Structural and thermodynamic characteristics of ionic associates in vapors over sodium bromide and iodide

Nonempirical methods are used to calculate the geometric parameters, the frequencies of normal vibrations, and thermochemical characteristics of ions existing in saturated vapors over sodium bromide and iodide: Na₂X⁺, NaX₂⁻, Na₃X₂⁺, and Na₂X₃⁻ (X = Br, I). According to the calculations, Na₂X⁺ and NaX₂⁻ triatomic ions have a linear equilibrium configuration of D _∞h symmetry. Pentaatomic ions can exist in the form of three isomers: linear with D _∞h symmetry, planar cyclic with C _2v symmetry, and bipyramidal with D _3h symmetry. At a temperature of ∼1000 K, Na₃X₂⁺ and NaX₃⁻ pentaatomic ions are shown to be present in vapor mainly in the form of linear isomers. The energies and enthalpies of ion molecular reactions with the participation of the above ions are calculated, and the formation enthalpies of the ions are determined, Δ _f H ^o(0 K): 293±2 kJ/mol (Na₂Br⁺), 354±2 kJ/mol (Na₂I⁺), −536±2 kJ/mol (NaBr₂⁻, −458±2 kJ/mol (NaI₂⁻, 24±5 kJ/mol (Na₃Br₂⁺, 143±5 kJ/mol (Na₃I₂⁺, −810±5 kJ/mol (Na₂Br₃⁻, and −675±5 kJ/mol (Na₂I₃⁻.     

The thermodynamic characteristics of resolvation of calcium and cadmium ions in aqueous-ethanolic mixtures

The real and chemical thermodynamic characteristics of resolvation of calcium and cadmium ions in water-ethanol mixtures were determined by measuring the compensation voltages of Volta circuits by the Kenrick method at 295.15 K. The Harkins method was used to substantiate the assumption of a constant surface potential at the nonaqueous solvent/gas phase interface starting with a certain nonaqueous component concentration.     

Analysis of the thermodynamic characteristics of solvation of individual ions in ethanol using model concepts

Solvation of individual ions in ethanol are given by the sum of various contributions: electrostatic, cavity formation, interactions of ions with the solvent, and structural rearrangement of the solvent itself. An analysis of the influence of all these contributions on the temperature dependence of solvation of ions is presented.     

Molecular dynamic study of thermodynamic properties of water clusters containing sodium or chlorine ions

The molecular dynamic method with design is used to calculate the thermodynamic properties of isolated water clusters containing Na⁺ or Cl^-. The number of water molecules in the clusters is from 2 to 14. The size dependence of the microscopic analogs of pressure, isothermal compressibility, and surface tension of the clusters is determined. The effect of positive and negative ions on heteromolecular nucleation in vapor is discussed. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 6, pp. 1092–1102, November–December, 1997.     

The thermodynamic characteristics of complex formation between calcium ions and L-leucine in aqueous solution

Complex formation of L-leucine with calcium ions in aqueous solution was studied by potentiometric titration at 298.15 K and ionic strength values I = 0.5, 1.0, and 1.5 (KNO₃). The formation of the CaL⁺ and CaHL²⁺ complex particles was established and their stability constants were determined. The enthalpies of protolytic equilibria of leucine and formation of calcium ion complexes with leucine were determined calorimetrically at 298.15 K and I = 0.5 (KNO₃). The thermodynamic characteristics of complex formation between calcium ions and L-leucine were calculated.     

