, record-high capacities of O₂ can be delivered to hypoxic red blood cells, highlighting one potential application of this new class of microporous liquids for physiological gas transport.

Effect of the Topology on Wetting and Drying of Hydrophobic Porous Materials

Establishing molecular mechanisms of wetting and drying of hydrophobic porous materials is a general problem for science and technology within the subcategories of the theory of liquids, chromatography, nanofluidics, energy storage, recuperation, and dissipation. In this article, we demonstrate a new way to tackle this problem by exploring the effect of the topology of pure silica nanoparticles, nanotubes, and zeolites. Using molecular dynamics simulations, we show how secondary porosity promotes the intrusion of water into micropores and affects the hydrophobicity of materials. It is demonstrated herein that for nano-objects, the hydrophobicity can be controlled by changing the ratio of open to closed nanometer-sized lateral pores. This effect can be exploited to produce new materials for practical applications when the hydrophobicity needs to be regulated without significantly changing the chemistry or structure of the materials. Based on these simulations and theoretical considerations, for pure silica zeolites, we examined and then classified the experimental database of intrusion pressures, thus leading to the prediction of any zeolite's intrusion pressure. We show a correlation between the intrusion pressure and the ratio of the accessible pore surface area to total pore volume. The correlation is valid for some zeolites and mesoporous materials. It can facilitate choosing prospective candidates for further investigation and possible exploitation, especially for energy storage, recuperation, and dissipation.

Subnanometer Topological Tuning of the Liquid Intrusion/Extrusion Characteristics of Hydrophobic Micropores

Intrusion (wetting)/extrusion (drying) of liquids in/from lyophobic nanoporous systems is key in many fields, including chromatography, nanofluidics, biology, and energy materials. Here we demonstrate that secondary topological features decorating main channels of porous systems dramatically affect the intrusion/extrusion cycle. These secondary features, allowing an unexpected bridging with liquid in the surrounding domains, stabilize the water stream intruding a micropore. This reduces the intrusion/extrusion barrier and the corresponding pressures without altering other properties of the system. Tuning the intrusion/extrusion pressures via subnanometric topological features represents a yet unexplored strategy for designing hydrophobic micropores. Though energy is not the only field of application, here we show that the proposed tuning approach may bring 20-75 MPa of intrusion/extrusion pressure increase, expanding the applicability of hydrophobic microporous materials.

Bowtie-Shaped Deformation Isotherm of Superhydrophobic Cylindrical Mesopores

Deformation of superhydrophobic cylindrical mesopores is studied during a cycle of forced water filling and spontaneous drying by in situ small-angle neutron scattering. A high-pressure setup is put forward to characterize the deformation of ordered mesoporous silanized silica up to 80 MPa. Strain isotherms of individual pores are deduced from the shift of the Bragg spectrum associated with the deformation of the hexagonal pore lattice. Due to their superhydrophobic nature, pore walls are not covered with a prewetting film. This peculiarity gives the ability to use a simple mechanical model to describe both filled and empty pore states without the pitfall of disjoining pressure effects. By fitting our experimental data with this model, we measure both the Young's modulus and the Poisson ratio of the nanometric silica wall. The measurement of this latter parameter constitutes a specificity offered by superhydrophobic nanopores with respect to hydrophilic ones.

ResponZIF Structures: Zeolitic Imidazolate Frameworks as Stimuli-Responsive Materials

Zeolitic imidazolate frameworks (ZIFs) have long been recognized as a prominent subset of the metal-organic framework (MOF) family, in part because of their ease of synthesis and good thermal and chemical stability, alongside attractive properties for diverse potential applications. Prototypical ZIFs like ZIF-8 have become embodiments of the significant promise held by porous coordination polymers as next-generation designer materials. At the same time, their intriguing property of experiencing significant structural changes upon the application of external stimuli such as temperature, mechanical pressure, guest adsorption, or electromagnetic fields, among others, has placed this family of MOFs squarely under the umbrella of stimuli-responsive materials. In this review, we provide an overview of the current understanding of the triggered structural and electronic responses observed in ZIFs (linker and bond dynamics, crystalline and amorphous phase changes, luminescence, etc.). We then describe the state-of-the-art experimental and computational methodology capable of shedding light on these complex phenomena, followed by a comprehensive summary of the stimuli-responsive nature of four prototypical ZIFs: ZIF-8, ZIF-7, ZIF-4, and ZIF-zni. We further expose the relevant challenges for the characterization and fundamental understanding of responsive ZIFs, including how to take advantage of their flexible properties for new application avenues.

