Water Molecules Research Articles

Fluorinated metal-organic framework aerogels with enhanced moisture resistance and efficient gas diffusion for CO2 capture

Escalating CO2 levels in confined spaces threatens human well-being and safety, underscoring the urgent need for effective CO2 mitigation strategies. Metal-organic frameworks (MOFs), with their large surface areas and capacious pores, represent a promising avenue for CO2 capture. However, their application is often limited by susceptibility to moisture and lack of uniformity in mesopore distribution. Here, we present a novel approach involving ligand fluorination modification and pore reconstruction to fabricate ultralight, highly moisture resistant, and hierarchically porous UiO-66-NH2-F4 aerogels. This design ingenuity promotes the formation of an efficient gas transport network and generates an abundance of micropores decorated with hydrophobic units, effectively shielding CO2 adsorption from the interference of water molecules. The UiO-66-NH2-F4 aerogels exhibit an impressive CO2 adsorption capacity of 5.18 mmol/g (dry) and 4.48 mmol/g (70 % RH) at 298 K, a remarkable CO2/N2 selectivity of 29, and exceptional cyclability. Density functional theory (DFT) calculations further elucidate the robust binding interactions of these CO2-selective UiO-66-NH2-F4 aerogels, attributing them to the tailed pore environment created by fluorinated hydrophobic groups and the accessibility of adsorption sites. This work charts a promising course for developing moisture-resistant and hierarchically porous MOF aerogels, which are poised to revolutionize CO2 adsorption practices in confined environments.

Density functional theory (DFT) study of water autoionization in solvated clusters.

We have implemented a cluster-continuum method using density functional theory to model water clusters and various charged species derived from water. The two aims of this study are to determine the minimal basis required for proper modeling of water autoionization and to determine the minimum number of explicit water molecules required to properly model the energetics of solvation. The thermodynamics of water autoionization converge following a modified power law to deliver chemically accurate values of the Gibbs energy change for autoionization with tractably small clusters. Convergence is slower and not exponential as assumed in previous work. We identify the n = 21 set of clusters as the first effectively bulk water like clusters that can capture the energetic influence of both the first and second solvation shells. In this cluster, a water molecule is encapsulated in the center of a closed shell of other water molecules that hydrogen bond to form five-membered rings. The total energy change for clusters with n ≥ 21 calculated using both the RPBE-D3 and ωB97X-D exchange-correlation functionals with the 6-311+G** basis set is shown to deliver good approximations to the free energy change at 298K. This is true even though neither functional models the individual enthalpy or entropy contributions particularly well.

A theoretical simulation of carbon dioxide adsorption capture on amine-functionalized graphene oxides

Amine-functionalized graphene oxides (AFGOs) are potential candidates for CO2 capture applications, as the amine functional groups could act as effective sites, promoting CO2 adsorption strength and selectivity. However, the CO2 adsorption mechanism on AFGO appears to be ambiguous, particularly with no explicit identification of the chemically bonded species. In this work, density functional theory and microkinetic simulations were carried out to investigate the physical and chemical adsorption mechanisms and behavior between AFGO and CO2. Our findings indicate that the van der Waals and hydrogen bonding (HB) interactions constitute physical binding modes between the aminated GO and CO2. The chemical adsorption pathways on AFGO materials have been revealed by the transition state search method, in which the relevant CO2 adsorption species, including carbamic acid and carbamate, and their formation conditions were theoretically identified on AFGOs. The chemisorption mechanisms have been recognized to be the nucleophilic interaction between GO-supported amines and CO2, simultaneously assisted by the cooperative effect of multiple HB interactions. More specifically, the surface hydroxyl groups and water molecules can serve as proton transfer media and offer HB interactions, thus regulating the reaction activity and chemical adsorption species. This theoretical work presents a new insight into CO2-AFGO interaction mechanisms, which is valuable for designing aminated GO adsorbents.

Water Networks within Metal Organic Nanotubes: Assessment of Techniques to Understand Structure and Properties

Hybrid materials, such as metal organic nanotubes (MONTs) can possess nanoconfined water molecules within their pore space and the overall behavior of the water within the material may be tuned based upon interactions with the inner channel walls. We have previously developed a range of methods (electron density mapping, kinetic models, and water interaction enthalpies) to evaluate water behavior under nanoconfinement using a uranium‐based metal organic nanotube (UMONT) but have not explored their applicability across a range of materials. In the current study, we test our methodologies on two additional MONT materials (LaMONT and Cu‐LaMONT) to determine if the techniques can be utilized in other systems to predict behavior within complex hybrid materials. In addition, we explored how to use Hirshfeld surface maps generated by the CrystalExplorer software in the visualization and prediction of water behavior within complex hybrid materials.

