Molecular modelling of ionic liquids in the ordered mesoporous carbon CMK-5

We performed classical molecular dynamics simulations of the ionic liquids (ILs) [dmim⁺][Cl^?] and [emim⁺][NTf₂^?], confined in a model CMK-5 material, which consists of amorphous carbon nanopipes (ACNPs) arranged in a hexagonal array. We compare our findings against the behaviour of the same ILs inside an isolated ACNP (i.e. no IL adsorbed on the outer surface of the ACNP) and inside a model CMK-3 material (which is similar to CMK-5, but is formed by amorphous carbon nanorods). Our results indicate that the presence of IL adsorbed in the outer surface of an uncharged ACNP in CMK-5 affects the dynamics and the density of an IL adsorbed inside the ACNP and vice versa. ILs adsorbed outside the nanopipes in CMK-5 (i.e. with IL also adsorbed inside the nanopipes) have faster dynamics and remain closer to the carbon surfaces when compared to the same ILs adsorbed on CMK-3 materials. The trends are IL-specific: [dmim⁺][Cl^?] has slower dynamics when inside an isolated ACNP than when inside the ACNPs in CMK-5, but in contrast, [emim⁺][NTf₂^?] moves faster when it is inside an isolated ACNP than when it is inside the ACNPs in CMK-5 (i.e. with IL adsorbed outside the nanopipes).     

Differences in the adsorption and diffusion behaviour of water and non-polar gases in nanoporous carbon: role of cooperative effects of pore confinement and hydrogen bonding

We investigate the effect of pore confinement and molecular geometry on the adsorption and self-diffusion of H₂O, CO₂, Ar, CH₄, C₃H₆, SF₆ and C₅H₁₂, in a realistic model of nanoporous silicon carbide derived carbon (SiC-DC), constructed using hybrid reverse Monte Carlo simulation. Adsorption isotherms, adsorbate–adsorbate and adsorbate–adsorbent contributions to the isosteric heat of adsorption are determined to study the effect of pore confinement, microporosity and molecular geometry on adsorption of these gases. We describe the cooperative effect of pore confinement and hydrogen bonding on the formation of water clusters and anomalous adsorption behaviour of water compared with non-polar gases. We find that, in contrast to literature results based on the slit-pore model, pore-filling does not occur below the saturation pressure in hydrophobic amorphous carbon materials such as SiC-DC and activated carbon fibre. We also compare self-diffusivities and activation energy barriers of water and non-polar gases in the microporous structure of SiC-DC to identify underlying correlations with molecular properties. We demonstrate that the self-diffusivity of water deviates considerably from the correlation between diffusivity and molecular kinetic diameter observed for non-polar gases. This is attributed to the reduced diffusivity of water, and its relatively large energy barrier at high loadings despite its small kinetic diameter, which is due to the blocking effect of water clusters at pore entries.     

激光极化技术制备核磁共振探针的方法和应用原理

通过激光增强核自旋极化技术,可以得到很高的非热平衡极化度和长驰豫时间的惰性气体（129Xe和3He),其作为核磁共振探针,具有广泛的生物医学应用价值,该文探讨了激光增强129Xe和3He核自旋极化的磁共振成像和波谱学原理,介绍了激光极化惰性气体的设备和方法,以及极化气体的收集贮存和输入过程,最后对氢核及极化核的磁共振成像,极化气体129Xe和3He的应用进行了比较性总结。     

Determination of Natural In Vivo Noble-Gas Concentrations in Human Blood

Although the naturally occurring atmospheric noble gases He, Ne, Ar, Kr, and Xe possess great potential as tracers for studying gas exchange in living beings, no direct analytical technique exists for simultaneously determining the absolute concentrations of these noble gases in body fluids in vivo. In this study, using human blood as an example, the absolute concentrations of all stable atmospheric noble gases were measured simultaneously by combining and adapting two analytical methods recently developed for geochemical research purposes. The partition coefficients determined between blood and air, and between blood plasma and red blood cells, agree with values from the literature. While the noble-gas concentrations in the plasma agree rather well with the expected solubility equilibrium concentrations for air-saturated water, the red blood cells are characterized by a distinct supersaturation pattern, in which the gas excess increases in proportion to the atomic mass of the noble-gas species, indicating adsorption on to the red blood cells. This study shows that the absolute concentrations of noble gases in body fluids can be easily measured using geochemical techniques that rely only on standard materials and equipment, and for which the underlying concepts are already well established in the field of noble-gas geochemistry.     

