Formation Of Chemical Bonds Research Articles

Crystal structure-dependent evaluation of chloride ions resistance of iron passivation film in reinforced concrete at the nanoscale

In reinforced concrete, the passivation film refers to the spontaneous formation of an ultra-thin film of corrosion product on a metal's surface that acts as a barrier to additional chemical reactions and is extraordinarily helpful in mitigating corrosion damage. However, the Cl ions can significantly harm the completeness of the passivation film and thus facilitate the corrosion process. In view of the complexity of the passivation film composition, in this work, the Cl attack process of typical structures of passivation films (α-Fe2O3, β-Fe2O3 and γ-Fe2O3) is studied by Bonn-Oppenheimer molecular dynamics and density functional theory. It is found that the β-Fe2O3 and γ-Fe2O3 have the best and worst resistance of Cl ions, respectively. The reduced gradient density analyses indicate that the Cl ions can break chemical bonds of passivation film by creating Cl–Fe bonds. The details of binding energy and electron localization analyses prove that the rank of the strength of binding is γ-Fe2O3 > α-Fe2O3 > β-Fe2O3. Moreover, Different from the surface adsorption of α-Fe2O3 and β-Fe2O3, γ-Fe2O3 adsorbs Cl ions into the matrix, which also results in the formation of more chemical bonds. Further analyses of the density of states also confirm the bonding formation in the Cl ions attack process. This work suggests that the increase in the proportion of β-Fe2O3 in the composition of the passivation film is a potential approach for inhibiting corrosion. And also an effective method for evaluating the performance of passivation films is also proposed, especially since it is extremely difficult to evaluate given its nanometer characteristics.

Poly-ε-caprolactone-based granules with allylisothiocyanate for controlling of golden cyst potato nematode

In this study, the characteristics of extruded granules based on biodegradable poly-ε-caprolactone and montmorillonite deposited with allylisothiocyanate and their effect on Globodera rostochiensis RoI were investigated. The prepared granules were characterized using Fourier-transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy. It was shown that encapsulation efficiency of allylisothiocyanate in montmorillonite depends on the conditions of complex preparation and ranges from 0.76 to 29.83%, and in poly-ε-caprolactone/montmorillonite/allylisothiocyanate granules after thermal processing it decreases down to 1.06 %. According to the results of Fourier-transform infrared spectroscopy it was found that allylisothiocyanate inclusion did not result in formation of new chemical bonds, but significantly affected the temperature of poly-ε-caprolactone degradation that decreased from 537 to 472 °С. In comparison with the thermogram of montmorillonite, the weight loss corresponding to dehydration at 100 °С decreased by 2.9 %, which probably means that part of the intramolecular water was replaced by allylisothiocyanate molecules encapsulated in montmorillonite. In the experiment with two potato varieties infested with nematode cysts it was shown that soil treatment with allylisothiocyanate solutions allows to decrease number of cysts of Globodera rostochiensis RoI compared to positive control (non-treated infested potato) in 1.5–3.0 times depending on the variety. Moreover, in contrast to allylisothiocyanate solutions, poly-ε-caprolactone/montmorillonite/allylisothiocyanate granules are more effective that makes them promising for applications in Globodera rostochiensis RoI control.

Hydrostatic pressure-induced transformations and multifunctional properties of Francium-based halide perovskite FrCaCl3: Insights from first-principles calculations

Under varying hydrostatic pressures ranging from 0 to 150 GPa, first-principles calculations were conducted to investigate the structural, electronic, bonding, optical, elastic, and mechanical characteristics of the Lead (Pb)-free halide perovskite FrCaCl3 using both the GGA and hybrid HSE06 parameterized density functional theory (DFT). Since the FrCaCl3 cubic perovskite has not yet been synthesized experimentally, its structural and thermodynamic stabilities are confirmed by the Goldschmidt tolerance factor, the octahedral factor, and the formation energy. The induction of pressure has caused a simultaneous decrease in both the lattice parameters and the electronic band gap. Applying the hybrid HSE06 potential refines the accuracy of the band gap, with values decreasing from 5.705 to 2.618 eV from 0 to 150 GPa pressure, suggesting improved optoelectronic attributes. Employing pressure facilitates the formation of stronger chemical bonds characterized by reduced bond lengths. The investigation of optical functions demonstrates that with increased pressure ranging to 150 GPa, the optical conductivity along with the absorption coefficient is oriented towards the low-energy region. The FrCaCl3 perovskite has the prospect to be used in X-ray imaging and other fields of nuclear medicine and diagnostics as it contains the radioactive element Francium (Fr). Additionally, it is found via the study of mechanical characteristics that FrCaCl3 is mechanically stable under various applied pressure, and adding pressure makes it more ductile as well as more anisotropic.

