Improved structure stability and performance of a LiFeSO4F cathode material for lithium-ion batteries by magnesium substitution.

Tavorite LiFeSO4F with high Li-ion conductivity has been considered a promising alternative to LiFePO4. However, its poor cycle stability and low electronic conductivity limit the practical application of Tavorite LiFeSO4F. In the present study, we employ a solvothermal method to produce magnesium-substitution LiMgxFe1-xSO4F (x = 0, 0.02, 0.04) cathode materials in which the Mg substitutes the Fe(2) sites. The first-principles calculations demonstrate that Mg-substitution could reduce the bandgap of LiFeSO4F and increase its electronic conductivity to 2.5 × 10-11 S cm-1. Meanwhile, CI-NEB and BV calculations reveal that the diffusion energy barrier of lithium along the (100) direction after Mg substitution is lower than the pristine sample, and the electrochemical inactive Mg2+ could improve the structure stability. The results show that the Mg-substituted LiFeSO4F exhibits enhanced cycle stability and rate performance compared with the pristine LiFeSO4F, suggesting that the use of electrochemically inactive ion substitution may be critical for the development of high-performance LiFeSO4F cathode materials for lithium-ion batteries.

Coalescence and Break-Up Behaviors of Nanodroplets under AC Electric Field.

Water must be separated from water-in-oil (W/O) emulsion because of the corrosion it brings to the relative equipment in the process of transportation and storage. It is an effective method to apply external electric field to achieve high performance of separating small, dispersed water droplets from W/O emulsion; however, the coalescing micromechanism of such small salty droplets under AC electric field is unclear. In this paper, molecular dynamics simulation was adopted to investigate the coalescence and separation process of two NaCl-aqueous droplets under AC electric field and discuss the effect of AC electric field frequency, as well as the time required for contacting, the critical electric field strength, the dynamic coalescence process and the stability of the final merged droplet. The results show that the critical electric field strength of the droplet coalescence increases with the increase of frequency, while the time required for droplet contacting becomes shorter. The shrinkage function curve was applied to characterize the droplet coalescence effect and it was found that the droplets coalescence and form a nearly spherical droplet under the AC electric field with a frequency of 1.25 GHz and strength of 0.5 V/nm. When the electric field frequency is 10 GHZ, the merged droplet presents a periodic fluctuation with the same period as the AC electric field, which mainly depends on the periodic movement of cations and anions under the AC electric field. The results can provide theoretical basis for the practical application of electrostatic demulsification technology in the petroleum or chemical industry from the microscopic perspective.

Influences of Divalent Ions in Natural Seawater/River Water on Nanofluidic Osmotic Energy Generation.

Besides the dominant NaCl, natural seawater/river water contains trace multivalent ions, which can provide effective screening of surface charges. Here, in both negatively and positively charged nanopores, influences from divalent ions as counterions and co-ions have been investigated with respect to the performance of osmotic energy conversion (OEC) under natural salt gradients. As counterions, trace Ca2+ ions can suppress the electric power and conversion efficiency significantly. The reduced OEC performance is due to the bivalence and low diffusion coefficient of Ca2+ ions instead of the uphill transport of divalent ions discovered in the previous work. Effectively screened charged surfaces by Ca2+ ions induce an enhanced diffusion of Cl- ions which simultaneously decreases the net ion penetration and ionic selectivity of the nanopore. As co-ions, Ca2+ ions have weak effects on the OEC performance. The promotion from charged exterior surfaces in OEC processes for ultrashort nanopores is also studied, with an effective region of ∼200 nm in width beyond pore boundaries independent of the presence of Ca2+ ions. Our results shed light on the physical details of the nanofluidic OEC process under natural seawater/river water conditions, which can provide a useful guide for high-performance osmotic energy harvesting.

Phase-field numerical study on the dynamic process of thermocapillary patterning.