High‐Temperature Solid‐State Post‐synthetic Modification of Highly Luminescent Cu(I) Metallacycles toward New Luminescent Thermic Tracers

A new luminescent Cu(I) tetrametallic metallacycle B is reported, featuring very rare semi‐bridging aqua ligands. As heated markedly above room temperature, this compound undergoes in the solid‐state a post‐synthetic transformation, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single crystals X‐ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, however caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye‐perceived green (B) to blue (C) room temperature luminescence for these molecular solids. Photophysical measurements and time‐dependent density‐functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, suggesting that Thermally Actived Delayed Fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering the access to stimuli‐sensitive materials that can witness a posteriori the exceeding of critical temperatures in their environment.

High‐Temperature Solid‐State Post‐synthetic Modification of Highly Luminescent Cu(I) Metallacycles toward New Luminescent Thermic Tracers

A new luminescent Cu(I) tetrametallic metallacycle B is reported, featuring very rare semi‐bridging aqua ligands. As heated markedly above room temperature, this compound undergoes in the solid‐state a post‐synthetic transformation, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single crystals X‐ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, however caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye‐perceived green (B) to blue (C) room temperature luminescence for these molecular solids. Photophysical measurements and time‐dependent density‐functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, suggesting that Thermally Actived Delayed Fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering the access to stimuli‐sensitive materials that can witness a posteriori the exceeding of critical temperatures in their environment.

Electron transfer in the respiratory chain at low salinity

Recent studies have established that cellular electrostatic interactions are more influential than assumed previously. Here, we use cryo-EM and perform steady-state kinetic studies to investigate electrostatic interactions between cytochrome (cyt.) c and the complex (C) III2-IV supercomplex from Saccharomyces cerevisiae at low salinity. The kinetic studies show a sharp transition with a Hill coefficient ≥2, which together with the cryo-EM data at 2.4 Å resolution indicate multiple cyt. c molecules bound along the supercomplex surface. Negatively charged loops of CIII2 subunits Qcr6 and Qcr9 become structured to interact with cyt. c. In addition, the higher resolution allows us to identify water molecules in proton pathways of CIV and, to the best of our knowledge, previously unresolved cardiolipin molecules. In conclusion, the lowered electrostatic screening renders engagement of multiple cyt. c molecules that are directed by electrostatically structured CIII2 loops to conduct electron transfer between CIII2 and CIV.

Halogen Bonding on Water─A Drop in the Ocean?

Halogen bonding is a valuable interaction in drug design, offering an unconventional way to influence affinity and selectivity by leveraging the halogen atoms' ability to form directional bonds. The present study evaluates halogen-water interactions within protein binding sites, demonstrating that targeting a water molecule via halogen bonding can in specific cases contribute beneficially to ligand binding. In solving and examining the crystal structure of 2-cyclopentyl-7-iodo-1H-indole-3-carbonitrile bound to DYRK1a kinase, we identified a notable iodine-water interaction, where water accepts a halogen bond with good geometric and energetic features. This starting point triggered further investigations into the prevalence of such interactions across various halogen-bearing ligands (chlorine, bromine, iodine) in the PDB. Using QM calculations (MP2/TZVPP), we highlight the versatility and potential benefits of such halogen-water interactions, particularly when the water molecule is a stable part of the binding site's structured environment. While the interaction energies with water are lower compared to other typical halogen bond acceptors, we deem this different binding strength essential for reducing desolvation costs. We suggest that "interstitial" water molecules, as stable parts of the binding site engaging in multiple strong interactions, could be prime targets for halogen bonding. Further systematic studies, combining high-resolution crystal structures and quantum chemistry, are required to scrutinize whether halogen bonding on water is more than a "drop in the ocean".

Water structures and anisotropic dynamics at Pt(211)/water interface revealed by machine learning molecular dynamics

Abstract Water molecules at solid–liquid interfaces play a pivotal role in governing interfacial phenomena that underpin electrochemical and catalytic processes. The organization and behavior of these interfacial water molecules can significantly influence the solvation of ions, the adsorption of reactants, and the kinetics of electrochemical reactions. The stepped structure of Pt surfaces can alter the properties of the interfacial water, thereby modulating the interfacial environment and the resulting surface reactivity. Revealing the in situ details of water structures at these stepped Pt/water interfaces is crucial for understanding the fundamental mechanisms that drive diverse applications in energy conversion and material science. In this work, we have developed a machine learning potential for the Pt(211)/water interface and performed machine learning molecular dynamics simulations. Our findings reveal distinct types of chemisorbed and physisorbed water molecules within the adsorbed layer. Importantly, we identified three unique water pairs that were not observed in the basal plane/water interfaces, which may serve as key precursors for water dissociation. These interfacial water structures contribute to the anisotropic dynamics of the adsorbed water layer. Our study provides molecular-level insights into the anisotropic nature of water behavior at stepped Pt/water interfaces, which can influence the reorientation and distribution of intermediates, molecules, and ions—crucial aspects for understanding electrochemical and catalytic processes.

