Presence Of Covalent Bonds Research Articles

Different Approaches for the Preparation of Composite Ionic Liquid-Based Membranes for Proton Exchange Membrane Fuel Cell Applications—Recent Advancements

The use of ionic liquid-based membranes as polymer electrolyte membranes for fuel cell applications increases significantly due to the major features of ionic liquids (i.e., high thermal stability and ion conductivity, non-volatility, and non-flammability). In general, there are three major methods to introduce ionic liquids into the polymer membrane, such as incorporating ionic liquid into a polymer solution, impregnating the polymer with ionic liquid, and cross-linking. The incorporation of ionic liquids into a polymer solution is the most common method, owing to easy operation of process and quick membrane formation. However, the prepared composite membranes suffer from a reduction in mechanical stability and ionic liquid leakage. While mechanical stability may be enhanced by the membrane’s impregnation with ionic liquid, ionic liquid leaching is still the main drawback of this method. The presence of covalent bonds between ionic liquids and polymer chains during the cross-linking reaction can decrease the ionic liquid release. Cross-linked membranes reveal more stable proton conductivity, although a decrease in ionic mobility can be noticed. In the present work, the main approaches for ionic liquid introduction into the polymer film are presented in detail, and the recently obtained results (2019–2023) are discussed in correlation with the composite membrane structure. In addition, some promising new methods (i.e., layer-by-layer self-assembly, vacuum-assisted flocculation, spin coating, and freeze drying) are described.

Fluorescent probe for detection of formaldehyde based on UiO-66-NH2

Formaldehyde (FA) is a harmful substance commonly found in the environment, so effective testing is essential. Consequently, we designed and synthesized a novel fluorescent probe, which is based on covalent post-synthetic modification (PSM) of UiO-66-NH2 by naphthylamide (NI). The combination of the metal-organic frameworks (MOFs) and the covalent PSM makes the probe with large amount of hydrazine group exposed on the surface, which results excellent FA sensing performance like an ultrafast response time (20 ​s) and a low detection limit of 0.167 ​μM. The presence of covalent bonds can lock the fluorophore and prevent strong π-π interactions to exhibit aggregation-induced emission (AIE). Further we confirmed that such probe can be applied to real water samples and portable test strip experiment. In addition, experimental evidence such as fluorescence lifetime decay curves suggest that the main mechanism of fluorescence quenching is Förster resonance energy transfer (FRET).

Bioinspired graphene oxide nanofiltration membranes with ultrafast water transport and selectivity for water treatment

The weak interlayer interactions and poor mechanical stability of graphene oxide (GO) membranes pose significant challenges in any practical application environment. Moreover, the layered stacked GO membranes have low water permeability and low selectivity for small molecules due to their inherent nano-interlayer channels. In order to solve the problems of mechanical properties and size control of interlayer nanochannels, we synthesized GO-chitosan (GO-CS) composites with covalent bonds by a one-step hydrothermal method, and prepared a series of composite membranes by vacuum filtration. The GO-CS composite membrane resulted in a widened and defined two-dimensional (2D) channel due to the intercalation of GO-CS. The prepared GO-CS composite membrane has the highest water permeability of 107.4 L·m−2·h−1·bar−1, which is nearly 4 times that of the GO membrane. Almost complete of methylene blue (MnB), Congo red CR), rhodamine B (RB) dye molecules, with excellent molecular sieving ability. Furthermore, the presence of covalent bonds between GO and CS resulted in significantly enhanced membrane stability. We aim to provide potential applications for next-generation nanofiltration membranes by tuning the microstructure of the membranes.

Hydrogen bonding interactions on molecular properties of pesticidal compound 4-nitrophthalic acid: experimental density functional theory computations, electron localized function, localized orbital locator analysis and molecular docking scrutiny

Nitroaromatic compounds are expansively used in the manufacturing of pharmaceuticals, pesticides, plasticizers, azo dyes and explosives. Nitrophenolic compounds can accumulate in the soil as a result of hydrolysis of several insecticides such asparathion, methyl parathion, and fenitrothion which can cause propitious effects to the biological systems. In optimized geometry hyperconjugation of nitro and dicarboxylic acid groups with the ring system increases the electron density of the aromatic system. Electron localized Function and Localized orbital Locator Analysis has value in a region wherein indicates high localization of electrons due to the presence of covalent bond, a lone pair of electrons or a nuclear shell in that region. In Nuclear Magnetic Resonance analysis signals in highly deshielded region clearly indicates the presence of carboxylic carbon in the compound. From vibrational assignments, broadening and red shifting depicts the level of intermolecular hydrogen bonding. In-vitro antimicrobial activity results clearly reveal that in all the four strains, activity increases with increase in compound concentration and the inhibition of the strains also increases. From both molecular docking and In-vitro Antimicrobial Activity the title compound can be certified that it inhibits antimicrobial activity and it is a pesticide.