Control over structural-dimensional characteristics of tungsten disulfide particles in aerosol-assisted chemical vapor deposition

Solutions of ammonium thiotungstate in dimethylformamide were used to synthesize spherical tungsten disulfide particles with average radius of 500–100 nm by the method of aerosol-assisted chemical vapor deposition. Nanoparticles with composition close to stoichiometric tungsten disulfide are formed at pyrolysis temperatures not lower than 800°C. It was found that the average particle radius linearly decreases as the reagent concentration in solution becomes lower, and the nebulizer power has no effect within the range under study on the size characteristics and structure of the particles obtained. It was demonstrated that the particles have a layered structure that is formed in all probability by S–W–S packets, which must provide high antifriction properties of the material in its use as a high-temperature solid lubricant. The results obtained indicate that the size of tungsten disulfide particles can be controlled in a wide range in the course of the aerosol-assisted chemical vapor deposition. This may be of interest for developing a technology for creating high-temperature wear-resistant antifriction coatings.     

The thermodynamic properties of liquid and vapor in the cadmium-thallium-lead system

The liquid-vapor phase transitions boundaries were calculated on the basis of the values of vapor pressure of the components in the lead-bismuth system during the stepwise pressure decrease by one order of magnitude from 10⁵ down to 1 Pa. The emergence of azeotropic liquid under pressure lower than 19.3 kPa was ascertained. The emergence of azeotropic mixture near the lead edge of the phase diagram was concluded to be the reason for technological difficulties in the distillation separation of the system into the components in a vacuum.     

Heat capacity and thermodynamic functions of cadmium fluoride

Adiabatic calorimetry was used to measure heat capacities of cadmium fluoride in the range 5–340 K. Spline smoothing of the heat capacity versus temperature data allowed thermodynamic functions to be calculated within the range of the measurement temperatures. The thermal behavior of CdF₂ was studied and showed no phase transitions within 300–723 K.     

Improvement in thermodynamic characteristics of sodium acetate trihydrate composite phase change material with expanded graphite

In this study, three different volume expansion ratios of expanded graphite (EG) are prepared and investigated to enhance the heat transfer efficiency of the sodium acetate trihydrate (SAT) composites. A series of SAT composite phase change materials (CPCMs) with EG were prepared. The influence of volume expansion ratio and mass fraction of EG on thermodynamic characteristics of SAT CPCMs was examined, including thermal conductivity, phase change temperature, enthalpy, latent heat storage and release time, and the degree of supercooling. Results showed that SAT CPCMs can be absorbed adequately by EG, and EG could enhance the heat transfer efficiency effectively. But it also brought some problems with the addition of all the three volume expansion ratios of EG, such as the poor enthalpy and serious supercooling. Particularly, the situation gets worse with the increase in mass and expansion ratio of EG. Therefore, it is better to choose the EG with proper expansion ratio or reduce the proportion of the EG which possesses higher expansion ratio. Besides, thermal cycling test and thermogravimetric analysis revealed that the SAT CPCMs with 3 mass% EG showed a good thermal stability.

     

Optical-fibre sensing of fluoride ions in a flow-stream

The Alizarin Fluorine Blue method for the determination of fluoride has been adapted for use with optical fibres. The reagent was immobilized on a polymer matrix, Amberlite XAD-2, and the reflectance of this reagent phase was measured as a function of fluoride concentration by use of a flow-cell assembly and a bifurcated fibre-optic system. A linear response was obtained for 0.16-0.95mM fluoride at a pH of 4.1, with a response time of approximately 12 min.     

Sodium fluoroaluminates formed in the reaction between aluminum fluoride solution and crystalline sodium fluoride

The reaction of aluminum fluoride solution with crystalline sodium fluoride was investigated. Conditions for the formation of Na₃AlF₆ (cryolite), Na₅Al₃F₁₄ (chiolite) and NaAlF₄.H₂O were established. The hitherto presumed to be unstable NaAlF₄.H₂O was isolated and its X-ray diffraction data as well as thermal behavior were determined. The possibility to convert these compounds one into the other was outlined.     

