Showing papers in "Russian Journal of Physical Chemistry A in 2005"

Sorption properties of calcium nitrate dispersed in silica gel : The effect of pore size

Abstract: New composite Ca(NO 3 ) 2 -in-silica gel mesopore sorbents of water were synthesized and analyzed. Sorption isobars at a water vapor pressure of 17 mbar and various sizes of silica gel pores (6, 10, 12, and 15 nm) were measured over a temperature range of 30 to 160°C. It was demonstrated that the composite sorbs water by three mechanisms: (a) heterogeneous adsorption on silica gel, (b) the formation of salt dihydrate, and (c) formation of salt aqueous solution in silica gel pores. The modification of silica gel with the salt increased significantly its sorption capacity for water (from 0.04-0.06 to 0.2-0.3 g/g). The sorption properties of the composite were found to deviate from those predicted by a linear combination of the properties of silica gel and bulk calcium nitrate. The temperature of conversion of the water-free salt into its crystalline dihydrate shifts increases with decreasing size of the pores in the host matrix. Near this transition, a sorption hysteresis was observed, which narrowed with increasing dispersity of Ca(NO 3 ) 2 . The properties of a solution in the pores were found to be identical to those of the corresponding bulk solution, with no hysteresis observed. Measurements with the use of differential solvation, X-ray diffraction, and differential scanning calorimetry demonstrated that the properties of the salt change due to its being dispersed into nanosized grains rather than due to its interaction with the matrix.

Kinetics of Aggregation in Micellar Solutions

Abstract: Small parameters were introduced for describing the kinetics of aggregation of nonionic surfactants in solutions in an analytical form. The direct and reverse transition of molecular aggregates over the activation barrier of aggregation at a surfactant concentration between the first and second critical micellation concentra- tions was described. The kinetics of the initial stages of micellation was considered. Expressions for the char- acteristic times of attainment of the quasi-equilibrium concentrations of molecular aggregates in the micellar, subcritical, and supercritical regions, as well as the time of attainment of the quasistationary concentration of molecular aggregates in the near-critical size region were derived. The times of rapid and slow relaxation of the solution were examined. The total time it takes to establish the state of complete equilibrium in the solution was determined. It was demonstrated that this time decreases with increasing concentration of micelles. A probabi- listic-statistical interpretation of the mean time interval between two consecutive emissions of a surfactant monomer from a micelle, the mean life time of a surfactant monomer in the micelle, and the mean lifetime of a micelle was proposed. The hierarchy of characteristics times of aggregation in micellar solutions, which char- acterize a complex multistage character of the nucleation and decomposition of micelles and the establishment of equilibrium in micellar solutions, was discussed. It was demonstrated that these hierarchy arise due to the existence of small parameters in the kinetic theory of aggregation.

A 1H NMR study of the association of caffeine with flavin mononucleotide in aqueous solutions

Abstract: One- and two-dimensional 1 HNMR spectroscopy (500 MHz) was used to study the heteroassociation of caffeine (CAF), a hydrotropic agent, with flavin mononucleotide (FMN) in an aqueous solution in order to gain insights intothe molecular mechanism of solubilization in the FMN-CAF system. The equilibrium constants, induced proton chemical shifts and thermochemical parameters (AH and AS) for the heteroassociation of the molecules were determined from the concentration and temperature dependences of the proton chemical shifts for interacting CAF and FMN molecules. An analysis of the results obtained demonstrated that CAF-FMN heteroassociates are formed due to the stacking-interaction between the aromatic chromophores of these molecules. The most probable structure of the 1 : 1 CAF-FMN complex was determined from molecular dynamics simulations with the use of the X-PLOR program and an analysis of the induced proton chemical shifts for the molecules under study. Calculations of the relative content of homo- and heterocomplexes at various values of the ratio r of the concentrations CAF and FMN demonstrated that, at r > 10, the contribution from the CAF-FMN heterocomplexes is predominant in the equilibrium distribution of associates in aqueous solutions. It was concluded that the formation of stack-type intermolecular associates of hydrotropic agents (caffeine and nicotinamide) with bioactive compounds is the governing factor in the solubilization of such compounds.

