Crystalline Urea Research Articles

Electrodeposition of NiCo on stainless steel substrate using deep eutectic solvent for efficient hydrogen evolution and methanol oxidation reactions

The electrodeposition of NiCo on a stainless-steel substrate using Deep eutectic solvent (DES) was carried out using constant current methods. A 1:2 mol ratio of ChCl (Choline chloride) and crystalline urea was used to prepare DES. The XRD analysis of the fabricated NiCo@SS showed a highly crystalline nature, and the SEM image revealed the fabricated electrode surface morphology. The composition and oxidation state of the deposited material were confirmed by EDAX and XPS techniques. The prepared electrodes were subjected to LSV and CV studies with 1 M KOH using a three-electrode system to analyse its catalytic ability in the Hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR). Among all the deposited electrocatalysts, NiCo@SS demonstrated the highest catalytic activity and excellent stability for the hydrogen evolution reaction (HER), achieving an overpotential of 257 mV at a current density of 100 mA/cm2. Additionally, NiCo@SS exhibited a remarkable current density of 240 mA at a potential of 0.8 V towards the MOR. The synthesized NiCo@SS possessed excellent low overpotential, low charge transfer resistance, high current density, and large surface area. The explored catalyst of NiCo@SS was examined with different concentrations of methanol using the CV technique. Finally, the stability of NiCo@SS, as examined by chronoamperometry and LSV with 3000 cycles, revealed it to be a robust, stable catalyst towards HER and OER.

N-representable one-electron reduced density matrix reconstruction with frozen core electrons.

Recent advances in quantum crystallography have shown that, beyond conventional charge density refinement, a one-electron reduced density matrix (1-RDM) satisfying N-representability conditions can be reconstructed using jointly experimental X-ray structure factors and directional Compton profiles (DCP) through semidefinite programming. So far, such reconstruction methods for 1-RDM, not constrained to idempotency, have been tested only on a toy model system (CO2). In this work, a new method is assessed on crystalline urea [CO(NH2)2] using static (0 K) and dynamic (50 K) artificial experimental data. An improved model, including symmetry constraints and frozen core-electron contribution, is introduced to better handle the increasing system complexity. Reconstructed 1-RDMs, deformation densities and DCP anisotropy are analysed, and it is demonstrated that the changes in the model significantly improve the reconstruction quality, even when there is insufficient information and data corruption. The robustness of the model and the strategy are thus shown to be well adapted to address the reconstruction problem from actual experimental scattering data.

Study on the Purification Process of Used Motor Oils in the Presence of Crystalline Urea

The paper deals with the results of the study on the purification process of used mineral and semi-synthetic motor oils in the presence of crystalline urea. The effect of the process main factors (the amount of the main reagent, time, and temperature) on the performance indicators of purified motor oils was studied. Used mineral and semi-synthetic motor oils for gasoline and diesel engines were purified under the established optimal conditions. The results of X-ray-fluorescence and IR-spectroscopic studies on the chemical composition of motor oils purified in the presence of urea are presented.

USE OF UREA FOR CLEANING USED SEMI-SYNTHETIC MOTOR OILS

During the operation of motor vehicles with gasoline or diesel internal combustion engines, motor oil is exposed to high temperatures, is constantly in contact with metals, its hydrocarbon part undergoes chemical transformations, is contaminated by wear products of metal surfaces, decomposition of additives, and residues of incomplete combustion of fuel. As a result, motor oil undergoes irreversible changes in its high-quality chemical composition and loss of operational properties. The paper describes the results of the study of the process of cleaning exhausted semi-synthetic motor oils in the presence of crystalline urea. Castrol 10W-40 oil, widely used in truck diesel engines, and ELF Evolution 700 STI oil, used in passenger car gasoline engines, were chosen as used semi-synthetic motor oils. The influence of the main factors of process control (the amount of urea, duration, and temperature) on the change in the functional properties of refined motor oils was studied. It was established that the lowest values of defective indicators (acid number, water content, ash content, content of mechanical impurities) were obtained at 140 ºС, duration of 80 min., and 5 mass. % of crystalline urea. In addition, there is a slight increase in the values of kinematic viscosity and viscosity index. The method of X-ray fluorescence analysis confirmed that the content of metals decreased in urea-treated oils compared to used ones. IR spectroscopic research demonstrated that under the established optimal conditions of the purification process with crystalline urea, the content of oxygen-containing "aging" products (aldehydes, ketones, alcohols, ethers, esters, organic acids) in purified semi-synthetic motor oils significantly decreased. It is proposed to use the process of cleaning used semi-synthetic motor oils in the presence of crystalline urea as an intermediate stage of the combined technological cycle of regeneration of used motor oils.

