Vol 78, No 5 (2016)

Colloidal silver nanocubes (NCs) were successfully synthesized by reduction of silver nitrate with ethylene glycol and polyvinylpyrrolidon as capping agent. The effect of solar light irradiation on the formation and morphology of silver NCs was investigated. Moreover, altering the amount of sodium sulfide was used to control the morphology and shape of primary silver seeds. Scanning electron microscopy, transmitting electron microscopy, X-ray diffraction and UV-vis spectroscopy were used to characterize silver NCs. The samples prepared under the solar light irradiation do not possess cubic shape while highly monodispersed silver NCs were obtained in dark room conditions. For dark room synthesis, a decrease of the amount of Na₂S by only 10 μL resulted in formation of mixture of silver nanospheres and nanowires in addition to NCs instead of the monodispersed silver NCs. However, similar increase of the amount of sodium sulfide results in distortion of cubic geometry of particles. The results suggest that solar light has a negative effect on the shape evolution of the primary silver seeds.

It has been shown that silica container particles containing a large amount (~1 g/g of SiO₂) of a corrosion inhibitor, catamine AB, can be obtained, with the inhibitor being loaded at the stage of the sol–gel synthesis of the particles by using catamine micelles as a template. Being introduced into an H₂S-containing aggressive aqueous medium, such containers protect rather efficiently carbon steel from hydrogen-sulfide corrosion and hydrogenation. The protection effect seems to be realized via not only the release (desorption) of catamine AB molecules from the containers, but also the adsorption of the products of gradual dissolution of silica matrix on the surface of metal being protected.

Possible variants of the rheological behavior of silica model dispersions have been analyzed. Different types of interaction between the particles and a dispersion medium make it possible to obtain different systems from low-viscosity sols to gels. Proton-donor (water) and aprotic (dimethyl sulfoxide) media have been used for comparison. Dispersions in the aprotic medium behave as non-Newtonian viscous fluids exhibiting shear thinning or shear thickening depending on deformation rate. Aqueous dispersions are viscoelastic and viscoplastic objects that exhibit the shear thickening at stresses higher than the yield stress. The introduction of small amounts of poly(ethylene oxide) into the organic dispersion medium initiates gelation. An increase in the polymer content in the dispersion medium above the concentration corresponding to the formation of a macromolecular network promotes an increase in stiffness and strength of the gels. The rheological behavior of gels is influenced by the polymer molecular mass and its affinity for a solvent.

The aggregation kinetics of OX50 sols in aqueous NaCl solutions (10^–4–2 × 10^–1 M) has been studied for 15 days or more by dynamic light scattering. The following set of characteristics has been considered to quantitatively estimate the coagulation intensity in the disperse systems: the particle size corresponding to the maximum in the differential particle size distribution curve, the height of the maximum, the polydispersity index, and the average diameter of the intensity distribution. It has been found that slow sol coagulation proceeds via the barrierless mechanism in secondary potential minimum, which arises from the predominance of the dispersion attractive forces over structural and electrostatic repulsive forces.

A full-atomic molecular dynamics simulation has been performed for a ligand shell of colloidal cadmium selenide quantum dots. Trioctylphosphine, trioctylphosphine oxide, octadecylphosphonic acid, and hexadecylamine have been used as ligands. For a mixture of the two former ligands, the effect of surface curvature on the fraction of surface ions of quantum dots bonded to ligands has been studied. It has been shown that, for particles with radii of 1.9 and 4.5 nm, every second and approximately third cadmium atom, respectively, is bonded to trioctylphosphine oxide. Partial introduction of octadecylphosphonic acid and hexadecylamine may increase the fraction of bonded surface atoms by more than two times.

Nonequilibrium processes have been considered on interfaces between media, with one medium being a permeable membrane. The nonequilibrium processes have been described in terms of nonequilibrium thermodynamics combined with the Fokker−Planck kinetic equation. Phenomenological equations derived within the framework of the combined approach have been discussed. It has been shown that the formal use of linear equations of nonequilibrium thermodynamics may be, in some cases, reduced to nonlinear equations for heat and mass transfer through an interface. It has been noted that the asymmetry of transport characteristics of asymmetric membranes may be explained in terms of the derived relations.

