Understanding the effects of concentration on the solvation structure of Ca2+ in aqueous solutions.: II:: Insights into longer range order from neutron diffraction isotope substitution

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, J. L.; Badyal, Y. S.; Simonson, J. M.; Heald, S. M. J. Phys. Chem. A 2003, 107, 4688-4696). The common goal of both studies was to develop a much clearer and more consistent picture of the effects of concentration on the Ca2+ solvation structure. In particular, we have tried to elucidate the microscopic basis of a thermodynamic anomaly reported for ambient aqueous solutions of CaCl2 at concentrations greater than approximately 4.0 m. By measuring the neutron scattering from isotopically distinct (Ca-nat/Ca-44) pairs of CaCl2 aqueous solutions at two concentrations (4.0 and 6.4 m) in both light and heavy water, the uniquely detailed Ca-H and Ca-X (X = O, Cl, or Ca) pair distribution functions, and the changes in them with concentration, were determined. This type of second-order isotope difference experiment has only been applied successfully before to two other aqua-ions, Ni2+ and Cr3+. Our findings confirm the lack of substantial change in the nearest atomic neighbor Ca-O environment and the virtual absence of Ca2+- Cl- contact ion pairing even at high concentration, as first indicated by the preceding EXAFS investigation. Instead, the principal change with concentration appears to be the entry of significant numbers of Cl- ions into the second hydration shell around Ca2+ and the resulting formation of Ca2+-OH2-Cl- solvent-shared ion pairs. Our results provide compelling evidence for this picture, in particular the impact on the water hydrogen structure in the second hydration shell and the apparent effects on the tilt angle distribution of water molecules in the first hydration layer. The average number of water molecules in the first hydration shell of Ca2+ is close to 7 at both concentrations (although a small decrease with concentration is noticeable). Given the great rigor and consistency required for second-order isotope difference experiments, our values for coordination number and other structural parameters should be regarded as the benchmarks for this system. The pair distribution functions will also serve as an exacting test of theoretical and simulation models.