Human glabrous skin contains crystallized urea dendriform structures in the stratum corneum which affect the hydration levels.

Glabrous skin is hair-free skin with a high density of sweat glands, which is found on the palms, and soles of mammalians, covered with a thick stratum corneum. Dry hands are often an occupational problem which deserves attention from dermatologists. Urea is found in the skin as a component of the natural moisturizing factor and of sweat. We report the discovery of dendrimer structures of crystalized urea in the stratum corneum of palmar glabrous skin using laser scanning microscopy. The chemical and structural nature of the urea crystallites was investigated in vivo by non-invasive techniques. The relation of crystallization to skin hydration was explored. We analysed the index finger, small finger and tenar palmar area of 18 study participants using non-invasive optical methods, such as laser scanning microscopy, Raman microspectroscopy and two-photon tomography. Skin hydration was measured using corneometry. Crystalline urea structures were found in the stratum corneum of about two-thirds of the participants. Participants with a higher density of crystallized urea structures exhibited a lower skin hydration. The chemical nature and the crystalline structure of the urea were confirmed by Raman microspectroscopy and by second harmonic generated signals in two-photon tomography. The presence of urea dendrimer crystals in the glabrous skin seems to reduce the water binding capacity leading to dry hands. These findings highlight a new direction in understanding the mechanisms leading to dry hands and open opportunities for the development of better moisturizers and hand disinfection products and for diagnostic of dry skin.

Biological and Urea Treatments Reduce the Primary Inoculum of Red Leaf Blotch of Almond Caused by Polystigma amygdalinum.

Red leaf blotch (RLB), caused by Polystigma amygdalinum, is considered the most prevalent foliar disease in both traditional and new intensive almond-growing areas in Spain. Since the disease is monocyclic, its control must be based on the reduction of the only source of inoculum-the leaves infected in the previous season and fallen to the ground in autumn. Thus, this study aimed to determine the effect of two microorganisms and urea on RLB inoculum reduction by evaluating different application modes to fallen leaves in field conditions. Leaves of almond cv. Guara showing symptoms of RLB were collected in autumn, placed into nylon mesh bags, and treated by dipping or spraying with conidial suspensions of Myrothecium inundatum or the nonpathogenic strain Fusarium oxysporum FO12. The bags were exposed on the ground or buried in an experimental almond field for 6 months in each experimental year. Bags treated with crystalline urea solution at 46% N or not treated were included as controls. The primary inoculum (number of ascospores per gram of leaf) and the development of fruiting bodies (maturity stages of perithecia) were monitored in the fallen leaves for each experimental treatment combination. M. inundatum significantly reduced the primary inoculum in comparison with the nontreated control or F. oxysporum FO12, showing a similar effect to that observed for urea in the 2 experimental years. The type of application (spraying or dipping) did not show any significant effect, whereas the inoculum was significantly reduced in buried leaves in comparison with leaves maintained on the ground for all the treatments tested. This study represents the first report evaluating management strategies against RLB based on the reduction of the primary inoculum of P. amygdalinum.