It is well known that surface tension is dependent on temperature, and thus a nonuniform temperature may cause thermocapillary flow which is referred to as the Marangoni effect. For a thin liquid-air film confined between a flat hot plate and a topographical cold template, it undergoes deformation due to thermocapillary flow. This phenomenon is termed as thermocapillary patterning, and has been used to fabricate micro- and nanostructure in polymer films. In most cases, the obtained structure conforms to the template; i.e., it can be considered as a replication technique. In this paper, we developed a two-phase flow numerical model based on the phase field to study the dynamic process of thermocapillary patterning. As a remeshing-free method, the phase field enables the incorporation of thermal field and multiphase flow with free surface deformation. The numerical model was employed to study the dynamic process of thermocapillary patterning. Meanwhile, the effects of some parameters, e.g., temperature, geometry parameters, and contact angle, were also investigated.

Effective Charged Exterior Surfaces for Enhanced Ionic Diffusion through Nanopores under Salt Gradients.

High-performance osmotic energy conversion requires both large ionic throughput and high ionic selectivity, which can be significantly promoted by exterior surface charges simultaneously, especially for short nanopores. Here, we investigate the enhancement of ionic diffusion by charged exterior surfaces under various conditions and explore corresponding effective charged areas. From simulations, ionic diffusion is promoted more significantly by exterior surface charges through nanopores with a shorter length, wider diameter, and larger surface charge density or under higher salt gradients. Effective widths of the charged ring regions near nanopores are reversely proportional to the pore length and linearly dependent on the pore diameter, salt gradient, and surface charge density. Due to the important role of effective charged areas in the propagation of ionic diffusion through single nanopores to cases with porous membranes, our results may provide useful guidance to the design and fabrication of porous membranes for practical high-performance osmotic energy harvesting.

Physical Properties of Betaine-1,2-Propanediol-Based Deep Eutectic Solvents.

Due to their splendid advantages, deep eutectic solvents have attracted high attention and are considered as analogues of ionic liquids. Deep eutectic solvents (DESs) are homogeneous mixtures formed by two or three green and cheap components through hydrogen bond, which is divided into hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD). Recently, Betaine has been widely used as a hydrogen bond acceptor. In this work, four DESs were synthesized by blending betaine as HBA and 1,2-propanediol as HBD in four molar ratios (1:3.5, 1:4, 1:5, 1:6). Then, the physical properties of these DESs were measured. The density values were measured within the temperature range (293.15 K to 363.15 K) at atmospheric pressure, whereas the surface tension and viscosity data were determined in four and seven temperatures between 293.15 K and 353.15 K. The relationship between the density and surface tension with temperature have been analyzed and have been fitted as a linear function. The commonly used Arrhenius model was used to describe the dependence between viscosity and temperature. The results of this study are important not only for the DESs' industrial applications but also for the research on their synthesis mechanism and microstructure.

Deformation hysteresis of a water nano-droplet in an electric field.

Electric field is an effective method to manipulate droplets in micro/nano-scale, and various physical phenomena have been found due to the interaction of electric field and fluid flow. In this study, we developed a molecular dynamic model to investigate the deforming behavior of a nano-droplet in a uniform electric field. The nano-droplet was initially confined between two plates and then wetted on the lower plate (i.e., substrate) until an equilibrium state, after that a uniform electric field in vertical direction was imposed to the system. Due to the electrical force, the droplet started to deform until achieving a new equilibrium state and the dynamic process is recorded. By comparing the equilibrium state under different electric field strength, we found a deformation hysteresis phenomenon, i.e., different deformations were obtained when increasing and decreasing the electric field. To be specific, a large electric field (E = 0.57 V ·nm^-1) is needed to stretch the nano-droplet to touch the upper plate, while a relatively lower field (E = 0.40 V ·nm^-1) is adequate to keep it contacting with the plate. Accompanied by the deformation hysteresis, a distribution hysteresis of the average dipole orientations of water molecules in the nano-droplet is also found. Such a hysteresis phenomenon is caused by the electrohydrodynamic interactions between droplet and plates, and the findings of this study could enhance our understanding of droplet deformation in an electric field.

Molecular Dynamics Simulation on the Electrowetting Behaviors of the Ionic Liquid [BMIM][BF4] on a Solid Substrate.