The special features of the thermodynamic characteristics of hydration of univalent ions according to the reference interaction site model

The dependence of the Gibbs energy of hydration of univalent ions was calculated on the basis of integral equations for a molecular liquid. Calculations were performed in the approximation of the reference interaction site model (RISM). The relation between the energy of hydration of ions and local electrostatic field microinhomogeneities in solution was studied. It was shown that the best variant was the RISM with the hypernetted chain closure, partial inclusion of orientation effects, and semiempirical corrections for the excluded volume and the presence of H-bonds with solutes. Predictions based on the selected variant of the model were in close agreement with the experimental data. The analysis performed indicated directions for the improvement of the RISM method.     

Removal of fluoride ions using cuttlefish bones

Because of the high toxicity of fluoride to mankind, there is an urgent need to treat fluoride-contaminated drinking water to make it safe for human consumption. This work investigated the possibility of eliminating, by sorption, the excess of fluoride in overloaded water according to World Health Organization WHO recommendations. We tested the cuttlefish bone as an adsorbent material (available in Tunisia) for the defluoridation of water. Initially, we determined the optimal conditions of use (contact time, pH effect, adsorbent dose, initial fluoride concentration) of the cuttlefish bone on synthetic solutions of sodium fluoride. The second step was to verify the effectiveness of the sorption process on the cuttlefish bone by testing it on natural waters loaded with fluoride. The results obtained showed that sorption on the cuttlefish bone could be an effective method for the removal of fluoride. The efficacy of cuttlefish bone to remove fluoride from water was found to be 80% at pH 7.2, 1 h contact time, 15 g L⁻¹ adsorbent dose and 5 mg L⁻¹ initial fluoride concentration. Despite the different anions (Cl⁻ and SO₄²⁻) generally present in natural waters, a fluoride concentration in agreement with the norm (<1.5 mg L⁻¹) could be reached whatever the water treated. The regeneration of the cuttlefish bone was performed with a NaOH solution (10 g of cuttlefish bone/1000 mL NaOH 3 M). After 1 h of agitation, 95% of fluorides were desorbed. Following regeneration, the adsorbent can be used for further removal of fluoride.     

Adsorption of fluoride ions onto carbonaceous materials

The characteristics of fluoride ion adsorption onto carbonaceous materials were derived as adsorption isotherms at different temperatures and in different pH solutions. The fluoride ion was adsorbed into pores in carbonaceous materials produced from wood; the larger the specific surface area, the more fluoride ions adsorbed. Bone char was the most effective adsorbent. The composition of bone char includes calcium phosphate, calcium carbonate, and so on. This suggests that the phosphate ion in bone char was exchanged with a fluoride ion. Moreover, the mechanism of fluoride ion adsorption onto bone char is clearly chemical in nature because the amount of fluoride ion adsorbed onto bone char increased with increasing temperature and decreasing pH. The amount of fluoride ion adsorbed onto bone char was also shown to depend on the concentration of sodium chloride in solution because of the "salting-out" effect. The adsorption of fluoride ion onto bone char is endothermic. Bone char can be utilized to remove fluoride ions from drinking water.     

Calculation of thermodynamic solution functions in gas-liquid chromatography

The solubility of m- and p-isomers of xylene and fluorotoluene in aromatic stationary phases is investigated. It is shown that orientation interaction can be calculated from a lattice model, with the results compared with experiment. By calculating the orientation and dispersion forces it is possible to construct an enthalpy diagram for molecular rotation, on the basis of which the differences in heats of solution and rotational entropies of the isomers can be determined. An example is cited in which hydrogen bonding was found in systems containing fluorotoluenes. From increments in the heats of solution and rotational entropies of the homologous n-paraffins it is possible to determine the structural characteristics of the solvent, and the accuracy of the proposed structure can be evaluated by calculating the individual heats of solution. The presence of aromatic nuclei and aliphatic radicals in the molecule of the stationary phase can be determined from characteristic variations in the heat-of-solution increments. The size of the natural cavity in the solvent structure can be determined from the rotational entropy increment. It is shown that the selectivity of the stationary phase with respect to the n-paraffin homologs does not remain constant. Aromatic solvents give the highest selectivity.