Sustainable bio-based dialdehyde cellulose for transforming crystalline urea–formaldehyde resins into amorphous ones to improve their performance

Sustainable bio-based dialdehyde cellulose (DAC) was employed to transform crystalline urea–formaldehyde (UF) resins into amorphous ones for simultaneouly improving their adhesion strength and formaldehyde emission. Serial samples of the UF resins modified with DAC during the resin synthesis were extracted to understand the chemical reactions between the DAC and UF species. Fourier transform infrared, 1H-nuclear magnetic resonance (NMR), and 13C NMR spectroscopies confirmed the occurrence of reactions between the DAC and UF species. As the synthesis proceeded, the crystallinity of the modified UF resins decreased from 51.7% to 17.4%, transforming the crystalline domains into amorphous ones. Thermograms showed that the DAC in the modified UF resins was decomposed at temperatures over 200 °C as degraded form, resulting in a lower cross-linking density than that of the neat UF resins. The adhesion strength of the modified UF resins was statistically similar to that of the neat UF resins, and the formaldehyde emission of the modified UF resins dramatically decreased to ∼ 64.6%. These results evidence the significant application potential of bio-based DAC in improving the sustainability and performance of UF resins.

Two-Dimensional Nuclear Magnetic Resonance Analysis of Hydrogen-Bond Formation in Thermosetting Crystalline Urea–Formaldehyde Resins at a Low Molar Ratio

Two-Dimensional Nuclear Magnetic Resonance Analysis of Hydrogen-Bond Formation in Thermosetting Crystalline Urea–Formaldehyde Resins at a Low Molar Ratio

Infrared spectra of amorphous and crystalline urea ices.

Urea is a molecule of great interest in chemistry and biology. In particular, it is considered a key building block in prebiotic chemistry on Earth. The hypothesis of its possible exogenous origin has been reinforced after the recent detection of this molecule in the interstellar medium, where it is believed to form in the ice mantles of dust grains. In this work the infrared spectra of urea ices and urea:H2O ice mixtures have been studied both experimentally and theoretically. Urea ices were generated by vapour deposition at temperatures between 10 K and 270 K. It was found that an amorphous phase is formed at temperatures below 200 K. A theoretical modelling of crystalline urea and of a tentative amorphous urea solid phase, as well as of amorphous urea:H2O ice mixtures, has been performed. The corresponding infrared spectra were simulated with density functional theory. The main spectral features observed in the various solid samples are interpreted with the help of the theoretical results. Infrared band strengths are also provided for amorphous and crystalline urea. The infrared spectroscopic information given in this work is expected to be useful for the detection and quantification of urea in astrophysical ices.

Wet synthesis of high purity crystalline urea phosphate from untreated Moroccan industrial phosphoric acid

The phosphorus-nitrogen fertilizer family improves the availability of phosphorus and nitrogen for efficient crop growth. Urea phosphate 17-44-0 (N-P 2 O 5 -K 2 O%) is considered as a binary fertilizer characterized by its high purity and it is easily soluble in water. In addition, urea phosphate can be used in other fields as a high-quality food additive, finishing agent for metals and fermentation food. As a fertilizer, it is suitable for alkaline soils. In this work, a new process for the synthesis of urea phosphate by the wet route is developed. It consists in directly producing crystalline urea phosphate by the chemical reaction between Moroccan industrial phosphoric acid (54 % P 2 O 5 ) and solid urea (46 %). The synthesis is carried out by mixing the solid urea with a stoichiometric amount of untreated Moroccan industrial phosphoric acid. The chemical reaction is complete for optimal conditions which are a temperature of 50 °C and a duration of 90 min. After cooling, the urea phosphate crystals are formed without any additional treatment and are easily recovered. Analysis of the residual liquor after recovery of the urea phosphate crystals confirms that the majority of impurities which may be harmful to the environment pass into the liquid phase

A selectable approach for polarity-fixed and polarity-controllable polymer films with hexagonal columnar structures

Electroresponsive columnar liquid crystalline urea compounds, N,N’-bis(3,4,5-trialkyloxyphenyl)ureas 1 and 2 possessing six and four terminal double bonds, were photopolymerized by UV irradiation in the liquid crystal phases to give polymerized compounds, labeled P-1-Colh and P-2-Colh, that possessed a hexagonal columnar structure. Furthermore, the photo polymerization of 1 and 2 sandwiched between two indium tin oxide-deposited glass plates with applying an electric field (20 V μm−1), produced polymerized compounds, P-1-Colh-E and P-2-Colh-E, and polarized light optical microscopy clarified that the columns polymerized were aligned in parallel to the applied electric field. In addition, by the phenomenon that polar substances absorb laser light (λ = 1064 nm) and emit second harmonic generation light (λ = 532 nm), the difference in electro-response between P-1-Colh-E and P-2-Colh-E has been revealed. The former was a polymer whose macropolarity was completely fixed, and the latter was a polymer whose macropolarity was controllable by applying an external voltage.