Suspensions of lyophobic nanoporous particles as smart materials for energy absorption

HYPOTHESIS

Suspensions of nanoporous particles in non-wetting fluids (lyophobic nanoporous suspensions, LPNPS) are explored as energy absorbing materials for shock absorbers, bumpers, and energy storage. Upon application of pressure, the non-wetting fluid invades the pores transforming the impact energy into the interfacial energy that can be stored and released on demand.

EXPERIMENTS

Here, we present a comprehensive experimental study of the dynamics of LPNPS compression within a wide range of shock impact energy for three types of mesoporous materials (Libersorb 23, Polysorb-1, and Silochrome-1.5) with water and Wood alloy as non-wetting fluids.

FINDINGS

Three different regimes of the LPNPS compression-expansion cycle in response to the shock impact are distinguished as the impact energy increases: without fluid penetration into the pores, with partial penetration, and with complete pore filling. In two latter regimes, the suspension compressibility in the process of rapid compression increases by 2-4 decimal decades. This giant effect is associated with the onset of penetration of the non-wetting fluid into the nanopores upon achievement of a certain threshold pressure. The dynamic threshold pressure exceeds the threshold pressure of quasistatic intrusion and does not depends on the impact pressure, temperature, and suspension composition. A dynamic model of suspension compression is suggested that allows to separate the effects of the fluid intrusion into the pores and the elastic deformation of the system.

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances

Zeolites have been game-changing materials in oil refining and petrochemistry over the last 60 years and have the potential to play the same role in the emerging processes of the energy and environmental transition. Although zeolites are crystalline inorganic solids, their structures are not perfect and the presence of defect sites - mainly Brønsted acid sites and silanols - influences their thermal and chemical resistance as well as their performances in key areas such as catalysis, gas and liquid separations and ion-exchange. In this paper, we review the type of defects in zeolites and the characterization techniques used for their identification and quantification with the focus on diffraction, spectroscopic and modeling approaches. More specifically, throughout the review, we will focus on silanol (Si-OH) defects located within the micropore structure and/or on the external surface of zeolites. The main approaches applied to engineer and heal defects and their consequences on the properties and applications of zeolites in catalysis and separation processes are highlighted. Finally, the challenges and opportunities of silanol defect engineering in tuning the properties of zeolites to meet the requirements for specific applications are presented.

Absorption of mechanical energy via formation of ice nanotubes in zeolites

Molecular dynamics simulations are carried out for a heterogeneous system composed of bulk water and pure-silica zeolites of the AFI type. My simulations show, for the first time, the spontaneous crystallization of water in hydrophobic zeolite channels by compression, while the water outside remains liquid. The formation of ice nanotubes results in a molecular bumper behavior in the absence of chemical reactions, although the mechanism has been explained by the appearance of silanol defects. In contrast, the same zeolite-water system exhibits a weak shock-absorber behavior at higher temperatures. My study shows that the phase transitions of confined water dramatically change its intrusion/extrusion behavior and alter the energetic performance by varying the temperature alone. The results offer a new perspective for a better design of hydrophobic nanoporous materials utilized with water.

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

With their strong confining porosity and versatile surface chemistry, zeolitic imidazolate frameworks-including the prototypical ZIF-8-display exceptional properties for various applications. In particular, the forced intrusion of water at high pressure (∼25 MPa) into ZIF-8 nanopores is of interest for energy storage. Such a system reveals also ideal to study experimentally water dynamics and thermodynamics in an ultrahydrophobic confinement. Here, we report on neutron scattering experiments to probe the molecular dynamics of water within ZIF-8 nanopores under high pressure up to 38 MPa. In addition to an overall confinement-induced slowing down, we provide evidence for strong dynamical heterogeneities with different underlying molecular dynamics. Using complementary molecular simulations, these heterogeneities are found to correspond to different microscopic mechanisms inherent to vicinal molecules located in strongly adsorbing sites (ligands) and other molecules nanoconfined in the cavity center. These findings unveil a complex microscopic dynamics, which results from the combination of surface residence times and exchanges between the cavity surface and center.

