Anomalous x-ray diffraction studies of Sr2+ hydration in aqueous solution

Anomalous x-ray diffraction experiments were carried out at two energies below the K edge of strontium on a 3.5 molal solution of strontium chloride in water. The first order difference method was applied to the corrected and normalized data sets, and a difference function for the Sr2+ coordination obtained. Fourier transformation of this function gave a modified total radial distribution function for Sr2+, which when analyzed showed that there are eight to nine nearest neighbor water molecules sited at 2.67 A from the Sr2+ ion. A second coordination shell is also apparent in the range 4.5less than or equal tor(Angstrom)less than or equal to5.5 which can accommodate Cl- counterions and a second shell of water molecules. The results obtained for the distance between Sr2+ and O are in excellent agreement with those obtained from standard x-ray diffraction, extended x-ray absorption fine structure, and neutron diffraction isotopic substitution. The hydration structure obtained for Sr2+ is compared with the structure of two other ions in the alkaline earth series (Mg2+ and Ca2+). (C) 2003 American Institute of Physics.