Compared with traditional aqueous solutions, ionic liquids have important application prospects in the field of wetting and electrowetting due to the advantages of high electric conductivity, long liquid range, and low volatility. In this paper, molecular dynamics method was employed to investigate the wetting and electrowetting behaviors of the nanodroplet of ionic liquid on a solid substrate, as well as the distribution of ionic groups. The ionic liquid is 1-butyl-3-methyl tetra-fluoroborate and coarse grained to simplify the molecular simulation model. The results show that the anion and cation groups are distributed in layers above the wall, and the peaks are different corresponding to different ionic groups. Due to the attraction of the solid substrate and the electrostatic force between anions and cations, the contact angle tends to increase slightly with the increase of ionic liquid pairs. To investigate the electrowetting behaviors of ionic liquid droplet, several electric fields of different strengths and directions have been applied to the system, respectively. The results show that the static contact angles decrease obviously with the increase of electric field, and the ionic liquid droplet wets the solid surface asymmetrically under electric fields in positive and negative directions due to different diffusion abilities of cationic and anionic coarse particles. However, for a hydrophilic surface (ε = 2.0 kcal/mol), the ionic liquid droplet wets symmetrically under the electric field E = ±0.18 V/Å because of the strong interaction from the solid surface. Thus, the wetting and electrowetting behaviors are determined by the combine effect of electric field, interaction among cationic/anionic coarse particles, and solid substrates.

Wetting Behaviors of a Nano-Droplet on a Rough Solid Substrate under Perpendicular Electric Field.

Molecular dynamic simulations were adopted to study the wetting properties of nanoscale droplets on rough silicon solid substrate subject to perpendicular electric fields. The effect of roughness factor and electric field strength on the static and dynamic wetting behaviors of a nano-droplet on a solid surface was investigated at the molecular level. Results show that the static contact angle tends to decrease slightly and show small difference with the increase of roughness factor, while it shows an obvious increase for the ramp-shaped surface because the appearing bottom space reduces the wettability of solid surface. Additionally, under the electric field, a nano-droplet was elongated in the field direction and the equilibrium contact angle increases with the increase of electric field strength. The nano-droplet was completely stretched to be column-shaped at a threshold value of the field. Besides, accompanied by the shape variation of water droplets, the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields.

Electro-wetting of a nanoscale water droplet on a polar solid surface in electric fields.

Molecular dynamics simulations were applied to study the wetting properties of nanoscale droplets on a polar silica solid substrate subjected to constant and alternative electric fields with various field frequencies. Results show that the external applied electric fields have significant effects on the wetting of the nanoscale droplet on a polar solid substrate. The droplet spreads asymmetrically under the effect of the external applied field, and this asymmetry culminates to the maximum when the electric field equals to 0.45 V nm-1. For an electric field of 1.0 V nm-1, the dynamic electro-wetting process undergoes two stages even with a symmetric equilibrium spreading state. The stage A-B transition happens suddenly when molecules on the leading edge drop onto the solid surface due to the strong attraction of the solid substrate. Furthermore, under the alternative electric field with a different GHz frequency range, it was observed that the spreading asymmetry was weakened by increasing the field frequency and the nanoscale water droplet shape changes very slightly above a threshold frequency. Accompanied by the shape variation of water droplets, the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields. In addition, the polar solid surface has significant effects on the rearrangement of water molecules compared with a single droplet. Thus, the electro-wetting behaviors of water droplets on a silica solid surface are determined by the competing intermolecular forces among water, solid and the electric field.

Parametric Study on Electric Field-Induced Micro-/Nanopatterns in Thin Polymer Films.

Electric field-induced micro-/nanopatterns in thin polymer films, sometimes referred as electrohydrodynamic patterning, is a promising technique to fabricate micro-/nanostructures. Extensive attention has been attracted because of its advantages in microcontact (easy demolding) and low cost. Although considerable work has been done on this technique, including both experimental and theoretical ones, there still appears a requirement for understanding the mechanism of electrohydrodynamic patterning. Thus, we systematically studied the effect of different parameters on electrohydrodynamic patterning with a numerical phase field model. Previous researchers usually employed lubrication approximation (i.e., long-wave approximation) to simplify the numerical model. However, this approximation would lose its validity if the structure height is on the same scale or larger than the wavelength, which occurs in most cases. Thus, we abandoned the lubrication approximation and solved the full governing equations for fluid flow and electric field. In this model, the deformation of polymer film is described by the phase field model. As to the electric field, the leaky dielectric model is adopted in which both electrical permittivity and conductivity are considered. The fluid flow together with electric field is coupled together in the framework of phase field. By this model, the effect of physical parameters, such as external voltage, template structure height, and polymer conductivity, is studied in detail. After that, the governing equations are nondimesionalized to analyze the relationship between different parameters. A dimensionless parameter, electrical Reynolds number ER, is defined, for which, a large value would simplify the electric field to perfect dielectric model and a small value leads it to steady leaky model. These findings and results may enhance our understanding of electrohydrodynamic patterning and may be a meaningful guide for experiments.