High-rate nanofluidic energy absorption in porous zeolitic frameworks

Optimal mechanical impact absorbers are reusable and exhibit high specific energy absorption. The forced intrusion of liquid water in hydrophobic nanoporous materials, such as zeolitic imidazolate frameworks (ZIFs), presents an attractive pathway to engineer such systems. However, to harness their full potential, it is crucial to understand the underlying water intrusion and extrusion mechanisms under realistic, high-rate deformation conditions. Here, we report a critical increase of the energy absorption capacity of confined water-ZIF systems at elevated strain rates. Starting from ZIF-8 as proof-of-concept, we demonstrate that this attractive rate dependence is generally applicable to cage-type ZIFs but disappears for channel-containing zeolites. Molecular simulations reveal that this phenomenon originates from the intrinsic nanosecond timescale needed for critical-sized water clusters to nucleate inside the nanocages, expediting water transport through the framework. Harnessing this fundamental understanding, design rules are formulated to construct effective, tailorable and reusable impact energy absorbers for challenging new applications.

Compact Thermal Actuation by Water and Flexible Hydrophobic Nanopore

Efficient and compact energy conversion is at the heart of the sustainable development of humanity. In this work it is demonstrated that hydrophobic flexible nanoporous materials can be used for thermal-to-mechanical energy conversion when coupled with water. In particular, a reversible nonhysteretic wetting-drying (contraction-expansion) cycle provoked by periodic temperature fluctuations was realized for water and a superhydrophobic nanoporous Cu2(tebpz) MOF (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate). A thermal-to-mechanical conversion efficiency of ∼30% was directly recorded by high-precision PVT-calorimetry, while the operational cycle was confirmed by in operando neutron scattering. The obtained results provide an alternative approach for compact energy conversion exploiting solid-liquid interfacial energy in nanoscopic flexible heterogeneous systems.

Dynamics of heterogeneous wetting in periodic hybrid nanopores

We present experimental and theoretical results concerning the forced filling and spontaneous drying of hydrophobic cylindrical mesopores in the dynamical regime. Pores are structured with organic/inorganic moieties responsible for a periodicity of the surface energy along their axis. We find that the forced intrusion of water in these hydrophobic pores presents a slow dynamics: the intrusion pressure decreases as the logarithm of the intrusion time. We find that this slow dynamics is well described quantitatively by a classical model of activated wetting at the nanoscale, giving access to the structural length scales and surface energies of the mesoporous material.

The Effect of Surface Entropy on the Heat of Non-Wetting Liquid Intrusion into Nanopores

On-demand access to renewable and environmentally friendly energy sources is critical to address current and future energy needs. To achieve this, the development of new mechanisms of efficient thermal energy storage (TES) is important to improve the overall energy storage capacity. Demonstrated here is the ideal concept that the thermal effect of developing a solid-liquid interface between a non-wetting liquid and hydrophobic nanoporous material can store heat to supplement current TES technologies. The fundamental macroscopic property of a liquid's surface entropy and its relationship to its solid surface are one of the keys to predict the magnitude of the thermal effect by the development of the liquid-solid interface in a nanoscale environment-driven through applied pressure. Demonstrated here is this correlation of these properties with the direct measurement of the thermal effect of non-wetting liquids intruding into hydrophobic nanoporous materials. It is shown that the model can resonably predict the heat of intrusion into rigid mesoporous silica and some microporous zeolite when the temperature dependence of the contact angle is applied. Conversely, intrusion into flexible microporous metal-organic frameworks requires further improvement. The reported results with further development have the potential to lead to the development of a new supplementary method and mechanim for TES.

Crystals springing into action: metal-organic framework CUK-1 as a pressure-driven molecular spring

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction. The near-absence of hysteresis upon cycling exhibited by this robust MOF, akin to an ideal molecular spring, is associated with a constant work energy storage capacity of 40 J g⁻¹. Molecular simulations were further deployed to uncover the free-energy landscape behind this unprecedented pressure-responsive phenomenon in the area of compliant hybrid porous materials. This discovery is of utmost importance from the perspective of instant energy storage and delivery.

