Vol 81, No 3 (2019)

The kinetics of spontaneous boiling of liquid methane saturated with helium has been studied in experiments on measuring the lifetime of the superheated liquid. The temperature dependence of nucleation frequency J has been determined in a range from 10⁴ to 10⁸ s^–1 m^–3 at pressures \(p\) = 1.6 and 2.0 MPa and helium concentrations in liquid methane \(x\) = 0.06 and 0.10 mol %. The experimental results have been compared with the data of the classical nucleation theory. As in the case of pure methane, the solution superheating temperatures reached in the experiments at J > 4 × 10⁶ s^–1 m^–3 appear to be systematically lower (by 0.6–1.0 K) than their theoretical values. It has been shown that the discrepancy between the theoretical and experimental data is associated with the size dependence of the surface tension at the critical bubble–solution interface.

Reducing and stabilizing abilities of three poly(ethylene glycol) (PEG) samples modified with primary amino groups in one or two terminal positions of the polymer chains, as well as with dendrons based on L-aspartic acid in both terminal positions of the polymer chains, have been studied. Stable dispersions of silver nanoparticles have been formed at room temperature in aqueous solutions of AgNO₃ in the presence of the modified PEGs without additional reducing agents. Spectrophotometric examinations have shown that an increase in the number of amino groups per polymer molecule results in accelerating the formation of nanoparticles and improving the stabilizing ability of the modified PEGs. Molecular hydrodynamic methods (analytical centrifugation and dynamic light scattering) have been used to determine the absolute values of the molecular mass of silver nanoparticles stabilized with dendronized PEGs and the hydrodynamic sizes of the particles. Molecular hydrodynamics and electron microscopy have yielded interconsistent estimates of silver nanoparticle sizes.

Homogeneous–heterogeneous bulk condensation of potassium sulfate vapor has been numerically simulated in a dusty vapor–gas flow of coal combustion products upon their cooling along a technological path. A closed model that we have proposed for the formation of submicron particles in coal combustion products has been employed. Data have been obtained on the concentration and size distribution of particles formed at varied parameters of heterogeneous condensation sites and rates of variations in the temperature of the flow. Variations in the relative contributions of the homogeneous and heterogeneous mechanisms with variations in flow dustiness have been considered. A criterion enabling one to judge the effect of flow dustiness on the bulk condensation process has been proposed. This criterion takes into account both dust parameters and rate of temperature variations in a condensation zone. Data have been presented on the influence of coagulation processes on the parameters of submicron particles resulting from coal combustion.

Composite poly(vinyl alcohol) (PVA) cryogels comprising dispersed porous cellulose-containing fillers (microcrystalline cellulose, wood sawdust) and salting-out electrolytes (Na₂SO₄, NaF) have been prepared by freezing at –20°C, incubation in the frozen state for 12 h, and defrosting at a rate of 0.03°C/min. The influence of the chemical nature and concentration of the soluble additives and fillers on the rheological behavior of initial suspensions, as well as the morphology of macropores and physicochemical and thermophysical properties of corresponding composite cryogels, has been studied. Viscometric studies have shown that, because of the salting-out action of the electrolytes, the viscosity of PVA solutions decreases due to the compaction of macromolecular coils. However, in the case of filler suspensions, the viscosity increases owing to the enhancement of the adhesion interactions between the discrete and continuous phases, thereby affecting the rigidity and heat endurance of filled cryogels resulting from the freezing–defrosting of such suspensions. The most pronounced increase in the compression elasticity modulus and heat endurance of the composites takes place upon the combined incorporation of a porous filler and a salting-out electrolyte into the cryogel matrix. The microstructure of both unfilled and filled cryogels has been studied by scanning electron microscopy. Substantial changes have been revealed in the macroporous morphology of these objects upon the incorporation of a porous dispersed cellulose-containing filler or a salting-out electrolyte separately and, especially, upon their combined incorporation into the matrix of the PVA cryogel.

The Du Nouy ring detachment, pendant drop, and maximum bubble pressure methods have been employed to study the tensiometric (surface tension) and rheological (viscoelasticity modulus and phase angle) characteristics of solutions of dicationic and monocationic imidazolium oximes at a liquid–gas interface. It has been revealed that dicationic oxime (1,3-bis(3'-cetylimidazolium-1'-yl)-2-oxyminopropane dichloride) has high surface activity and viscoelasticity modulus values. The experimental dependences of the dynamic surface (interfacial) tension and the rheological parameters of the surface layers of imidazolium oximes are adequately described in terms of the reorientational model. Ideas have been suggested on a possible reason for the high values of the viscoelasticity modulus of cetyl-substituted dicationic oxime at the liquid–gas interface.

Size-exclusion chromatography has been employed to determine the sizes of palladium and silver nanoparticles synthesized by the radiation-chemical method in micellar solutions of a surfactant (AOT) at different degrees of hydration ω₀ = [H₂O]/[AOT]. Silver and palladium nanoparticle sizes are in the ranges of 1.5–5.3 and 1.5–3 nm. The results obtained have been compared with the data of atomic force microscopy. It has been shown that the sizes of nanoparticles synthesized in micellar solutions must be corrected with allowance for the contribution of micelle shell thickness.

The Monte Carlo method has been employed to simulate the growth of condensed phase nuclei on a silver iodide crystal surface with parallel nanoscopic grooves having different depths and profiles. The growth thermodynamics has been studied in terms of the free energy, entropy, formation work, and Gibbs free energy calculated for attachment reactions by the bicanonical statistical ensemble method. The grooves decrease the chemical potential of first water molecules attached to the surface by 20k_BT. The grooves present on the surface shift the onset of the nucleation to the region of a rarefied vapor by nearly nine orders of magnitude and increase the thermodynamic stability of the nuclei; however, in the region of moderate values of density, the effect of the grooves is weakened and directed toward the deceleration of the condensation. Fundamental equations have been derived, which represent a generalization of Clapeyron–Clausius equation to the case of condensed phase of nonmacroscopic sizes. Using the aforementioned equations, physical reasons for the regularities observed in the computer simulation have been revealed and the agreement between the numerical results and universal relations that follow from the equations has been analyzed.