A new method for analyzing intermolecular interactions in the structure of crystals : Saturated hydrocarbons

Abstract: A new method for analyzing intermolecular interactions is considered for the example of 17 saturated hydrocarbons, including saturated and carcass ones. The method uses the characteristics of Voronoi-Dirichlet polyhedra of atoms in the structure of crystals. The crystal structural data are used to determine the area S m of Voronoi-Dirichlet polyhedron faces corresponding to all intermolecular contacts of one molecule, the total volume of the pyramids V m whose bases are such faces and vertices are occupied by the nuclei of the atoms that participate in intermolecular contacts, and the total solid angle Ω m at which the intermolecular faces of Voronoi-Dirichlet polyhedra are seen from the corresponding nuclei of molecule atoms. The dependences of the enthalpy of sublimation of saturated hydrocarbons on the S m , V m , and Ω m integral parameters were found to be linear. The obtained correlation dependences were used to calculate the thermodynamic parameters of several saturated hydrocarbons with known crystal structures but unknown enthalpies of sublimation. The conclusion that the thermochemical data on cubane were incorrect was substantiated.

Correction of the retention factor in gas chromatography with consideration given to the volume of the extracolumn communications

Abstract: A method for measuring the effective components of the extracolumn volume of a chromatographic system during the determination of the physicochemical characteristics of sorption is proposed. This method makes it possible to determine the retention factor in accordance with its physical meaning. Experimental data obtained show that the distribution constant of a volatile compound in the nonvolatile liquid-gas system calculated by using the corrected retention factor virtually coincides with that calculated from the specific retention volume.

Heat effects of acid-base interactions in aqueous solutions of glycylglycine

Abstract: The heat effects of interaction between solutions of glycylglycine and HNO 3 , HCl, KOH, and LiOH were measured by the direct calorimetric method at 298.15 K and several ionic strength values against KNO 3 , LiNO 3 , and KClas background electrolytes. The heats of step dissociation of the peptide in a standard solution were found by extrapolation to zero ionic strength. The standard thermodynamic characteristics of acid-base interaction reactions in solutions of the peptide were calculated.

Formation of free radicals during the photolysis and radiolysis of cellulose

Abstract: EPR spectroscopy in the 2-mm band was used to demonstrate that virtually all of the singlet spectra of γ- and UV-irradiated cellulose belong to paramagnetic centers with conjugated carbon-carbon bonds (no centers with thelocalization of the unpaired electron on O atoms were observed using EPR). The formation of the conjugated radicals occurs immediately during irradiation at 77 K (15-20%) and continues during the post-effect due to secondary concerted reactions of dehydration of the primary radicals. The main process during the γ- and UV-irradiation of cellulose is the dehydrogenation of H-C. =O bonds of carbon atoms 1-4 in the pyranose cycle with the formation of single-type alkyl radicals in various amounts, depending on the nature of radiation. In addition, the photolysis is accompanied by the rupture of C5-C6 bonds with the formation of formyl radicals (H-C. =O). EPR measurements show that the multiplet spectra of alkyl radicals give way the spectra of allyl and then singlet spectra of dienyl and polyene radicals. The process results in the release of low-molecular-weight destruction products and the formation of conjugated segments in polymer chains. Photolysis was found to be more effective in producing polyconjugated structures.

Rheological properties and density of liquid-crystal 4-(ω-hydroxyalkyloxy)-4'-cyanobiphenyls

Abstract: The temperature dependences of the density and dynamic viscosity for the nematic and isotropic phases of 4-(ω-hydroxyalkyloxy)-4'-cyanobiphenyl (n = 3, 6, 8) and 4-alkoxy-4'-cyanobiphenyl (n = 6, 8) liquid crystals were obtained. The terminal hydroxy group was found to affect the molecular packing and viscosity of nematic cyanobiphenyls.

Solvation of glycine and its oligomers in a water-acetonitrile binary solvent at 298.15 K

Abstract: The enthalpies of solution ΔH° s o l of glycine, glycylglycine, and diglycylglycine in the water-acetonitrile (0-0.4 mole fractions) binary solvent at 298.15 K were determined for the first time by calorimetry. The enthalpy of solvation (ΔH° s o l v of glycine, the enthalpy of transfer (ΔH° t r ) of the above compounds from water to the binary solvent, and the pair-interaction enthalpy coefficients h X Y were calculated. The behavior of ΔH° s o l = f(x 2 ) function is largely determined by the energy of water-organic component interaction. The strength of the intermolecular interaction with acetonitrile increases in the series glycine < glycylglycine < diglycylglycine. An increase in the electron-donor ability of the organic component in the aqueous-organic solvent causes a decrease in the degree of solvation.

