Dynamics of Water Protons in Concentrated Ga³⁺, A1³⁺, 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⁺, 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 n_h = 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 n_h = 6, the data are better represented if a degree of hydrolysis is assumed. This gives n_h > 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.