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Interfacial free energy is a quantitative basis for explaining and predicting interfacial behavior that is ubiquitous in nature. The contact angle (CA) method can determine the surface free energy (γ) as well as Lifshitz-van der Waals (γ

The facile exfoliation of a two-dimensional metal-organic nanosheet of {[Co(HL)(H

Positive cooperative binding, a phenomenon prevalent in biological processes, holds great appeal for the design of highly sensitive responsive molecules and materials. It has been demonstrated that metal-organic frameworks (MOFs) can show positive cooperative adsorption to the benefit of gas separation, but potential binding cooperativity is largely ignored in the study of sensory MOFs. Here, we report the first demonstration of positive cooperative protonation of a MOF and the relevant pH response in fluorescence and proton conduction. The MOF is built of Zr-O clusters and bipyridyl-based tetracarboxylate linkers and has excellent hydrolytic stability. It shows a unique pH response that features two synchronous abrupt turn-off and turn-on fluorescent transitions. The abrupt transitions, which afford high sensitivity to small pH fluctuations, are due to cooperative protonation of the pyridyl sites with a Hill coefficient of 1.6. The synchronous dual-emission response, which leads to visual color change, is ascribable to proton-triggered switching between (n, π*) and (π, π*) emissions. The latter emission can be quenched by electron donating anion-dependent through photoinduced electron transfer and ground-state charge transfer. Associated with cooperative protonation, the proton conductivity of the MOF is abruptly enhanced at low pH by two orders, but overhigh acid concentration is adverse because excessive anions can interrupt the conducting networks. Our work shows new perspectives of binding cooperativity in MOFs and should shed new light on the development of responsive fluorescent MOFs and proton conductive materials.

Sensory materials that show color and/or fluorescence changes in response to specific gases or vapors have important applications in many fields. Here, we report the postsynthetic preparation of novel sensory metal-organic frameworks (MOFs) and their multiple responsive properties. Through postsynthetic N-amination, the 2,2'-bipyridyl spacers in a Zr(IV) MOF are partially transformed into N-aminobipyridinium. The new MOF (Zr-bpy-A) shows chromic behavior toward ammonia and amines because the electron-deficient pyridinium groups form charge-transfer complexes with amino moieties. It also shows a unique chromic response to formaldehyde owing to the Schiff-base condensation with the N-amino groups. Furthermore, the N-amino group can be used to graft different polycyclic aromatic hydrocarbons, which endow the MOF with strong fluorescence of variable colors and afford a high-contrast fluorescence response to ammonia/amines and formaldehyde associated with the chromic response. The presence of the unquaternized bipyridyl group also leads to a fluorescence response to HCl. The multiple responsive behaviors hold appeal for applications in sensing, switching, and antifake marking, which are illustrated with a test paper and writing ink.

Solid catalysts for CO2 cycloaddition to epoxides often require homogeneous halide anions in large amounts, which sacrifice the heterogeneity and ecofriendliness. Here, we demonstrate a novel appro...

Remote stereocontrolled 1,8-addition of heteroatom nucleophiles to 6-methylene-6H-indoles generated in situ from 6-indolylmethanols has been developed for the first time. With the aid of a chiral phosphoric acid, 6-indolylmethanols reacted with benzotriazoles to furnish 1,8-adducts with a nitrogen-containing tertiary carbon stereocenter in 54-80% yield with 76-92% ee. Importantly, the stereoselective 1,8-addition of benzotriazoles featured N2 selectivity. Furthermore, using thioacids as nucleophiles enabled the formation of 1,8-adducts with a sulfur-containing tertiary carbon stereocenter in 70-78% yield with 75-94% ee.

Conventional ultrafiltration (UF) technology suffers from membrane fouling and limited separation performance. This work demonstrates a novel electrical tuning strategy to improve the separation efficiency of the UF process. An electrically enhanced UF (EUF) system with two sets of oppositely placed membrane-electrode modules was set up. A series of multicycle treatment experiments were conducted to reveal the performance and tuning mechanism of the EUF system. The applied electrical tuning operation brought about an up to 68% reduction of average transmembrane pressure increasing rate (

A novel metal-organic framework (MOF), formulated as [Cd2(TTVTC)Cl2(H2O)3]·2H2O (1), was synthesized from a tetracarboxylate ligand ([TTVTC]2-) functionalized with the thiazolothiazole extended viologen (TTV2+) fluorophore. The MOF features three-dimensional (10,3)-d frameworks with 6-fold interpenetration. The MOF exhibits reversible photochromism, due to photoinduced electron transfer from carboxylate to TTV2+. The photoactivity benefits from the electron donor-acceptor contacts enabled by mutual interpenetration of the frameworks. This is the first demonstration of photochromism in TTV2+ derivatives. In addition, the fluorescence arising from the TTV2+ fluorophore can be reversibly modulated during the photochromic process. The work demonstrates the great potential of extended viologen based ligands in the construction of MOFs with dual photomodulable optical properties, which could find future applications in photoelectronics.