Can We Predict the Pressure Induced Phase Transition of Urea? Application of Quantum Molecular Dynamics.

Crystalline urea undergoes polymorphic phase transition induced by high pressure. Form I, which is the most stable form at normal conditions and Form IV, which is the most stable form at 3.10 GPa, not only crystallize in various crystal systems but also differ significantly in the unit cell dimensions. The aim of this study was to determine if it is possible to predict polymorphic phase transitions by optimizing Form I at high pressure and Form IV at low pressure. To achieve this aim, a large number of periodic density functional theory (DFT) calculations were performed using CASTEP. After geometry optimization of Form IV at 0 GPa Form I was obtained, performing energy minimization of Form I at high pressure did not result in Form IV. However, employing quantum molecular isothermal–isobaric (NPT) dynamics calculations enabled to accurately predict this high-pressure transformation. This study shows the potential of different approaches in predicting the polymorphic phase transition and points to the key factors that are necessary to achieve the success.

Multipole electron densities and atomic displacement parameters in urea from accurate powder X-ray diffraction.

Electron density determination based on structure factors obtained through powder X-ray diffraction has so far been limited to high-symmetry inorganic solids. This limit is challenged by determining high-quality structure factors for crystalline urea using a bespoke vacuum diffractometer with imaging plates. This allows the collection of data of sufficient quality to model the electron density of a molecular system using the multipole method. The structure factors, refined parameters as well as chemical bonding features are compared with results from the high-quality synchrotron single-crystal study by Birkedal et al. [Acta Cryst. (2004), A60, 371-381] demonstrating that powder X-ray diffraction potentially provides a viable alternative for electron density determination in simple molecular crystals where high-quality single crystals are not available.

Bond-Dependent Thole Model for Polarizability and Spectroscopy.

We present a new model for the calculation of molecular polarizabilities from effective atomic polarizabilities. This model is based on the Thole modified dipole interaction model for molecular polarizabilities, where the total polarizability is computed as a sum of effective atomic polarizabilities modified by dipole-dipole interactions. We extend this model by making the atomic polarizabilities explicit functions of the interatomic distances, scaling them by the radius of the volume an atom occupies in a molecule. We use the SCAN functional to show that this model, denoted TholeL, yields accurate molecular polarizabilities with little dependence on the training set. We also demonstrate that the TholeL model yields accurate polarizabilities for configurations far from the ground state structure for a wide range of molecules. Finally, we show that the TholeL model can be used to generate accurate Raman spectra for water, crystalline urea, and urea in water from ab initio molecular dynamics simulations.

Intermolecular vibrational modes and H-bond interactions in crystalline urea investigated by terahertz spectroscopy and theoretical calculation

The characteristic absorption spectra of crystalline urea in 0.6–1.8 THz region have been measured by terahertz time-domain spectroscopy at room temperature experimentally. Five broad absorption peaks were observed at 0.69, 1.08, 1.27, 1.47 and 1.64 THz respectively. Moreover, density functional theory (DFT) calculation has been performed for the isolated urea molecule, and there is no infrared intensity in the region below 1.8 THz. This means that single molecule calculations are failure to predict the experimental spectra of urea crystals. To simulate these spectra, calculations on a cluster of seven urea molecules using M06-2X and B3LYP-D3 are performed, and we found that M06-2X perform better. The observed THz vibrational modes are assigned to bending and torsional modes related to the intermolecular H-bond interactions with the help of potential energy distribution (PED) method. Using the reduced-density-gradient (RDG) analysis, the positions and types of intermolecular H-bond interactions in urea crystals are visualized. Therefore, we can confirm that terahertz spectroscopy can be used as an effective means to detect intermolecular H-bond interactions in molecular crystals.