Greenly engineered bimetallic Ag-ZnO nanohybrids for synergistic antibacterial enhancement

The escalating issue of infectious agents developing resistance to traditional antibiotics has spurred ongoing research into effective and broad-spectrum antimicrobial solutions. This study focuses on the fabrication of silver-zinc oxide Nanohybrids (A-ZnO-NHs) with varying silver content (1 %, 3 %, 5 %) using a simplified modified sol–gel method, further enhanced by a green synthesis approach utilizing orange peel extract for the generation of silver nanoparticles. The physicochemical characteristics of the A-ZnO-NCs were thoroughly examined. X-ray diffraction analysis verified the presence of Ag nanoparticles within the zinc oxide and scanning electron microscopy revealed the nanoscale silver particles uniformly distributed on the spherical zinc oxide nanoparticles. Transmission electron microscopy indicated that the Ag-ZnO-NCHs ranged in size from 10 to 20 nm. X-ray photoelectron spectroscopy analysis confirmed the formation of strong chemical bonds between the silver and zinc oxide surfaces in the nanohybrids. This study explored into the structural, morphological, and antimicrobial characteristics of the A-ZnO-NHs at different compositions. The bactericidal efficiency of the A-ZnO-NHs was assessed against both gram-positive and gram-negative bacterial strains. The impact of the A-ZnO-NHs on the cellular structure and chemical composition of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) was also explored. The findings revealed that the A-ZnO-NHs with 3 % silver content demonstrated higher antimicrobial activity against E. coli and S. aureus compared to other compositions and pure zinc oxide nanoparticles. The antimicrobial activity decreases when the concentration of silver increases because the silver particles agglomerate together, reducing their surface area and lessening their effectiveness as antibacterial agents, despite their potential for various applications.

Development of Foam Composites from Flax Gum-Filled Epoxy Resin

In the present work, an innovative range of foams based on flax gum-filled epoxy resin was developed, reinforced or not by flax fibers. Foams and composites with different gum and epoxy resin contents were produced and their mechanical and thermal performances were characterized. To enhance the organic flax gum filler’s cross-linking, we exploited the oxidized components’ reactivity with the amine hardener (isophorone diamine). We compared the materials obtained with those derived from the native components. The flax gum and fibers were primarily characterized by chemical analysis, NMR, and FTIR to evaluate the mild oxidation of the native materials. The formation of chemical bonds between the oxidized polymer chains, epoxy resin, and hardener was evidenced by FTIR, and the materials were then studied by SEM and X-ray computed micro-tomography (CT) and submitted to mechanical and thermal tests. The relevance of the oxidation treatment was highlighted through a significant increase in density and mechanical performance (+36% and +81%, respectively, for the 100% flax gum material). The positive effect of the flax fibers on homogeneity evidenced through micro-CT analysis was also clearly addressed. This set of promising results paves the way for the future development of fully flax-based insulation composite materials.

What Is the Real Origin of Single-Walled Carbon Nanotubes for the Performance Enhancement of Si-Based Anodes?

A large amount of lithium-ion storage in Si-based anodes promises high energy density yet also results in large volume expansion, causing impaired cyclability and conductivity. Instead of restricting pulverization of Si-based particles, herein, we disclose that single-walled carbon nanotubes (SWNTs) can take advantage of volume expansion and induce interfacial reactions that stabilize the pulverized Si-based clusters in situ. Operando Raman spectroscopy and density functional theory calculations reveal that the volume expansion by the lithiation of Si-based particles generates ∼14% tensile strains in SWNTs, which, in turn, strengthens the chemical interaction between Li and C. This chemomechanical coupling effect facilitates the transformation of sp2-C at the defect of SWNTs to Li-C bonds with sp3 hybridization, which also initiates the formation of new Si-C chemical bonds at the interface. Along with this process, SWNTs can also induce in situ reconstruction of the 3D architecture of the anode, forming mechanically strengthened networks with high electrical and ionic conductivities. As such, with the addition of only 1 wt % of SWNTs, graphite/SiOx composite anodes can deliver practical performance well surpassing that of commercial graphite anodes. These findings enrich our understanding of strain-induced interfacial reactions, providing a general principle for mitigating the degradation of alloying or conversion-reaction-based electrodes.