The structure and sorption properties of 5-hydroxy-6-methyluracil

Abstract: The crystalline phase of 5-hydroxy-6-methyluracil isolated from aqueous solutions was studied by X-ray diffraction analysis. The elementary unit of the structure of 5-hydroxy-6-methyluracil was found to be the dimeric molecule. It was assumed that first dimers and then tetramers were formed in self-association, and subsequent self-assembly led to the association of tetramers. The resulting network structure contained alternating hydrophilic and hydrophobic cavities. The oxymethyluracil molecule was shown to have three bidentate "recognition" centers, two of which participated in tetramer formation and one, in the association of tetramers. The ability of 5-hydroxy-6-methyluracil to participate in hydrophilic and hydrophobic interactions was estimated by the method of reversed-phase gas chromatography for the example of the sorption of n-alkanes C 6 H 1 4 -C 1 0 H 2 2 and n-alcohols CH 3 OH-C 4 H 9 OH. The dispersion properties of the surface of the adsorbent were found to depend on temperature and decrease as the temperature increased. Alcohols virtually did not form H-bonds with 5-hydroxy-6-methyluracil, and the increase in adsorption as the number of carbon atoms grew can be explained by the predominance of hydrophobic interaction between alcohol molecules and the surface of the adsorbent.

The influence of plasma chemical treatment of copper-containing complex zirconium phosphates on the catalytic transformations of butanol-2

Abstract: The catalytic activity of sodium-zirconium phosphates of the Nasicon family in reactions of butanol-2 was studied before and after their treatment in a glow-discharge oxygen plasma. The catalysts had the composition Na 1 - 2 x Cu x Zr 2 (PO 4 ) 3 , where x = 0, 0.15, 0.25, and 0.35. Plasma chemical treatment was found to increase the yield of butenes in the dehydration reaction by a factor of 1.5-3 at a 100% selectivity. The strongest effect of plasma chemical treatment was observed for the sample free of copper (x = 0). The activation energy of dehydration decreased for untreated samples and increased for samples subjected to plasma chemical treatment as the content of copper grew. The acidity of the surface of catalysts was studied spectrophotometrically using test adsorption of pyridine. The adsorption value changed insignificantly after plasma chemical treatment but depended on x and the temperature of adsorption.

Permeability of monolithic capillary columns in gas chromatography

Abstract: The permeability of a capillary column with a monolithic layer of porous silica gel was investigated using helium as a carrier gas. It was shown that the porous structure of silica gel synthesized immediately in the capillary, which occupied the entire internal space of the capillary, exhibits resistance to a carrier gas flow as if the capillary were closely packed with spherical particles 27 μm in diameter. Based on an electron microscopy analysis of the structure of the monolithic silica gel layer, according to which it is built of particles no larger than 2 μm in diameter, it was concluded that the monolith layer is characterized by a very high porosity. This conclusion was supported by direct measurements of the interstitial volume, which accounts for 84% of the entire geometrical volume of the layer; for comparison, the interstitial volume for closely packed spherical particles amounts only ∼40% of the total volume.

Calculating the atomic and electronic structure and magnetic properties of inorganic fullerenes

Abstract: The atomic models of inorganic fullerenes, that is, hollow carcass nanoparticles with polyhedral morphology based on layered Fe, Ni, and Cd dichlorides, Ti, Zr, Nb, and Mo disulfides, and Ti dioxide, were suggested. A comparative analysis of the electronic structure, magnetic properties, chemical bonding, and charge distributions was performed for the fullerenes of octahedral morphology specified above by the self-consistent discrete variational method within the framework of the local spin density formalism. The magnetic properties of the NbS 2 , MoS 2 , FeCl 2 , and NiCl 2 fullerenes were predicted. The interrelation between the special features of interatomic interactions and the composition of fullerene-like metal dichloride molecules is discussed.

Volumetric properties of ethylene glycol solutions of tetramethylurea

Abstract: The densities of ethylene glycolic solutions of tetramethylurea were measured with a precise vibrating-tube densimeter over the accessible concentration range and temperatures of 288 to 318 K, with the focus on low tetramethylurea concentrations. The slope of the density vs. concentration dependence changes near a ratio between the components of 1: 1. The apparent and partial molar volumes of tetramethylurea were calculated. Extremums in the dependences of these parameters on the tetramethylurea near 4 mol % were explained by the solvophobic nature of the solvation of tetramethylurea by ethylene glycol.

Heat capacity and thermodynamic functions of DyLiMgMn2O6 manganite in the temperature range 298.15 to 673 K

Abstract: The heat capacity of the DyLiMgMn 2 O 6 ternary manganite was measured by dynamic calorimetry. The C° p vs.f(T) dependence exhibits a λ-shaped peak associated with a second-order phase transition at 423 K. Expressions for the temperature dependence of C° p were derived. The S°(T), H°(T) - H°(298.15), and Φ**(T) thermodynamic functions were calculated.

