A. I. Bulavchenko

Electrokinetic Potential of Nanoparticles in Reverse AOT Micelles: Photometric Determination and Role in the Processes of Heterocoagulation, Separation, and Concentration

A simple photometric method for determining the electrophoretic mobility of nano- and microparticles in reverse micelles and in solvents with a low dielectric permittivity (2-5) has been developed. The method is based on the use of a thermostatically controlled diaphragm-based optical cell (length 2 cm) with three vertical plane-parallel electrodes (2 x 3 cm; interelectrode gap, 0.3 cm) placed into a standard photocolorimeter. When an electrostatic field (100-600 V) is applied, the particles begin to move away from the electrode of the same polarity. The path traveled by the particles for a given time (2-30 s) is calculated from the change in the optical density of the solution in the near-electrode zone. The electrophoretic potential of nanoparticles in the model systems, calculated from the values of electrophoretic mobility by Huckel-Onsager theory, varied from 70 (Ag nanoparticles in AOT micelles in decane) to -73 mV (aggregated SiO(2) nanoparticles in a decane-chloroform mixture). Calculations by the classical Deryaguin-Landau-Verwey-Overbeek (DLVO) theory determined the contribution of the electrostatic interaction to the stability of the studied systems. We have shown that the surface charge of nanoparticles permits: (1) an electrophoretic concentration of the charged nanoparticles (Ag) with an enrichment factor of up to 10(4), (2) the separation of nanoparticles with zero (C(60)) and a high (Ag) electrokinetic potentials, and (3) the formation of electrostatically bound aggregates (Ag-SiO(2)) through the heterocoagulation of oppositely charged particles.

Colloidal solutions of niobium trisulfide and niobium triselenide

Exfoliated nanomaterials, such as graphene and related few-layered materials, are now widely studied for electronic devices, electrodes and composites, so it is desirable to demonstrate exfoliation of a wider number of layered materials. We have shown that bulk niobium trichalcogenides NbS3 and NbSe3 may be stably dispersed in a number of common organic solvents by ultrasonic treatment. The most concentrated dispersions are obtained in alcoholic media (up to ∼0.443 g L−1). The colloids contain thin well-crystallized nanoribbons of NbS3 and NbSe3. Filtration or spraying of the colloids produces strongly textured thin films with good conducting properties.

The kinetics of synthesis and mechanism of coagulation of gold nanoparticles in Triton N-42 reverse micelles

The reduction of Au(III) with hydrazine monohydrate in micellar Triton N-42 solutions was shown to be an autocatalytic reaction. Its rate constants were calculated. The growth of a gold nucleus proceeded as a result of surface reduction until the polar micelle nanocavity was completely filled. Calculations according to the Derjaguin-Landau-Verway-Overbeck theory showed that the fate of nanoparticles formed depended on interparticle interaction energy. At a small radius of particles, high surface potential, and fairly thick surfactant surface layer, stable systems were formed. The coagulation zones were calculated depending on the structural parameters of nanoparticles and micelles. If a nanoparticle grew larger than 6.1 nm at a surface potential lower than 10 mV and surface layer thickness ∼1.6 nm, the potential well depth exceeded 3/2 kT in magnitude, and coagulation occurred in the system.

Metal Concentration by Reversed Micelles

Abstract A new method of metal concentration by reversed micelles is described. It differs from traditional solvent extraction concentration in the procedure of metal stripping from an organic phase. Reversed micelles are broken by heating or by adding a polar organic diluent. A concentrated metal solution is produced at surfactant disaggregation: there is no need to use a water-stripping solution. Coefficients of concentration for Fe(III), Pt(IV), and Pd(II) chloride complexes as high as 150 to 1800 have been seen when nonionic oxyethylated surfactants are used.

Colloidal dispersions of tantalum trisulfide: syntheses and characteristics

Stable colloidal solutions with the TaS3 content ≈ 1 mmol L–1 were obtained by the ultrasonic treatment of TaS3 in MeCN and DMSO. The detailed analysis of the particle size distribution in colloidal dispersions and their stability was carried out using data of atomic force microscopy and photon correlation spectroscopy for TaS3/MeCN. The data of powder diffractometry and Raman spectroscopy show that the films prepared from colloidal dispersions of TaS3 retain the crystal structure of the initial tantalum trisulfide.

Kinetics of dissolution of silver nanoparticles inside triton N-42 reversed micelles

Our spectrophotometric study of the kinetics of dissolution of silver nanoparticles by nitric acid inside inverted micelles of Triton N-42 (a nonionic surfactant) verified the universal character of the mechanism for this type of process, which includes the interaction of surface metal atoms with an oxidizer in two routes: either with (autocatalysis) or without newly formed ionic species of the oxidized metal. Effective rate constants for both routes are independent of the value of solubilization capacity (Vs/Vo is the ratio of the volume of the dispersed aqueous phase to the volume of the micellar solution); the solubilization capacity is useful to control the micelle and particle sizes: k1 = 0.018±0.003, 0.010±0.003, and 0.012±0.003 s−1 and k2 = 2.5±0.8, 1.5±1.1, and 1.4±0.4 L/(mol s) for 100 Vs/Vo = 1, 2, and 3%, respectively.