Morphological, TGA, and FTIR on Rigid Polyurethane Composite Laminated with Untreated and Treated Bamboo Fiber Roof Insulation

The performance of roof insulation such as polyurethane decreased due to problems such as insufficient absorption and poor thermal insulation performance, especially during rainstorms. The aims of this study are to investigate the physical property and its potential reinforced material such as rigid polyurethane doped with treated and untreated bamboo fiber composite (RPU-BF) at different ratios of 0, 25, 30, 35, and 40% of bamboo fibers as an insulation material for roof applications. The bamboo fibers were treated by using silane coupling agent treatment. The rigid polyurethane composite samples were prepared and then laminated bamboo fiber to overcome the sound problem in roofs. The physical characterization was investigated by Water Contact Angle (WCA), the morphological by Scanning Electron Microscopy (SEM), Thermo-gravimetric Analysis (TGA), and Fourier Transform Infrared Spectroscopy (FTIR) Analysis. The results showed that the treated bamboo fiber had a 192.5° water contact angle as a super hydrophobic property due to the presence of the chemical bonds Si-O-Si and Si-O-C in the silane coupling agent treatment. The morphology showed that 30% ratios of RPU-BF-T30 give the smallest pore diameter size. The peak of thermal degradation temperature of untreated and treated bamboo fiber was increased from 320°C to 350 °C with a weight loss of 80% to 50%. The treated bamboo fiber exhibited peaks at 3010–3040 cm-1 were associated with stronger Si-O-Si bonding, indicating the formation of new chemical bonds between bamboo fiber and silane coupling agent due to the ester bond from the cellulose, lignin, and hemicellulose. Thus, there was a similar trend peak in the functional chemical group in the FTIR spectrum of the RPU-BF composite. This result shows that RPU-BF composite had the potential of the optimum ratio of bamboo fiber as an insulation material for local communities and beneficial to the bamboo industry.

Some issues in reactive wetting at high temperatures described by reaction product control theory

The phenomenon of reactive wetting at high temperature and its special issues are systematically discussed, focusing on the wetting behavior in metal/metal and metal/ceramic systems. Especially, based on the reaction product control (RPC) theory, the influence of wetting mechanism, spreading dynamics and interfacial reactivity on wettability and interfacial structure is deeply analyzed. The limitations of the classical RPC model under certain conditions are pointed out. In the ultra-fast spreading systems, the spreading dynamics cannot satisfy the prediction of RPC model, which is related to the formation of chemical bonds at the solid/liquid interface. Among them, the Gibbs free energy of the interface reaction does not directly point to the difference of wettability, but is reflected in the thickness of the reaction product layer. The greater the absolute value of Gibbs free energy of interface reaction, the thicker the thickness of precipitation layer. The mechanism leading to wetting is not always consistent with the spreading dynamics, the former is related to the bonding properties, and the latter is related to the slowest link leading to spreading.

Improvement in the Sustained-Release Performance of Electrospun Zein Nanofibers via Crosslinking Using Glutaraldehyde Vapors.

Volatile active ingredients in biopolymer nanofibers are prone to burst and uncontrolled release. In this study, we used electrospinning and crosslinking to design a new sustained-release active packaging containing zein and eugenol (EU). Vapor-phase glutaraldehyde (GTA) was used as the crosslinker. Characterization of the crosslinked zein nanofibers was conducted via scanning electron microscopy (SEM), mechanical properties, water resistance, and Fourier transform infrared (FT-IR) spectroscopy. It was observed that crosslinked zein nanofibers did not lose their fiber shape, but the diameter of the fibers increased. By increasing the crosslink time, the mechanical properties and water resistance of the crosslinked zein nanofibers were greatly improved. The FT-IR results demonstrated the formation of chemical bonds between free amino groups in zein molecules and aldehyde groups in GTA molecules. EU was added to the zein nanofibers, and the corresponding release behavior in PBS was investigated using the dialysis membrane method. With an increase in crosslink time, the release rate of EU from crosslinked zein nanofibers decreased. This study demonstrates the potential of crosslinking by GTA vapors on the controlled release of the zein encapsulation structure containing EU. Such sustainable-release nanofibers have promising potential for the design of fortified foods or as active and smart food packaging.

