Pulsed neutron diffraction study on lithium (I) hydration in supercooled aqueous chloride solutions

Abstract

Pulsed neutron diffraction measurements have been performed on aqueous LiCl solutions (mole ratio D2O:LiCl=5) in the supercooled state (258, 213, and 173 K) and at room temperature (295 K). An isotopic substitution method with respect to Li has been used to derive the Li+-related radial distribution function. The neutron diffraction data at all the temperatures have shown the Li–O and Li–D distances to be 2.02±0.05 and 2.61±0.05 Å, respectively, and the most likely orientation of the coordinated water molecules has been found to be such that the four atoms in a Li+–D2O unit is pyramidal. It has also been found that at temperatures from 295 K down to 213 K the Li+ hydration is represented mostly by the primary hydration shell of about four coordinated water molecules, whereas at 173 K, about 40 K above the glass transition temperature, the definite second hydration shell of Li+ is formed. The obtained structural characteristics of the supercooled solutions are discussed in connection with nucleation of ice, an isotropic reorientational motion of water, crystallization, and the partial recovery of hydrogen bonds in the supercooled solution.