The dynamics of aqueous Zn²⁺ solutions: A study using incoherent quasi-elastic neutron scattering

The method of high-resolution incoherent quasi-elastic neutron scattering (IQENS) is used to study the proton diffusion and cation to water-proton binding time in 1.98 mol al (mol. kg^-1) and 2.58 molal aqueous Zn(ClO ₄)₂ solutions and a 1.96 molal aqueous ZnCl₂ solution at room temperature. In the perchlorate solutions a lower limit of tau ₁>10^-10 s is found for the Zn²⁺ to water-proton binding time and an upper limit of tau ₁<5* 10^-9 s is estimated. The chloride solution cannot be described by a single proton population on the IQENS observation time scale. Furthermore, it cannot be described by a model which assumes that only those protons bound to free Zn²⁺ ions are in slow exchange, the fraction of these ions being taken from the published sets of stability constants used to describe the equilibria between the various zinc complexes present in solution.