Physical properties of LiXH4 (X= B, Al) hydrogen storage materials: ab-initio study

The mechanical and thermodynamical features of LiBH4 and LiAlH4 complex hydrides in the α, β and γ phases are evaluated using density functional theory. To our knowledge, thermal parameters such as, the Grüneisen parameter γ, the heat capacity, and thermal expansion coefficient of LiXH4 (X = B, Al) complex hydrides in α, β and γ phases are computed for the first time. The parameters are determined in terms of pressure (0–6 GPa) and temperature (0–600) K using the quasi-harmonic Debye model. The computed stress–strain relationships effectively yield the elastic constants of a single crystal. The estimated elastic constants of α-LiBH4 compound are noticeably higher than those of several closely similar ABH4 (A = Na, K, Rb, and Cs) borohydrides. In the β-LiAlH4 tetragonal structure, the [001] direction is readily compressible as compared to the [100] direction. The compressibility modulus of the LiBH4 compound is greater than that of the LiAlH4. However, the distance between B and H is less than the distance between Al and H. This might be explained by the presence of covalent bonds in BH4 and AlH4 in LiXH4 (X = B, Al). The melting temperatures of LiXH4 (X = B, Al) are used to predict the temperatures of hydrogen decomposition. The obtained melting temperature of α-LiBH4 is higher than that of β-LiAlH4. Thus, LiAlH4's decomposition temperature at which hydrogen is emitted from a fuel cell is anticipated to be lower than that of the LiBH4 compound. The bonding behavior of α-LiBH4 is more directed than that of β-LiAlH4. All the LiXH4 (X = B, Al) are found to be ductile except α-LiAlH4 which is found to be fragile. LiBH4 experiences greater volume change during uniaxial deformation than LiAlH4 and both are centrally strong solids. Our PBE calculations yield the linear compressibility and Young's modulus orientation dependent. The orthorhombic LiBH4 and tetragonal LiAlH4 are isotropic in bulk and anisotropic in Young's modulus.

Crosslinked poly (vinyl alcohol): Structural, optical and mechanical properties

Poly (vinyl alcohol) (PVA) is a water-soluble, transparent, semi-crystalline polymer with excellent mechanical properties, chemical resistance, and biocompatibility. The crystallinity, optical and mechanical properties of PVA can be modified by the crosslinking process. In this work, a simple heat treatment method is used to fulfil this purpose. Prepared films are heated at different temperatures and time intervals, and their various properties were studied. Due to heating, PVA becomes more crystalline, and as a result, some weak peaks become more pronounced in X-ray diffraction data. Fourier transform infrared studies are incorporated to show structural modification and the presence of covalent bonds. Smooth surface morphology of PVA films is obtained from scanning electron microscopy images, except for the film treated at a higher temperature. Optical properties of pure and heat-treated films are explored using UV-Vis absorption and photoluminescence spectroscopy. UV-Vis absorption peaks of pure PVA are located at around 284 nm and 325 nm, and the corresponding fluorescence emission is observed at around 416 nm. From all the experimental observations, an improved crosslinked structure is obtained for the film heated at 140°C for 40 min. In this condition, the improved mechanical property is also obtained from the stress-strain curves. This crosslinked PVA prepared by simple heat treatment can have potential applications in the textile industry, food packaging, etc.

Mechanical properties of graphene-reinforced aluminium composite with modified substrate surface: a molecular dynamics study

Due to its extraordinary properties, graphene has been widely used as reinforcing nanofillers to enhance the mechanical properties of polymer- or metal-based composites. However, the weak interfacial interaction between the matrix and graphene is still a major bottleneck that considerably hinders its reinforcing effectiveness and efficiency. This study presents an atomistic study via molecular dynamics simulation on a chemical modification strategy where the aluminium (Al) substrate is modified with Al2O3 (with or without covalent bonds formed between Al2O3 and graphene) or Al4C3 to achieve significantly improved interfacial shear strength and overall mechanical properties of graphene-reinforced aluminium (Al/Gr) composites. Numerical results show that this strategy works very well and among the three cases considered, modifying Al substrate by Al2O3 without covalent bonds formed at the interface between Al2O3 and graphene produces the strongest interfacial interaction and the best mechanical properties. In the presence of covalent bonds, however, the reinforcing effect is adversely affected due to the sp2–sp3 bond transformation which partially degrades graphene. The present work provides, for the first time, valuable insight into the role of substrate surface modification on the mechanical performance of Al/Gr nanocomposites.