Mechanical and Water Barrier Properties of Inhomogeneous Clay Nano-Particles Reinforced Thermoplastic Starch

This research investigated the development of biodegradable bioplastic as a possible replacement for petroleum-based plastics, which constitute a serious environmental hazard. These hazards include but are not limited to flooding resulting from blocked sewage and danger to aquatic life in marine environments. The solution casting method was used to blend inhomogeneous kaolinite clay nano-particles with distilled water, starch, dilute acetic and nitric acids to produce different compositions of thermoplastic starch (TPS)/Clay composites with clay reinforcements ranging from 2.5 to 10 wt.%. The composites were characterized using an X-ray diffraction (XRD), and the mechanical and water absorption properties were determined. The result revealed a 9-fold improvement in the tensile strength (0.72 MPa), flexural strength increased 5-fold (3.34 MPa), and hardness increased 2-fold (23.56 HVN) as well as a reduction in water absorption by 3-fold (6.63%) when compared to the control. Furthermore, the 10 wt.% clay content composite showed the highest mechanical properties. The significant improvement in the listed properties was attributed to a reduction in crystallinity and the formation of new chemical bonds between the thermoplastic starch and the nano-clay. It was observed that the properties of the composites can be further enhanced if a synchronized machine blender (such as an extruder) is employed.

Powdered Cogon Grass (Imperata cylindrica) as a Biosorbent for the Removal of Iron (II)

Powdered Cogon Grass (Imperata cylindrica) as a Biosorbent for the Removal of Iron (II)

Critical Sensing Modalities for Hydrogen: Technical Needs and Status of the Field to Support a Changing Energy Landscape.

Decarbonization of the energy system is a key aspect of the energy transition. Energy storage in the form of chemical bonds has long been viewed as an optimal scheme for energy conversion. With advances in systems engineering, hydrogen has the potential to become a low cost, low emission, energy carrier. However, hydrogen is difficult to contain, it exhibits a low flammability limit (>40000 ppm or 4%), low ignition energy (0.02 mJ), and it is a short-lived climate forcer. Beyond commercially available sensors to ensure safety through spot checks in enclosed environments, new sensors are necessary to support the development of low emission infrastructure for production, transmission, storage, and end use. Efficient scalable broad area hydrogen monitoring motivates lowering the detection limit below that (10 ppm) of best in class commercial technologies. In this perspective, we evaluate recent advances in hydrogen gas sensing to highlight technologies that may find broad utility in the hydrogen sector. It is clear in the near term that a sensor technology suite is required to meet the variable constraints (e.g., power, size/weight, connectivity, cost) that characterize the breadth of the application space, ranging from industrial complexes to remote pipelines. This perspective is not intended to be another standard hydrogen sensor review, but rather provide a critical evaluation of technologies with detection limits preferably below 1 ppm and low power requirements. Given projections for rapid market growth, promising techniques will also be amenable to rapid development in technical readiness for commercial deployment. As such, methods that do not meet these requirements will not be considered in depth.

Synergistic inhibition of Mikania micrantha extract with iodide ion on the corrosion of cold rolled steel in trichloroacetic acid medium

The corrosion inhibition behavior of Mikania micrantha extract (MME) and iodide ion in a synergistic system consisting of 0.10 M trichloroacetic acid (Cl3CCOOH, TCA) on cold rolled steel (CRS) materials and its mechanism was fully investigated by weightlessness, electrochemical, and surface analysis measurements. The results show that MME/I− performs higher inhibition than either MME or I− with a peak inhibition efficiency of 96.3 %. The adsorption of MME, I− and MME/I− on CRS follow with Langmuir isotherm. MME/I− retards both anodic and cathodic reactions. With the MME/I− complex inhibitor, the capacitance arc radius reaches a maximum. The results of scanning electron microscopy (SEM), atomic force microscopy (AFM), and confocal laser scanning microscopy (CLSM) showed that there were obvious adsorbent films on the surface of the suppressed CRS. Compositional analysis of surface adsorbates on CRS by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS) revealed that the main components of the adsorption film on the surface of CRS contained a large number of polar compounds, notably the formation of chemical bonds by iron atoms.

