Showing papers in "Russian Journal of Physical Chemistry A in 2002"

Thermodynamic and topological analysis of heterogeneous equilibrium diagrams of multicomponent mixtures

Abstract: Works on the thermodynamic and topological analysis of stationary vector fields of conodes and scalar fields of temperature (pressure) for heterogeneous equilibria in two-phase and multiphase systems with and without reversible chemical reactions were surveyed.

Vapor pressure over a charged drop

Abstract: A model of condensation of polar molecules on ions was proposed. The model predicts that, under conditions of significant ionization, a gas consisting of polar molecules condenses into nanoparticles more rapidly than a nonpolar gas. The equilibrium diameter of particles formed via the charge-dipole mechanism is substantially higher than the equilibrium diameter calculated by the Kelvin equation from the vapor pressure over the charged particle under the same conditions.

Influence of solution pH on the change from synergistic to competitive sorption of lysozyme and ribonuclease on carboxylic cationite

Abstract: Conditions of maximal binding of lysozyme and ribonuclease by carboxylic cationite were determined over a wide range of pH values. It was shown that under conditions close to the conditions of maximal binding, the ascending and descending branches of the sorption isotherms exhibit a hysteresis; i.e., there was no shift in equilibrium when the concentration of a protein in the solution decreases. For the consecutive sorption of the two proteins, the second protein was sorbed due to protein-protein interactions without displacement of the first one. A decrease in the degree of ionization of the polyelectrolyte chains or a change in the charge of the protein caused by a change in the pH value reduce the electrostatic protein-sorbent interaction, giving rise to the competitive mode of protein sorption.

Electron-beam modification of the surface of oxide materials (SiO2 and BaTiO3)

Abstract: Variations in the functional composition under the action of accelerated electrons was analyzed for the surface of oxide materials (A-175 Aerosil and barium titanate). The dependence of the content of Lewis and Broensted sites and their mutual conversion on the absorbed dose exhibit extremal behavior. The mechanisms of the variation of the functional composition of the surface were established to be related to the following rearrangements of the surface layer structure under the action of increasing absorbed dose. At doses below 100 kGy, the physically sorbed molecules of water dissociate into radicals and react with siloxane groups to form hydroxyls. At higher doses, a dehydration of the surface occurs. Electron-beam processing was demonstrated to be a promising method for controlled modification of the surface of oxides.

Dispersing effects on the donor-acceptor properties of the surface of ferroelectrics

Abstract: The distributions of donor-acceptor centers on the surface of dispersed barium titanate and a VS-1 ceramic material based on barium titanate and distribution changes after treatment with a dispersing agent and during dispersion were studied by adsorption of acid-base indicators. The obtained data can be used to rationally select the type of dispersing agents and determine the nature of interphase interactions during dispersion.

1H NMR analysis of the self-association of riboflavin-mononucleotide and its complexation with nicotinamide in an aqueous solution

Abstract: One- and two-dimensional 1 H NMR spectroscopy (500 MHz) were used to study the self-association of 7,8-dimethyl-10-(1-D-ribityl)-isoalloxazin-5'-phosphate sodium salt (riboflavin-mononucleotide, RMN), a derivative of vitamin B 2 , and its heteroassociation with nicotinamide (NA), a hydrotropic agent, in an effort to elucidate the molecular mechanism of solubilization in the RMN-NA system. The equilibrium constants of the reactions, induced chemical shifts for protons, and thermodynamic parameters (AH and ΔS) for the self- and heteroassociation were determined from the temperature and concentration dependences of the chemical shifts for the protons of the interacting aromatic molecules. An analysis of the results obtained demonstrated that RMN-NA heteroassociation complexes are formed due the stacking interaction between the aromatic chromophores. The relative contents of the self- and heteroassociation complexes at various ratios r p of the concentrations of NA and RMN in aqueous solutions were calculated. It was demonstrated that, at r P > 30, the contribution from the heteroassociation complexes is predominant, a factor that plays an important role in the molecular mechanism of the hydrotropic action of nicotinamide on riboflavin.