The Mechanisms of the Interaction of Stable Isotopes with Biological Objects in the Presence of an Uncompensated Neutron in Chemical Bonds

The physical regularities that lead to isotope fractionation and to the accumulation of certain isotope forms in the intercellular and intracellular space at various levels of biological objects were studied. A new hypothesis of fractionation of stable isotopes in biological objects through the neutron effect was discussed. The following feature is noted: it is very likely that the development of isotope shock in living beings is observed mainly in the presence of covalent bonds between atoms with an uncompensated neutron. One possible explanation for this phenomenon can be associated with a change in the physical parameters of the following events: the interactions of the magnetic moments of valence electrons with the magnetic moments of atomic nuclei and the interactions of magnetic moments of atomic nuclei, which result in a change in the distance between atomic nuclei.

Improvement of Electrical Conductivity of Polyamide 6/Polyaniline Blends by Graphene

Polymer composites of polyaniline (PANI)/polyamide 6 (PA6) blends and reduced graphene oxide (rGO) were prepared via the in situ synthesis method in order to improve formability and electrical properties. Polymer blends and composites were characterized by the Fourier transform infrared spectroscopy, scanning electron microscopy, and thermal gravimetric analysis. It was found that the composites prepared by the in situ synthesis method have better compatibility between polymer blends and matrix than that prepared by the conventional dry-mix method as investigated by scanning electron microscope (SEM). The Fourier-transform infrared (FT-IR) spectrograms indicate the presence of covalent bonds between functional groups between polyaniline and reduced graphene oxide. The PA6/PANI-rGO films show the electrical conductivity of 2.970×10-6 S/cm while PA6/PANI-GtO show electrical conductivity of 4.082×10-7 S/cm. Thermal stability of polymer blend was characterized by the thermogravimetric analysis. The thermal stability of polyamide 6 after blending is not changed significantly.

Silica‐filled S‐SBR with epoxidized soybean oil: Influence of the mixing process on rheological and mechanical properties of the compound

ABSTRACTThe incorporation of epoxidized soybean oil (ESO) in a silica‐filled tread rubber was studied. The aim of this research is to determinate the influence of ESO addition stage on rheological, thermal, mechanical, and viscoelastic properties in the rubber compound. The results suggested that ESO may compete with the silanes in the reaction with silanol groups on the silica surface, interfering with the silanization process. Therefore, a series of ESO‐functionalized silica particles were prepared in which the ESO addition at different conditions were compared to the compound mixing conditions. The presence of covalent bonds by ESO onto silica surface and the changes originated by the silane incorporation were confirmed by Fourier‐transform infrared spectroscopy and thermogravimetric analysis. Consequently, it can be affirmed that the effect of reaction conditions and the sequence of silane/ESO mixing have a strong influence on the final properties of the rubber compound. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48504.

Oxygen adsorption on Graphene/GaN (0001) surface: A first-principles study

Graphene or reduced-graphene-oxide are used in gallium nitride-based electronic devices because of similar properties such as transparency and high thermal conductivity. To date, there is not a detailed theoretical discussion of the oxygen adsorption on Graphene/GaN (0001) surface in the Literature. In this study, using the Density Functional Theory, we investigate the changes in the interactions between carbon and gallium atoms caused by the adsorption of oxygen atoms on the 3√3 × 3√3 graphene/ 4 × 4 GaN(0001) surface. Based on analysis of Electron Localization Function we have determined that the presence of covalent bonds between carbon and gallium atoms, in the surface without oxygen adsorbed, that produced a vertical buckling in graphene of 0.54 Å and 0.22 Å in the topmost gallium layer. The adsorption of oxygen on the surface caused the increment of the vertical buckling on both values; In this case, with 11 oxygen atoms adsorbed, it increases up to 1.01 Å in graphene and 0.38 Å in the topmost gallium layer. This effect causes a reordering of carbon-gallium bonds and modifies its bond length. Furthermore, based on the obtained formation energies, when the surface is exposed to oxygen-poor conditions, it remains without oxygen atoms adsorbed; in contrast, when the surface is exposed to rich oxygen conditions, it favors the adsorption of oxygen starting from 7 atoms. After this point, to increase the oxygen adsorption on the surface is required to apply energy to the system.

