Russian Journal of Physical Chemistry A

The cross section of the phase diagram of the Mn(CH₃SO₃)₂–CH₃SO₃H–H₂O system at 298.15 K is obtained by the isothermal solubility method. The Mn(CH₃SO₃)₂∙2H₂O dihydrate is shown to be in equilibrium with solutions containing from 0 to 58 wt% acid. In this ternary system, the bulk properties of solutions at 298.15 K are obtained for a number of compositions, and in its subsystem Mn(CH₃SO₃)₂–H₂O they are obtained in the temperature range 288.15 K – 323.15 K. A semi-empirical Laliberté approach is used to describe the bulk properties. The water activities in the temperature range 288.15-323.15 K are calculated by the static method using the measurement results of the saturated vapor pressure. In the Mn(CH₃SO₃)₂–CH₃SO₃H–H₂ system, the possibility of estimating the solubility and activity of water using only the binary parameters of the Pitzer-Simonson-Clegg model for the liquid phase and the known solubility constant at 298.15 K is checked. It is shown that only binary parameters are insufficient for adequate prediction of solubility, it is necessary to take into account at least one parameter of ternary interaction.

The influence of the second modifying metal (Rh) on the acid and catalytic properties of Mg/HZSM-5 in conversion of dimethyl ether to light olefins and the state of active components (Mg, Rh) on the zeolite surface are studied. It is shown that when it is introduced into Mg/HZSM-5, rhodium significantly increases stability of the catalyst operation while maintaining the selectivity for light olefins at the level of 75 wt. %. It is found that various oxocationic or oxide forms of magnesium are formed on the zeolite surface in the monometallic sample of Mg/HZSM-5, and introduction of rhodium contributes to stabilization of magnesium mainly in the form of Mg²⁺ cations while the strength of Lewis acid properties of magnesium cations decreases, which all together makes catalyst deactivation slow down.

The heat effects of dissolution of 1-aza-18-crown-6 in mixed water-ethanol and water-dimethylsulfoxide solvents at 298.15 K are determined by the calorimetric method. Dissolution of 1-aza-18-crown-6 is found to be exothermic in water and in water-organic solvents with initial co-solvent additions. For co-solvent contents greater than 0.2 mole fraction, dissolution of 1-aza-18-crown-6 is accompanied by an endo effect, which increases with the increasing content of both ethanol and dimethyl sulfoxide. The enthalpies of transfer of 1-aza-18-crown-6 from water to aqueous solutions of ethanol and dimethyl sulfoxide are calculated. The enthalpic characteristics of solvation of crown ethers in aqueous-organic solvents are compared.

Quantum-chemical calculations of binary complexes with the intermolecular bond С...H−Cl formed by methane, ethane, and propane molecules with a chlorine hydride molecule were carried out by the MP2/aug-cc-pVTZ method. It is shown that the bonding of hydrocarbon with an HCl molecule is possible at different mutual orientation of monomers; at that, the properties of the formed complexes are similar to the properties of molecular systems with a typical hydrogen (H-) bond. Upon complexation, elongation of the covalent bond H−Cl is observed with a frequency shift of the respective IR band of the valence vibration to the long-wave region, as well as a chemical shift on the bridging hydrogen atom characteristic of H-bonded complexes. Analysis of the nature of intermolecular bonding included decomposition of the binding energy into components, as well as NBO analysis and study of the electron density topology by the AIM method of the Bader theory. Potential curves of intermolecular interaction and electron density shift maps when the complex is formed out of monomers are were plotted.

The influence of additives of some organic substances on the crystallization onset temperatures of kerosene fractions (KF) obtained from crude oil (straight-run or SRKF) and in the process of catalytic cracking of heavy oil residues (HKF) is studied by the method of thermodynamic modeling. Normal paraffins C_nH2_n+2 (n = 9, 11, 16) are used as additives to the KFs, and m-ethylbutylbenzene is used as an aromatic hydrocarbon. It is shown that using the UNIFAC and UNIQUAC models, one can reproduce the experimental data presented in publications and indicating that the addition of normal paraffins to HKF noticeably increases the freezing point when n is 11 and greater. For SRKF, a similar increase occurs starting from n = 16. According to the calculation results, the addition of m-ethylbutylbenzene practically does not affect the crystallization onset temperature.

The problem of escape of a Brownian particle from a cylindrical cavity through a hole on the surface of one of the cylinder ends is considered. Using the method of surface homogenization, a one-dimensional description of the process is proposed. The solution obtained with its help allows finding the average lifetime of a particle in such a cavity with any size of the hole. Its qualitative difference from the well-known solution for the mean lifetime of a particle diffusing in an isometric (sphere-like) cavity is that the previously obtained result depends only on the volume of the cavity while the solution found in this work depends both on the volume and on the length of the cylinder.