The temperature dependence of internal pressure in liquids

Abstract: Internal pressures were calculated for a large variety of liquids in a wide temperature range from abundant experimental data on isothermal compressibility and thermal volumetric expansion coefficients obtained on a unique piesometric unit. The temperature coefficient of internal pressure was shown to be a structural characteristic of liquids. This coefficient was found to be positive for weakly associated liquids whose structure approximated close packing and negative for liquids with a spatial network of H-bonds. The largest negative temperature coefficient of internal pressure was obtained for water, whose spatial network of H-bonds was most perfect.

Thermodynamic perturbation theory calculations of interphase tension in small objects

Abstract: The size dependence of the specific surface Gibbs energy for Lennard-Jones small objects and water droplets was studied based on thermodynamic perturbation theory. For essentially nonspherical molecules, the reference interaction site model was used; according to this model, the radial distribution function of an unperturbed system is determined for fragments (groups) constituting molecules rather than for molecules themselves. For all systems studied in this work, the linear Rusanov and the Tolman formulas were found to hold at small and large radii, respectively. The method for calculating the surface tension of microdrops in a vapor-gas medium was extended to emulsions, that is, microdrop-liquid dispersion medium systems. Calculations performed for the water drop-hexane system showed that, at small radii (up to 1 nm), interphase tension linearly increased with R. The specific interphase Gibbs energy then reached an asymptotic macroscopic value, which was in close agreement with the tabulated data.

Anionic transport in simple fluoride structures

Abstract: Recent studies on anionic transport in crystals of simple fluorides of metals of Groups I-IV were reviewed. The effects of the formation of defects in MF n (n = 1-4) crystals and of the structure of the crystals on the characteristics of anionic conduction was analyzed.

Sorption of water vapor and acetonitrile by human serum albumin

Abstract: The adsorption of water and acetonitrile (AN) vapor on a human serum albumin (HSA) from water-acetonitrile mixtures was measured by IR spectroscopy at 298 K in the water activity range from 0 to 0.98. It was established that the ability of the protein to bind water markedly decreases in the presence of acetonitrile. The dry protein sorbs ∼180 mol AN/mol HSA. The amount of AN sorbed linearly decreases with increasing humidity of the protein. An analysis of the absorption band belonging to the stretching vibration of the nitrile group of AN suggests that the environment of sorbed AN molecules changes from predominantly peptide to predominantly aqueous. The degree of helicity of proteins significantly increases in the presence of vapors of organic compounds. For the desorption branch, the difference in the degrees of helicity of a protein that sorbed pure water and a water-acetonitrile (AN) mixture (excess helicity) depends on the humidity, linearly increasing in passing from pure water to pure acetonitrile. During subsequent sorption-desorption cycles when the water is completely removed without removing AN, the excess helicity is independent of what branch is used, being determined only by the amount of AN in the protein sample.

Simulation of phase equilibria in the water-helium and water-neon systems

Abstract: Fragments of the H 2 O-He and H 2 O-Ne phase diagrams including regions of ice Ih-inert gas solid solutions were constructed by thermodynamically modeling phase equilibria in these systems in the temperature and pressure ranges 250-273 K and 1-1500 bar respectively. The solubilities of helium and neon in ice were found to be higher than in liquid water.

Effect of the molecular structure on the enthalpies of hydration of amino acids and oligopeptides

Abstract: The enthalpies of solution Δ s o l H° of eight amino acids and oligopeptides in water at 298.15 K were determined by the calorimetric method for the first time. The enthalpies of hydration Δ h H° were calculated. The dependences of Δ h H° on the molecular weight, dipole moments, and charges on the molecules were established. An equation for calculating the heats of sublimation Δ s u b H of various amino acids from quantum-chemical data was proposed.

Conditions for the applicability of a thermodynamic description of highly disperse and microheterogeneous systems

Abstract: Systems comprising "small objects" in a homogeneous medium are considered. A quantitative criterion was obtained for distinguishing between heterogeneous highly disperse systems and systems which are microheterogeneous atthe structural level but homogeneous from the thermodynamic point of view. It was shown that the method of Gibbs surface phases could be extended to highly disperse and microheterogeneous systems if the specific Gibbs surface energy was treated as a function of the radius of small objects. In certain instances (for instance, for nanocrystals and cylindrical small objects), linear and point nonautonomous phases should be considered in addition to separating surfaces.

