The embedded atom model for liquid metals: Liquid gallium and bismuth

A method for calculating embedded atom potentials in liquid metals is suggested. The method uses diffraction structural data, density, bulk compression modulus, and thermal expansion coefficient close to the melting point. The method was applied to liquid gallium and bismuth at temperatures from their melting to critical points. The critical temperatures of these metals were estimated at 4940 and 4225 K. The other critical parameters were also determined. The self-diffusion coefficients were found to increase almost linearly as the temperature grew. The model allows changes in the structural characteristics of the metals when the temperature increases by several hundred kelvin units to be correctly described.     

Embedded atom model for liquid metals: Liquid iron

A method for calculating the embedded atom model potential suggested earlier for liquid Ga and Bi uses data on the structure and thermodynamic properties of metals close to their melting points. This method was applied to liquid iron at temperatures and pressures up to 5000 K and 360 GPa. Several iron models with the potential of the embedded atom model were constructed by the method of molecular dynamics at temperatures from 1820 to 5000 K and densities from 8.00 to 12.50 g/cm³. The thermodynamic, structural, diffusion, and viscosity properties of iron were calculated. The self-diffusion coefficients decreased almost linearly as the volume of the system became smaller. The conclusion is drawn that iron in the external region of the Earth’s core behaves as a liquid with self-diffusion coefficients of about ～10^-5 cm²/s and viscosity ～10^-3?10^-2 Pa s. At the boundary between the external and inner core regions, at densities of 11–12 g/cm³, iron has the properties of an amorphous phase and its self-diffusion coefficient becomes too low to be estimated by the method of molecular dynamics. Under the Earth’s inner core conditions, the embedded atom model of iron spontaneously crystallizes.     

The embedded atom model of liquid cesium

The embedded atom potential was calculated for cesium over the temperature range 323–1923 K at pressures up to 9.8 GPa from the diffraction data on the structure of the metal close to the temperature of fusion (T _f). The parameters of the embedded atom potential were adjusted using the data on the thermodynamic properties and structure of liquid cesium. The embedded atom potential well predicts the structural and thermodynamic characteristics of the liquid metal as the temperature increases along the liquid-vapor equilibrium line and under strong compression. The calculated potential energy and structure of liquid cesium closely agree with the experimental data at temperatures up to 1373 K. The calculated bulk compression modulus is close to its experimental values at all temperatures except 323 K. The self-diffusion coefficients increase as the temperature grows by a power law with an exponent close to 2 and satisfy the Stokes-Einstein equation. Deviations from experimental data at temperatures above 1400 K are explained by the metal-nonmetal transition that occurs as the density decreases.     

Modeling the molecular dynamics of liquid metals at high pressures: Liquid potassium

A way of building potassium models by molecular dynamics using the embedded atom model (EAM) is developed. The contribution from pairwise interaction is presented as power series at an interpartial distance. Embedded potential parameters are determined by the experimental dependence of pressure on volume for a static compression of potassium at 300 K to a pressure of 53 GPa (potential A). By using potential A to describe shock compression and choosing the appropriate temperature at given degree of compression (up to 40000 K at compression to 0.29 of initial volume) it is shown that the model pressure can be made equal to the pressure indicated by the Rankine-Hugoniot relations. The model energy is lower than the actual energy determined by the relations, and the difference in energies increases with temperature almost linearly; such growth corresponds to an excess average heat capacity of about 11.6 J/(mol K), compared to the model heat capacity. It is established that the reasons for this divergence are the inability of the EAM potential to describe the temperature dependency of metal properties precisely, and the appearance of an energy contribution upon heating that is dependent on temperature but not on atom coordinates. Adding another summand to the potential energy (which is dependent on temperature only) allows us to match the heat capacities of real potassium and the models. The dependence of potassium’s melting temperature on pressure is calculated. The calculated melting temperature at 41.2 GPa is 1231 K. Additional data (e.g., the actual temperature on the Rankine-Hugoniot curve and precise quantum mechanics calculations of heat capacity at extreme conditions) is required to eliminate potential ambiguity.     

The embedded atom model of molten lead

The method for calculating the embedded atom potential for liquid metals from the diffraction structural data close to the melting point was applied to lead at temperatures from 613 to 20000 K. The embedded atom potential parameters were adjusted using the data on the lead structure at 613–1173 K, the thermodynamic properties of lead over the temperature range 613–2000 K, and the results of shock wave experiments. The embedded atom potential and molecular dynamics method allowed the structural characteristics of the liquid metal to be successfully predicted up to 1173 K. The calculated bulk compression modulus at 613 K was close to its actual value. The self-diffusion coefficients along the liquid-vapor equilibrium line increased as the temperature rose following the power law with the exponent close to 2.03. The properties of lead under extremal conditions were calculated up to the temperature 20000 K and density 20.721 g/cm³. At 1000 K and a density of 18.156 g/cm³, close agreement with the experimental pressure (101.5 GPa) was obtained. The potential found fairly well described the properties of crystalline lead. At the same time, the embedded atom potential adjusted to describe the properties of the crystalline phase only poorly described the properties of liquid lead at increased densities.     

