Russian Journal of Physical Chemistry

Phase equilibria of the liquid–solid system at 298.15 K (25°C) and liquid–vapor equilibria at 298.15–308.15 K were investigated for the oxalic acid (H₂Ox) – citric acid (H₃Cit) – water system. Stability regions of the crystal hydrates H₃Cit·H₂O and H₂Ox·2H₂O were determined. Experimental water activities were obtained for solutions of the ternary system oxalic acid – citric acid – water; saturated vapor pressures over the H₃Cit – H₂Ox – H₂O solutions were measured by the static method, and water activities were determined by the dew-point method. It was shown that water activity depends only weakly on temperature within the range 298.15–308.15 K. Using the Pitzer model, the solubility products of H₃Cit·H₂O and H₂Ox·2H₂O were calculated. A model was developed that describes the thermodynamic properties and phase equilibria of the three-component H₃Cit – H₂Ox – H₂O system with accuracy comparable to that of experimental data.

Recently, “green fuel” based on highly concentrated hydrogen peroxide solutions has become increasingly popular for use in rocket and space production; its main advantages are low toxicity, versatility, and economic efficiency. With the development of modern technologies in the rocket and space industry, where high reliability and operational safety are of primary importance, the use of additive technologies (3D printing) is a challenge. In this regard, the study of compatibility of rocket fuel components (solutions of highly concentrated hydrogen peroxide) with finished products is very promising. The catalytic activity and chemical (corrosion) resistance of materials in hydrogen peroxide solutions has been analyzed. Various polymer materials with metal powder fillers, used in 3D printing, were considered and studied. These technologies are characterized, and their advantages are shown.

The viscosity properties of binary systems containing the basic amino acid L-arginine (Arg) and nicotinic acid (NA) in water and in a phosphate buffer were compared at various concentrations over the temperature range 288.15–313.15 K. For the first time, viscosities of Arg and NA solutions in phosphate buffer (pH 7.4) were measured. Concentration dependencies of the viscosity for the studied systems follow the Jones–Dole equation for dilute solutions; the obtained Bη coefficients decrease with increasing temperature, reflecting the kosmotropic effect of Arg and NA on the solvent. The Arrhenius equation was used to describe the temperature dependence of viscosity at fixed solute composition. The activation energies of viscous flow (E_η^≠) of the studied systems depend linearly on solute concentration, allowing the contributions of solvent and solute to E_η^≠ to be determined. Solutions of Arg and NA in water exhibit lower activation energies compared to those in buffer. In the studied concentration range, the E_η^≠ parameter is primarily determined by the structural contribution of the solvent in which solvated particles of the solutes move.

This review presents recent advances in the application of variational, projector, and diffusion Monte Carlo methods (VMC, PMC, and DMC, respectively). The prospects of using the path-integral Monte Carlo method (PIMC) for modeling molecular systems are also discussed. It is emphasized that modern machine-learning techniques effectively meet the requirements for parametrizing both wavefunctions in the target solution space for a wide range of quantum molecular modeling problems and the corresponding observables’ functionals.

N-, P-, and Si-doped few-layer graphitic fragments (FGFs) were synthesized by pyrolysis of acetonitrile, triphenylphosphine, and tetramethylsilane in the presence of an MgO template. The obtained N-, P-, and Si-FGFs were consolidated by spark plasma sintering (SPS) at 1100°C and 30 MPa and also treated in a high-frequency inductively coupled plasma. The materials were examined using transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. SPS transformed the N-, P-, and Si-FGFs into a morphologically complex and heterogeneous material with substantially reduced heteroatom content compared to the unsintered samples. It was shown that pressure during SPS plays a significantly smaller role than resistive heating. Upon heating, electrical breakdown occurs, generating plasma that promotes phase transformations. Plasma treatment of N-, P-, and Si-FGFs in a high-frequency inductively coupled discharge results in their conversion into N-doped onion-like carbon particles (OLCs) with sizes around ~10 nm and into amorphous P- and Si-doped carbon.

The behavior of a mixture of NH₄VO₃, NH₄H₂PO₄, and NaF components in hexane during short-term mechanochemical activation (10 min) in a Pulverisette 7 planetary mill was studied. It was shown that under these conditions, mechanochemical reactions proceed intensively, forming a strongly aggregated powder containing a mixture of various sodium vanadates and ammonium phosphate-vanadates, which upon thermal treatment yields single-phase Na₃V₂(PO₄)₂F₃. The addition of the dispersing agent SPAN-80 inhibits the intensive mechanochemical reactions but reduces the Na₃V₂(PO₄)₂F₃ phase content in the product to 75 wt. %. Increasing the target phase content to 92 wt. % is achieved by adding paraffin to the reagent mixture alongside SPAN-80, followed by thermal treatment using a two-step scheme including compaction of the intermediate product. Optimal additive contents were determined. The morphology of products synthesized via one- and two-step schemes and the electrochemical properties of the obtained cathode material were studied.

An Al/Fe-silicate composite material was obtained by intercalating Al/Fe polyoxocomplexes into the structure of montmorillonite clay followed by heating at 350°C. The composite material was characterized using energy-dispersive analysis, X-ray fluorescence (XRF), scanning electron microscopy (SEM), and low-temperature nitrogen adsorption. It was shown that the applied modification led to an increase in Al and Fe content, as well as in pore volume and specific surface area compared to the original clay. For the first time, the kinetics and equilibrium of adsorption of the azo dye “Eriochrome Blue SE” on the Al/Fe composite material were investigated. The adsorption capacity of the composite material for the dye exceeded that of the clay by up to five times. It was established that the main physicochemical parameters affecting dye adsorption are the pH of the dye solution, sorbent dosage, and the initial dye concentration. The adsorption capacity of the material increased from 5.2 to 26.1 mg/g as the initial dye concentration rose from 15 mg/L to 200 mg/L. The dye removal efficiency increased with the material dosage, reaching 85% at 20 g/L. An increase in pH from 4.0 to 6.5 led to a decrease in adsorption, which was attributed to changes in the effective surface charge of the material. According to the analysis using various adsorption models, the adsorption isotherm of the dye on the composite material fits the Langmuir model (R2 = 0.991, Q_max = 36.1 mg/g). Comparison of the adsorption kinetic data with pseudo-first-order, pseudo-second-order, and intraparticle diffusion models showed that the adsorption kinetics is satisfactorily described by a pseudo-second-order model.