Luminescent metal–organic frameworks (LMOFs) are newly emerged sensory materials, but rational design of LMOFs for specific analytes still faces many challenges for lack of sufficient knowledge abo...

This study systematically investigated the correlations between fluorescence distributions characterized by the excitation-emission matrix (EEM) and hydrophobic/hydrophilic composition of dissolved organic matter (DOM) in membrane bioreactors (MBRs). On the basis of samples from 10 full-scale MBRs, we performed point-to-point comparisons among different components using an EEM fluorescence quotient (FQ) method and obtained a hydrophobic/hydrophilic fluorophore distribution map via Wilcoxon signed rank test. Hydrophobic acids/bases (HOA/HOB) concentrated in the low-wavelength region [excitation wavelength (Ex) 235 nm [especially with emission wavelength (Em) = 300-360 nm]. Quantitatively, EEM regional contribution to whole wavelength fluorescence was found to significantly correlate with the hydrophobic/hydrophilic proportions of DOM, with Pearson's coefficients of 0.94 and 0.78 ( p < 0.01) for HOA and HIS, respectively. We established a linear regression model showing the HOA proportion as a function of the EEM regional contribution at (Ex, Em) = (200-285, 340-465 nm), with R2 = 0.876, which was validated via leave-one-out cross-validation and Monte Carlo simulation. This study shows a statistically hydrophobicity-dependent fluorescence property across different MBRs, and it might be applied to provide a quick estimation of hydrophobic/hydrophilic composition of DOM in wastewater treatment systems based on EEM monitoring.

We report our observations of the transient formation of the translational and rotational defects in the host lattice of methane hydrates. We perform molecular dynamics simulations of a fully occupied methane hydrate system and find that the hydrogen bonds of a water molecule can be thermally broken, and the water molecule can then rotate more freely. We observe the formation of transient Bjerrum defects around the water molecule, and the two hydrogens of the water molecule can be interchanged when the transient defects anneal. The 95% confidence interval for the rate of the hydrogen interchange is estimated to be 1.3 × 105–2.4 × 105 s–1 per water molecule, and the 95% confidence interval for the associated free energy of activation is estimated to be 38.2–39.4 kJ/mol at 270 K. We also observe the transient formation of vacancy-interstitial water defects. The formation and annealing of these vacancy-interstitial defects can result in the interchange of two or three neighboring water molecules on the gas h...

Gas diffusion is considered a rate-limiting step in the formation of gas hydrates, yet its molecular mechanisms remain unclear. In this work, we present the molecular mechanisms of the CO2 cage-to-cage transport in gas hydrates, as directly observed from molecular dynamics simulations performed at elevated temperatures. We found that at least one water vacancy is required for the CO2 molecules to pass through five-membered water rings, while only the distortion of the local ring structure is required for the CO2 molecules to pass through the six-membered water rings. We used the transition-state theory to estimate the relevant kinetic parameters associated with the CO2 diffusion in gas hydrates. The calculated free energy of activation is about 44 ± 6 kJ/mol, and the diffusion coefficient is in the range of 1.0 × 10–16∼2.0 × 10–14 m2/s, for the CO2 diffusion at 270 K, in close agreement with previous experiments. This work suggests that the presence of empty cages is crucial for the CO2 cage-to-cage trans...

Conversion of natural gas hydrates into CO2 hydrates is well-known as an innovative and eco-friendly method of both future energy production and greenhouse gas control. To increase the total CH4 production rate by using such a method, experiments were carried out by replacing CH4 with gas mixtures containing CO2 and N-2 from naturally occurring CH4 hydrates at 273.9 K and a pressure ranging from 2.50 to 6.67 MPa. Raman spectroscopic analysis was performed to examine the structure and gas distributions in the hydrate phase. Results showed no structural transition in the hydrate phase. CO2 was found to prefer replacing the CH4 in the large hydrate cages, whereas N-2 was found to prefer replacing the CH4 in the small hydrate cages. The total CH4 production was found to increase with addition of N-2. The experimental pressure showed little effect on the CH4 recovery rate with pure CO2. However, with addition of N-2, the CH4 recovery rate was found to decrease and became dependent on the experimental pressure. The transportation of CO2 molecules in the hydrate phase was suggested to be the rate-limiting step, while N-2 captured in hydrate cages slowed the diffusion rate of CO2 in hydrate phase, leading to a lower CH4 recovery rate.