The micellization theory previously elaborated based on the determination of critical micelle concentration using the constant of the law of mass action is refined for ionic surfactants. The degree of micellization is used as a parameter to obtain implicit dependences of monomer and micelle concentrations on overall surfactant concentration in an ideal micellar solution. As expected, the counterion concentration increases monotonically, while the surface-active ion concentration passes through a maximum immediately after the critical micelle concentration. Repeated calculations performed at different degrees of counterion binding show that, as this parameter is decreased, the maximum becomes sharper, disappearing when the parameter becomes equal to unity. The complex character of the exact analytical description of the concentration functions for monomers and micelles is in contrast to their simple graphical representations. This fact makes it possible to derive simple analytical approximations for them in the form of explicit functions useful for calculations.

A method has been developed for obtaining Cu–Pd hydrosols via catalytic reduction of copper ions by hydrogen on seed palladium nanoparticles 2.5 ± 0.3 nm in size. It has been found that reduction of Cu²⁺ to metal proceeds stage-by-stage through the formation of an intermediate Cu⁺ ion. Cu–Pd hydrosol remains stable with respect to sedimentation and aggregation for several weeks. The hydrodynamic size of Cu–Pd nanoparticles increases proportionally to the copper content in particles. It has been shown that this is due to an acidification of the solution as a result of H⁺ ions formation via the reaction of reduction of Cu²⁺ ions by hydrogen.

The small-angle X-ray scattering (SAXS) in micellar sodium dodecyl sulfate solutions has been studied in the range of overall concentrations c from 8 mM (CMC₁) to 300 mM and the absolute values of scattering vector q from 0.07 to 3.0 nm^–1. The total intensity of isotropic scattering has been revealed to increase with solution concentration. At c > 27 mM, the SAXS spectra have been found to exhibit an interference peak, which testifies a correlation in the arrangement of micelles in the bulk solution. This peak corresponds to the magnitude of q close to 1.55 nm^–1. Using the position of this maximum, average distance r₀ between the centers of micelles has been determined, which is equal to 4.1 nm and remains almost unchanged upon an increase in the overall concentration of sodium dodecyl sulfate. The observed regularities have been explained in terms of the DLVO theory taking into account the electrostatic and molecular intermicellar interaction.

Temperature-modulated differential scanning calorimetry has been employed to analyze the structure-related thermal properties of petroleum bitumen. This method enables one to distinguish between “order–disorder” and glass transitions, thereby making it possible to monitor and identify structure-related phase transformations, the signals from which are invisible or overlapped in the thermograms of conventional differential scanning calorimetry. Bitumen has been shown to be a colloidal disperse system only under certain temperature–time conditions. Its dispersed phase may be represented by aggregates of two types with colloidal sizes. Saturated hydrocarbons form a solid crystalline phase in accordance with the regularities of first-order structural phase transitions and nucleation mechanism of phase separation. Asphaltenes and resins form a solid amorphous phase for a relatively long time as a result of a structural relaxation glass transition by the spinodal mechanism of phase separation.

Vapor mass flux density has been calculated on the surface of spherical droplets under almost isothermal conditions. The calculations have been performed at droplet radii corresponding to the free-molecular, intermediate, and continuum flow regimes. The study has been performed by the direct numerical solution of the Boltzmann kinetic equation. The method of characteristics has been adapted for describing transfer processes in spherically symmetric systems. The applicability of different simplified approaches to the calculation of mass flux density on droplet surface has been estimated.

Stable colloidal dispersions of fullerene С₆₀ in water free of organic solvents have been obtained. The addition of a С₆₀ solution to a water–acetone mixture has been shown to cause a solvatochromic effect.

The average activity coefficients of ionic surfactants as a whole and their monomeric forms are compared. The concept of an aggregative activity coefficient is introduced.