X-ray absorption fine structure (XAFS) spectroscopy was used to probe the effects of
concentration on the first-shell structure of Ca2+ in aqueous solution. Measurements were
carried out under ambient conditions using a bending magnet beamline (sector 20) at the
Advanced Photon Source, Argonne. The Ca K-edge EXAFS spectrum for 6 m CaCl2 yielded
no evidence for the formation of significant numbers of Ca2+− Cl-contact ion pairs even at
such high concentration, a result confirmed by comparison with the data for a dilute (0.2 m) …

Although the Ca2+ aqua-ion is of great importance to biology and a key component of
natural groundwaters, many details of the solvation structure and its behavior have
remained either unexplored or controversial. We report the findings of a new neutron
diffraction study, building upon the results of an earlier, complementary investigation with
EXAFS (Fulton, JL; Badyal, YS; Simonson, JM; Heald, SMJ Phys. Chem. A 2003, 107, 4688−
4696). The common goal of both studies was to develop a much clearer and more consistent …