Vol 79, No 5 (2017)

Surface dilatational dynamic elasticity ε of films formed by cross-linked poly(N-isopropylacrylamide) microparticles on a water/air interface has been determined as a function of polymer surface concentration Γ. The experimental dependences exhibit two maxima of ε at surface pressures π of nearly 6 and 35 mN/m. In the region of the second maximum, the pattern of the dependence is governed by a method used to vary Γ. At π > 25 mN/m, film compression leads to the formation of a metastable monolayer, while the gradual addition of a microparticle dispersion results in the establishment of equilibrium between the monolayer and surface aggregates. In the region of π values corresponding to the second maximum of ε, slow relaxation processes with a characteristic time substantially longer than 10 s occur in the system. At π > 35 mN/m, the film collapses due to the displacement of microgel particles from the water surface.

It has been shown that the rheological properties and resistance to sedimentation of aqueous poly(vinyl acetate) (PVA) latexes, which are used for the preparation of coatings and adhesives, can be controlled by adding sodium montmorillonite (MMT). The adding of MMT initiates gelation of the PVA dispersions, which manifests itself as the appearance of the yield stress, thixotropy, and viscoelasticity. X-ray diffraction analysis of the complex dispersions and films based on them has shown a transition from exfoliated to intercalated clay tactoids with an increase in their content. In view of the complex composition of the PVA latexes, which contain a stabilizer (poly(vinyl alcohol)) and a plasticizer (dibutyl phthalate), the components that are predominantly intercalated into the MMT interplanar space, have been identified. The highest yield stress and rigidity of the structural network, which arises in a sample as a result of the joint coagulation of PVA and MMT particles, are observed upon the incorporation of 1.2 wt % MMT into the latex.

Features of the evolution of ultrafine gold nanoparticles synthesized by the Duff method with temperature and time have been studied in relation to their use as seeds for the formation of plasmonic nanoshells. A quantitative relation has been revealed between the duration of preheating of such particles at a preset temperature and their size. The obtained relation indicates that Au nanoparticles grow mainly via the Ostwald ripening mechanism. Using anisotropic composite FeOOH/Ag particles as an example, it has been shown that the obtained information may be used to substantially decrease the duration of the synthesis of metal nanoshells on diverse cores.

Amphiphilic complexes of 1-hexadecyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide (D-16) with nitrates of transition metals (La³⁺, Cu²⁺, Ni²⁺, and Co²⁺) have been synthesized and characterized. Absorption spectrophotometry in the UV, visible, and IR regions, as well as ¹Н NMR spectroscopy, have been employed to study their spectral properties in water and organic media. Adsorption properties and micelleforming and solubilizing abilities of the amphiphilic complexes have been investigated in aqueous solutions by the methods of tensiometry and conductometry, as well as by solubilization of a water-insoluble dye (Orange OT). The values of the critical micelle concentration, adsorption parameters at a water/air interface, and solubilization capacity S of complex micelles have been determined. It has been shown that the most pronounced decrease in the critical micelle concentration (as large as two to three times) and differences in the adsorption characteristics and the S values as compared with those of ligand D-16 are observed for complexes of copper and lanthanum. The S values for these complexes are 1.3−2.5 and 3−6.5 times higher than those for D-16 and cetyltrimethylammonium bromide, respectively.

Composite poly(ethylene terephthalate) track membranes containing immobilized silver nanoparticles with the aim of using them for surface-enhanced Raman scattering spectroscopy have been obtained and studied. A dispersion of negatively charged silver nanoparticles has been synthesizes by the method of pulsed electrical discharge between silver electrodes immersed in distilled water. To ensure the electrostatic deposition of nanoparticles onto the track membrane surface, it has been modified with polyethyleneimine. The composition and morphology of the surface of the obtained composite membranes have been studied by X-ray photoelectron spectroscopy and scanning electron microscopy. Aggregation of nanoparticles on the surface has been analyzed. The coefficient of Raman-scattering enhancement has been determined by the example of rhodamine 6G molecules adsorbed on a membrane with immobilized silver nanoparticles.

Variations in the aggregation number of spherical micelles are considered within the micellization theory based on the law of mass action. The mechanism of micellization in a polydisperse aggregated system and the transition to a monodisperse model are explained. A relation between aggregation numbers and chemical potentials of molecules or ions is determined using the curve for equilibrium distribution of aggregates over the aggregation numbers. It is shown that the aggregation numbers of nonionic surfactants unambiguously grow with concentration; however, such a conclusion cannot be drawn for ionic surfactants. For the explicit concentration dependence of the aggregation number, two versions of an analog of the Langmuir equation are proposed to be used, i.e., versions comprising the total surfactant concentration and the concentration of monomers. Comparison with experimental data is carried out by the example of conventional surfactants, namely, sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.

The physicochemical properties and structure of moxifloxacin‒methyl-β-cyclodextrin complex have been studied by UV spectroscopy, FTIR spectroscopy, and computer simulation. The optimal conditions for the formation of the complex have been determined, and the dissociation constant of the complex in acidic media (K_dis = (5.0 ± 0.3) × 10^–5 М) has been obtained. It has been found that complexation significantly slows down the release of the drug in acidic media. Experimental results are in good agreement with computer simulation data. The following mechanism of complex formation has been proposed: the incorporation of the aromatic fragment of moxifloxacin into the cavity of methyl-β-cyclodextrin is followed by additional stabilization of the complex via multiple hydrophobic interactions and hydrogen bonding.

The Monte Carlo method has been employed to simulate the nucleation of condensed water phase from vapor at 260 K on a crystalline silver-iodide surface containing a defect in the form of a nanoscopic spot with a random distribution of ions. The free energy and work of formation of a nucleus have been calculated in the bicanonical ensemble at the molecular level as functions of nucleus size; computer images and spatial correlation functions of molecules have been obtained. The presence of a defect with a disordered (amorphous) structure, on the one hand, entails local destructions of a monomolecular film, but, on the other hand, shifts the onset of the adsorption process toward lower vapor pressures by several orders of magnitude. Under the conditions of a growing condensate film, the defect leads to its thermodynamic stabilization and a decrease in the barrier of the formation of subsequent layers, thereby weakening the known effect of the hydrophobicity of monomolecular films on crystalline surfaces with hexagonal structures. The factors that predetermine the abnormally high efficiency of silver-iodide particles as stimulators for atmospheric-moisture nucleation at negative Celsius temperatures seem to be the presence of extended defects on the surface of aerosol particles in combination with the hexagonal structure of their crystal lattice, the optimum magnitude of direct interactions between water molecules with ions of the crystal surface layer, and the collective domain-formation effects that result from a relatively high polarizability of iodine ions.

The application of surfactants in the chemical etching of track membranes enables one to control their pore shape. To find out the mechanism of the surfactant action on the track etching in the nanometer range of pore sizes, the adsorption of a nonionic surfactant (polyoxyethyelene-4-nonylphenyl ether) on porous and nonporous poly(ethylene terephthalate) films has been studied. The experimental results have been analyzed in comparison with the data previously obtained on the adsorption of an anionic surfactant on similar films. It has been concluded that the behaviors of anionic and nonionic surfactants in negatively charged pores about 100 nm in radius are strongly different due to the electrostatic exclusion of co-ions from the pores, which is of significance only for compounds dissociating into ions.