Quantum crystallographic charge density of urea.

Standard X-ray crystallography methods use free-atom models to calculate mean unit-cell charge densities. Real molecules, however, have shared charge that is not captured accurately using free-atom models. To address this limitation, a charge density model of crystalline urea was calculated using high-level quantum theory and was refined against publicly available ultra-high-resolution experimental Bragg data, including the effects of atomic displacement parameters. The resulting quantum crystallographic model was compared with models obtained using spherical atom or multipole methods. Despite using only the same number of free parameters as the spherical atom model, the agreement of the quantum model with the data is comparable to the multipole model. The static, theoretical crystalline charge density of the quantum model is distinct from the multipole model, indicating the quantum model provides substantially new information. Hydrogen thermal ellipsoids in the quantum model were very similar to those obtained using neutron crystallography, indicating that quantum crystallography can increase the accuracy of the X-ray crystallographic atomic displacement parameters. The results demonstrate the feasibility and benefits of integrating fully periodic quantum charge density calculations into ultra-high-resolution X-ray crystallographic model building and refinement.

Computation of Second Harmonic Generation for Crystalline Urea and KDP. An ab Initio Approach through the Coupled Perturbed Hartree-Fock/Kohn-Sham Scheme.

The electronic second harmonic generation (SHG) tensor, d, of crystalline urea and potassium dihydrogen phosphate (KDP) is evaluated as a function of frequency using a Gaussian type basis set and the Coupled Perturbed Hartree-Fock (CPHF) and Kohn-Sham (CPKS) schemes as implemented in the CRYSTAL code. The results of various functionals, including LDA, GGA (PBE), and global and range-separated hybrids (B3LYP, PBE0, LC-BLYP), as well as Hartree-Fock, are compared. It is found that the calculated SHG intensity always decreases as the percentage of exact exchange increases. The hybrid functionals turn out to provide results that agree well with experiment. For urea and KDP the percentage of exact exchange determined by the inverse dielectric constant is too large. At 1064 nm the vibrational contribution for urea is found to be less than 5% of the total value. To the authors' knowledge, this is the first coupled (self-consistent) calculation of SHG for any periodic system.

(Solid + Liquid) Phase Equilibrium in the Aqueous Ternary System NaH2PO4 + CO(NH2)2 + H2O at (308.15, 328.15, and 348.15) K

In this study, the solubility of the ternary system (NaH2PO4 + CO(NH2)2 + H2O) at (308.15, 328.15, and 348.15) K were determined, using isothermal solution saturation. The equilibrium solid phases were analyzed by the Schreinemaker’s method of wet residues and verified by X-ray diffraction (XRD). According to the experimental data, the phase diagrams were plotted and the crystallization areas were determined. The phase equilibrium at different temperatures were compared and discussed. This research provides an accurate answer for whether crystalline hydrate and urea phosphate exist in this system or not. All results can offer fundamental data for crystallization processes and further theoretical studies.

IR Spectral Study of Mesoscale Process during Urea Crystallization from Aqueous Solution

Unravelling the mesoscale process and the dynamic heterogeneous structures that appear on the mesoscale in a crystallization system is important in designing and fabricating functional crystalline materials. Recent experimental observations show the existence of stable clusters and amorphous intermediates before the formation of a crystalline solid, which seems to contradict classical nucleation theory. Here we show by in situ infrared spectroscopy and theoretical calculation that the liquid/solid phase transformation of urea proceeds through the agglomeration of primary clusters. The phase transformation pathway of urea in solution has been identified, in which urea molecules initially aggregate into one-dimensional (1D) molecular chains, and then these 1D molecular chains assemble to 2D plane-like and 3D net-like clusters. Crystalline urea with P–421m symmetry can be formed when these 3D net-like clusters overcome a critical size. Both experimental and calculated results demonstrate that the liquid/soli...

The applicability of the dimeric heterosynthon concept to molecules with equivalent binding sites. A DFT study of crystalline urea–H2O2

The limited applicability of the dimeric heterosynthon concept to a two-component urea–H2O2crystal is reported.