Although the melting of gas hydrates tends to be heterogeneous in most cases, in principle, this process can also be homogeneous when surface melting is properly inhibited. In this work, we investigated the molecular mechanisms of the homogeneous melting of superheated methane hydrates by means of molecular dynamics simulations. The homogeneous melting processes were found to be stochastic with varied induction times. We observed the formation of structural defects within the hydrogen-bonded water lattices of hydrate crystals during the induction times. The methane molecules were found to be relatively more stable within the gas hydrate phases, which might be responsible for the high stability of the superheated metastable methane hydrates. Although the melting processes involve the collective motion of water and methane molecules, the current work suggests that the migration and aggregation of methane molecules are critical in initiating the homogeneous melting of gas hydrate crystals.

The architecture and performance of microbial desalination cell (MDC) have been significantly improved in the past few years. However, the application of MDC is still limited in a scope of small-scale (milliliter) reactors and high-salinity-water desalination. In this study, a large-scale (10 L) stacked MDC packed with mixed ion-exchange resins was fabricated and operated in the batch mode with a salt concentration of 0.5 g/L NaCl, a typical level of domestic wastewater. With circulation flow rate of 80 mL/min, the stacked resin-packed MDC (SR-MDC) achieved a desalination efficiency of 95.8% and a final effluent concentration of 0.02 g/L in 12 h, which is comparable with the effluent quality of reverse osmosis in terms of salinity. Moreover, the SR-MDC kept a stable desalination performance (93%) when concentrate volume decreased from 2.4 to 0.1 L (diluate/concentrate volume ratio increased from 1:1 to 1:0.04), where only 0.875 L of nonfresh water was consumed to desalinate 1 L of saline water. In addition, the SR-MDC achieved a considerable desalination rate (95.4 mg/h), suggesting a promising application for secondary effluent desalination through deriving biochemical electricity from wastewater.

A vinylogous tetrathiafulvalene (TTFV) monomer was prepared and copolymerized with fluorene to give a conformationally switchable conjugated copolymer. This copolymer was shown to undergo a conformational change upon protonation with trifluoroacetic acid (TFA). When mixed with single-walled carbon nanotubes (SWNTs), this TTFV-fluorene copolymer exhibited strong interactions with the SWNT surface, leading to stable, concentrated nanotube dispersions in toluene. Photoluminescence excitation mapping indicated that the copolymer selectively disperses low-diameter SWNTs, as would be expected from its ability to form a tightly coiled conformation on the nanotube surface. Addition of TFA to the copolymer-SWNT dispersion resulted in a rapid conformational change and desorption of the polymer from the SWNT surface, resulting in precipitation of pure SWNTs that were completely free of polymer. Importantly, the nanotubes isolated after dispersion and release by the TTFV-fluorene copolymer were more pure than the original SWNTs that were initially dispersed.

An efficient, concise, and transition metal-free synthesis of functionalized sulfonylated five-membered heterocyclic compounds via an S(N)Ar reaction has been developed. Using commercially available sodium sulfinates as sulfonylation reagents, various five-membered heterocyclic sulfones were obtained in good yields.

Polyvinylidene fluoride (PVDF) has drawn much attention as a predominant ultrafiltration (UF) membrane material due to its outstanding mechanical and physicochemical properties. However, current applications suffer from the low fouling resistance of the PVDF membrane due to the intrinsic hydrophobic property of the membrane. The present study demonstrates a novel approach for the fabrication of a highly hydrophilic PVDF UF membrane via postfabrication tethering of superhydrophilic silica nanoparticles (NPs) to the membrane surface. The pristine PVDF membrane was grafted with poly(methacrylic acid) (PMAA) by plasma induced graft copolymerization, providing sufficient carboxyl groups as anchor sites for the binding of silica NPs, which were surface-tailored with amine-terminated cationic ligands. The NP binding was achieved through a remarkably simple and effective dip-coating technique in the presence or absence of the N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) cross-linking process. The properties of the membrane prepared from the modification without EDC/NHS cross-linking were comparable to those for the membrane prepared with the EDC/NHS cross-linking. Both modifications almost doubled the surface energy of the functionalized membranes, which significantly improved the wettability of the membrane and converted the membrane surface from hydrophobic to highly hydrophilic. The irreversibly bound layer of superhydrophilic silica NPs endowed the membranes with strong antifouling performance as demonstrated by three sequential fouling filtration runs using bovine serum albumin (BSA) as a model organic foulant. The results suggest promising applications of the postfabrication surface modification technique in various membrane separation areas.