Mechanical properties of hollow and water-filled graphyne nanotube and carbon nanotube hybrid structure.

By performing molecular dynamics simulations, a GNT/CNT hybrid structure constructed via combing (6, 6) graphyne nanotube (GNT) with (6, 6) carbon nanotube (CNT) has been designed and investigated. The mechanical properties induced by the percentage of GNT, water content and electric field were examined. Calculation results reveal that the fracture strain and strength of hollow hybrid structure are remarkably smaller than that of perfect (6, 6) CNT. In addition, the Young's modulus decreases monotonously with the increase of percentage of GNT. More importantly, the tunable mechanical properties of hybrid structure can be achieved through filling with water molecules and applying an electric field along tensile direction. Specifically, increasing water content from 0.0 to 8.70 mmol g-1 in the absence of electric field could result in fracture strain and strength reducing by 15.09% and 12.87%, respectively. Besides, enhancing fracture strain and strength of water-filled hybrid structure with water content of 8.70 mmol g-1 can also be obtained with rising electric field intensity. These findings would provide a valuable theoretical basis for designing and fabricating a nanodevice with controllable mechanical performances.

Simultaneous Determination of Nitrofuran Metabolites and Chloramphenicol in Shrimp with a Single Extraction and LC-MS/MS Analysis.

A method has been developed to quantify the nitrofuran metabolites 3-amino-5-morphorinomethyl-1,3-oxazolidinone, 3-amino-oxazolidinone, 1-aminohydantoin, and semicarbazide, as well as chloramphenicol in shrimp with a single extraction procedure followed by LC-MS/MS analysis. Dynamic selected reaction monitoring with positive and negative ionization mode switching was used. The method LODs were 0.5 ng/g for the nitrofuran metabolites and 0.3 ng/g for chloramphenicol.

Screening for multiple weight loss and related drugs in dietary supplement materials by flow injection tandem mass spectrometry and their confirmation by liquid chromatography tandem mass spectrometry.

A new method has been developed using flow injection tandem mass spectrometry to semi-quantitatively screen for weight loss drugs, including sibutramine, N-desmethylsibutramine, N-didesmethylsibutramine, and phenolphthalein in dietary supplements. Positive identification of these drugs in samples was further confirmed and quantified by liquid chromatography tandem mass spectrometry. The degradation products of sibutramine were observed and identified by LC-MS/MS which include N-desmethylsibutramine, N-didesmethylsibutramine, N-formyldesmethylsibutramine, and N-formyldidesmethylsibutramine.

Screening for multiple phosphodiesterase type 5 inhibitor drugs in dietary supplement materials by flow injection mass spectrometry and their quantification by liquid chromatography tandem mass spectrometry.

A flow injection tandem mass spectrometry method (FI-MS/MS) has been developed to detect enzyme phosphodiesterase type 5 inhibitors, including tadalafil, sildenafil, and vardenafil. Multiple reaction monitoring (MRM) was used to detect the drugs and product ion ratios were used for identification. FI-MS/MS was used for semi-quantification and liquid chromatography tandem mass spectrometry (LC-MS/MS) was used for further confirmation and quantification. One of 13 samples has been found to be adulterated with prescription levels of tadalafil and also low level of sildenafil. The method can be used for screening large numbers of herbal products for adulteration since it takes less than 1 min without chromatographic separation on a column.

Fast screening of lovastatin in red yeast rice products by flow injection tandem mass spectrometry.