The results of liquid chromatography of a complex mixture of unsaturated lipid molecules as the basis of the hydrophobic matrix of biomembranes are summarized. The data of relative retention of such lipids, which included residues of the most important fatty acids, allowed calculating the most characteristic general parameters that satisfactorily determine their behavior when silver salt is introduced into a planar or column liquid chromatographic system in order to drastically increase the selectivity of separation of unsaturated lipid molecules from each other. A variant of quantitative estimation of the relationship between the level of selectivity of separation of particular molecules of natural lipids from each other and the proposed parameters of their constituent fatty acid residues, which are calculated on the basis of variations in the chemical potential of such molecules when silver appears in this system, is proposed.

In rocket launchers, where operational safety and high reliability are key factors, environmentally friendly propellants, in particular, hydrogen peroxide solutions, are widely used as fuel. The authors study the composition of highly concentrated hydrogen peroxide as an oxidizer for liquid rocket fuels. Different areas of using chromatograph mass spectrometry in the aerospace industry are considered. A comparative analysis is performed for ways of detecting and measuring impurities in highly concentrated hydrogen peroxide by means of chromatograph mass spectrometry.

The physicochemical properties (specific ion conductivity, viscosity, and density) of 1.0M solutions of LiSO₃CF₃ in sulfolane – 1.3-dioxolane mixtures in the temperature range of 30–50°C are studied. The specific ion conductivity isotherms is shown to pass through their maximum at a 1.3-dioxolane content of about 60 mol % (1.75×10^–3 Ω^–1 cm^–1, 30°C). It is found that the viscosity and corrected (for viscosity) conductivity of the studied solutions decrease as the 1.3-dioxolane content increases and the temperature grows. It is concluded that the activation energies of the conductivity and viscous flow, as well as their ratio, decrease as the 1.3-dioxolane content increases. Self-diffusion coefficients of all components of the studied electrolyte solutions are estimated by NMR spectroscopy, and lithium cation transport numbers are calculated. The transport number of lithium cation is found to vary nonlinearly depending on the solution composition, viz. the maximum value (0.56) is reached when the ratio of sulfolane:1.3-dioxolane ≈ 2:3, which correlates with the position of the maximum on the conductivity isotherm. The melting points of 1.0M LiSO₃CF₃ solutions in mixtures of sulfolane with 1.3-dioxolane are shown to decrease as the content of the latter increases. It is noted that when the content of 1.3-dioxolane is more than 50 mol %, electrolyte solutions are in the liquid phase state at temperatures below –70°С.

Fries photo-rearrangement of the thioester of para-methyl benzocarbothiol in a number of solvents is studied using CIDNP technique. It is shown that the reaction occurs with the formation of radical pairs in the singlet state. The elementary stages of the reaction are determined.

The possibility of obtaining a dense material of the Ti-B-Fe system in one stage during self-propagating high-temperature synthesis is studied. The influence of some process parameters on the densification of reaction products of the Ti-B-Fe system, which has high physical and mechanical characteristics, is studied. The maximum temperature developed in the combustion wave, using ferroboron alloys as initial reagents, annealing of initial powders, and preheating of the charge before self-propagating high-temperature synthesis are established to have the greatest influence on the formation of a nonporous product during the combustion of the Ti-B-Fe system.

The identification of the characteristic structural conformations of enzymes and proteins, especially key titratable amino acids, may become a necessary stage of further research in the implementation of natural and computational experiments, which are carried out by varying the pH values, charges and concentrations of the water-salt environment. In this work, computer molecular dynamics (MD) and experimental studies of the enzyme alcohol dehydrogenase and its cofactor (ADH+NAD) solvated with water on a graphite carbon surface are carried out. The snapshots of the adsorption process of ADH+NAD on the surface of a graphitic carbon surface during long-term 100 nanosecond dynamical conformational and rotational changes are obtained. The MD-analysis provides mapping of the ADH+NAD enzyme orientation adsorption, thereby allowing for the detailed observation of changes in protein conformation in the region of titratable amino acid residues of ADH. Next, based on an extension of the MD-model implementation, the mechanism of conformational changes in the entire system (ADH+NAD + water / graphitic carbon surface), as well as the orientation adsorption of the entire protein system along with key titratable amino acids are considered and the MD simulation data are compared with the experimental observations.

The present state of equilibrium thermodynamics is analyzed from the position of using the original Clausius statement for the second law δS ≥ δQ/T when describing four directions of its applications, viz. phase states of the solid body, states of small systems (i.e., surfaces and dispersed phases), for methods of statistical physics and chemical kinetics in non-ideal systems. It is noted that the symbol ≥ simultaneously reflects both the process of striving to reach equilibrium in a closed system and this limit equilibrium state itself. The first two directions were stated by Gibbs in the limit form of the second law as the equality δS ≥ δQ/T; the same equality sign was used in the development of the third direction. The fourth direction was developed on the basis of the Gibbs chemical potential. The results of using the full statement of Clausius’s second law for each of the above directions are discussed. It is shown that for correct calculation of equilibrium characteristics one should take into account the differences in relaxation times of thermodynamic parameters and the self-consistent description of equilibrium and kinetics in molecular approaches.