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

The results of a molecular dynamics simulation of a 1.1 molal aqueous MgCl₂ solution are com-pared 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.