M. G. Demidova

ICP AES determination of platinum group elements and gold in collective extract and strip product solution in analysis of geological samples

ICP AES techniques for the determination of all platinum group elements (PGEs) and gold in extract and collective strip product solution have been developed on the basis of the interrelation between the main stages of the analysis. The ICP AES permissible contents of base metals interfering with the PGE and gold determination have been established. The extraction system (a mixture of p-alkyl aniline with petroleum sulfides in o-xylene or in the post-xylene fraction) is shown to be applicable to ICP and ensures a 10– to 20–fold concentration of PGEs and gold and the required (106) degree of decontamination of the extract from the matrix elements. A new two-step method for a collective back extraction of PGEs and gold with ammonia and thiourea solutions is proposed. The validity of the techniques is confirmed by the analysis of six standard samples and by the agreement of the results with those obtained by the independent ETAAS method. The techniques were tested on raw materials and copper-nickel plant products to determine all PGEs, including osmium, and gold over the range of their contents from 10−7 to 10−4%.

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.

Synthesis and electrophoretic concentration of nanoparticles of CdS in reversed micellar solutions

Stable organosols of cadmium sulfide are obtained via the ion exchange reaction of cadmium nitrate with sodium sulfide in reversed micellar solutions of Brij-30 in n-decane and are characterized by means of spectrophotometry, luminescence, photon correlation spectroscopy (PCS), and transmission electron microscopy (TEM). It is established that adding anionic surfactant AOT to organosols produces double electric layers on the surfaces of nanoparticles and contributes to an additional 50-fold electrophoretic concentration. Electrophoretic concentrates of cadmium sulfide nanoparticles (0.5 M) are obtained in cells with vertical orientation of the electrodes and periodic changes in polarity. The average diameter of the nanoparticles according to TEM data is 5 nm, considerably less than the hydrodynamic diameter found by PCS (70 nm), testifying to the complex structure of a mixed adsorption layer surrounding a nanoparticle.

The size, charge, and electrokinetic potential of gold nanoparticles in solutions of sodium bis(2-ethylhexyl) sulfosuccinate in n-decane-chloroform mixtures

The hydrodynamic diameters and electrokinetic potentials of gold nanoparticles in n-decane, chloroform, and mixtures thereof have been studied as functions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) concentration. At AOT concentrations in n-decane below 2 mM, gold nanoparticles are covered with AOT monolayer and do not reveal electrophoretic mobility. At a concentration of 0.3 M that, according to the data of photon-correlation spectroscopy, corresponds to the onset of adsorption of AOT micelles on the nanoparticle surface, the ζ-potential reaches a maximum value of 190 mV; then, it decreases with increasing surfactant concentration. A rise in the volume fraction of chloroform in decane-chloroform mixtures drastically increases the ζ-potential even at low AOT concentrations. The obtained results have been analyzed in terms of the Debye-Hückel theory.

Microemulstion synthesis of powders of water-soluble energy-saturated salts

The feasibility of preparing energy-saturated salts (NH4NO3, KNO3, and NaBH4) in powders with various particle sizes in microemulsion systems based on oxyethylated surfactant Tergitol NP-4 has been demonstrated. Powders were isolated by destroying microemulsions with acetone. The regions of micellar synthesis have been determined depending on the solubilization capacities and concentrations of the reagents and salts at a fixed Tergitol NP-4 concentration (0.25 mol/L). The morphologies and particle sizes of the thus-prepared salt powders were characterized by scanning electron microscopy (SEM), X-ray powder diffraction, differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetry; the hydrodynamic radii of microemulsions were characterized by photon-correlation spectroscopy.

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.

Generation of silver and lead chromate nanoparticles in Triton N-42 and AOT reversed-micellar solutions

A process is proposed for manufacturing silver and lead nanoparticles in Triton N-42 and AOT solutions in decane. The completion of nanoparticle formation and solution stability were studied spectrophotometrically as functions of the parameters of reversed-micellar systems (the nature and proportion of the surfactant, solubilization capacity, reagent concentrations, and their ratios). Atomic absorption spectroscopy (AAS) and X-ray powder diffraction (XRD) showed that the nanoparticles isolated from Triton N-42 and AOT solutions compositionally correspond to and are isostructural to Ag2CrO4 and PbCrO4. Mean nanoparticle sizes are 15–30 nm as determined by transmission electron microscopy (TEM).

Mechanism of Formation of Cadmium Sulfide Nanoparticles on Polystyrene Supports from Ammonia—Thiourea Solutions

The nucleation of CdS particles on the surface of polystyrene supports from aqueous ammonia—thiourea solutions was studied. The increment in the mass of nanoparticles on the support with time was determined. The degree of binding of Cd(II) ions into sulfide on the surface (support area 14 cm2, solution volume 3.7 cm3) after 3 h at 50°C was 2.2% of their total amount. The results agreed well with those of model calculations for spherical (for small times) and ellipsoid particles at a constant surface concentration. The nanoparticles that formed in solution were not involved in the formation of surface films on polystyrene supports. The photoluminescent properties of the obtained films were studied.

A study of sodium bis(2-ethylhexyl) sulfosuccinate adsorption on silica from silver and gold organosols

Adsorption of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT) and silver and gold nanoparticles on SiO2 from heptane solutions has been investigated by spectophotometery and CHN analysis. The adsorption isotherm of Aerosol OT is described by the Langmuir equation. Sorption capacity (2.4 × 10–4 mol/g), sorption constant (9.0 m3/mol), and area per Aerosol OT molecule in an adsorption layer (0.83 nm2) have been determined. It has been shown that, at Aerosol OT concentrations lower than 6 × 10–4 M, gold and silver ((0.3–6) × 10–4 M) are simultaneously extracted by 98%; however, they have no effect on the adsorption and determination of the surfactant.