The dynamics of water molecules in ionic solution. I. The application of quasi-elastic neutron scattering to the study of translational diffusive proton motion

The use of quasi-elastic neutron scattering (QENS) as a probe for the translational diffusive motion of water protons in aqueous ionic solutions is considered. A scattering law is given that takes into account the exchange of water molecules between the primary hydration shell of an ion and the remaining water. The restrictions on the experimental parameters are then discussed and the choice of a back-scattering spectrometer with energy resolution of around 1 mu eV is justified. The effect that a typical instrumental resolution function and multiple scattering of neutrons in an aqueous solution have on the shape of the derived scattering law is described, and the sensitivity of the technique to the magnitude of the cation-proton binding time tau 1 is examined. It is shown that the QENS technique will enable the proton population to be observed in ‘fast exchange’ if tau 1<or approximately=10^-10 s, ‘intermediate exchange’ if 10^-10< tau 1<5*10^-9 s and ‘slow exchange’ if tau 1>or approximately=5*10^-9 s.