Hydration Environment of Complexes and Their Dynamics in Aqueous Zinc Chloride Solutions

Aqueous ZnCl₂ serves as a model electrolyte where the zinc ion exhibits variable coordination geometries and strongly polarizes water in its solvation shells. Molecular dynamics simulations using a machine-learned interatomic potential trained at the revPBE-D3 level of density functional theory reveal its structure, dynamics, and electrostatics. Dilute solutions feature the dominant octahedral [Zn(H₂O)₆]²⁺ complex, which adapts to trigonal bipyramidal and tetrahedral geometries to accommodate chloride ions. Relative to bulk water, water molecules in zinc's first solvation shell in the complexes exhibit an enhancement of their dipole moment by 15-30%, and a retardation of their residence and reorientational time scales. The residence time displays a nonmonotonic dependence on the number of participating chloride ions; a minimum of 96 ps for [ZnCl]⁺ and maxima of 550 ps for Zn²⁺ and [ZnCl₃]^- are observed. Exchange and reorientation time scales in the second solvation shell decrease monotonically with chloride inclusion in the complex.