Liquid—liquid extraction of transition metals with acylthioacetamides

The extraction behaviour of acylthioacetamides, R¹—CO—CH₂—CS—NR²R³, was studied for a series of transition metals. The highest distribution ratios were obtained with benzoylthioacetanilide. Complexes with acylthioacetamides which were completely substituted on the amide group were extracted slowly. The ranking of the extraction characteristics is related to the different pK_a values of the compounds used. Polymerization and solvation reactions can be excluded at metal concentrations of ? 10^-3 mol dm^-3 and extractant concentrations of ?10^-1 mol dm^-3, respectively. Decreasing extractability, Cu(II) > Zn(II) > Ni(II) > Co(II) > Cd(II), corresponds to the stability constants of the extracted metal chelates. Iron(III) cannot be extracted at pH ?3; Hg(II), Pd(II), and Au(III) are extracted readily over a reasonably wide pH range. The different extraction constants for zinc(II) and cadmium(II) permit an effective separation of these ions with benzoylthioacetanilide.     

Liquid—liquid sandwich-transfer extraction: optimization and analytical use

The feasibility of using liquid-liquid sandwich-transfer extraction methods as fast analytical tools was examined. The effect of surfactant, acid and base concentrations on the stability of the sandwich membrane was tested. The possibility of using this method in pre-concentration processes was investigated for chromium(VI). The rate of transfer extraction of radioactive ⁵¹Cr from aqueous sulphuric acid to sodium hydroxide solution was found to be rapid at reasonable concentrations (1%) of the sandwich reagent tri-n-octylamine.     

Account for electron contributions in embedded atom model for iron and nickel in molecular dynamics simulation

Potentials of embedded atom model (EAM) are calculated in analytical form for liquid iron and nickel. A series of models of these metals is created up to the temperature of 4000–4200 K. It is shown that these potentials enable us to obtain good agreement between the density of models and real metals; upon heating, however, the energy of the models is reduced relative to the real energies. It is established that the divergence of energies at ～4000 K is 32 kJ/mol for iron and 33 kJ/mol for nickel. It is shown that most of this divergence is determined by the contribution from the heat energy of electrons; after subtracting this contribution, a divergence of energies of 8.1 kJ/mol for iron and 14.3 kJ/mol for nickel remains. It is concluded that this divergence could be due partly to the insufficient adequacy of EAM potential in describing broad temperature ranges.     

The repulsive part of the effective interatomic potential for liquid metals

Structure factors obtained by neutron and X-ray measurements on five liquid metals, Na, K, Pb, Ga, and Zn, have been analyzed to extract information about the repulsive part of the effective interionic potential. It is shown that the repulsive part of the pair potential between the ions in liquid metals can be obtained from that part of the structure factor curve lying beyond the first diffraction maximum. A general form for a repulsive potential is presented. It is found that within this form a wide range of potentials will produce structure factor curves which are in agreement with the experimental data at large k values. This analysis uses an extension of the blip-function method for the calculation of the structure factor of a fluid whose interatomic potential is purely repulsive and short ranged. Several properties of the potentials found for each liquid metal studied are consistent with corresponding physical properties of the solids. The large uncertainty in the repulsive potentials extracted from the large k portion of the structure factors make it seem unlikely that quantitative information about the attractive and long range parts of the potential can be extracted from the small k portion of the structure factor.     

Collective excitations of embedded potassium clusters

The collective surface-plasmon excitation of potassium clusters embedded in different dielectric matrices has been studied within the time-dependent local-density-approximation and the jellium model. The matrix has been taken into account through a static dielectric constant in both the cluster ground state and in the determination of the dynamical susceptibility. We conclude that the change in the residual electron-electron interaction when the cluster is introduced in a matrix is a basic effect for the determination of the static polarizabilities and plasma frequencies. As we increase the surface-plasmon energy is shifted to the red and a tendency to saturation is obtained. The red shift is in good agreement with electron-energy-loss spectroscopy experiments.     

Corresponding-states correlation for the compressed liquid density of metals

Applying the previous correlation for the saturated liquid density of metals, we have developed a corresponding-states correlation for the prediction of their compressed liquid densities. The correlation needs the values of the melting and boiling point parameters of metals plus an adjustable parameter, used to predict the saturated liquid densities. In this work, we have shown that by employing the Tait equation together with the previous correlation, for saturated liquid densities, it is possible to develop an accurate method for the prediction of the compressed liquid density of metals. The agreement of the predicted density values with the experimental ones for alkali metals, mercury, bismuth, tin, and lead over a wide range of temperatures, from melting point up to several hundred degrees above the boiling point, and pressures ranging from the vapor pressure up to 4000 bar, is quite good. From 821 data points examined for the aforementioned metals the average absolute deviation for the calculated densities compared with experiment is 0.72%. The correlation is also examined against a number of existing regularities for the liquid phase.     