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction.

Thermodynamics of interfaces extended to nanoscales by introducing integral and differential surface tensions

As a system shrinks down in size, more and more molecules are found in its surface region, so surface contribution becomes a large or even a dominant part of its thermodynamic potentials. Surface tension is a venerable scientific concept; Gibbs defined it as the excess of grand potential of an inhomogeneous system with respect to its bulk value per interface area [J. W. Gibbs, "The Collected Works" in Thermodynamics (1928), Vol. 1]. The mechanical definition expresses it in terms of pressure tensor. So far, it has been believed the two definitions always give the same result. We show that the equivalence can break down for fluids confined in narrow pores. New concepts of integral and differential surface tensions, along with integral and differential adsorptions, need to be introduced for extending Gibbs thermodynamics of interfaces. We derived two generalized Gibbs adsorption equations. These concepts are indispensable for an adequate description of nanoscale systems. We also find a relation between integral surface tension and Derjaguin's disjoining pressure. This lays down the basis for measuring integral and differential surface tensions from disjoining pressure by using an atomic force microscope.

The Formation and Decay of an Unstable State of a Suspension of Hydrophobic Nanoporous Particles under Rapid Compression

The study of non-wetting liquid transport in a nanoporous medium is stimulated by the possible use of this process to absorb or accumulate mechanical energy. The filling of nanopores of suspended particles with a non-wetting liquid under decay of the unstable state, when the pressure increase rate is much higher than the rate of volume change, is studied. Based on the new experimental data and a theoretical model of the interacting modes of the spontaneous filling and filling under rapid compression, a picture of the percolation transition and a mechanism of liquid transport under such conditions are proposed. It is shown that a new dynamic filling threshold P0 is reached. It is shown that the filling of the porous medium is the result of the slow mode of impact compression when the fast mode of spontaneous filling is continuously adjusted to the slow mode on a small time scale. The theoretical model of the interacting modes is based on the solving of a system of kinetic equations for the distribution functions f(n,t) and F(n,t) clusters of filled pores under rapid compression, respectively. It is shown that filling at P=const corresponds to the non-dissipative transport of liquid on a time scale smaller than the characteristic filling time. The proposed model quantitatively describes the experimental data. So, the response of suspension to impact is characterized by the positive feedback.

Water Adsorption in Soft and Heterogeneous Nanopores

ConspectusLiquids under confinement differ in behavior from their bulk counterparts and can acquire properties that are specific to the confined phase and linked to the nature and structure of the host matrix. While confined liquid water is not a new topic of research, the past few years have seen a series of intriguing novel features for water inside nanoscale pores. These unusual properties arise from the very specific nature of nanoporous materials, termed "soft porous crystals"; they combine large-scale flexibility with a heterogeneous internal surface. This creates a rich diversity of behavior for the adsorbed water, and the combination of different experimental characterization techniques along with computational chemistry at various scales is necessary to understand the phenomena observed and their microscopic origins. The range of systems of interest span the whole chemical range, from the inorganic (zeolites, imogolites) to the organic (microporous carbons, graphene, and its derivatives), and even encompass the hybrid organic-inorganic systems (such as metal-organic frameworks).The combination of large scale flexibility with the strong physisorption (or even chemisorption) of water can lead to unusual properties (belonging to the "metamaterials" category) and to novel phenomena. One striking example is the recent elucidation of the mechanism of negative hydration expansion in ZrW₂O₈, by which adsorption of ∼10 wt % water in the inorganic nonporous framework leads to large shrinkage of its volume. Another eye-catching case is the occurrence of multiple water adsorption-driven structural transitions in the MIL-53 family of materials: the specific interactions between water guest molecules and the host framework create behavior that has not been observed with any other adsorbate. Both are counterintuitive phenomena that have been elucidated by a combination of experimental in situ techniques and molecular simulation.Another important direction of research is the shift in the systems and phenomena studied, from physical adsorption toward studies of reactivity, hydrothermal stability, and the effect of confinement on aqueous phases more complex than pure water. There have been examples of water adsorption in highly flexible metal-organic frameworks being able to compete with the materials' coordination bonds, thereby limiting its hydrothermal stability, while tweaking the functional groups of the same framework can lead to increased stability while retaining the flexibility of the material. However, this additional complexity and tunability in the macroscopic behavior can occur from changes in the confined fluid rather than the material. Very recent studies have shown that aqueous solutions of high concentration (such as LiCl up to 20 mol L^-1) confined in flexible nanoporous materials can have specific properties different from pure water and not entirely explained by osmotic effects. There, the strong ordering of the confined electrolyte competes with the structural flexibility of the framework to create an entirely new behavior for the {host, guest} system.