Chemistry of Formate and Water Ligands on Metal Oxide Cluster Nodes of Metal-Organic Framework hcp Hf-UiO-66: Keys to Understanding Reactivity of Paired μ2-OH and Defect Sites.

Many metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters, and ligands that have been observed on these nodes include formates, acetates, water, hydroxyl groups, and others, all of which are potentially important in affecting reactivities for applications in separations, catalysis, and sensing. Formate is a common node ligand, arising from formic acid used as a modulator and from N,N-dimethylformamide used as a solvent in MOF syntheses. Yet only little work has been reported characterizing the reactivities of node formate ligands. Infrared spectra reported here show that formate bonds to two types of sites on the paired Hf6O8 nodes of hcp UiO-66, namely, defect and μ2-OH sites. Quantifying the number of formate ligands by 1H NMR spectroscopy of digested samples showed an almost equal number of formate ligands on the two sites, indicating the likelihood that they neighbor each other. These formate ligands interact with water molecules, reversibly switching their bonding from bidentate to monodentate. The formates on μ2-OH sites of hcp Hf-UiO-66 interact much more strongly with water than those on defect sites of the same node, and both interact more strongly than isolated defect sites of Hf-UiO-66. Correspondingly, the catalytic activities of hcp UiO-66 determined as turnover frequencies on each site are approximately twofold higher than those on UiO-66, bolstering the inference that methanol dehydration is catalyzed by a node defect site and a neighboring node μ2-OH site. The results show how MOFs, with their well-defined node structures, provide unprecedented opportunities to understand details of reactivities and catalysis on metal oxide clusters, in contrast to bulk metal oxide surfaces.

Multi-Modal Sensing Ionogels with Tunable Mechanical Properties and Environmental Stability for Aquatic and Atmospheric Environments.

Ionogels have garnered significant interest due to their great potential in flexible iontronic devices. However, their limited mechanical tunability and environmental intolerance have posed significant challenges for their integration into next-generation flexible electronics in different scenarios. Herein, the synergistic effect of cation-oxygen coordination interaction and hydrogen bonding is leveraged to construct a 3D supramolecular network, resulting in ionogels with tunable modulus, stretchability, and strength, achieving an unprecedented elongation at break of 10800%. Moreover, the supramolecular network endows the ionogels with extremely high fracture energy, crack insensitivity, and high elasticity. Meanwhile, the high environmental stability and hydrophobic network of the ionogels further shield them from the unfavorable effects of temperature variations and water molecules, enabling them to operate within a broad temperature range and exhibit robust underwater adhesion. Then, the ionogel is assembled into a wearable sensor, demonstrating its great potential in flexible sensing (temperature, pressure, and strain) and underwater signal transmission. This work can inspire the applications of ionogels in multifunctional sensing and wearable fields.

Manifold Fluorescence Enhancement of a Styryl(pyridinium)-chromene Hybrid Dye upon Binding with an Elongated β-Cyclodextrin Cavity.

The interaction of a styryl(pyridinium)-chromene hybrid dye (DSP-C) with the 2-hydroxypropyl-β-cyclodextrin (HPβCD) macrocycle leads to a remarkably large increase (∼310-fold) in its fluorescence intensity, in contrast to the relatively smaller (∼45-fold) enhancement observed with native β-cyclodextrin (βCD). Both macrocycles (βCD and HPβCD) bind with the styryl(pyridinium) as well as the chromene fragments of the hybrid dye, with the simultaneous formation of 1:1 and 2:1 host:guest complexes. However, the binding constant (Keq1) is more than an order of magnitude higher for HPβCD than for βCD. The improved binding affinity of HPβCD is attributed to its elongated and deeper hydrophobic cavity. This is supported well by the optimized geometries of the host-guest complexes. Theoretical calculations also reveal that the energy change due to the release of high-energy water molecules from the host nanocavity is more favorable for HPβCD than βCD, resulting in greater stability of the HPβCD:DSP-C complex. Interestingly, though the fluorescence of DSP-C arises from its styryl(pyridinium) fragment, the complexation of the chromene unit with the host plays a major role in augmenting the fluorescence enhancement of the hybrid dye. The large fluorescence change in the HPβCD:DSP-C system has been utilized for the detection of bile salts by the indicator displacement strategy.

