Neutron Diffraction Study on the Structure of Hydrated Li+ in Dilute Aqueous Solutions.

Neutron diffraction measurements have been carried out for 6Li/7Li isotopically substituted aqueous 1.0 mol % (0.5 mol/kg) LiCl and 1.1 mol % (0.56 mol/kg) LiClO4 solutions in D2O to obtain structural insight concerning hydration structure of Li+ in more dilute electrolyte solutions. The first-order difference function, ΔLi(Q), was analyzed by means of the least squares fitting procedure to obtain short-range structural parameters around the Li+. It was revealed that the nearest neighbor Li+···O(D2O) distance, rLiO, and the coordination number, nLiO, for the aqueous 1.0 mol % LiCl solution are 2.01 ± 0.02 Å and 5.9 ± 0.1, respectively. The values, rLiO = 1.97 ± 0.02 Å and nLiO = 6.1 ± 0.1, are obtained for aqueous 1.1 mol % LiClO4 solution. These results indicate that the hydration number of Li+ in a dilute solution is close to 6, which is much larger than 4, which has long been believed. A possible explanation is that the hydration number of Li+ varies with the solute concentration.