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.

Structural interpretation of the energetic performances of a pure silica LTA-type zeolite

The high pressure intrusion-extrusion process of different electrolyte aqueous solutions (NaCl and CaCl2, 2 M and 3 M) in a hydrophobic pure-silica LTA zeolite was investigated for energetic purposes by means of in situ X-ray powder diffraction, porosimeter tests, thermogravimetric analysis and NMR spectroscopy. The intrusion pressure of the saline solutions was proved to be higher than that of pure water, with the highest value measured for CaCl2, thus increasing the energetic performance of the system. The intrusion of NaCl solutions was irreversible (bumper behavior), whereas that of CaCl2 solutions is partially reversible (shock absorber behavior). The structural investigation allowed interpreting these results on the basis of the different intrusion mechanisms, in turn induced by the different nature of the cations present in the electrolyte solutions. When Si-LTA is intruded by NaCl solution, firstly H2O molecules penetrate the pores, leading to higher silanol defect formation followed by the solvated ions. With CaCl2, instead, due to a higher solvation enthalpy of Ca2+, a higher pressure is required for intrusion, and both H2O and ions penetrate at the same pressure. The structural refinements demonstrate (i) a different arrangement of the extraframework species in the two systems, (ii) the intrusion of the salt solutions occurs through strong desolvation of the ions and (iii) the salt/H2O ratios of the intruded species are higher than those of the starting electrolyte solutions.

Hydrophobic Metal-Organic Frameworks: Assessment, Construction, and Diverse Applications

Tens of thousands of metal-organic frameworks (MOFs) have been developed in the past two decades, and only ≈100 of them have been demonstrated as porous and hydrophobic. These hydrophobic MOFs feature not only a rich structural variety, highly crystalline frameworks, and uniform micropores, but also a low affinity toward water and superior hydrolytic stability, which make them promising adsorbents for diverse applications, including humid CO2 capture, alcohol/water separation, pollutant removal from air or water, substrate-selective catalysis, energy storage, anticorrosion, and self-cleaning. Herein, the recent research advancements in hydrophobic MOFs are presented. The existing techniques for qualitatively or quantitatively assessing the hydrophobicity of MOFs are first introduced. The reported experimental methods for the preparation of hydrophobic MOFs are then categorized. The concept that hydrophobic MOFs normally synthesized from predesigned organic ligands can also be prepared by the postsynthetic modification of the internal pore surface and/or external crystal surface of hydrophilic or less hydrophobic MOFs is highlighted. Finally, an overview of the recent studies on hydrophobic MOFs for various applications is provided and suggests the high versatility of this unique class of materials for practical use as either adsorbents or nanomaterials.

The effect of nanostructures on high pressure intrusion–extrusion of water and electrolyte solutions in hierarchical nanoboxes of silicalite-1

A drastic effect of the presence of cavities on high pressure intrusion–extrusion of LiCl aqueous solution in silicalite-1 nanoboxes is observed.

