Molecular dynamics simulations of monoclinic calcium apatites: A universal equation of state

Molecular dynamics simulations of monoclinic (P2₁/b) hydroxy- and chlorapatite were undertaken in the range 498 K < T < 1298 K, and for pressures up to 7.5 GPa. The all-atom Born–Huggins–Mayer force field, that had been previously used to successfully describe the room temperature isotherms of both compounds, was also used in this work. The isothermal sets of p–V data generated by simulation were each fitted to the three-parameter form of the isothermal Parsafar and Mason equation of state (EoS) with an accuracy better than 0.07%. Taking the temperature dependence of the coefficients into account, it was found that the MD data are satisfactorily reproduced by the universal EoS. The isothermal compressibility coefficient dependence with pressure can be described by a linear relation.     

Structural and vibrational studies of Li[Kx(NH4)1−x]SO4 and Li2KNH4(SO4)2 mixed crystals

Mixed crystals of Li[K_x(NH₄)_1−x]SO₄ have been obtained by evaporation from aqueous solution at 313 K using different molar ratios of mixtures of LiKSO₄ and LiNH₄SO₄. The crystals were characterized by Raman scattering and single-crystal and powder X-ray diffraction. Two types of compound were obtained: Li[K_x(NH₄)_1−x]SO₄ with x?0.94 and Li₂KNH₄(SO₄)₂. Different phases of Li[K_x(NH₄)_1−x]SO₄ were yielded according to the molar ratio used in the preparation. The first phase is isostructural to the room-temperature phase of LiKSO₄. The second phase is the enantiomorph of the first, which is not observed in pure LiKSO₄, and the last is a disordered phase, which was also observed in LiKSO₄, and can be assumed as a mixture of domains of two preceding phases. In the second type of compound with formula Li₂KNH₄(SO₄)₂, the room-temperature phase is hexagonal, symmetry space group P6₃ with cell-volume nine times that of LiKSO₄. In this phase, some cavities are occupied by K⁺ ions only, and others are occupied by either K⁺ or NH₄⁺ at random. Thermal analyses of both types of compounds were performed by DSC, ATD, TG and powder X-ray diffraction. The phase transition temperatures for Li[K_x(NH₄)_1−x]SO₄x?0.94 were affected by the random presence of the ammonium ion in this disordered system. The high-temperature phase of Li₂KNH₄(SO₄)₂ is also hexagonal, space group P6₃/mmc with the cell a-parameter double that of LiKSO₄. The phase transition is at 471.9 K.     

Improved generator coordinate Hartree–Fock method applied to generate Gaussian basis sets for the isoelectronic series of the atoms He to Ne

We have generated Gaussian basis sets (GBSs) for the neutral and the first 20 cations members of the isoelectronic series of each ground state atom from He to Ne with the improved generator coordinate Hartree–Fock (IGCHF) method. For all atomic species studied here, our total energy errors are smaller than those calculated by the original GCHF method using GBSs of the same sizes. The largest difference between our total energy results and those computed with a numerical Hartree–Fock approach is equal to 215 μhartree for Co¹⁷⁺. We also compare the ionization potentials obtained with the IGCHF with the corresponding experimental values. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 88: 252–262, 2002     

Comparative study on the use of Ir, W and Zr-coated graphite tubes for the determination of chromium in slurries of human scalp hair by electrothermal atomic absorption spectrometry

The use of Ir, W and Zr-coated graphite tubes, as permanent chemical modifiers, was studied for the determination of chromium in human scalp hair by ETAAS using the slurry sampling technique. The use of Mg(NO₃)₂ and Pd, as aqueous chemical modifiers, was also investigated and compared to the use of the permanent chemical modifiers. Scalp hair samples were pulverized using a Zr vibrational ball mill, (mean particle diameter of 0.8 μm) and suspended in ultrapure water. The lowest limit of detection, 44.5 μg kg^–1,was achieved for the use of W-coated graphite tubes. The repeatability of the overall procedures (slurry preparation and ETAAS determination) were 15.7, 14.5 and 16.7% for W- and Zr-coated graphite tubes and Mg(NO₃)₂, respectively. The methods were applied to several reference materials, CRM 397 (human hair), DOLT-1 (dogfish liver) and DORM-1 (dogfish muscle), and the results obtained were in agreement to the certified values. Finally, the methods were applied to several human scalp hair samples from healthy adults.     

