X-ray absorption spectroscopy and molecular dynamics studies of Zn²⁺ hydration in aqueous solutions

The hydration of Zn²⁺ ions in aqueous solutions was studied at room temperature by x-ray absorption spectroscopy and molecular dynamics (MD) simulation. The extended x-ray absorption fine structure (EXAFS) above the Zn K-edge was interpreted using the multiple-scattering approach by taking into account only one coordination shell composed of 6 ± 0.2 water molecules at A(Zn-O) = 2.06± 0.02 Å with a mean square relative displacement (MSRD) σ² = 0.009 ± 0.002 Ų. No evidence of significant contributions from the second hydration shell to the EXAFS signal was found in the solutions. This is explained by the cancellation interference effect between double-scattering and single-scattering EXAFS signals in the second shell due to large thermal/static disorder (σ² ∼ 0.1 Ų) as predicted by our MD simulation and by known results of diffraction techniques.