Interfacially Fabricated Covalent Organic Framework Membranes for Film‐based Fluorescence Humidity Sensors and Moisture Driven Actuators

Rapid, on‐site measurement of ppm‐level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β‐ketoenamine‐linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5‐tris‐(4‐aminophenyl)triazine (TTA) with 1,3,5‐tri‐formylphloroglucinol (TP). Based on the super‐sensitive and reversible response of the COF membrane to water vapor, we developed a high‐performance film‐based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000‐time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on‐site and real‐time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra‐sensitive film‐based fluorescent sensors (FFSs) and high‐performance actuators.

From Brownian motion to virtual biopsy: a historical perspective from 40years of diffusion MRI.

Diffusion MRI was introduced in 1985, showing how the diffusive motion of molecules, especially water, could be spatially encoded with MRI to produce images revealing the underlying structure of biologic tissues at a microscopic scale. Diffusion is one of several Intravoxel Incoherent Motions (IVIM) accessible to MRI together with blood microcirculation. Diffusion imaging first revolutionized the management of acute cerebral ischemia by allowing diagnosis at an acute stage when therapies can still work, saving the outcomes of many patients. Since then, the field of diffusion imaging has expanded to the whole body, with broad applications in both clinical and research settings, providing insights into tissue integrity, structural and functional abnormalities from the hindered diffusive movement of water molecules in tissues. Diffusion imaging is particularly used to manage many neurologic disorders and in oncology for detecting and classifying cancer lesions, as well as monitoring treatment response at an early stage. The second major impact of diffusion imaging concerns the wiring of the brain (Diffusion Tensor Imaging, DTI), allowing to obtain from the anisotropic movement of water molecules in the brain white-matter images in 3 dimensions of the brain connections making up the Connectome. DTI has opened up new avenues of clinical diagnosis and research to investigate brain diseases, neurogenesis and aging, with a rapidly extending field of application in psychiatry, revealing how mental illnesses could be seen as Connectome spacetime disorders. Adding that water diffusion is closely associated to neuronal activity, as shown from diffusion fMRI, one may consider that diffusion MRI is ideally suited to investigate both brain structure and function. This article retraces the early days and milestones of diffusion MRI which spawned over 40years, showing how diffusion MRI emerged and expanded in the research and clinical fields, up to become a pillar of modern clinical imaging.

Thermal Stability Improvement of Cu-Based Catalyst by Hydrophobic Modification in Methanol Synthesis

Water can cause the growth and oxidation of Cu nanoparticles on the surface of Cu-based catalysts, leading to their deactivation. However, during methanol synthesis process from syngas on Cu-based catalysts, water is inevitably produced as a by-product due to the presence of CO2. Therefore, enhancing the stability of Cu-based catalysts during the reaction, particularly in the presence of water, is crucial. In this study, Cu/ZnO/Al2O3 was first subjected to wet etching and then hydrophobically modified using the sol–gel method with methyltrimethoxysilane (MTMS) and the grafting method with 1H,1H,2H,2H-perfluoroalkyltriethoxysilanes (PFOTES) as modifiers. These modifications aimed to mitigate the impact of water on the catalyst and improve its stability. After modification, the catalysts exhibited excellent hydrophobicity and enhanced catalytic activity in the methanol synthesis process. The surface physical properties, composition, and thermal stability of the catalysts before and after hydrophobic modification were characterized by SEM, FT-IR, BET, XRD and TGA. Additionally, molecular dynamics simulations were employed to compare the diffusion behavior of water molecules on the catalyst surfaces before and after hydrophobic modification. The results indicated that the modified catalyst surface formed a micro/nano structure composed of nanosheets and nanosheet clusters, while the hydrophobic modification did not alter the structure of the catalyst. According to the results of simulations, the hydrophobic layers on the modified catalysts were able to expel water quickly from the surfaces and reduce the relative concentration of water molecules at the active sites, thereby improving the stability of the catalyst. Notably, the thermal stability and hydrophobicity of the PFOTES-modified catalyst were superior to those of the MTMS-modified catalyst, resulting in a more significant enhancement in catalyst stability, which aligned with the experimental results.

Interfacially Fabricated Covalent Organic Framework Membranes for Film-based Fluorescence Humidity Sensors and Moisture Driven Actuators.