Structure and thermal stability of the hydration shells of chloride ions in the polar stratosphere as modeled by the Monte Carlo method

Abstract: The characteristics of Cl - (H 2 O) N clusters with N from 1 to 60 were numerically simulated at 298 and 210 K. The Gibbs free energy, work of formation, and equilibrium cluster size were calculated. The interactions in thesystem were described by a combined model potential constructed from the ST2 and SPC potentials. The combined potential allows explicitly for the polarizabilities of the ion and water molecules in the electric field of the ion. Optimal values of the parameters of the potential were obtained from experimental data on the Gibbs free energy of addition of the first few water molecules to the cluster and from correlation functions for aqueous electrolytes. The hydration shell was found to retain the stability in a supersaturated vapor. The thermal fluctuations destroy ring molecular structures; this process is also advanced by the strong electric field of the ion. At low temperatures, the chloride ion is forced out onto the cluster surface.

The standard enthalpy of formation of liquid germanium tetrachloride

Abstract: The enthalpies of solution of germanium metal and liquid germanium tetrachloride in solutions of sodium hydroxide containing hydrogen peroxide were measured calorimetrically. The enthalpies of dilution of solutions of hydrogen peroxide and sodium hydroxide and the enthalpies of solution of crystalline sodium chloride in solutions of NaOH containing the corresponding components were determined. The experimental data were used to calculate the standard enthalpy of formation of liquid GeCl 4 .

A description of adsorption equilibria for nitrogen-oxygen mixtures on zeolite NaX in a wide range of composition, pressure, and temperature variations in terms of the stoichiometric adsorption model

Abstract: The stoichiometric competitive adsorption model was shown to satisfactorily describe the vast array of experimental data on adsorption equilibria for nitrogen-oxygen mixtures on zeolite NaX. The ratio between the stoichiometric coefficients of the adsorption of oxygen and nitrogen exactly equaled the ratio between the molar volumes of oxygen and nitrogen in the condensed state. The selectivity of adsorption of air components continuously decreased as the pressure and temperature of the mixtures increased.

The thermodynamic properties of iron(II) bromide and perchlorate complexes with 4-amino-1,2,4-triazole

Abstract: The heat capacity of iron(II) bromide and perchlorate complexes with 4-amino-1,2,4-triazole with the 1 A 1 ⇔ 5 T 2 spin transition was studied in the temperature ranges 8-346 and 57-331 K, respectively. Measurements wereperformed on a vacuum adiabatic calorimeter. The thermodynamic parameters of the phase transition in the complexes were determined.

The screening effect in interatomic interactions

Abstract: The quasi-classical approximation of quantum mechanics was used to derive an equation for the screening radius of valence electrons. The screening effect of adsorbent surface postulated in the Brunauer-Emmett-Teller theory was explained. Examples of screening radius calculations whose results agree with the available experimental data are considered. Screening of electrons caused by molecular symmetry with respect to the inversion of coordinates was shown to occur along covalent bonds about which molecular fragments rotated.

Simulation of the combustion of thermite compositions in mechanochemical reactors for the example of the Zn-Sn-S system

Abstract: The principal parameter of self-propagating syntheses in mechanochemical reactors, the induction period of the ignition of combustion, was numerically calculated on the basis of known and newly suggested model concepts taking into account the heat effects of chemical reactions. Accompanying phenomena were studied. The results were compared with experimental data on the Zn-Sn-S system.

Description of adsorption in the Henry region in terms of generalized charge

Abstract: The concept of the generalized charge of an atom (molecule or its rigid fragment) was introduced based on the theory of a multicomponent gas of electrons. This generalized charge is an effective characteristic of interatomic interactions. The additivity rule for the generalized charges of the elements of a rigid structure is of the power type. In the proposed model ofadsorption,the generalized charge appears not only in the expression for the potential energy but also in the expressions for the van der Waals radius and specific heat. A derivation of these expressions is presented. The van der Waals interaction between a homogeneous surface and a molecule was also described. The main parameters of molecular adsorption on graphite were calculated.