The use of High-Performance Liquid Chromatography for the Speciation of Organotin Compounds

This review of the use of high-performance liquid chromatography (HPLC) for the speciation of organotin compounds which are primarily of significance in the marine environment is divided into sections on the basis of the different HPLC modes of separation. However, it should be noted that such a classification does not exist in reality. For instance, in an ion-pair reversed-phase system the separation mechanism for the ionic solutes may be ion-pair partitioning, or ion exchange, or both. The relevant practical information (e.g. column type, mobile phase, method of detection and detection limit) is presented in tabular form. A brief overview of the reported detection methods is included, because the delay in development of an easily interfaced, specific and sensitive detector has hindered the use of HPLC for organotin speciation studies. The literature reviewed covers publications from 1977, the year of the first application of HPLC to organotin speciation, to April 1995.     

The use of the microtron for the activation analysis of pure metals and alloys

Oxygen and carbon concentrations up to 0.06 ppm and 0.03 ppm respectively were determined in high purity metals such as Fe, Cu, Nb, Mo, In and some others. Analytical procedures and results of the determination of alloying additives in a number of rare metal alloys are given. The relative standard deviation constitutes 0.02 to 0.04. Accuracy of the procedures used is confirmed by a comparison of experimental results with those of the other analytical methods. Experimental data on the sensitivity of determining 35 elements are reported on irradiating them with thermal neutrons and a bremsstahlung emitted by the microtron. For most elements (for radioactive isotopes having a half-life of above 2 min.) sensitivity of the analysis is 10⁻⁶ to 10⁻⁸, g, at the microtron current of 30 μA and irradiation time of less than 10 min.     

Preparation of a new C18 stationary phase containing embedded urea groups for use in high-performance liquid chromatography

A new C18 urea stationary phase was prepared by a single-step modification process through the reaction of ProntoSil spherical silica (3 microm, Bischoff) with the trifunctional alkoxysilane (CH3CH2O)3-Si-(CH2)3-NH-C(O)-NH-(CH2)17-CH3, prepared in our laboratory. The phase was characterized by elemental analysis and solid-state 29Si and 13C nuclear mangnetic resonance spectrometry. Chromatographic evaluations of the new phase in 150 x 3.9 mm HPLC columns involving the separation of different test mixtures, indicate good performance for both polar and basic mixtures and also show promising results for further applications.     

Liquid crystal-based sensors for the detection of heavy metals using surface-immobilized urease

In this study, a new method for the detection of heavy metals in aqueous phase was developed using liquid crystals (LCs). When UV-treated nematic LC, 4-cyano-4'-pentyl biphenyl (5CB) that was confined in the urease-modified gold grid was immersed in a urea solution, an optical response from bright to dark was observed under a polarized microscope, indicating that a planar-to-homeotropic orientational transition of the LC occurred at the aqueous/LC interface. Since urease hydrolyzes urea to produce ammonia, which would be ionized into ammonium and hydroxide ions, the main product of the photochemically degraded 5CB, 4-cyano-4'-biphenylcarboxylic acid (CBA), was deprotonated and self-assembled at the interface, inducing the orientational transition in the LC. Due to the high sensitivity and rapid response of this system, detection of heavy metal ions was further exploited. The divalent copper ion, which could effectively inhibit the activity of urease, was used as a model heavy metal ion. The optical appearance of the LC did not change when urea was in contact with the copper nitrate hydrate-blocked urease. After the copper-inhibited urease was reactivated by EDTA, a bright-to-dark shift in the optical signal was regenerated, indicating an orientational transition of the LC. This type of LC-based sensor shows high spatial resolution due to its optical characteristics and therefore could potentially be used to accurately monitor the presence of enzyme inhibitors such as heavy metal ions in real-time.     

The use of potassium alkynyltrifluoroborates in Mannich reactions

Potassium alkynyltrifluoroborates react with amines and salicylaldehydes in the presence of benzoic acid to generate highly functionalized amines. Ionic liquids such as butylmethylimidazolium tetrafluoroborate (BmimBF₄) are suitable solvents for the reaction.     

New generation of sterically protected C18 stationary phases containing embedded urea groups for use in high-performance liquid chromatography

New monofunctional C18 urea stationary phases with sterically protecting dimethyl and diisopropyl groups were prepared by a single step modification process. ProntoSil spherical silica (3 microm) was chemically modified with the monofunctional ethoxysilanes, [(3-octadecylurea)propyl]dimethyl and [(3-octadecylurea)propyl]diisopropyl ethoxysilanes. The phases were characterized by elemental analysis, infrared and solid-state 29Si and 13C NMR spectroscopies. Chromatographic characterizations of the new phases in 50x3.9 mm HPLC columns were performed by the separation of two different test mixtures, containing nonpolar, polar and highly basic compounds.