The structure of water in the hydration shell of cations from x-ray Raman and small angle x-ray scattering measurements

X-ray Raman scattering (XRS) spectroscopy and small angle x-ray scattering (SAXS) are used to study water in aqueous solutions of NaCl, MgCl 2 , and AlCl 3 with the particular aim to provide information about the structure of the hydration shells of the cations. The XRS spectra show that Na + weakens the hydrogen bonds of water molecules in its vicinity, similar to the effect of increased temperature and pressure. Mg 2+ and Al 3+ , on the other hand, cause the formation of short and strong hydrogen bonds between the surrounding water molecules. The SAXS data show that Mg 2+ and Al 3+ form tightly bound hydration shells that give a large density contrast in the scattering data. From the form factors extracted from the SAXS data, we found that Mg 2+ and Al 3+ have, respectively, an equivalent of one and one and a half stable hydration shells that appear as a density contrast. In addition, we estimated that the density of water in the hydration shells of Mg 2+ and Al 3+ is, respectively, ∼61% and ∼71% higher than in bulk water.