Drug adulteration in dietary supplement materials is a world-wide problem and poses a regulatory challenge. Red yeast rice is a product used by consumers to lower blood levels of cholesterol. While most current methods to analyze red yeast rice are based on HPLC separation with a photo-diode array detector and/or a mass spectrometry detector, which takes 20-40min analysis time per sample, we developed a method to do fast screening of the active compound lovastatin by direct infusion into a mass spectrometer. This method takes under 1min per analysis on the instrument. By using multiple reaction monitoring with five product ions, all the ion ratios of the analyte in the samples are compared with those from the standards for qualitative analysis. The results from this method were compared to the result from the liquid chromatography tandem mass spectrometry, which uses retention time and one ion ratio as the confirmation criteria. No false positives or false negatives were found among the 12 samples tested. The method also seems to be effective in measuring the lovastatin in red yeast rice semi-quantitatively. This kind of method could be adapted to the screening of other dietary supplement products.

"Cross-talk" in scheduled multiple reaction monitoring caused by in-source fragmentation in herbicide screening with liquid chromatography electrospray tandem mass spectrometry.

Interferences, or "cross-talk", have been found for the liquid chromatorgraphy mass spectrometry (LC-MS) determination of chlorophenoxy acid (CPA) herbicides. The time-scheduled multiple reaction monitoring (MRM) of m/z 161.0→125.0 and 163.0→127.0 transitions were produced by 2,4-dichlorophenol and 2,4-dichlorophenoxyacetic acid. Although MRM reduces the possibility of false positives when two transitions for LC/MS-MS are used for quantification and qualitative confirmation, in the case of the structurally related CPAs here, false positives still occurred when using a mixture of standards to identify the residues. It was necessary to analyze pure individual standards to compare with the extracted retention times of the candidate CPAs in food samples.

Chromatographic fingerprint analysis of Pycnogenol dietary supplements.

The bark of maritime pine (Pinus pinaster Aiton) has been widely used as a remedy for various degenerative diseases. A standard high-performance liquid chromatographic (HPLC) procedure for Pycnogenol analysis is a method specified in the United States Pharmacopeia (USP) monograph, which requires measurement of peak areas and identification of four components of the extract: caffeic acid, catechin, ferulic acid, and taxifolin. In this study, a fingerprint analysis using an HPLC method based on the USP monograph has been developed to provide additional qualitative information for the analysis of Pycnogenol-containing dietary supplements (PDS). Twelve commercially available PDS samples were purchased and analyzed along with a standard Pycnogenol extract. Their chromatographic fingerprints were analyzed using principal component analysis. The results showed that two of the samples were not consistent with the standard reference Pycnogenol extract. One contained other active ingredients in addition to Pycnogenol, and the other may have resulted from a quality control issue in manufacturing.

A study of noncovalent protein complexes by matrix-assisted laser desorption/ionization.

A sample preparation method has been developed for detection of noncovalent protein-protein complexes by MALDI in this work. An aqueous solution of the matrix at pH 7 allows the reproducible detection of a protein dimer, a protein tetramer, and a heterodimer. The signals are stable under long irradiation and can be detected at wide ranges of concentrations and with different laser intensities.

The origins of high-affinity enzyme binding to an extrahelical DNA base.

Base flipping is a highly conserved strategy used by enzymes to gain catalytic access to DNA bases that would otherwise be sequestered in the duplex structure. A classic example is the DNA repair enzyme uracil DNA glycosylase (UDG) which recognizes and excises unwanted uracil bases from DNA using a flipping mechanism. Previous work has suggested that enzymatic base flipping begins with dynamic breathing motions of the enzyme-bound DNA substrate, and then, only very late during the reaction trajectory do strong specific interactions with the extrahelical uracil occur. Here we report that UDG kinetically and thermodynamically prefers substrate sites where the uracil is paired with an unnatural adenine analogue that lacks any Watson-Crick hydrogen-bonding groups. The magnitude of the preference is a striking 43000-fold as compared to an adenine analogue that forms three H-bonds. Transient kinetic and fluorescence measurements suggest that preferential recognition of uracil in the context of a series of incrementally destabilized base pairs arises from two distinct effects: weak or absent hydrogen bonding, which thermodynamically assists extrusion, and, most importantly, increased flexibility of the site which facilitates DNA bending during base flipping. A coupled, stepwise reaction coordinate is implicated in which DNA bending precedes base pair rupture and flipping.