Computer simulation of gas adsorption in modified COF-108: the impregnation of C60 into COF-108

Gas adsorption and separation performance of COF-108 framework impregnated by C₆₀ clusters were simulated. The adsorption properties of pure CO₂, the mixtures of CO₂/CH₄, CO₂/N₂ and N₂/O₂ were investigated. The simulated results of the adsorption isotherms, the adsorption quantity, the density fields, the isosteric heats and the selectivity in COF-108s were obtained. It is shown that the impregnation of C₆₀ can enhance the adsorption capacity of CO₂, N₂ and O₂, and the selectivity of CO₂/CH₄, CO₂/N₂ and N₂/O₂ in COF-108. The impregnation of C₆₀ can increase the surface area COF-108 but decrease its free volume and the pore diameter. At low adsorption pressures, the monolayer surface adsorption is dominant. With the increase in adsorption pressure, the dominant factor is changed into the free volume of COF-108 by the multilayer adsorption. The impregnation of C₆₀ plays different roles for the polar or non-polar gases at different pressures.     

Prediction of adsorption equilibria of water–methanol mixtures in zeolite NaA by molecular simulation

Predictions for the adsorption of mixtures of water and methanol in zeolite NaA are reported. The pressure dependence of the adsorption properties such as equilibrium amounts of adsorption and isosteric heats of adsorption are calculated at 378 K by molecular simulations using effective pair potential models. These data are also determined for the adsorption from liquid mixtures. The models predict selectivity inversion in the investigated range of pressure. The change in adsorption ratios can partly be explained by the structural characteristics of the system.     

Predicting helium and neon adsorption and separation on carbon nanotubes by Monte Carlo simulation

The adsorption of helium and neon mixtures on single-walled carbon nanotubes (SWCNTs) was investigated at various temperatures (subcritical and supercritical) and pressures using canonical Monte Carlo (CMC) simulation. Adsorption isotherms were obtained at different temperatures (4, 40, 77 and 130 K) and pressures ranging from 1 to 16 MPa. Separation factors and isosteric enthalpies of adsorption were also calculated. Moreover, the adsorption isotherms were obtained at constant specific temperatures (4 and 40 K) and pressures (0.2 and 1.0 MPa) as a function of the amount adsorbed. All of the adsorption isotherms for an equimolar mixture of helium and neon have a Langmuir shape, indicating that no capillary condensation occurs. Both the helium and the neon adsorption isotherms exhibit similar behavior, and slightly more of the helium and neon mixture is adsorbed on the inner surfaces of the SWCNTs than on their outer surfaces. More neon is adsorbed than helium within the specified pressure range. The data obtained show that the isosteric enthalpies for the adsorption of neon are higher than those for helium under the same conditions, which means that adsorption of neon preferentially occurs by (15, 15) SWCNTs. Furthermore, the isosteric enthalpies of adsorption of both gases decrease with increasing temperature.     

Simulation Study of Sorption of CO2 and N2 with Application to the Characterization of Carbon Adsorbents

Abstract

The Monte Carlo method was used in its grand ensemble variant (GCMC) in combination with CO₂ and N₂ experimental isotherm data at low (77 and 195.5 K) and ambient temperatures (298 and 308 K), in order to characterize microporous carbons and obtain the corresponding pore size distribution (PSD). In particular, the CO₂ and N₂ densities and the isosteric heats of adsorption inside single, slit shaped, graphitic pores of given width were found on the basis of GCMC for pre-defined temperatures and different relative pressures. In a further step, we determined the optimal PSD for which the best match is obtained between computed and measured isotherms. Comparisons were made between the PSDs found for the same carbon sample at low and ambient temperatures for different gases, and conclusions concerning the applicability of the method and the reliability of the resulting micropore size distributions were drawn.     