Concentration of Platinum(IV) From Acidic Chloride andSulfate–Chloride Aqueous Media With Reversed Micellar Solutions ofOxyethylated Surfactant

The possibility of employing reversed micelles of oxyethylated surfactants to concentrate platinum from acidic aqueous media was investigated. A procedure for reversed micellar concentration is described. Platinum concentration was effected by means of desolubilization in a non-traditional back-extraction stage by dilution of the reversed micellar solution with chloroform or a mixture of chloroform with hexane. In acidic sulfate–chloride media the distribution ratio of Pt IV increased to about 10 2 –5 × 10 3 in the presence of Br - and I - as complexing agents. In chloride media the distribution ratio could be increased to about 2 × 10 3 by using SnCl 2 . The maximum recovery of platinum from the extract did not exceed 85%. With I - as a complexing agent, no back-extraction could be performed. With this back-extraction procedure, the Pt IV concentration factor varied from about 10 2 to 10 3 depending on the aqueous feed composition. It was shown spectrophotometrically that Pt IV complex species were the same in the feed, extract and desolubilized aqueous solution. The possibility of the spectrophotometric determination of platinum with SnCl 2 directly in the reversed micellar solution is demonstrated.

Properties of Conducting Films of Electrophoretic Concentrates of Silver and Gold Nanoparticles in AOT Surfactant

Liquid concentrates of silver and gold nanoparticles with 1–2 M metal concentrations were isolated by electrophoresis in a capacitor-type cell from AOT reverse micellar solutions in n-decane. The electrophoretic concentrates and the starting micellar solutions were characterized by nonaqueous electrophoresis, transmission electron microscopy, photon correlation spectroscopy (dynamic light scattering), and spectrophotometry. The hydrodynamic diameter of silver and gold nanoparticles was 13.2 and 8.6 nm, respectively; the ζ-potential was 70 and 13 mV. The drying of the concentrates on glass and silicon substrates and subsequent treatment with a 30% solution of water in ethanol gave mirror conducting Ag, Au, Ag-Au, and Au/Ag films containing on the average 80% metal and 20 wt % AOT. The film structure, morphology, and composition were studied by scanning electron microscopy (SEM) and energy dispersion analysis (EDX).

Kinetics of oxidative dissolution of gold nanoparticles in triton N-42 reversed micelles

The dissolution of gold nanoparticles as a result of a reaction with dispersed aqueous solution of Cl− and H2O2 solubilized in Triton N-42 reversed micelles (an oxyethylated surfactant) in n-decane was studied photocolorimetrically. An adequate description of the process kinetics is provided by the following autocatalytic scheme (in a Cl− + H2O2 excess): Au0 → Au+, Au0 + Aun+ → 2Au3+, n = 1 and 3. The reactions involve the formation and redox decomposition of intermediate complexes with the oxidizer on the surfaces of metallic particles. Dimensional factors associated with gold particles (the surface and reactivity of gold particles change during the process) and micelles (as nanoreactors) affect the process kinetics. The first effect is taken into account when data processing is performed in terms of the current number and effective charge density of surface gold atoms; therefore, it does not affect the observed rate constants kd1 and kd2. The second effect makes kd1 and kd2 dependent on the micelle size through changing the quality of the aqueous medium and the reactivity of the reaction components, mainly the activity of water and its hydration and adsorption abilities.

Effect of the organic solvent polarity on the structure of the gold nanoparticle adsorption layer as observed by photon-correlation spectroscopy

Spherical gold nanoparticles with the metal core of 5nm in diameter (according to the data of the transmission electron-microscopy) are prepared by the reduction of chloroauric acid with hydrazine monohydrate in reverse micelles of sodium bis-(2-ethylhexyl sulfosuccinate) (AOT). The photon correlation spectroscopy (PCS) method is used to determine the structure of the gold nanoparticle adsorption layer after their concentration and redispersion in AOT solutions in organic solvents with different polarity (in a series of decane, toluene, and chloroform). It is shown that in case of the abovementioned solvents at low AOT concentrations a gold nanoparticle is surrounded by a monolayer consisting of AOT molecules. When the concentration increases up to 1 M, the absorption layer thickness sharply increases; an increase in the organic solvent polarity enhances this increase. An increase in the temperature (from 20°C to 55 °C) leads to a partial and reversible decrease in the absorption layer thickness.