The reactivity of lignin carbohydrate complex (LCC) during manufacture of dissolving sulfite pulp from softwood

The presence of covalent bonds between lignin and polysaccharides was investigated in dissolving pulps made with one-stage and two-stage acidic sulfite pulping for 100% pine heartwood raw material. The covalent bonds between lignin and pulp polysaccharides occurred mainly to xylan and glucomannan and were of the phenyl glycosides and γ–esters types. The α-ethers that are common in wood were missing in the studied pulp samples. Based on these findings and known lignin reactions during sulfite pulping, a mechanism explaining the absence of the α-ethers is discussed. It is suggested that the lignin carbohydrate bonds may play a vital role in lignin recalcitrance.

Stable NCNgNSi (Ng=Kr, Xe, Rn) Compounds with Covalently Bound C-Ng-N Unit: Possible Isomerization of NCNSi through the Release of the Noble Gas Atom.

Although the noble gas (Ng) compounds with either Ng-C or Ng-N bonds have been reported in the literature, compounds containing both bonds are not known. The first set of systems having a C-Ng-N bonding unit is predicted herein through the analysis of stability and bonding in the NCNgNSi (Ng=Kr-Rn) family. While the Xe and Rn inserted analogues are thermochemically stable with respect to all dissociation channels, but for the one producing CNSiN and free Ng, NCKrNSi has another additional three-body dissociation channel, NCKrNSi→CN+Kr+NSi, which is exergonic by -9.8 kcal mol-1 at 298 K. This latter dissociation can be hindered by lowering the temperature. Moreover, the NCNgNSi→Ng+CNSiN dissociation is also kinetically prohibited by a quite high free energy barrier ranging from 25.2 to 39.3 kcal mol-1 , with a gradual increase in going from Kr to Rn. Therefore, these compounds are appropriate candidates for experimental realization. A detailed bonding analysis by employing natural bond orbital, electron density, energy decomposition, and adaptive natural density partitioning analyses indicates that both Ng-N and C-Ng bonds in the title compounds are covalent in nature. In fact, the latter analysis indicates the presence of delocalized 3c-3e σ-bond within the C-Ng-N moiety and a totally delocalized 5c-2e σ-bond in these compounds. This is an unprecedented bonding characteristic in the sense that the bonding pattern in Ng inserted compounds is generally represented as the presence of covalent bond in one side of Ng, and the ionic interaction in the other side. Further, the dissociation of Ng from NCNgNSi facilitates the formation of a higher energy isomer of NCNSi, CNSiN, which cannot be formed from bare NCNSi as such, because of the very high free energy barrier associated with the isomeric transformation. Therefore, in the presence of Ng atoms it might be possible to detect the high energy isomer.

Nanostructured organic electrode materials grown on graphene with covalent-bond interaction for high-rate and ultra-long-life lithium-ion batteries

Nanostructured organic tetralithium salts of 2,5-dihydroxyterephthalic acid (Li4C8H2O6) supported on graphene were prepared via a facile recrystallization method. The optimized composite with 75 wt.% Li4C8H2O6 was evaluated as an anode with redox couples of Li4C8H2O6/Li6C8H2O6 and as a cathode with redox couples of Li4C8H2O6/Li2C8H2O6 for Li-ion batteries, exhibiting a high-rate capability (10 C) and long cycling life (1,000 cycles). Moreover, in an all-organic symmetric Li-ion battery, this dual-function electrode retained capacities of 191 and 121 mA·h·g–1 after 100 and 500 cycles, respectively. Density functional theory calculations indicated the presence of covalent bonds between Li4C8H2O6 and graphene, which affected both the morphology and electronic structure of the composite. The special nanostructures, high electronic conductivity of graphene, and covalent-bond interaction between Li4C8H2O6 and graphene contributed to the superior electrochemical properties. Our results indicate that the combination of organic salt molecules with graphene is useful for obtaining high-performance organic batteries.

Molecular dynamics simulations of incipient carbonaceous nanoparticle formation at flame conditions