The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channel regulate thermosensory neuron gene expression and function in C. elegans

The cultivation temperature (T(c)) modulates the thermosensory responses exhibited by C. elegans on thermal gradients. The AFD sensory neurons are essential for thermosensory behaviors, but the molecular mechanisms by which temperature is sensed and the memory of the T(c) is encoded are unknown. Here, we show that the CMK-1 Ca2+/calmodulin-dependent protein kinase I (CaMKI) and the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons. Mutations in cmk-1 and tax-4 result in temperature-dependent defects in AFD-specific gene expression, and TAX-4 functions are required during larval stages to maintain gene expression in the adult. CMK-1 and TAX-4 act cell autonomously to regulate AFD-mediated thermosensory behaviors. The molecular requirements for CMK-1 activity in the AFD neurons appear to be distinct from those previously described. We propose that the activation of distinct programs of AFD-specific gene expression at different temperatures by CMK-1 and TAX-4 enables C. elegans to sense and/or encode a memory for the T(c).     

Growth responses of Neurospora crassa to increased partial pressures of the noble gases and nitrogen

Buchheit, R. G. (Union Carbide Corp., Tonawanda, N.Y.), H. R. Schreiner, and G. F. Doebbler. Growth responses of Neurospora crassa to increased partial pressures of the noble gases and nitrogen. J. Bacteriol. 91:622-627. 1966.-Growth rate of the fungus Neurospora crassa depends in part on the nature of metabolically "inert gas" present in its environment. At high partial pressures, the noble gas elements (helium, neon, argon, krypton, and xenon) inhibit growth in the order: Xe > Kr> Ar > Ne > He. Nitrogen (N(2)) closely resembles He in inhibitory effectiveness. Partial pressures required for 50% inhibition of growth were: Xe (0.8 atm), Kr (1.6 atm), Ar (3.8 atm), Ne (35 atm), and He ( approximately 300 atm). With respect to inhibition of growth, the noble gases and N(2) differ qualitatively and quantitatively from the order of effectiveness found with other biological effects, i.e., narcosis, inhibition of insect development, depression of O(2)-dependent radiation sensitivity, and effects on tissue-slice glycolysis and respiration. Partial pressures giving 50% inhibition of N. crassa growth parallel various physical properties (i.e., solubilities, solubility ratios, etc.) of the noble gases. Linear correlation of 50% inhibition pressures to the polarizability and of the logarithm of pressure to the first and second ionization potentials suggests the involvement of weak intermolecular interactions or charge-transfer in the biological activity of the noble gases.     

Methane storage in homogeneous armchair open-ended single-walled boron nitride nanotube triangular arrays: a grand canonical Monte Carlo simulation study

The physisorption of methane in homogeneous armchair open-ended SWBNNT triangular arrays was evaluated using grand canonical ensemble Monte Carlo simulation for tubes 11.08, 13.85, 16.62, and 19.41 ? [(8,8), (10,10), (12,12), and (14,14), respectively] in diameter, at temperatures of 273, 298, 323, and 373 K, and at fugacities of 0.5-9.0 Mpa. The intermolecular forces were modeled using the Lennard-Jones potential model. The absolute, excess, and delivery adsorption isotherms of methane were calculated for the various boron nitride nanotube arrays. The specific surface areas and the isosteric heats of adsorption, Q(st), were also studied, different isotherm models were fitted to the simulated adsorption data, and the model parameters were correlated. According to the results, it is possible to reach 108% and 140% of the US Department of Energy's target for CH(4) storage (180 v/v at 298 K and 35 bar) using the SWBNNT array with nanotubes 16.62 and 19.41 ? in diameter, respectively, as adsorbent. The results show that for a van der Waals gap of 3.4 ?, there is no interstitial adsorption except for arrays containing nanotubes with diameters of >15.8 ?. Multilayer adsorption starts to occur in arrays containing nanotubes with diameters of >16.62 ?, and the minimum pressure required for multilayer adsorption is 1.0 MPa. A brief comparison of the methane adsorption capacities of single-walled carbon and boron nitride nanotube arrays was also performed.     