A complex study of the phase diagram of the Sn-Sb system

Abstract: The Sn-Sb system was studied by the electromotive force method and the microstructural, X-ray diffraction, differential thermal, and microprobe analysis methods. Phenomenological crystal chemical models of the β phase were suggested. The models were based on hexagonal polymorph transformation into a quasi-cubic polymorph with a change in crystal symmetry corresponding to a second-order phase transition. The thermodynamic properties of the system were optimized.

A software-hardware unit for measuring signals from a 1000D derivatograph and computer-assisted processing of thermogravimetric data

Abstract: A soft hardware unit for measuring and recording output signals from the transducers of a 1000D derivatograph (MOM, Hungary) was designed and a program package for quantitative processing of derivatograms (T, TG, DTG, andDTA curves) was developed.

The surface energy of nanocrystals

Abstract: An equation for the surface energy a as a function of the size and shape of an n-dimensional nanocrystal was obtained. The nanocrystal had the form of an n-dimensional parallelepiped with a square base; the ratio between the side edge length and the base edge length f determined the shape of the system. The σ value was shown to decrease as the number of atoms in the nanocrystal N decreases, and the larger the difference between the shape parameter f and one, the stronger the a(N) dependence. At high temperatures, the surface Helmholtz energy decreased as the temperature T increased, and the smaller the size of the nanocrystal of the given shape or the stronger the deviation of the shape of the nanocrystal at the given N from the thermodynamically stablest shape of the n-dimensional cube, the larger was the -(∂σ/∂T) value. Nanocrystals were shown to melt when their surface energy decreased to a certain value independent of their size and shape. A polymorphic phase transition to a less dense structure decreased the surface energy. The conditions under which the inequality σ(N,f; T, p) < 0 was valid, that is, crystal size fragmentation or shape dendritization could be observed, were considered.

Electrical conductivity of aqueous solutions of polyethylene glycol

Abstract: New data on the electrical conductivity and viscosity of aqueous solutions of polyethylene glycol were obtained and used as reference data in studying the ionic channels of biological membranes. Over wide ranges of concentration and polymer molecular weight, conductivity was independent of the molecular weight for long chains and weakly depended on the molecular weight for short chains. Several models suggested in the literature for solving the general problem of the diffusion of a tracer in polymeric solutions are discussed. The processes responsible for a decrease in the mobility of ions were qualitatively analyzed to explain the weak conductivity sensitivity to the length of polymer chains. It is shown that experiments can be interpreted using the microviscosity concept. Microviscosity increases with the addition of a polymer much less rapidly than usual (macroscopic) viscosity. A simple empirical formula describing the dependence of conductivity on the polymer concentration is suggested.

Chemical equilibria in the gas phase of the Ge-Br system

Abstract: Thermodynamic function values were refined for two independent chemical reactions in the gas phase of the Ge-Br system.

Phase equilibria and the structure of individual phases in the Sr-Co-Ni-O system at 1100°C in air

Abstract: Solid solutions SrCo 1 - x Ni x O 2 . 5 + δ (0 ≤ x ≤ 0.15) and Sr 3 Co 2 - x Ni x O 7 - δ (0 ≤ x ≤ 1.1) were prepared by solid-state synthesis and using citrate technology. Annealing at the final stage was performed at 1100°C in air. The phase composition was controlled by X-ray diffraction. The structure parameters of single-phase samples were calculated by the full-profile Rietveld method. A projection of the isobaric-isothermal (1100°C, air) phase diagram of the Sr-Co-Ni-O system onto the triangle of compositions (metallic component concentrations) was constructed on the basis of the X-ray powder patterns of the samples.

Theoretical estimation of conditions for the preparation of nanoscale systems in mechanochemical reactors

Abstract: The solid-state mechanochemical synthesis of a nanoscale desired product based on the method of diluting the initial reagent powder mixture by another exchange reaction product was studied theoretically. The suggested trimodal particle-size distribution in the mechanically activated system was used to calculate the optimum molar ratios between mixture components that provided the existence of shock-frictional contacts of reagent particles and prevented the aggregation of nanoscale desired product particles. A kinetic equation was derived for mechanochemical reactions of the preparation of nanosystems by the dilution method; the equation was used to calculate mechanical activation times necessary for the completion of exchange reactions. The theoretical results were in agreement with the available experimental data.

