A. T. Arymbaeva

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.

Extraction-Electrophoretic Concentration of Gold by Reverse Mixed Micelles of Triton N-42 and AOT

A new method of gold concentration with a total enrichment factor of ∼104 has been developed. The method includes a liquid-liquid extraction of by reverse micelles; reduction of gold (III) to Au nanoparticles (with hydrazine monohydrate as a reducing agent) in the extracts and an electrophoretic concentration of the particles in a cell with parallel-plane electrodes. With increasing sodium sulfate concentration in the feed there was a decrease of water content for ordinary micelles and an increase of gold extraction. At a constant acid-salt background (3 mol/L HCl + 3.5 mol/L Na2SO4) concentration, AOT introduction into micelles led to a decrease in extraction. The obtained regularities are consistent with a hydrate-solvate mechanism. The electrophoretic mobility of gold nanoparticles in extracts has been determined. When AOT was introduced into the extracts after the stage of reduction the electrophoretic mobility of nanoparticles was considerably increased (from −1.8 × 10−11 to 1.8 × 10−10 m2/(V × s)) which made possible an additional stage of electrophoretic concentration with an enrichment factor of ∼1.5 × 103.

Temperature structural transformations of reverse micelles of oxyethylated surfactants

The dependence of the hydrodynamic radius of reverse micelles of Tergitol NP-4 on temperature (20–50°C), solubilization capacity (0–7 vol %), electrolyte composition (NH4NO3, KNO3, HNO3, NH4OH, and KOH), and electrolyte concentration (0–8 mol/l) in an aqueous pseudophase was studied by photon-correlation spectroscopy. The hydrodynamic radius of micelles was shown to decrease with increasing temperature regardless of the concentration and type of electrolyte, and spherical micelles were formed in the process. It was ascertained that temperature aligns the features of interaction between solubilisate and micelles due to dehydration of the surfactant molecules: the radius of spherical micelles at 50°C depended only on solubilization capacity.

Isothermal mass crystallization of ammonium and potassium nitrates from Tergitol NP-4-based water-in-oil microemulsions

A procedure is elaborated for preparing powders of ammonium and potassium nitrates with microsized particles and stable sols with particle sizes of 7–16 nm based on isothermal mass crystallization from Tergitol NP-4-stabilized water-in-oil microemulsions in decane. The crystallization process is studied by turbidimetry and photon-correlation spectroscopy. The isolated powders are characterized by scanning electron microscopy, while sols and microemulsions are studied by photon-correlation spectroscopy. Evaporation of water from microemulsion droplets upon stirring water-in-oil micellar solutions at 25–45°C is shown to be the reason for the salt crystallization. It is ascertained that the time of the onset of crystallization and the morphology of resulting particles depend on temperature, content of an aqueous pseudophase, and the nature of a salt. A desolubilization-emulsion hypothesis is proposed for explaining the formation of nanoparticle powders and organosols.

The preparation of gold nanoparticles in triton N-42 reverse micelles after preliminary concentration from acid sulfate-chloride solutions

The distribution of Au(III) between an acid (3 mol/l HCl) aqueous phase and a micellar Triton N-42 solution in n-decane was studied as a function of the concentration of Na2SO4 (0–3.55 mol/l). The high distribution coefficients (200–500) allow 50-fold absolute concentration to be performed at a 90% extraction. Au0 nanoparticles were obtained by the injection solubilization of a reducing agent (hydrazine) into micellar extracts. At low solubilization capacities (≤1 vol %) and high reducing agent concentrations (≥0.2 mol/l) in Triton N-42 micelles, stable systems of Au0 nanoparticles suitable for the spectrophotometric determination of gold were obtained. At high solubilization capacities and low hydrazine concentrations, reduction was accompanied by coagulation and sedimentation processes. These processes were studied by spectrophotometry and the static and dynamic light scattering methods.