Confining a Protein-Containing Water Nanodroplet inside Silica Nanochannels

Incorporation of biological systems in water nanodroplets has recently emerged as a new frontier to investigate structural changes of biomolecules, with perspective applications in ultra-fast drug delivery. We report on the molecular dynamics of the digestive protein Pepsin subjected to a double confinement. The double confinement stemmed from embedding the protein inside a water nanodroplet, which in turn was caged in a nanochannel mimicking the mesoporous silica SBA-15. The nano-bio-droplet, whose size fits with the pore diameter, behaved differently depending on the protonation state of the pore surface silanols. Neutral channel sections allowed for the droplet to flow, while deprotonated sections acted as anchoring piers for the droplet. Inside the droplet, the protein, not directly bonded to the surface, showed a behavior similar to that reported for bulk water solutions, indicating that double confinement should not alter its catalytic activity. Our results suggest that nanobiodroplets, recently fabricated in volatile environments, can be encapsulated and stored in mesoporous silicas.

Pore Morphology Determines Spontaneous Liquid Extrusion from Nanopores

In this contribution we explore by means of experiments, theory, and molecular dynamics the effect of pore morphology on the spontaneous extrusion of nonwetting liquids from nanopores. Understanding and controlling this phenomenon is central for manipulating nanoconfined liquids, e. g., in nanofluidic applications, drug delivery, and oil extraction. Qualitatively different extrusion behaviors were observed in high-pressure water intrusion-extrusion experiments on porous materials with similar nominal diameter and hydrophobicity: macroscopic capillary models and molecular dynamics simulations revealed that the very presence or absence of extrusion is connected to the internal morphology of the pores and, in particular, to the presence of small-scale roughness or pore interconnections. Additional experiments with mercury confirmed that this mechanism is generic for nonwetting liquids and is rooted in the pore topology. The present results suggest a rational way to engineer heterogeneous systems for energy and nanofluidic applications in which the extrusion behavior can be controlled via the pore morphology.

Liquid Intrusion into Zeolitic Imidazolate Framework-7 Nanocrystals: Exposing the Roles of Phase Transition and Gate Opening to Enable Energy Absorption Applications

Liquid intrusion into zeolitic imidazolate framework 7 (ZIF-7) has been observed for the first time. Among the three typical phases of ZIF-7, we discover that only the guest-free ZIF-7-II structure can be intruded by mechanical pressure, and intriguingly, this pressurized liquid intrusion behavior is detected only in nanocrystals, indicating the crystal size effect. Because of its unique combination of non-outflow property and high intrusion pressure, water intrusion into ZIF-7-II generates a marked energy dissipation capacity of ∼2 J/g despite its limited pore volume. We present several strategies that can be easily implemented to tune its intrusion pressure and energy dissipation and accomplish material reusability. Remarkably, we found that the pore cavities of ZIF-7-II can accommodate water molecules without experiencing any phase transition, which is entirely different from other solvents whose incorporation will trigger a spontaneous conversion into ZIF-7-I. Our pressure-vs-volume data further reveal that the process of water infiltration and retainment is controlled by the gate-opening/closing mechanism, which has enabled us to probe the viscoelasticity of ZIF-7 via cyclic liquid intrusion experiments. This study has deepened our understanding of the time-dependent mechanical properties of ZIFs and shed new light on the structural flexibility central to the novel applications of metal-organic framework materials.

Ru/UiO-66 Catalyst for the Reduction of Nitroarenes and Tandem Reaction of Alcohol Oxidation/Knoevenagel Condensation

A 3.1% Ru/UiO-66 material was prepared by adsorption of RuCl₃ from ethyl acetate on to MOF UiO-66, followed by reduction with NaBH₄. The presence of acid-base and ox-red sites allows this 3.1% Ru/UiO-66 material acting as a bifunctional catalyst for the reduction of nitroarenes and tandem reaction of alcohol oxidation/Knoevenagel condensation. The high efficiency of 3.1% Ru/UiO-66 was demonstrated in the reduction of nitroarenes to amines. This system can be applied as a catalyst for at least six successive cycles without loss of activity. The 3.1% Ru/UiO-66 catalyst also was active in the tandem aerobic oxidation of alcohols/Knoevenagel condensation with malononitrile. However, the activity of this catalyst strongly decreased in the second cycle. A combination of physicochemical and catalytic experimental data indicated that Ru nanoparticles are the active sites both for the catalytic reduction of nitro compounds and the aerobic oxidation of alcohols. The activity for the Knoevenagel condensation reaction was from the existence of the "Zr ⁿ⁺-O^2- Lewis acid-base" pair in the framework of UiO-66.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.