Direct electron transfer and electrocatalysis of hemoglobin adsorbed on mesoporous carbon through layer-by-layer assembly

Using chitosan as an effective linker between CMK-3 and glassy carbon electrode surface, {Hb/CMK-3}n multilayer film-modified electrodes were constructed through layer-by-layer assembly. The morphology of thus-formed {Hb/CMK-3}n film was characterized by scanning electron micrographs, and the interaction of hemoglobin (Hb) with CMK-3 was studied by UV-vis spectroscopy and electrochemical methods. Under optimal conditions, {Hb/CMK-3}6 film showed a couple of stable and well-defined redox peaks at about -377 and -296 mV in pH 7.0 buffers. Furthermore, the {Hb/CMK-3}6 film displayed excellent electrocatalysis to the reduction of both H2O2 and O2. Based on thus-formed film and its direct electron transfer behavior, a novel biosensor was presented for the determination of H2O2 ranging from 1.2 to 57 muM with the detection limit of 0.6microM at S/N=3. CMK-3 provided a desirable matrix for protein immobilization and biosensor preparation.     

Moisture Sorption Characteristics of Starchy Products: Oat Flour and Rice Flour

Moisture sorption isotherms for oat flour and rice flour were determined at 5, 23, and 45 °C using a gravimetric technique in an a _w range of 0.08–0.98. The results obtained showed that, for a _w values lower than around 0.75, in both flours, the sorption capacity decreased with increasing temperature, while the opposite behavior was observed at a _w greater than this value. It was found that the experimental data in the water activity range considered were better represented by the four-parameter Peleg equation. The monolayer water contents for both materials were determined by the BET and GAB models. The net isosteric heats of sorption were estimated using the Clausius–Clapeyron equation. For both materials, the monolayer water content and the isosteric heat of sorption were found to decrease with increasing temperature and increasing moisture content, respectively.     

The role of molecular modelling and simulation in the discovery and deployment of metal-organic frameworks for gas storage and separation*

ABSTRACT

Metal-organic frameworks (MOFs) are highly tuneable, extended-network, crystalline, nanoporous materials with applications in gas storage, separations, and sensing. We review how molecular models and simulations of gas adsorption in MOFs have informed the discovery of performant MOFs for methane, hydrogen, and oxygen storage, xenon, carbon dioxide, and chemical warfare agent capture, and xylene enrichment. Particularly, we highlight how large, open databases of MOF crystal structures, post-processed to enable molecular simulations, are a platform for computational materials discovery. We discuss how to orient research efforts to routinise the computational discovery of MOFs for adsorption-based engineering applications.     

3D Carbon Materials for Efficient Oxygen and Hydrogen Electrocatalysis

Sustainable energy production at an acceptable cost is key for its widespread application. At present, noble metals and metal oxides are the most widely used for electrocatalysis, but they suffer from low selectivity, poor durability, and scarcity. Because of this, metal‐free carbons have become the subject of great interest as promising alternative electrocatalysts for energy conversion and storage devices, and remarkable progress has been accomplished in the advance of metal‐free carbons as electrocatalysts for renewable energy technologies. Particularly interesting are 3D porous carbon architectures, which exhibit outstanding features for electrocatalysis applications, including broad range of active sites, interconnected porosity, high conductivity, and mechanical stability. This review summarizes the latest advances in 3D porous carbon structures for oxygen and hydrogen electrocatalysis. The structure–performance relationship of these materials is consequently rationalized and perspectives on creating more efficient 3D carbon electrocatalysts are suggested.     

Effect of Intrinsic Defects of Carbon Materials on the Sodium Storage Performance

Due to their high conductivity and low cost, carbon materials have attracted great attention in the field of energy storage, especially as anode material for sodium ion batteries. Current research focuses on introducing external defects through heteroatom engineering to improve the sodium storage performance of carbon materials. However, there is still a lack of systematic investigation of the effects of intrinsic defects prevalent in carbon materials on sodium storage performance. Herein, template‐assisted method was used to design carbon materials with different degrees of intrinsic defects and explore their sodium storage properties. The experimental results show that the intrinsic defects in the carbon materials facilitates the adsorption behavior of Na⁺ during the surface induction capacitance process. Among them, the best carbon anode material exhibits high reversible capacity (221 mAh g^?1 at 1 A g^?1) and excellent rate performance. In addition, the density functional theory calculations also show that the existence of intrinsic defects can optimize the distribution of electron density, thereby increasing the Na‐adsorption capacity. This work makes an important contribution to understanding the role of intrinsic defects in the sodium storage performance of carbon materials.     