Role of Dioctyl Sulfide in Micellar Preconcentration, Synthesis, and Coagulation of Gold Nanoparticles

Extraction preconcentration of gold(III) by mixed Triton N-42 plus dioctyl sulfide (DOS) micelles from acid sulfate-chloride solutions was studied. The distribution ratio increases from 200 to 1200 as the DOS concentration rises from 0 to 0.25 mol/L (positive synergetic effect). Analysis of extraction equilibria shows that mixed micelles are key contributors into gold(III) extraction compared to Triton N-42 micelles and DOS molecular species in n-decane. Introduction of DOS at the synthesis stage increases the rate constants for gold reduction by hydrazine and enhances the coagulation of nascent nanoparticle

Preparation and characterization of water-in-oil decane/AOT microemulsions containing silver and gold nanoparticles and large amounts of water

Stable microemulsions with water contents as high as 10 vol % have been obtained, including those additionally containing silver and gold nanoparticles. Especial attention has been focused on the influence of water and stabilizer contents on the structure of adsorption layers on nanoparticles. The properties of nanoparticles obtained via the traditional microemulsion synthesis have been compared with the properties of nanoparticles that have preliminarily been concentrated with the help of electrophoresis and dried. The electrophoretic concentration and drying of nanoparticles have been shown to improve the stability of their microemulsions. Microemulsions with the highest content of water have been studied to determine the occurrence of percolation and the influence of nanoparticles on their percolation temperature and electrical conductivity.

Solvent replacement (decane → hexane) in concentrated silver organosols stabilized with sodium bis(2-ethylhexyl) sulfosuccinate

The structure-related properties of silver nanoparticles synthesized in reverse micellar solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been studied in the processes of electrophoretic concentraton of nanoparticles, drying of concentrates, and redispersion in n-hexane and n-decane. It has been shown that the dispersity of nanoparticles and stability of organosols to aggregation remain preserved upon the replacement of a low-volatile solvent (decane) by a high-volatile solvent (hexane). The developed procedures enable one to easily regulate important technological characteristics of metal-based inkjet “nanoinks,” such as concentration and size of particles, as well as viscosity and drying rate of the inks.

Synthesis and Concentration of Organosols of Silver Nanoparticles Stabilized by AOT: Emulsion Versus Microemulsion

In this work, we tried to combine the advantages of microemulsion and emulsion synthesis to obtain stable concentrated organosols of Ag nanoparticles, promising liquid-phase materials. Starting reagents were successively introduced into a micellar solution of sodium bis-(2-ethylhexyl)sulfosuccinate (AOT) in n-decane in the dynamic reverse emulsion mode. During the contact of the phases, Ag+ passes into micelles and Na+ passes into emulsion microdroplets through the cation exchange AOTNaOrg + AgNO3Aq = AOTAgOrg + NaNO3Aq. High concentrations of NaNO3 and hydrazine in the microdroplets favor an osmotic outflow of water from the micelles, which reduces their polar cavities to ∼2 nm. As a result, silver ions are contained in the micelles, and the reducing agent is present mostly in emulsion microdroplets. The reagents interact in the polar cavities of micelles to form ∼7 nm Ag nanoparticles. The produced nanoparticles are positively charged, which permitted their electrophoretic concentration to obtain liquid concentrates (up to 30% Ag) and a solid Ag-AOT composite (up to 75% Ag). Their treatment at 250 °C leads to the formation of conductive films (180 mOhm per square). The developed technique makes it possible to increase the productivity of the process by ∼30 times and opens up new avenues of practical application for the well-studied microemulsion synthesis.

Silver and gold nanoparticles in AOT and Brij-30 reverse microemulsions in saturated hydrocarbons: Synthesis and characterization

The effect of the properties of solvent on the characteristics of microemulsions is studied in anionic (AOT) and nonionic (Brij-30) surfactants, which are widely used in reverse micelle synthesis of silver and gold nanoparticles. The model solvents used are normal alkanes having six to 16 carbon atoms, which differed considerably from one another in the key parameters of dispersion medium for metal-based ink, namely, viscosity and volatility. The prepared organosols of nanoparticles were characterized using spectrophotometry, transmission electron microscopy, photon correlation spectroscopy, and laser electrophoresis.