Bound water fractions from quasi-elastic neutron scattering

Quasi-elastic neutron scattering measurements of AlCl₃ aqueous solution are analysed to derived the ratio of bound to solvent water molecules. We find that the ratio depends on the nature of the binding of the water molecules in the hydration shell and in the solvent. If the details of the binding are not known it is shown that such measurements must be carried out at small neutron wavevector transfers to obtain quantitative results.