Hydration Shell Structures in an MgCl₂ Solution from X-Ray and MD Studies

Abstract

The results of a molecular dynamics simulation of a 1.1 molal aqueous MgCl2 solution are compared with newly performed x-ray measurements. The structural properties of the solution are evaluated from the scattering data by a model fit to the experimental structure function. The comparison on the basis of fit parameters and partial structure functions shows an overall good agreement between experiment and simulation.Detailed information on the structure of the hydration shells is deduced from the simulation and shown in form of density maps and angular distributions. It is demonstrated that the octahedral arrangement of the water molecules in the first hydration shell of Mg+ + is strongly pronounced while it is only indicated in the case of Cl~. A preferential arrangement in tetrahedral directions has been found for the nearest neighbor water molecules around a central water molecule with an asymmetry in respect to lone pair and hydrogen atom directions. In addition, the probability of finding water molecules at a given number of symmetry sites at the same time has been calculated.

Hydration and Ion Pairing in Aqueous Mg2+ and Zn2+ Solutions: Force-Field Description Aided by Neutron Scattering Experiments and Ab Initio Molecular Dynamics Simulations

“…4,57,58 In contrast, experimental studies 38,44 show no sign of contact ion pairs between magnesium and chloride ions in MgCl 2 solutions; the first peak of the neutron signal ΔG Mg−W (r) can thus be readily assigned to the Mg−O distance. The presently obtained value of 2.05 Å, falls within the range of experimental values, 2.0 to 2.12 Å, previously reported in the literature, [42][43][44]59,60 but is shorter than the most recent neutron scattering estimate 44 of 2.10 Å. The position of the first peak is found here to be rather insensitive to the employed signal treatment.…”

“…We thus need to refine the original full charge force field by scaling the ionic charge and readjusting the ionic radius to obtain a scaled-charge ECC force field with the proper Mg–O distance. In a first ECC model, we tune σ to obtain a 2.10 to 2.12 Å Mg–O distance, following the estimates obtained in previous works, − labeling this model as “ECC big”. Figure b shows that this model improves the agreement with neutron scattering compared with a full charge force field.…”

“…We used in this work the previously developed ECC force field for the chloride anion, which was fitted against neutron scattering data of LiCl solutions. Two ECC force fields were developed for Mg 2+ here; the first was adjusted to reproduce the Mg 2+ –O distance previously reported in the literature (around 2.10 to 2.12 Å − ) and used for force-field parametrization, whereas the second was fitted to the measured neutron scattering signal, which required the use of a smaller Mg 2+ radius. A new ECC force field was also developed for Zn 2+ .…”

Ab initiomolecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…The literature on the hydration structure of Mg 2+ is relatively unambiguous. X-Ray Diffraction (XRD), 2,34 Neutron Diffraction (ND), 35 Raman spectroscopy, 36,37 Density Functional Theory (DFT), 38,39 MD 34,36,40 and AIMD [41][42][43][44] investigations all report a coordination number (CN) of 6 with the first shell Mg-O distance is found experimentally to be between 2.10 and 2.12 Å. [35][36][37][45][46][47][48] One gas phase DFT study investigating the successive binding energies of water to Mg 2+ by Pavlov et al 49 found stable structures up to a CN of 7.…”

Solvation Structures and Dynamics of the Magnesium Chloride (Mg2+–Cl–) Ion Pair in Water–Ethanol Mixtures

“…The solvation structures of MgCl 2 in aqueous solution have been studied − with the help of classical molecular dynamics simulations. The hydration structure of Mg 2+ has been investigated by many first principal molecular dynamics and Monte Carlo simulations. − It is observed from studies of solvation structures of magnesium chloride using ab initio molecular dynamics, density functional theory, and Car–Parrinello molecular dynamics (CPMD) that in the first solvation shell of magnesium ion there are six water molecules arranged in an octahedral arrangement around magnesium ion and the radius of first solvation shell ranges from 2.01 to 2.13 Å. − …”