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  • Author: P. Vogel x
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First results of a molecular dynamics study of an aqueous LiCl solution are reported. The system investigated consisted of 216 particles, 198 water molecules, 9 lithium ions, and 9 chloride ions. The calculations lead to fair agreement with the static properties of the first hydration shells of the ions as derived from X-ray and neutron diffraction studies, and with kinetic properties as derived from NMR measurements. A model for the motion of the water molecules in the first hydration sphere of Li+ is tentatively proposed.

Abstract

Results of a molecular dynamics study of aqueous solutions of LiJ, LiCl, NaCl, CsCl and CsF are reported. The basic periodic box contained 200 water molecules, 8 cations and 8 anions, equivalent to 2.2 molal solutions. Static properties of the first hydration shells of the ions are discussed in detail on the basis of radial pair correlation functions, average potential energies of the water molecules and pair interaction energy distributions. The calculations lead to the conclusion that in the first hydration shells a lone pair orbital of the water molecule is directed towards the cation while a hydrogen atom points towards the anion. In the five alkali halide solutions investigated ion pairing occurs only with CsF. The hydration numbers, when defined as the volume integrals of the ion-water radial pair correlation functions up to the first minimum, increase with increasing ion size and depend on the size of the counterion. The water-water interactions in the solutions show not only features of pure water at elevated temperatures but also of pure water under compresion. The agreement between calculated and measured self diffusion coefficients is still insufficient.

Abstract

Results of a molecular dynamics study of a 0.55 molal aqueous NaCl solution are reported. The basic periodic box contained 200 water molecules, 2 sodium ions and 2 chloride ions. The calculated properties of this solution are compared with those obtained previously for a 2.2 molal NaCl solution. The formation of second hydration shells, an increase of the number of water molecules in the first hydration shells, and a release of internal pressure are the main changes connected with a decrease of the concentration.

Abstract

Results of a molecular dynamics study of an aqueous CsCl solution are reported. The system consisted of 216 particles, 200 water molecules, 8 cesium ions and 8 chloride ions and was run over 8000 time steps equivalent of 9 · 10-13 sec. On the basis of radial pair correlation functions, average potential energy of the water molecules and pair interaction energy distribution the static properties of the first hydration shells of the ions are discussed in detail. The self diffusion coefficient for the water molecules is calculated and compared with NMR measurement as well as with molecular dynamics calculations for pure water at elevated temperatures and pressures.

The oxygen isotope fractionation factors between the hydration complex of the alkali ions in the gas phase and a free water molecule have been computed on the basis of the energy surfaces calculated by Kistenmacher, Popkie and Clementi for a water molecule in the field of an alkali ion. For comparison with recently measured oxygen isotope fractionation factors in aqueous alkali halide solutions, the gas phase values are multiplied with the corresponding separation factors between water vapor and liquid water thus relating the hydration complex in the gas phase with pure water. Qualitative agreement between computed and observed fractionation factors has been found for H2O and D2O even neglecting the isotope effect connected with the transfer of the hydration complex from the gas phase to the solution. This transfer effect is estimated for H2O by a quantitative comparison of computed and observed oxygen isotope fractionation factors.

Abstract

Blood distribution in the systemic circulation of bony fishes is effected by a dualistic construction of the vasculature: A secondary arterial system originates from the well known (primary) arteries by multiple root-like shunt vessels. These secondary arteries and the vessels they supply should no longer be regarded as lymphatics. After an intensive search for unequivocally lymphatic vessels in fish, it appears doubtful whether there are any at all in this class of vertebrates.

In vielen industriellen Anwendungen besteht ein großer Bedarf an einem rein optischen und damit korrosionsfesten Nachweis von Sauerstoff. Für diesen Zweck wurde ein hochempfindliches, extraktives, lasergestütztes Zweistrahl-Absorptionsspektrometer mit nur 200 mm Absorptionslänge zum O2-Nachweis im Bereich des A-Bandes bei 760 nm entwickelt und mit verschiedenen Lasertypen getestet. Mit einem Vertical-Cavity-Surface-Emitting-Laser mit 670 μW optischer Leistung wurde auf der R11 R11-Linie bei STP eine exzellente Linearität (R = 0,9999) und eine Nachweisgrenze von 35 ppmV O2 erreicht. Das entspricht einer optischen Dichte von 7 · 10−7 und ist nur noch einen Faktor 3 von der Schrotrauschgrenze entfernt. Die normierte Spektrometerempfindlichkeit beträgt damit 7,6 ppmV · m · mW1/2 · s1/2. Über Zeiträume von bis zu 10 Stunden betrug die Stabilität 65 ppmV. Das gleiche System wurde mit einem neuartigen Distributed-Feedback-Laser im externen Planresonator gekoppelt, um bei λ = 1,65 μm CH4 mit einer Empfindlichkeit von 600 ppb · m nachzuweisen.