Particle nucleation remains one of the most intriguing steps in the process of particle formation in flames. Although stacking/clustering of polycyclic aromatic hydrocarbons (PAHs) has been identified as a key step in the nucleation process, uncertainties remain about the size of the PAHs undergoing clustering, the organisation of the PAHs in the forming clusters and the factors affecting the relative amounts of order and disorder present in the texture organisation. A molecular dynamics approach is presented to study the influence of the molecular weight and structure of PAHs on the nucleation and coagulation processes and on the morphology of the formed clusters. Two different types of macromolecules have been analysed: pericondensed aromatic hydrocarbons (PCAHs) and aromatic aliphatic linked hydrocarbons (AALHs), known to be present in flame environments depending on flame temperature, equivalence ratio and fuel chemical structure. Clustering of six organic molecules has been analysed at two extreme temperatures, namely 500 K and 1500 K. The six molecules have been chosen on the basis of their molecular weight (pyrene and coronene) and molecular structure (single molecules, covalent-bonded dimers and covalent-bonded trimers). Nucleation of single pyrene molecules is negligible at low temperature and practically absent at high temperature. Conversely, nucleation of single coronene molecules involves 33% of the initial molecules at low temperature. Molecules within the clusters are highly mobile at flame temperatures and just 5% of the initial molecules are in clusters. The presence of covalent bonds among the aromatic sub-units improves clustering and growth, both for pyrene and coronene sub-units. Looking at the structure and the fractal dimensionality of the formed clusters, an enhancement of the disorder of the clusters is observed in the case of the AALHs (dimers and trimers) with an increased number of molecules not arranged in parallel planes. The same behaviour is found for the higher temperature.

The effect of hydrogen and acetylene mixing ratios on the surface, mechanical and biocompatible properties of diamond-like carbon films

In this study, diamond-like carbon (DLC) films were deposited on silicon substrates using different hydrogen and acetylene mixing ratios with the same total flow rate by RF plasma-enhanced chemical vapor deposition. By altering the ratios of ion and neutral radicals, different precursor combinations resulted in different film properties. To obtain the optimal precursor combinations, the morphological, structural, hydrophilic, mechanical, and biocompatibility properties were probed by AFM, Raman spectra, contact angle measurements, nanoindentation, and ECV-304 cell cultures, respectively. The results showed that, for DLC films with high hydrogen contents, the depleted carbon gas and high hydrogen etching resulted in a lower film thickness and smoother surface morphology. In terms of the film structure, the hydrogen-rich DLC films possessed a higher ID/IG ratio and an upward G-peak position shift due to an increase in the sp2 grain size and bond-angle order. The high hydrogen content also led to increase in the residual stress, hardness, and the Young's modulus. The primary mechanisms for this increase were determined to be due to extensive hydrogen bombardment and the presence of covalent bonds. In biocompatibility, the cell number for ECV-304 ascended if the hydrogen precursor ratio was increased. The mechanism for this increase was characterized by protein absorption associated with hydrophilicity. The results of this research demonstrated that the DLC deposited at moderate hydrogen levels might possess the best overall film properties.

Development of Biocomposites from Cocoa Pod Husk and Polypropylene: Effect of Filler Content and 3-Aminopropyltriethoxylsilane

Cocoa pod husk (CPH) is a waste material from cocoa industry. This work describes the development of biocomposites from polypropylene and cocoa pod husk. 3-aminopropyltriethoxysilane (3-APE) is used as coupling agent in filler treatment of CPH. The treated CPH with 3-APE had raised the stabilization torque of PP/CPH biocomposites. The results of tensile test and scanning electron microscope (SEM) indicated that 3-APE given coupling effect on PP/ CPH biocomposites. The presence of covalent bonds between CPH and 3-APE was confirmed by Fourier transmission infra-red (FTIR) analysis, while the improvement of thermal stability and crystallization were evidenced by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Enantioselective Assembly of a Ruthenium(II) Polypyridyl Complex into a Double Helix

AbstractEvolution can increase the complexity of matter by self‐organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self‐organizes into a continuous double helical structure, assembled by non‐covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs‐like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left‐handed double helical assembly of only the Λ‐enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.

Enantioselective assembly of a ruthenium(II) polypyridyl complex into a double helix.

Evolution can increase the complexity of matter by self-organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self-organizes into a continuous double helical structure, assembled by non-covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs-like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left-handed double helical assembly of only the Λ-enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.

Electrical conductivity of nanostructured and C60-modified aluminum

In this paper, we study the electrical conductivity of nanostructured C60-modified aluminum, and the possibility of optimizing its electrical and mechanical properties. The model proposed allows estimating the electrical conductivity of the material at low surface filling factor. A number of samples with different C60 mass fractions and aluminum crystallites sizes have been obtained and investigated; a mean crystalline size, conductivity, and hardness of these samples have been determined. A theoretical model has been compared to the experimental data. The model is in qualitative agreement with the experiment. The X-ray photoelectron spectroscopy and Raman spectroscopy studies of the material structure indicate the presence of covalent bonds between the aluminum in the clusters and the C60 molecules, and they are consistent with the proposed shell model.