Kinetic study of the 1,3,5-triphenyl-Δ2-pyrazoline/technetium(VII) system: Determination of Technetium

The interaction of technetium with 1,3,5-triphenyl-Δ²-pyrazoline (TPP) is examined and kinetic methods for the determination of technetium (0.01–1.2 mg l^?1) are developed; absorbances are measured at 678 nm. The optimal hydrochloric acid concentration for the reaction is 8.3 M, with a TPP concentration of 6.6 × 10^?5 M in 15 % (v/v) ethanol. Interferences by Cu(II), V(V) and Fe(III) are discussed. The method is applied to two synthetic nuclear fuels and to vegetation spiked with technetium.     

Comparative study of magnesium nitrate,palladium nitrate and reduced palladium for the direct determination of mercury in sea water by electrothermal atomization atomic absorption spectrometry

A comparative study of different chemical modifiers in graphite furnace atomic absorption spectrometry for the direct determination of mercury in sea water samples, in synthetic sea water sample of high (72.8%) and low 34.2%) salinity and in aqueous solutions, was carried out. The use of reduced palladium produces better results. The mixture of palladium nitrate and ascorbic acid, gives the best limit of detection (1.9 gl^–1). The use of reduced palladium and magnesium nitrate produced excellent recoveries (close to 100%) in the whole salinity range for all mercury concentration tested. The use of palladium nitrate alone or combined with magnesium nitrate gave good recoveries with respect to a real sea water sample for low salinities. The interference from the major components of sea water were completely removed by using reduced palladium and magnesium nitrate modifiers. Thus, a single calibration curve with synthetic sea water may be applied to the analysis of sea water samples of widely differing salinities.     

Nonequilibrium molecular dynamics of the rheological and structural properties of linear and branched molecules. Simple shear and poiseuille flows; instabilities and slip

Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edward's [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls.     

Direct determination of lead in blood by electrothermal atomic absorption spectrometry with L'vov platform and matrix modification

An electrothermal atomic-absorption procedure with the L'vov platform and a simple five- or ten-fold sample dilution with a matrix-modifier solution containing diammonium hydrogenphosphate, Triton X-100 and nitric acid, is described for the direct determination of relatively low levels of lead in heparinized blood. The graphite-furnace parameters and matrix-modifier composition are optimized. Sensitivity, imprecision, accuracy and detection limit are reported. Results obtained by standard addition for ten human blood samples (30–400 μg l^?1 lead) were confirmed by an extraction/flame atomic-absorption reference method. Differences in mean lead values ranged from 2 to 31 μg l^?1 with 5.1% mean relative difference. The mean relative standard deviations for consecutive and between-day determinations were 4.6 and 9%, respectively. Accuracy was verified by analyzing six bovine-blood standards certified for lead in the range 70–1100 μg l^?1; deviations of found concentrations from expected values ranged from 0 to 44 μg l^?1 with 4.3% mean relative error. Recovery experiments done with human blood gave 104% (90–121%) of the added lead. The method offers several advantages for routine application in comparison with the extraction/flame atomic-absorption procedure.     

Comparison of different chemical modifiers for the direct determination of arsenic in sea water by electrothermal atomic absorption spectrometry

A comparative study of different chemical modifiers for the direct determination of arsenic in real and in synthetic sea water samples of high (72.8 per thousand ) and low (34.2 per thousand ) salinity was carried out. The use of lanthanum chloride and magnesium nitrate (LOD=3.0 microg l(-1)) offers good recoveries for low salinities, while for high salinities an enhancement higher than 200% was obtained, rendering this method unsuitable for sea water samples. The use of silver nitrate (LOD=2.0 microg l(-1)) produces unfavourable analytical recoveries, around 150 and 300% for synthetic sea water of low and high salinity. Zirconium oxychloride (LOD=1.8 microg l(-1)) and palladium nitrate alone or combined with magnesium nitrate and reducing agents (LOD=1.1-1.3 microg l(-1)) produce analytical recoveries close to 100% for all arsenic concentration tried over all salinities; thus, a single calibration curve using aqueous standard solutions may be applied to the analysis of sea water samples over all salinities. Finally, the addition of reducing agents and magnesium nitrate to palladium does not improve the sensitivity.     

Mechanism of the oxidation of sulfides by dioxiranes: conformational mobility and transannular interaction in the oxidation of thianthrene 5-oxide

The detailed study of the oxidation of thianthrene 5-oxide (1) with methyl(trifluoromethyl)dioxirane (5b) in different solvents and in the presence of (18)O isotopic tracers is reported. Thianthrene 5-oxide (1) is a flexible molecule in solution, and this property allows for transannular interaction of the sulfoxide group with the expected zwitterionic 7 and hypervalent 10-S-4 sulfurane 9 intermediates formed in the oxidation and biases the course of the reaction toward the monooxygenation pathway.