Static and dynamic structural analysis of a saturated solution of ZnBr2 in water: Anomalous x-ray diffraction and molecular dynamics simulations

Abstract

Articles you may be interested inThe solvation of Cl−, Br−, and I− in acetonitrile clusters: Photoelectron spectroscopy and molecular dynamics simulations Constant pressure molecular dynamics simulations of the dodecamers: d(GCGCGCGCGCGC)2 and r(GCGCGCGCGCGC)2 Structure and dynamics of undercooled and glassy aqueous ionic solutions by NMR, Xray and neutron scattering AIP Conf.The supposedly very simple system of a saturated solution of ZnBr 2 in water exhibits unusually complex and therefore interesting structural behavior. Motivated by this, Mager did a detailed x-ray diffraction study ͑Th. Mager, PhD. thesis, Universität Würzburg, 1991͒, and we performed a long molecular dynamics ͑MD͒ simulation-using potential parameters from the general purpose GROMOS force field-of such a solution, which can be grossly characterized by the formula ZnBr 2 •3H 2 O. We start by calculating those properties that are directly accessible through the experiment from the MD simulation, in order to validate the physical relevance of the simulation. Seeing that the simulation delivers results that are compatible with those of the experiment, we proceed by analyzing the MD simulation in much more detail according to the static and dynamic structure of the system, thereby gaining insight into the structural behavior of ZnBr 2 •3H 2 O that is very difficult, if at all possible, to get from experimental studies. To this end we use the Voronoi algorithm to define coordination shells around atoms and ions in ZnBr 2 •3H 2 O. We study the time averaged as well as the time-resolved geometry and composition of these coordination shells and find that octahedral coordination of Zn 2ϩ ions is the dominant geometry in ZnBr 2 •3H 2 O, and that these octahedra are remarkably stable ͑after 1 ns only 10% decayed͒. We further find evidence for polymerlike Zn 2ϩ chains, where O atoms of water and Br Ϫ ions connect the Zn 2ϩ ions.

Local order in aqueous solutions of lanthanum chloride and bromide by x-ray diffraction, EXAFS and Raman spectroscopy

“…More recently, anomalous x-ray scattering studies on aqueous solutions have been allowed to obtain information, namely, on the hydration shells of the ions. These results are in reasonable agreement with those obtained previously by EXAFS, conventional x-ray studies and Raman spectroscopy [39][40][41][42][43][44].…”

Relaxation of Voronoi shells in hydrated molecular ionic liquids

“…This advantage has been extensively exploited in a series of studies performed in our group. [8][9][10][11][12][13][14] Thereby we could also establish a link to experimental studies of shell specific relaxation as described in detail in the review of Halle. 15 Up to now we have focused on the direct Voronoi analysis of data generated by computer simulation.…”

The frequency-dependent conductivity of a saturated solution of ZnBr2 in water: A molecular dynamics simulation

“…Recently, 14 we compared the static structure of ZnBr 2 •3H 2 O as this simulation suggests it to the results of anomalous x-ray diffraction experiments of ZnBr 2 •3H 2 O and found good agreement within the experimental uncertainties. Judged by Eqs.…”

“…In other words, independent motion of the ionic species in ZnBr 2 •3H 2 O does not exist. We have shown elsewhere 14 that the dominant coordination of Zn 2ϩ ions in ZnBr 2 •3H 2 O consists of a central Zn 2ϩ ion that is ͑octahe-drally, in the ideal case͒ surrounded by four O atoms of water molecules and two Br Ϫ ions. The fact that this coordination type is electrically neutral lends itself to a very natural explanation of the low static conductivity of ZnBr 2 •3H 2 O: In the presence of ͑a moderately strong͒ static electric field, these neutral associations of Zn 2ϩ and Br Ϫ ions remain stable and hence do not contribute to the static conductivity.…”