Vol 81, No 5 (2019)

The formation of equilibrium liquid–gas interfaces in single- and two-component Lennard-Jones systems has been reproduced by molecular-dynamics simulation. The second component in the two-component system is a volatile impurity. The initial state is created by bringing in contact homogeneous liquid and gas phases having equal temperatures, pressures, and chemical potentials. The times required to establish equilibrium values of pressure, composition, shape and thickness of an interfacial layer, relative adsorption, and surface tension have been evaluated by the simulation. The calculations have been carried out at a temperature close to the triple point temperature of a solvent. It has been found that, in the course of relaxation, the maximum dynamic surface tension exceeds the equilibrium value by a factor of 1.2–1.6, while the relaxation time increases from 10 to 100 ns as the concentration of the volatile component in the solution grows to 0.25. In the two-component system with a limited volume of the gas phase, an equilibrium interfacial layer is formed in two stages. At the first stage, the volatile component is transferred into the interfacial layer from the near-surface regions of the liquid and gas phases. When an equilibrium partial density of the volatile component in the gas phase is achieved, the second stage begins, at which the surface layer is mainly supplied with liquid-phase particles. As a result, the relaxation times of relative adsorption and surface tension substantially increase. The role of the dynamic surface tension in the process of nucleation has been discussed.

Although usual pressures have typically a weak effect on the properties of condensed phases and their surface layers, a parameter has been found in the surface physical chemistry—a contact angle at a three-phase contact line—that is rather sensitive to hydrostatic pressure. Experiments with an air bubble adhered to a solid surface immersed in water have shown that an increase in the hydrostatic pressure by less than two times causes a growth of the contact angle by more than 10°, if the angle is markedly smaller than 90°. Therewith, the three-phase contact line remains immobile, and only the liquid−gas interface changes its orientation. If the angle (no matter, acute or obtuse) is close to 90°, the three-phase contact line acquires mobility as an alternative way to reach an equilibrium . A thermodynamic theory has been developed on the basis of the generalized Young equation to explain these phenomena. It has been shown that, when the three-phase contact line is fixed, a growth of the pressure in a liquid always leads to a rise in the contact angle.

Analytical solutions have been obtained for ordinary, normalized, and higher-order (third- and fourth-order) central moments using the model of an equilibrium adsorption layer. The solutions are represented by one-term fractions and comprise the length of an adsorbent layer, the velocity of a mobile phase, the effective kinetic constant, and the Henry constant. Expressions for the third- and fourth-order moments have been obtained and analyzed in the case of the rectangular input signal . It has been shown that, when solving the inverse problem of chromatography, the calculation of the higher-order moments provides no new information on the kinetic or any other properties of an adsorbent layer.

Dispersions of molybdenum blues have been synthesized by reducing a molybdate with hydroquinone in an acidic medium. Spectrophotometry and photon-correlation spectroscopy have been employed to study the formation regularities of molybdenum blues particles (large molybdenum oxide clusters) under the conditions of varied reductant/molybdate ratio and dispersion medium pH.

Adsorption of some 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from water–acetonitrile solutions on Luna C₁₈ and Discovery C₁₈ silica gels with grafted octadecyl groups has been studied in a wide range of compositions at temperatures of 313.15–333.15 K. The comparative analysis of the liquid-phase adsorption of the heterocycles has revealed a number of differences between the mechanisms of binding molecules of the studied compounds by the surfaces of these octadecyl-derivatives of silica gel. The log–log dependences of the retention factor on the molar fraction of acetonitrile in the solutions are strictly linear only for adsorption on Luna C₁₈ silica gel, which is characterized by an increased hydrophobicity. For the adsorption of the studied heterocycles on Discovery C₁₈ silica gel, the concentration dependences of the retention at low and high acetonitrile concentrations in the solution are described by quadratic functions and characterized by higher retention values as compared with those for the linear dependence because of the lower hydrophobicity of this silica gel. It has been found that the surface layer of Luna C₁₈ modified silica gel is enriched with acetonitrile molecules to a higher extent than the surface layer of Discovery C₁₈ silica gel. It has been shown that one adsorbed molecule of 1,3,4-oxadiazoles and 1,2,4,5-tetrazines displaces three and two molecules of pre-adsorbed acetonitrile from the surfaces of Luna C₁₈ and Discovery C₁₈ silica gels, respectively. Linear correlation equations have been proposed for calculating the retention of the studied heterocycles on Luna C₁₈ and Discovery C₁₈ silica gels from mobile phases of arbitrary compositions at different temperatures of chromatographic columns.

