Volumes of aqua ions from measured neutron radial distribution functions

Abstract

Radial distribution functions (RDF) of aqua ions, obtained in neutron-scattering experiments, are shown to provide an explanation of the results of measurements of a macroscopic property of the ions, the partial molal volume. A simple analysis, based on evidence from the RDF that the aqua ion is spherical, directly gives a value for the volume of the ion's primary hydration sphere. Considering this experimentally derived hydration volume, and the known hydration number of the ion, then leads to a prediction of the ion's partial molal volume that is in good agreement with experiment. A corollary of this analysis is that electrostriction effects are shown to be small in the secondary hydration shell and beyond. The volume of the primary hydration shell of the ion and the average volume of the water molecules in the shell are also calculated; agreement is good between these calculations and the corresponding values derived from measured partial molal volumes. Results are given for 33 different cations. A comparison with published values of total hydration numbers, obtained from transport experiments for lanthanide and actinide ions, is also presented.