An Improved Li–SeS2 Battery with High Energy Density and Long Cycle Life

Selenium–sulfur solid solutions are a class of potential cathode materials for high energy batteries, since they have higher theoretical capacities than selenium and improved conductivity over sulfur. Here, a high‐performance cathode material by confining 70 wt% of SeS₂ in a highly ordered mesoporous carbon (CMK‐3) framework with a polydopamine (PDA) protection sheath for novel Li–Se/S batteries is reported. With a relatively high SeS₂ mass loading of 2.6–3 mg cm^?2, the CMK‐3/SeS₂@PDA cathode exhibits a high capacity of >1200 mA h g^?1 at 0.2 A g^?1, excellent C‐rate capability of 535 mA h g^?1 at 5 A g^?1, and prolonged life over 500 cycles. Benefitting from the unique advantages of SeS₂ and the rationally designed host framework, this new cathode material demonstrates a feasible strategy to overcome the bottlenecks of current Li–S systems for high energy density rechargeable batteries.     

Effect of temperature on adsorption of mixtures in porous materials

The effect of temperature on the adsorption of a simple mixture (Ar/Kr) in disordered porous materials is investigated by means of molecular simulation. In the larger mesopores of porous silica glasses, capillary condensation occurs upon decreasing the temperature. At temperatures above the capillary condensation temperature, Kr is preferentially adsorbed at the pore surface and Ar adsorption occurs in regions of low Kr density. For temperatures below the capillary condensation temperature, Ar density surprisingly increases as temperature increases, the behaviour that is consistent with an over-solubility effect. In contrast, in the disordered sub-nanoporous carbon, filling of the pores occurs in a reversible and continuous way upon decreasing the temperature, owing to the small size and amorphous shape of the pores. These results show that the crossover between capillary condensation and continuous reversible filling observed for pure fluids in pores also exists for mixtures. We also show that the Kr selectivity exhibits a minimum in the disordered porous silica that is located at the capillary condensation temperature. In contrast, in the disordered porous carbon where no capillary condensation occurs, the selectivity decreases monotonically with increasing the temperature. These results shed light on low-temperature adsorption of mixtures confined in porous materials and provide a guide to design efficient phase separation processes.     

Modeling protein-small molecule interactions: structure and thermodynamics of noble gases binding in a cavity in mutant phage T4 lysozyme L99A

The complexes of phage T4 lysozyme L99A with noble gases have been studied by molecular dynamics simulation. In a long simulation of the complex with one Xe atom, the structure was found to undergo global conformation change involving a reversible opening and closing of the entrance to the substrate-binding site, during which the conformations of the N and C-terminal domains varied little. The distributions of Xe positions sampled in dynamics simulations were refined in terms of anisotropic Gaussian distributions via least-squares minimization of the difference between Fourier transforms. In addition, molecular transformation simulations have been applied in order to calculate the binding free energies of Xe, Kr and Ar relative to a standard state at a pressure of 1 bar. A single bound Xe is found to assume an equilibrium distribution over three adjacent preferred sites, while in a two-Xe complex, the two Xe atoms preferentially occupy two of these. The positions of the three sites agree closely with the positions of bound Xe determined in the refined crystal structure of a complex formed at a pressure of 8 bar Xe, and the calculated affinities agree well with the observed partial occupancies. At a pressure of 8 bar, a mixture of one-Xe and two-Xe complexes is present, and similarly for complexes with Kr and Ar, with single occupancy relatively more prevalent with Kr and Ar. (Binding of a third Xe atom is found to be quite unfavorable.) A comparison with simulation results for the binding of benzene to the same site leads to the conclusion that binding of Xe within cavities in proteins is common because of several favorable factors: (1) Xe has a large atomic polarizability; (2) Xe can be applied at a relatively high pressure, i.e. high chemical potential; (3) an unfavorable entropic term related to the need to orient the ligand in the binding site is absent. Finally, it is found that the model's binding energy of a water molecule in the cavity is insufficient to overcome the unfavorable binding entropy.