Complex formation constants between iminodisuccinic acid and calcium ions in aqueous solutions

Abstract: The stability constants of calcium iminodisuccinates in aqueous solution at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 against the background of sodium chloride were determined by potentiometric titration. The formation of the CaL 2 - and CaHL - complexes was established. The heat effects of interaction between solutions of iminodisuccinic acid and calcium chloride at 298.15 K and several ionic strength values (0.5, 1.0, and 1.5; NaCl) were measured calorimetrically. The thermodynamic stability constants and the heat effects of formation of the normal and protonated calcium complexes with iminodisuccinic acid in salt solutions under standard conditions were determined by extrapolation to zero ionic strength according to an equation with one individual parameter. The standard thermodynamic characteristics of complex formation between iminodisuccinic acid and calcium ions at 298.15 K were calculated.

A conductometric study of ionic association and interparticle interactions in solutions of 1-1 electrolytes in ethyl acetate at 5-45°C

Abstract: The results of a conductometric study of solutions of LiClO 4 , NaClO 4 , NaBPh 4 , and Bu 4 NClO 4 in ethyl acetate over a temperature range of 5 to 45°C are presented. The Lee-Wheaton extended equation for describing electric conductivity in conjunction with a previously proposed method for processing conductometric data on 1-1 electrolytes in media with a low dielectric permittivity were used to determine the constant of association of ions into ion pairs and triple ions and the limiting molar electric conductivities of simple ions and triple ions. It was established that, in electrolytes containing bulky organic ions, contact triple ions with a certain degree of interpenetration of structural elements are predominantly formed. Depending on the nature of the ions, the non-Coulomb interaction between ions (during the formation of ion pairs) and between ion pairs and simple ions (during the formation of triple ions ) may result in either the attenuation or the strengthening of the Coulomb-force-controlled association of ions in the ethyl acetate media. The dynamic radii of the [Na 2 BPh 4 ] + and [Na(BPh4) 2 ] - ion exhibit an anomalous temperature dependence. The energetic characteristics of the interionic interactions that govern the formation of ion pairs and triple ions in solutions of NaBPh4 in ethyl acetate differ substantially.

Ozone reactions with lignins at various pH values

Abstract: The influence of pH on the kinetics of ozonization of aqueous solutions of sodium lignosulfonate (LS) in a bubbling reactor was studied. The effective rate constants and stoichiometric coefficients of the reaction betweenozone and LS were determined. The influence of pH on the effective rate constants and the amount of ozone consumed was characterized. The stoichiometric coefficients were shown to change from 1.3 to 5.6 as pH increased from 1.0 to 8.6. Simultaneously, the effective rate constants changed severalfold. The transformations of solid lignin and aqueous solutions of LS were studied by UV and 1 3 C NMR spectroscopy. The main stages of lignin ozonization were determined, and a mechanism for the reaction is suggested.

A new method for analyzing intermolecular interactions in unsaturated hydrocarbon crystals

Abstract: A new method for crystal chemical analysis of intermolecular interactions is considered for the example of unsaturated hydrocarbons. The method uses the characteristics of Voronoi-Dirichlet atomic polyhedra in the structure of substances. The area of Voronoi-Dirichlet polyhedron faces corresponding to all intermolecular contacts of one molecule, the total volume of pyramids whose bases are such faces and vertices contain nuclei of atoms participating in intermolecular contacts, and the common solid angle at which the "intermolecular" faces of Voronoi-Dirichlet polyhedra are seen from the corresponding nuclei of molecule atoms were determined from crystal structural data. The enthalpy of sublimation of hydrocarbons was shown to be a linear function of Voronoi-Dirichlet polyhedron parameters. The suggested method opens a possibility of calculating the enthalpy of sublimation of hydrocarbons and fullerenes directly from crystal structural data.

The influence of H-bonding on the enthalpies of solvation of proton acceptors in methanol

Abstract: The influence of cooperative phenomena on the solvation of proton acceptors in methanol was studied. It was shown that solvation processes should be considered taking into account the dependence of the cooperative effect on the ability of the proton acceptor to form H-bonds with the solvent. A scheme for calculating the enthalpies of H-bond formation between proton acceptors and methanol associates is suggested.

Optimization of the porous structure of a hydrophobic matrix for chromatomembrane mass-exchange processes

Abstract: Based on mercury porometry and microscopy data, the most suitable types of powdery polytetrafluoroethylene for preparing biporous hydrophobic matrices for chromatomembrane mass-exchange processes were chosen. It was foundthat there is no sharp boundary between the sizes of the pores through which the polar and nonpolar phases flow during a chromatomembrane process and that the pressure in the polar phase influences the volume fractions of these two phases in the biporous matrix.