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+, A13+, Fe3+, and Dy3+ aqueous solutions. The IQENS data for the Ga3+, A13+, and Fe3+ solutions are consistent with the model for slow exchange wherein the dynamic hydration number nh = 6. This gives a limit of τ1, ≿'; 5 × 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 τ1 > 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 τ1(2) < 5 × 10-9 s. A reduced limit of τ1,(2) ≾ 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 nh = 6, the data are better represented if a degree of hydrolysis is assumed. This gives nh > 6, a lower limit of Τ1, ≿ 5 × 10-9 s for protons in any metal ion hydrolysis products, and an upper limit of Τ1 > 5 × 10-9 s for any dissociated protons.
ASJC Scopus subject areas
- Colloid and Surface Chemistry