T. Radnai

The structure of aqueous sodium hydroxide solutions: A combined solution x-ray diffraction and simulation study

To determine the structure of aqueous sodium hydroxide solutions, results obtained from x-ray diffraction and computer simulation (molecular dynamics and Car-Parrinello) have been compared. The capabilities and limitations of the methods in describing the solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in describing the hydration spheres of the sodium ion and yield structural information on the anions hydration structure. Classical molecular dynamics simulations were not able to correctly describe the bulk structure of these solutions. However, Car-Parrinello simulation proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions and bulk structure of solutions. The results of Car-Parrinello simulations were compared with the findings of diffraction experiments.

X‐ray diffraction study of ferric chloride solutions and hydrated melt. Analysis of the iron (III)–chloride complexes formation

A series of concentrated ferric chloride solutions, both neutral and acidic, as well as the hydrated melt FeCl3⋅6H2O have been studied by x‐ray diffraction. The changes in the radial distribution curves by varying iron concentration or Cl−/Fe3+ ratio and least squares fitting of the structure functions for various solute models lead to an unambiguous conclusion with respect to the dominant species which are present in the solutions. The existence of monochloro, dichloro, trichloro, and tetrachloro complexes, predicted by literature information in dilute solutions, has been established in the different solutions investigated. Octahedral complexes, mainly Fe(H2O)4Cl+2 and Fe(H2O)3Cl3, are the principal species occurring in neutral and acidic solutions while in the hydrated melt tetrahedral FeCl−4 predominates. Species of the type FeCl5(H2O)2− or FeCl3−6 as well as polymeric network are excluded. Hydration water molecules around Cl− anions has been found for both free chloride and chloride ions bonded to iro...

Investigation of the uniqueness of the reverse Monte Carlo method: Studies on liquid water

Reverse Monte Carlo (RMC) simulation of liquid water has been performed on the basis of experimental partial pair correlation functions. The resulted configurations were analyzed in various aspects; the hydrogen bond angle distribution, three body correlation and orientational correlation were calculated. The question of the uniqueness of the RMC method was also examined. In order to do this, two conventional computer simulations of liquid water with different potential models were performed, and the resulted pair correlation function sets were fitted by RMC simulations. The resulted configurations were then compared to the original configurations to study how the RMC method can reproduce the original structure. We showed that the configurations produced by the RMC method are not uniquely related to the pair correlation functions even if the interactions in the original system were pairwise additive. Therefore the difference between the original simulated and the RMC configurations can be a measure of the uncertainty of the RMC results on real water. We found that RMC produces less ordered structure than the original one from various aspects. However, the orientational correlations were reproduced rather successfully. The RMC method exaggerates the amount of the close packed patches in the structure, although these patches certainly exist in liquid water.

Structure of water under subcritical and supercritical conditions studied by solution X-ray diffraction

Stuctures of water and aqueous electrolyte solutions under sub-and super-critical conditions studied mainly by X-ray diffraction and also by neutron diffraction are reviewed and the experimental results are compared with those reported by using computer simulations. Some Raman spectroscopic data are included for discussing the existence of hydrogen bonds in water at high temperature and high pressures (HTHPs).The authors propose a classification of supercritical water into three categories: (a) low density water, (b) medium density water, and (c) high density water, because density is a very important thermodynamic quantity to describe properties of sub- and super-critical water.From changes in the water–water intermolecular distance and the coordination number of water with temperature and pressure, and especially with density at HTHP, the authors conclude that the compact tetrahedral-like water structure is decomposed and long-distance water–water interactions increase with temperature and pressure, and they propose a model for water: under supercritical conditions water consists of small clusters, much smaller aggregates such as oligomers, and even monomeric gas-like water molecules.

Order phenomena in aqueous AlCl3 solutions

X‐ray diffraction data of two aqueous solutions of AlCl3 are interpreted using a model in which the Cl− ions have a coordination number of six and the Al(H2O)3+6 ions interact with twelve water molecules through short and linear H bonds, the water molecules nearest neighbor to the Al3+ ions having a trigonal orientation. Moreover, evidence is given that such a hydration structure tends to persist even when the total amount of water present in the system is rather small.

X-RAY DIFFRACTION STUDIES ON THE STRUCTURE OF WATER AT HIGH TEMPERATURES AND PRESSURES

X-Ray diffraction measurements of liquid water at high temperatures and pressures (up to 200 °C and 200 MPa) are reported. A diffractometer with a transmission geometry and a cylindrical sample holder made of beryllium were used, with a specially designed high pressure unit. The data reduction procedures are described in detail. The analysis of the structure functions and radial distribution functions show that the O-O distances shorten due to pressure but they are elongated with increasing temperature. The average contraction rate is 0·02 pm MPa-1. The average coordination number of the first neighbour molecules does not change significantly with increasing temperature but slightly increases with increasing pressure. The density dependence of the distances and coordination are discussed. The expansion of water is proportional to the expansion of first neighbour distances in the medium density region only (0·95-1·01 g cm-3), while the coordination number produces a minimum in the medium density region. Th...

On the orientation of water molecules in the hydration shell of the ions in a MgCl2 solution

The orientation of the water molecules in the hydration shells of Mg2+ and Cl− have been calculated from a molecular dynamics simulation of a 1.1 molal MgCl2 solution with the central force model for water. The results are compared with information obtained from neutron diffraction studies with isotopic substitution.

The structure of an aqueous ammonium chloride solution

X‐ray measurements have been performed on a 2 molal NH4 Cl solution at 25 °C. The scattering data have been evaluated with the first neighbor model (FNM). These results are compared with molecular dynamics (MD) simulations where the ST2 model for water is employed. The NH4+ is described by a rigid tetrahedron with point charges at the positions of the hydrogen atoms and the Cl− by a Lennard‐Jones sphere with an elementary charge at the center. Good agrement has been found between the measurements and the simulation. Contrary to previous results the nearest neighbor distance for NH4+–water is found to be significantly different from the one for water–water. The structure of the solvent described by the nearest neighbor distance and its root mean square deviation is in the limits of error the same as in pure water and significantly different from the solvent structure in alkali halide solutions.

Structure of liquid nitromethane: Comparison of simulation and diffraction studies

Simulation (molecular dynamics and Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]) and diffraction (x-ray and neutron) studies on nitromethane are compared aiming at the determination of the liquid structure. Beyond that, the capabilities of the methods to describe liquid structure are discussed. For the studied liquid, the diffraction methods are performing very well in the determination of intramolecular structure, but they do not give detailed structural information on the intermolecular structure. The good agreement between the diffraction experiments and the results of molecular dynamics simulations justifies the use of simulations for the more detailed description of the liquid structure using partial radial distribution functions and orientational correlation functions. Liquid nitromethane is described as a molecular liquid without strong intermolecular interactions such as hydrogen bonding, but with detectable orientational correlations resulting in preferential antiparallel order of the neighboring molecules.

Hydration of iron(II) ion in aqueous solutions

Abstract The hydration of several iron(II) compounds has been studied by the Mossbauer technique and the X-ray diffraction method. The similarity of the Mossbauer parameters (isomer shift, quadrupole splitting) of aqueous iron(II) salt solutions suggested that Fe 2+ contrary to Fe 3+ in the above solutions is present mainly in the form of the hexaaquo complex. Based on the analysis of X-ray data, however, the existence of moderate Fe(H 2 O) 5 A complex formation in addition to the dominating hexaaquo complexes in chloride and sulfate solutions could not be excluded.