Ion pairing in aqueous calcium chloride solution: Molecular dynamics simulation and diffraction studies

X-ray and neutron diffraction and molecular dynamics simulation studies were performed on 2.5 and 4.0 M aqueous solutions of calcium chloride. The aim of the work, beyond the determination of the solution structure, was to discuss the capability of each method to detect ion pair formation. It has been found that the diffraction methods are performing very well in the determination of Ca2+-O distances: the obtained values fell in the range 2.43-2.46 Å. Further on, by applying the X-ray diffraction method ion pairs could be observed in the case of higher concentrated solution, but for neutron diffraction study, the most adequate isotope substitution method has to be chosen with care in order to be able to describe ion pairs in solution. The results of diffraction studies were compared to molecular dynamics simulation results; the latter ones were found to be in general accordance with the experimental findings. The smaller discrepancies between the simulation and the experimental results are coming from small differences in the ion-water and ion-ion distances, and they may be due to the potential model applied in the simulation and to the experimental uncertainties.