Vol 80, No 6 (2018)

Using the modified Poisson–Boltzmann theory, which includes restrictions on the maximum attainable concentration of ionic species in the solution Cmax, determined by their effective dimensions, the spatial distributions of the electric potential and ions between spherical nanoparticles immersed in a 1 : 1 electrolyte solution have been investigated. It has been found that the pattern of the aforementioned distributions is governed by surface electric potential ψ_s of the particles, interparticle distance, and maximum concentration C_max. Between weakly charged particles, the electric potential profiles are almost independent of the value of C_max. As absolute value \(\left| {{{\psi }_{{\text{s}}}}} \right|\) increases, an effect associated with the ionic sizes manifests itself; i.e., for high \(\left| {{{\psi }_{{\text{s}}}}} \right|\) values, the electrical double layer expands into the solution, and, the lower C_max (the larger the effective size of counterions), the higher the degree of expansion. This behavior is related to the fact that, near a particle surface, the profile of counterion concentration is developed with the following distinctly pronounced regions: the region of a plateau with width l_c, which corresponds to the condensed part of the profile with C(x) = C_max for 0 < x < l_c, and the diffusion region with C(x) < C_max for x > l_c. When highly charged particles approach to each other up to such distances that their electrical double layers appear to be strongly overlapped, coions are completely displaced from the interparticle electrolyte layer, and only counterions are present in the layer.

Experimental data have been obtained on the complete diffusion evaporation of a sessile microdroplet of an aqueous 1-propanol solution on a hydrophobized polished quartz substrate in air at atmospheric pressure and room temperature. During the evaporation of the sessile droplet, time dependences have been determined for its key thermodynamic and geometric parameters, i.e., the contact angle, base surface area, and volume. It has been revealed that the character of time variations in the contact angle depends on the initial alcohol concentration in the droplet and air humidity. At a high alcohol concentration and a low air humidity, the droplet contact angle monotonically decreases throughout the evaporation process. The contact angle of a solution droplet with a prevailing content of water varies in several stages. In this case, the monotonic reduction in the contact angle is, at a certain moment, replaced by a stage of its growth. The comparison of the maximum contact angle of an evaporating droplet with the contact angle of a sessile droplet of pure water enables one to determine the amount of alcohol in a studied droplet by the end of this stage. The residual alcohol amount governs the subsequent evolution of the droplet up to its complete evaporation.

Results of calculating diffusion coefficients of ionic surfactants as functions of their concentration in micellar solutions have been analyzed within the framework of the quasi-chemical variant of the law of mass action. Diffusion coefficients have been evaluated for an ideal mixture of monomeric molecules and micelles, as well as taking into account the correction for the variability of the aggregation number of a surfactant using aqueous solutions of hexadecyltrimethylammonium bromide, hexadecylpyridinium chloride and bromide, and alkyltriphenylphosphonium bromides as examples.

Classical full-atomic molecular dynamics has been employed to study the effect of ligand environment on the dipole moments of colloidal quantum dots of cadmium selenide. Using trioctylphosphine oxide, hexadecylamine, and octadecylphosphonic acid as examples, it has been shown that the presence of uncompensated electrical charge at the polar group of a ligand molecule is the primary factor, while the distribution of point charges over the electrically neutral group is of secondary significance. Dipole–dipole interactions between CdSe nanoparticles make the greatest contribution upon the formation of dense layers, while, in solutions, this contribution is low as compared with the ordinary thermal motion.

The micellization theory that I previously developed on the basis of the definition of critical micelle concentration (CMC) using the constant of the law of mass action as applied to ionic surfactants (Colloid. J., 2016, vol. 78, p. 669) has been supplemented and refined. The problem concerning the relative positions of the maximum in the surface-active ion concentration and CMC as functions of aggregation number has been solved in the general form. It has been shown that the curves of these functions may intersect. On the side of smaller aggregation numbers relative to the intersection point, the maximum lies above the CMC, while, at larger aggregation numbers, it is, on the contrary, located below the CMC in the concentration axis. The influence of the type of an electrolyte on this effect has been studied by the example of an ionic surfactant. If the charge of a counterion is higher than the charge of a surface-active ion, the maximum is located above the CMC; otherwise, it is below the CMC. At the same time, the question of the formulation of the law of mass action for ionic surfactants with multivalent ions has been answered. It has been shown that the definition of CMC via the constant of the law of mass action must, in this case, comprise stoichiometric coefficients. The theory has been formulated under the condition of a constant aggregation number in the vicinity of CMC. In addition, an ideal behavior of a mixture of monomers and micelles is assumed, which is inherent in ionic surfactants with rather low CMCs.

Capillary electrokinetics has been employed to study the effect of salt cation charge on cationic polyelectrolyte adsorption by the negatively charged surface of fused quartz. It has been found that polyelectrolyte adsorption values decrease with a rise in the cation charge and increase with salt concentration. Salt cation charge influences the conformation of polyelectrolyte molecules in an adsorbed layer. High values of the charge reversal suggest a significant entropy contribution to the adsorption mechanism.

