DYNAMICS OF WATER PROTONS IN CONCENTRATED GA3+, AL3+, FE3+, AND DY3+ AQUEOUS-SOLUTIONS - A STUDY USING INCOHERENT QUASI-ELASTIC NEUTRON-SCATTERING

The method of high-resolution incoherent quasi-elastic neutron scattering (IQENS) is applied to investigate the translational diffusive motion of water protons and the cation to water proton binding time in concentrated Ga3+, Al3+, Fe3+, and Dy3+ aqueous solutions. The IQENS data for the Ga3+, Al3+, and Fe3+ solutions are consistent with the model for slow exchange wherein the dynamic hydration number n(h) = 6. This gives a limit of tau-1 greater-than-or-similar-to 5 X 10(-9)s for the binding time of protons in the hexa-aquo metal ion species. The IQENS data for the Dy3+ aqueous solution show that the water protons are in intermediate or slow exchange, which sets a lower limit of tau-1 > 10(-10)s. The second-shell water protons in the Ga3+, Al3+, and Fe3+ solutions are not observed to be in slow exchange with the cation which gives a binding time for these protons of tau-1(2) < 5 X 10(-9)s. A reduced limit of tau-1(2) less-than-or-similar-to 10(-10)s is, however, argued for. Although the IQENS data for the Ga3+ and Fe3+ solutions are, within the experimental uncertainties of the method, consistent with n(h) = 6, the data are better represented if a degree of hydrolysis is assumed. This gives n(h) < 6, a lower limit of tau-1 greater-than-or-similar-to 5 X 10(-9)s for protons in any metal ion hydrolysis products, and an upper limit of tau-1 < 5 X 10(-9) s for any dissociated protons.