The early stage of formation of gas hydrates has recently attracted attention as amorphous intermediate gas hydrate structures have been observed, apparently contrary to a classical model of nucleation and some experimental observations. To date, essentially all reported molecular simulations of the nucleation of gas hydrates have been under constant temperature conditions, which does not consider the possible impacts of heat transfer on the nucleation processes. Here we show, using constant energy molecular simulations, that the nuclei at an early stage of the hydrate formation have relatively more crystalline order in comparison with those observed in previous isothermal (NPT or NVT) work. The current work suggests a more transient role for intermediate amorphous structures during hydrate nucleation, thereby providing a stronger link between molecular simulation and experimental observations. Our NVE results nevertheless support the two-step nucleation mechanism proposed in previous simulation studies under constant temperature conditions which features the initial formation of amorphous hydrate-like structures.

A copper-catalyzed reaction providing direct access to 2-arylpyridines from acetophenones and 1,3-diaminopropane is described. A range of electronically diverse acetophenones undergo this transformation, affording 2-arylpyridines in good yields.

In this paper, we report a molecular simulation study exploring the crystal growth behavior of H(2)S hydrates within two-phase (H(2)S hydrate crystal and H(2)S aqueous solution) and three-phase (H(2)S hydrate crystal, liquid H(2)S, and H(2)S aqueous solution) systems. The microscopic mechanisms of growth, as well as the interfacial properties during the heterogeneous crystal growth process, are probed. We find that the H(2)S hydrate can be grown at a higher rate than methane hydrates under comparable conditions (Vatamanu, J.; Kusalik, P. G. J. Phys. Chem. B 2006, 110, 15896). The three-phase simulations, which also allow us to identify the simulation conditions on the experimental phase diagram, demonstrate that the present models reasonably reproduce the phase behavior of this system. We find that the crystal interface has a strong affinity for water molecules. We observed a relatively low level of defects in the newly formed H(2)S hydrate crystal.

The dynamic evolution of a gold nanojunction has been investigated by incorporation of molecular dynamics (MD) simulation and mechanically controllable break junction (MCBJ) in a wide range of strain rates, covering nonequilibrium, quasi-equilibrium, and equilibrium tensile states. Theoretical simulations corresponding to the nonequilibrium and quasi-equilibrium states demonstrated that the metallic nanojunction spontaneously grew in the time scale of ∼1 ns. In the final stage, the gap increment revealed a unique stepwise feature, corresponding to the atomic-resolved tip reconstruction. When strain rate varied from 489.6 to 0.245 m/s, the gap was reduced from 4.0 to 1.3 nm. In the regime of equilibrium stretching, i.e., several nanometers per second, the experimental MCBJ results gave a mean gap size of 0.4 ± 0.1 nm, which showed less strain rate dependence. This value corresponded well to the absolute value of the stepwise increment of the gap due to the local tip recrystallization as estimated by the MD...

Previous studies have indicated that CH4–CO2 hydrate replacement is closely related to the crystal dissociation and reformation processes. To further elucidate the replacement mechanism, the melting processes of mixed CH4–CO2 hydrates were characterized using in situ Raman spectroscopy and compared to that of pure CH4 and CO2 hydrates. Analysis results from powder X-ray diffraction showed that the crystalline structure of the mixed CH4–CO2 hydrates is strucutre I. The current study suggested that the hydrate crystal units collapse as an entity with no clear dependence upon gas distributions within the hydrate phase. The dissociation time was found to be stoichastic, consistent with the polycrystalline nature of the hydrate particles. Interestingly, for hydrates containing CH4, the Raman peaks of both CH4 and CO2 in the hydrate phase showed a temporary rise during the hydrate dissociation processes, suggesting the reformation of hydrates below the hydrate dissociation surface. A dissociation inside the hyd...

Using molecular dynamics (MD) simulation, we studied the structural transformation and breaking mechanism of a single crystalline copper nanowire under continuous strain. At a certain strain rate, an ensemble of relaxed initial states of the nanowire can preferentially go through one or more paths of deformation. In each deformation path, disordered atoms can be generated at the specific positions of the nanowire, where necking and breaking take place afterward. Such a breaking position is not predetermined; multiple initial states lead to a strain-rate-dependent, statistical distribution of breaking positions.