EMM-Promoted Pd-Catalyzed Solid State Intramolecular Heck-Type Cyclization/Boronation and Suzuki Couplings: Access to Functionalized Indolines.

EMM (electromagnetic mill)-promoted Pd-catalyzed solid state intramolecular Heck-type cyclization/boronation and Suzuki couplings are reported. Compared to previous mechanochemistry that constructed one chemical bond through a cross-coupling reaction, this strategy realizes cascade transformation along with multiple chemical bond formation. This conversion does not require organic solvents or additional heating, and it shows a good substrate scope and high functional group tolerance.

Construction of La2O3/AgCl S-scheme heterojunction with interfacial chemical bond for efficient photocatalytic degradation of bisphenol A

The formation of interfacial chemical bonds in S-scheme heterostructure is a crucial factor to realize efficient interfacial charge transfer. High-energy mechanical ball-milling is an effective method to form interfacial chemical bonds. In this work, an S-scheme La2O3/AgCl heterojunction catalyst was designed and prepared by ball-milling method. XPS, FTIR and Raman measurements showed that La-Cl chemical bond was formed at the interface of La2O3 and AgCl and an S-scheme charge transfer path mechanism existed in the La2O3/AgCl composite. The results of electrochemistry, photo-electrochemistry, capture experiments, free radical measurement and EPR verified that the S-scheme heterojunction and the interfacial La-Cl bond significantly accelerated the charge transfer rate between La2O3 and AgCl, and enhanced the photocatalytic capability of the composite catalysts. The degradation rate of bisphenol A by La2O3/AgCl heterojunction composite (LA50) reached 99 % in 20 min, and it also had excellent anti-interference ability and cycling stability. The toxicity of the degradation intermediates of bisphenol A was much less than itself, as calculated by TEST simulation. This work provided a promising strategy for improving the performance of S-scheme heterojunction catalysts by introducing interfacial chemical bonds as charge transfer channels.

Influence of Polymer Surface Roughness on the Fractions of Transmitted, Reflected and Absorbed Energy in Operation of Laser Transmission Welding

The study of energy fractions plays a fundamental role in laser joining operations: from their knowledge, it is possible to calculate the amount of laser beam energy that is effectively available during the formation of chemical and physical bonds, and how much energy is dissipated. This study examines semi-crystalline polymers of polyamide 6.6 (PA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and polypropylene (PP), semitransparent to light radiation, with the aim of studying the influence of surface roughness on the distribution of energy fractions, and in particular on the reflection portion. For this purpose, polymeric samples with different surface finishing were prepared and characterized by profilometric analysis. Subsequently, an experimental setup was implemented to directly measure the transmitted ratio, obtaining the reflected energy fraction from the Beer-Lambert law, and the absorbed ratio by energy balance. The results showed a decrease in the power transmitted by polymers subjected to surface treatment, due to an increase in the reflection fraction, a phenomenon particularly evident for PET, for which the reflection share increased from ~ 0.5% to ~ 15.3%, following P240 treatment. A lower influence was verified for PA and especially PTFE, due to a lower influence of the treatment on surface morphology. On the basis of the experimental results, it is hypothesised that roughening the lower section of the irradiated polymer could allow an increase in the total internal reflection fraction, favouring the joint at the interface point.Graphical

On the structural and mechanical properties of mixed coconut and olive oil oleogels and bigels

The interaction of monoglycerides and phytosterols in olive- and coconut oil on the structuring of oleogels was analyzed. Specifically, bigels with gelatin hydrogel in different ratios (40:60 and 60:40 w/w) were formed. The physicochemical and microstructural attributes of these systems were assessed. The olive oil to coconut oil ratio (0–100 w/w) and the added oleogelators affected the crystal structure and the mechanical properties of the oleogels. Polarized light microscopy revealed that the addition of coconut oil created a denser triglycerides crystal network and the presence of phytosterols created more needle-like crystals, enhancing the textural properties of the oleogels and of the resulting bigels. The hardness of the oleogels ranged from 0.50 N to 1.24 N and for bigels was 5.96–36.75 N. Bigels hardness decreased as the oleogel ratio in the bigel increased. Microscopy and FTIR revealed that the addition of coconut oil in oleogels hampered the formation of a distinct crystalline monoglycerides network. Also, the absence of new peaks in the bigels indicated that the two structured phases interact with each other mostly physically, without the formation of new chemical bonds. Consequently, the oleogels and bigels developed, comprise a promising hard fat substitute with improved nutritional profile.

