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.

The spectral densities of the hindered translations and librations for the ions and the water molecules in a 2.2 m LiI solution have been derived from an MD simulation with the ST2 model for water. The reorientational times of the dipole moment vector, the intramolecular proton–proton vector as well as the ion–oxygen and ion–hydrogen vectors are also reported. The separate calculation of various properties for the three water subsystems—hydration water of the cation, of the anion and bulk water—contributes significantly to the understanding of macroscopically measured quantities on a molecular level.

Indications from a molecular dynamics simulation of a 2.2 molal LiI solution of the existence of a second hydration shell of Li

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.

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