The synthesis, structure, and stability of N-cetylpyridinium interhalides: An experimental and quantum-chemical study

Abstract: N-Cetylpyridinium iodohalides N-C 1 6 H 3 3 P y XI 2 [X = Cl (I) and Br (II)] were synthesized for the first time. Interhalide of the composition N-C 1 6 H 3 3 P y I 3 : N-C 1 6 H 3 3 PyBrI 2 = 2: 1 (III) was prepared by double exchange of the bromide ion for iodide in the presence of excess alcali iodide. The quantum-chemical calculation data (gas phase, the HW basis set) and the spectrophotometric data on the stability of the compounds in chloroform solutions showed that the ability to retain molecular iodine was related to the nature of the anion. An X-ray structure study of N-cetylpyridinium polyhalides was performed. The crystal structure of salts I and II was, shown to be packing of uniformly alternating I 2 Cl - and I 2 Br - anions, respectively, and [(C 5 H 5 N)C 1 6 H 3 3 ] + cations. Cations and anions formed stacks of translationally equivalent molecules parallel to the a axis. The independent part of the unit cell of III contained two triiodide and one dibromide anions and three configurationally different N-cetylpyridinium cations.

Quantum-chemical ab initio calculations of the enthalpies of formation and hydrogenation of imines

Abstract: Quantum-chemical ab initio calculations of the structure and total energy of several aliphatic and aromatic imines and amines were performed. The method of isodesmic reactions with group balance and the G2MP2 procedure were used to calculate the enthalpies of formation (Δ f H o ) of these compounds and the enthalpies of hydrogenation (Δ h y d r /H o ) of imines. The additivity rule for Δ f H o within the framework of the Ben-son method was shown to hold, and group contributions to the enthalpies of formation of nitrogen derivatives of various structures were determined. The replacement of H atoms in H 2 C=NH by the Me, Et, Pr, i-Pr, Ph, and CH 2 =CH radicals was found to consistently change Δ h y d r H o . Corrections for substitution were determined, which allowed the enthalpies of hydrogenation of imines of various structures to be calculated with high accuracy.

The thermodynamic properties and structure of potassium-substituted natrolite in the phase transition region

Abstract: The thermodynamic properties of the potassium-substituted form of natural zeolite, natrolite, were studied in the temperature range 7.4-302 K in an adiabatic calorimeter. The experimental heat capacities and the principalthermodynamic functions calculated at several selected temperatures were tabulated. A first-order phase transition was observed at T t r = 250.32 K. The enthalpy and entropy of the transition were found to be 5263 ′ 41 J/mol and 21.02 ′ 1.2 J/(mol K), respectively. The structure of the substance was studied above and below the transition point. A possible mechanism of the phase transition was considered.

Kinetics of adsorption of beta-gal;actosidase on silica gel

Abstract: The specific activities of tetramers and dimers [0.67 and 0.2 (μM S)/(min mg E), respectively] and the equilibrium association constant (1.4 x 10 5 M - 1 ) for Penicillium Canescens β-galactosidase were determined. The kinetics of adsorption of β-galactosidase on silica gel was studied. It was shown that the association of the protein occurs through the solution. The adsorption was found to be a two-stage process: the protein is first weakly and then strongly bounded to the carrier. Three kinetic parameters of adsorption for the accepted scheme were calculated from the experimental data obtained. A kinetic analysis allowed us to calculate three rate constants for this mechanism. The β-galactosidase enzyme (KF 3.2.1.23) from Penicillium Canescens had a complex quaternary structure. Solutions of the enzyme were equilibrium mixtures of active dimers and tetramers.

Kinetics of ultrasonic inactivation of peroxidase in aqueous solutions

Abstract: The kinetics of the inactivation of horse-radish peroxidase in a 0.01 M phosphate citrate buffer (pH 5.4) under the action of high-frequency (2.64 MHz) ultrasound with a power input of 1 W/cm 2 was studied over a temperature range from 35.5 to 55.0°C and a concentration range from 10 to 100 nM and described by apparent first-order rate constants: k i n (total inactivation), k* i n (thermal inactivation), and k u s i n (ultrasound inactivation). These rate constants were demonstrated to be dependent on the temperature and pH value. As the concentration of peroxidase increases from 10 to 100 nM, the value of k i n changes only slightly, while k* i n decreases significantly. Poly(2-aminodisulfide-4-nitrophenol) and poly(gallic acid disulfide) cause a significant decrease in the degree of US inactivation of peroxidase: the presence of these compounds in micromolar concentrations completely stops the inactivation at 45°C. This result suggests that the HO and HO . 2 free radicals, which are generated by US cavitation, play the main role in the inactivation of the enzyme. Both these polysulfides of substituted phenols also cause a decrease in k* i n at 45°C.