Structure engineering and heteroatom doping-enabled high-energy and fast-charging dual-ion batteries

Sodium dual-ion batteries (SDIBs) have attracted extensive attention due to the high working voltage and low cost. Nevertheless, lack of robust anode materials that can achieve highly efficient insertion/extraction of sodium ions still remains a huge challenge to develop the high-performance SDIBs. Herein, we exhibited excellent long-cycling performance (capacity retention of 85 % over 2000 cycles) with large reversible capacity (1788 mA h g−1 under 0.2 A/g) as well as superior rate capability (721 mA h g−1 under 10 A/g) for sodium-ion storage by encapsulating red phosphorus within nitrogen-doped mesoporous graphene particles. And the structural stability of composite during sodiation was directly demonstrated using in-situ transmission electron microscopy. The results of density functional theory reveal that nitrogen doping and formation of PC chemical bonds within graphene matrix may improve the adsorption energy of P atoms and promote the sturdy contact. The adsorption energies of nitrogen-doped graphene for Na+ and PF6− are −2.12 and −3.62 eV, respectively, which shows that the doping of N element is beneficial to the adsorption of Na+ and PF6−. Upon pairing with nitrogen-doped mesoporous graphene particles as cathode, an innovative SDIBs with high energy-power-density of 324 W h kg−1 at 331 W kg−1 and 187 W h kg−1 at 2665 W kg−1 were manufactured. This study opens up an effective strategy towards high-performance SDIBs for electric vehicles and other applications.

Bond Formation at C8 in the Nucleoside and Nucleotide Purine Scaffold: An Informative Selection.

This paper presents methods for the introduction and exchange of substituents in a nucleobase and its nucleosides and nucleotides with emphasis on the C8-position in the purine skeleton. The nucleobase is open for electrophilic and nucleophilic chemistry. The nucleophilic chemistry consists mainly of displacement reactions when the C8-substituent is a good leaving group such as a halogen atom. The heteroatom in amines, sulfides, or oxides is a good nucleophile. Halides are good reaction partners. Metal-promoted cross-coupling reactions are important for carbylations. Direct oxidative metalation reactions using sterically hindered metal amides offer chemo- and regio-selectivity besides functional tolerance and simplicity. The carbon site is highly nucleophilic after metalation and adds electrophiles resulting in chemical bond formation. Conditions for metal-assisted reactions are described for nucleobases and their glycosides.

Mechanisms of acrylamide biosorption by Lactobacillus plantarum ATCC8014 peptidoglycan

This study extracted peptidoglycan (PG) from Lactobacillus plantarum ATCC8014, its acrylamide (AA) adsorption behavior was evaluated by creating adsorption models, and its binding characterizations were investigated by physicochemical methods. The kinetics and isothermal model revealed that the biosorption of AA by PG conformed to Langmuir, Freundlich equations, and the second-order kinetic model, confirming that the adsorption process involved both physisorption and chemisorption. Fourier transform infrared spectroscopy and chemical blocking of functional groups confirmed that carboxyl, amino and hydroxyl groups were involved. Meanwhile, a new peak was generated in the proton nuclear magnetic resonance spectrum of PG upon adsorption, and the water contact angle was significantly reduced, indicating that hydrogen bonds and hydrophobic force were involved in the biosorption. Besides, energy dispersive spectroscopy and X-ray photoelectron spectroscopy results have important findings, suggesting that amide bonds might be formed during the adsorption process. Subsequently, this speculation was confirmed by the identification results of amidase. Overall, our results indicated that the mechanism of adsorption is complex and involves hydrogen bonding, hydrophobic forces and the formation of chemical bonds.