Rapid, on-site measurement of ppm-level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β-ketoenamine-linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5-tris-(4-aminophenyl)triazine (TTA) with 1,3,5-tri-formylphloroglucinol (TP). Based on the super-sensitive and reversible response of the COF membrane to water vapor, we developed a high-performance film-based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000-time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on-site and real-time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra-sensitive film-based fluorescent sensors (FFSs) and high-performance actuators.

Facile Synthesis of Bis-Diphenylphosphine Oxide as a Flame Retardant for Epoxy Resins

A phosphorus-containing compound, (oxybis(4,1-phenylene))bis(phenylphosphine oxide) (ODDPO), was successfully synthesized and used as a flame retardant for epoxy resin (EP). The results demonstrated that EP/ODDPO, containing 1.2 wt% phosphorus, achieved a vertical burning V-0 rating, with a limited oxygen index value of 29.2%, indicating excellent flame retardancy. Comprehensive evaluations revealed that ODDPO exhibited both gas-phase and condensed-phase flame-retardant effects on EP, with a particularly notable barrier effect. In addition, the incorporation of ODDPO had a minimal negative impact on the glass transition temperature (Tg) and thermal stability of the EP matrix. Compared to unmodified EP (EP-0), the Tg value and initial decomposition temperature of EP/ODDPO-1.2 decreased by only 7.6 °C and 10.0 °C, respectively. Moreover, the introduction of ODDPO significantly improved the hydrophobicity and water absorption resistance of epoxy materials, which is attributed to ODDPO’s rigidity and symmetric structure, reducing water molecule permeation. Furthermore, the dielectric properties of ODDPO-modified EP samples were strengthened compared to EP-0, due to the ODDPO’s symmetric structure reducing the polarity of the matrix. The above results indicated that ODDPO serves as an excellent flame retardant while enhancing other properties of the EP matrix, thereby contributing to the preparation and application of high-performance epoxy materials.

Probing the Electric Field Response of a Water Molecule Confined in Small Carbon Nanocages: A Density Functional Theory Investigation.

We consider a water molecule under tight confinement in the small-sized fullerenes (C28, C30, C32) within the density functional theory (DFT) calculations with suitable exchange-correlation functionals. Such nanoscopic molecular cages provide an ideal setup to study their characteristic properties not present in the condensed phase. The water molecule entirely loses its feature of typical water when it is confined in small fullerenes of size equal to C30or smaller, in which the asymmetric O-H stretching vibration occurs at a lower wavenumber than the symmetric stretching. We study the response of the confined water molecule to the applied electric fields in terms of change in geometrical parameters, NMR spin-spin coupling constants, dipole moment, HOMO-LUMO (HL) gap, and vibrational frequency shift. The electric field shielding property of small-sized fullerene cages is explored and found to be strongly correlated with the HL gap. Since the electric field modulates the gap to decrease generally, shielding efficiency varies with field strength, thereby making large fields better shielded than small fields for the small penetration factor at large fields. The results that hold significance for technological applications are discussed.

About the structure of water

It is shown that during 10–10–10–11s, only clusters consisting of 4 or 3 water molecules can statistically form in water. It is shown that Brownian motion in water occurs as a result of collision of ice nanocrystals with Brownian particles. Brownian motion is an experimental confirmation of the nanocrystalline structure of water. It is shown that water consists of 13 % molecules and 87 % ice nanocrystals. Such a two‑phase structure provides water with structural properties, high fluidity and elasticity of steam.

Surface fluorination of BiVO4 for the photoelectrochemical oxidation of glycerol to formic acid

The C–C bond cleavage of biomass-derived glycerol to generate value-added C1 products remains challenging owing to its slow kinetics. We propose a surface fluorination strategy to construct dynamic dual hydrogen bonds on a semiconducting BiVO4 photoelectrode to overcome the kinetic limit of the oxidation of glycerol to produce formic acid (FA) in acidic media. Intensive spectroscopic characterizations confirm that double hydrogen bonds are formed by the interaction of the F–Bi–F sites of modified BiVO4 with water molecules, and the unique structure promotes the generation of hydroxyl radicals under light irradiation, which accelerates the kinetics of C–C bond cleavage. Theoretical investigations and infrared adsorption spectroscopy reveal that the double hydrogen bond enhances the C=O adsorption of the key intermediate product 1,3-dihydroxyacetone on the Bi–O sites to initiate the FA pathway. We fabricated a self-powered tandem device with an FA selectivity of 79% at the anode and a solar-to-H2 conversion efficiency of 5.8% at the cathode, and these results are superior to most reported results in acidic electrolytes.