An ion cyclotron resonance study of reactions between molybdenum oxide ionic clusters and ammonia

Abstract: Reactions between molybdenum oxygen-containing clusters Mo x O + y (x = 1-3, y = 1-9) and ammonia were studied by ion cyclotron resonance. The formation of several nitrogen-containing ions including the Mo 3 O 7 N + 2 complex with molecular nitrogen was observed. The upper and lower limits of bond energies in the complexes were estimated (kcal/mol): Mo + -NH ( 56), Mo 3 O + 8 -NH 3 (>58), Mo 3 O + 8 -NH (>44), Mo 3 O 7 (NH 3 ) + -NH 3 (>58), MO 3 O + 7 -2NH (>88), Mo 3 O + 7 -N 2 (>112) in the Mo=N-N=Mo structure, and Mo 3 O + 7 -N 2 (>56) in the Mo-N=N-Mo structure.

2-propanol-water binary solvent effects on the kinetics of 4-aminoazobenzene hydrogenation

Abstract: The influence of the composition of the 2-propanol-water binary solvent on the rate of liquid-phase hydrogenation of 4-aminoazobenzene on Raney nickel was studied. The reaction rate constant on an equiaccessible catalyst surface as a function of solvent composition passed an extremum caused by a change in the state of 4-aminoazobenzene in solution and hydrogen in the surface layer as a result of specific solvation interactions. The feasibility of controlling the rate of liquid-phase hydrogenation of substituted azobenzenes by varying solent composition was substantiated.

The heat capacity of zinc and cadmium chalcogenides (Znte, CDSe, and CDTe)

Abstract: The heat capacities of zinc telluride and cadmium selenide and telluride were measured on a DSM-2M calorimeter at 370-640 K (ZnTe) and 500-760 K (CdSe and CdTe). The literature data on the heat capacities of the specifiedsolids above 298 K were analyzed. The measurement results and the most reliable literature data on the heat capacities of these compounds were used to suggest new equations describing the temperature dependences of the heat capacities in the temperature ranges 220-1500 K (ZnTe and CdSe) and 220-1300 K (CdTe) and calculate the thermodynamic functions of the compounds.

The heats of adsorption of metal cations on silica gel with covalently fixed polyaspartic acid

Abstract: The heat effects of adsorption of ammonium and alkali, alkaline-earth, and transition metal cations on silica gel with covalently fixed polyaspartic acid were considered. The heats of adsorption of ammonium and alkali metal cations were determined with the use of 0.2 and 0.75 mM solutions of perchloric acid as eluents. The heats of adsorption of ammonium, and alkali metal cations on silica gel with covalently fixed polyaspartic acid with the use of a 0.2 mM solution of perchloric acid as eluent and ammonium, alkali, and alkaline-earth metal cations on Universal Cation and Selekton K adsorbents were determined with the use of a 2.75 mM solution of nitric acid as eluent. The retention mechanism was shown to be dominated by ion exchange on all adsorbents studied. Three types of dependences of cation retention on the temperature of the chromatographic column were found.

The thermodynamic functions of synthesis reactions and the thermodynamic properties of alkyl tert-alkyl ethers C-7 and C-8

Abstract: Saturated vapor pressures of ethyl tert-amyl ether were measured in the temperature and pressure ranges 318-374 K and 13.4-99.6 kPa. Equations describing the temperature dependence of vapor pressure and the enthalpy of vaporization were obtained. The normal boiling point of the compound, T b n = 374.82 K, was calculated. The enthalpy of vaporization was measured calorimetrically at 298.15 K, Δ v H = 38.19 ′ 0.16 kJ/mol. The density of the ether was determined in the temperature range 293-303 K. The experimental vapor pressures and densities were used to calculate the critical parameters of ethyl tert-amyl ether by the law of corresponding states. The enthalpies of vaporization and formation and the heat capacities of alkyl tert-alkyl ethers C 7 , C 8 obtained in this work and taken from the literature were used to calculate the equilibrium constants and the standard enthalpies and entropies of formation of the ethers in the temperature range 298-383 K. The results were compared with the available experimental data on equilibria in these reactions in the gas and liquid phases depending on temperature.