A cell model formulated in terms of the thermodynamics of nonequilibrium processes has been proposed for an ion-exchange membrane. The membrane is assumed to consist of an ordered set of porous charged spherical particles placed into spherical shells filled with a binary electrolyte solution. The problem of determining all the kinetic coefficients in the Onsager matrix has been set and the general solution of the boundary value problem has been obtained for the cell. The consideration has been realized within the framework of small deviations of system parameters from their equilibrium values upon imposition of external fields. The boundary value problem has been analytically solved for determining the hydrodynamic permeability of the membrane under the Kuwabara boundary condition imposed on the cell surface. It has been found that, when the volume charge disappears, the equation for the permeability is transformed into the equation derived previously for an uncharged membrane. It has been shown that the hydrodynamic permeability (direct kinetic coefficient) of a cation-exchange membrane grows, tending to its limiting value, with increasing electrolyte concentration and decreases with a rise in the exchange capacity of ion exchanger grains.

A procedure has been proposed for the self-assembly of carbon nanotubes into arrays with the use of coordinating molecules and the development of secondary porosity in the resulting supramolecular structures. Molecular dynamics has been employed to study the formation of such structures and determine the effective radius of pores formed in them. The average micropore sizes in the obtained supramolecular structures have been related to the sizes of coordinating molecules and their orientation with respect to nanotube surface. Adsorption of methane and hydrogen on such model systems has been calculated on the basis of the theory of volume filling of micropores. It has been shown that the porosity resulting from the organization of the nanotubes into arrays with the help of coordinating molecules makes it possible to accumulate methane and hydrogen at the level of the best model adsorbents.

It has been shown that the pretreatment of Celgard polyolefin membranes in the presence of peroxycarbonic acid, which is formed upon saturation of a hydrogen peroxide solution with carbon dioxide under high pressure, improves the characteristics of composites that are obtained on their basis by depositing a polydopamine layer via oxidative polymerization. As compared with the initial matrices that have not been subjected to the pretreatment, the pretreated matrices with the deposited polydopamine layer advantageously combine better wettability with polar media and an increased ion transport selectivity imparted to them in combination with the preserved high ionic conductivity. This effect is achieved due to a decrease in the effective pore diameter. The revealed positive effect of the pretreatment may be related to the oxidative activity of peroxycarbonic acid, which gives rise to the formation of anchor functional groups on the matrix surface. Apparently, these groups facilitate subsequent uniform deposition of polydopamine in all matrix regions, including deep and fine pores due to the high pressure applied.

Stabilization of Pickering emulsions with silica nanoparticles modified with nonionic surfactants, namely, Tween 20, Tween 40, and Tween 80; cationic cetyltrimethylammonium bromide; and anionic sodium oleate and oleic acid has been studied. The modification of positively charged Ludox CL nanoparticles and negatively charged Ludox HS-30 nanoparticles with sodium oleate and oleic acid has not led to the formation of stable emulsions at dispersion medium pH values of 2–8. The modification of Ludox CL nanoparticles with Tween 20, Tween 40, Tween 80, and cetyltrimethylammonium bromide also has not increased the stability of the emulsions. The stability of the emulsions stabilized with Ludox HS-30 nanoparticles has increased noticeably in the presence of Tween 20, Tween 40, Tween 80, or cetyltrimethylammonium bromide. These surfactants promote the formation of gel-like networks from aggregated Ludox HS-30 nanoparticles in the dispersion media of the emulsions, thereby leading to the formation of emulsions stable with respect to both coalescence and creaming.

Synthesis of silver nanoparticles using plant extract of Fumaria officinalis L., (AgNPs-E), as a reducing and capping agent, is presented in this study. Formation of AgNPs-E is monitored by measuring surface plasmon resonance (SPR) absorption band in the UV-VIS range 200–800 nm. Based on UV-VIS spectra, the SPR band of AgNPs-E obtained by synthesis at the room and boiling temperatures appeared at 438  and 440 nm, respectively. After 15 days of equilibration, the SPR band was slightly shifted by 6 nm indicating a good stability of the formed complex at room temperature. The bands in the range 200–400 nm, originating from π−π* and n−π* transitions in the extract, disappeared in the spectra of AgNPs-E after reduction. Morphology, size and shape of formed AgNPs-E were investigated by scanning electron microscopy. The presence of both elemental silver and its crystalline structure (face centered cubic type) in AgNPs-E was confirmed by the energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analyses. Particles size was determined by the XRD analysis: 20 ± 1 and 18 ± 1 nm at room (sample 1) and boiling temperature (sample 2), respectively. Fourier transform infrared spectroscopy (FTIR) assessments were carried out to identify biomolecules of F. officinalis responsible for the reduction and capping of the bioreduced silver nanoparticles and to predict their role in silver nanoparticles synthesis. FTIR spectroscopy indicated the activity of different functional groups during the synthesis by the changes in ν_C–OH, δ_OH, ν_C–O–C and (NH)C=O modes in the AgNPs-E spectrum relative to that of the extract. The AgNPs-E particles showed strong antibacterial activity against Staphylococcus aureus ATCC 25923, Bacillus cereus ATCC 11778, Bacillusluteus in haus strain, Bacillus subtilis ATTC 6633, Listeria monocytogenes ATCC 15313, Escherichia coli ATTC 25922, Pseudomonas aeruginosa ATTC 27853, Klebsiella pneumoniae ATTC 700603, Proteus vulgaris ATTC 8427, and antifungal activity against Candida albicans ATTC 2091.