The regularities of the mechanical activation of α-Bi₂O₃, the nature and thermal stability of defects resulting from the activation, and an increase in the reactivity of the oxide have been analyzed with the use of X-ray diffraction, measurement of specific surface area, and synchronous thermal analysis combined with mass spectrometry. The process of Bi₂O₃ mechanical activation may be divided into two stages. At the stage of the fracture of particles, their specific surface area grows to S = 3.2 m²/g, while the particle size and size L of the coherent-scattering region decrease to 100 and 40 nm, respectively. At the stage of friction, S somewhat decreases, while L remains unchanged. After grinding in air, a phase of Bi₂O₂CO₃ is observed in addition to the main phase of monoclinic α-Bi₂O₃, with the former phase resulting from sorption of CO₂ from air. When an activated sample is heated, bismutite decomposes with CO₂ liberation in a wide temperature range. For an activated sample of nanosized oxide, heat absorption due to the α-Bi₂O₃ → δ-Bi₂O₃ phase transition begins at a temperature that is 10°C lower than the usual one. The reactivity of activated Bi₂O₃ has been determined by the example of its reduction in the atmosphere of CO. The mechanical activation increases Bi₂O₃ conversion upon reduction at 600°C by 2.5 times and decreases the temperature of the reduction onset by nearly 100°C.

The influence of the surface chemistry of nonpolar sorbents (silica gel C18 and porous graphitized carbon), temperature, and liquid phase composition on the parameters of chromatographic retention of S,N-derivatives of 1,1-dimethylhydrazine (thiosemicarbazides) has been studied under the conditions of liquid chromatography. Experimental data have been compared with the results of preliminary estimates obtained for the adsorption characteristics of thiosemicarbazides using a molecular-statistical approache and determination of lipophility factor. Satisfactory agreement has been revealed between the experimental data and theoretical predictions. It has been shown that mixtures of ethyl-, allyl-, and phenylthiosemicarbazide are most efficiently separated by chromatography with the use of porous graphitized carbon packed into a Hypercarb column. The selectivity of this sorbent is 1.5 times higher than that of octadecyl silica gel packed into a Zorbax Eclipse XDB C18 column.

Employing the Onsager approach to nonequilibrium isothermal processes and the cell method, an algebraic equation has, for the first time, been derived for determining the electroosmotic permeability of an ion-exchange membrane at a preset constant current density. The cell model has been verified using experimental data on the electrical conductivity and electroosmotic permeability of an aqueous hydrochloric acid solution at preset current density and concentration through an initial MF-4SK cation-exchange membrane and that modified with halloysite nanotubes and platinum and iron nanoparticles. A good coincidence has been obtained between the theoretical and experimental data, and the physicochemical parameters of the model have been determined. The results of the calculations performed within the cell and homogeneous models of a fine-porous membrane have been compared. Their correspondence has been shown for all three samples of the MF-4SK sulfonic cation-exchange membrane.

Relative linear deformation of AUK microporous carbon adsorbent with a narrow pore size distribution has been measured upon n-octane adsorption at pressures of 1 Pa to 1.6 kPa and temperatures of 273, 293, 313, 353, and 393 K. Throughout the studied temperature range, adsorption-induced deformation curves exhibit a similar behavior. At low and moderate degrees of micropore filling, the adsorbent remains almost undeformed, while it dramatically expands in the region of high degrees of filling. Adsorption of n-octane molecules in a model slitlike pore, the average effective width of which corresponds to the width of micropores in the AUK adsorbent, has been numerically simulated by the molecular dynamics method. It has been shown that, in the initial and middle regions of micropore filling, the behavior of the deformation dependences is determined by the ratio between the sizes of n-octane molecules and micropores. At high degrees of filling, the formation of adsorption associates in the micropores leads to a rise in the internal pressure and a drastic expansion of the adsorbent. The compressibility of AUK has been calculated, and the results obtained correspond to the data published previously.