Solvation structure and dynamics of Ag⁺ in aqueous ammonia solutions: A molecular simulation study

We present an ab initio-based force-field for silver ion interactions with water and ammonia. Compared to quantum calculations, our model allows for rather large-scale molecular dynamics simulations of silver solutions of aqueous ammonia. For a series of NH₃:H₂O ratios ranging from 1 to 20 mol. %, Ag⁺ ions were mainly found as octahedral [Ag(NH₃)_x(H₂O)]_6-x⁺ coordination complexes with preferential values of x ranging from 0 to 3. In the first coordination structure, water ↔ ammonia exchanges occur within a 1-3 ps time scale and, depending on the NH₃ concentration, imply significant fluctuations of x covering the whole range from 0 to 6. Based on ns-scale molecular dynamics simulations, chemical potentials are derived for all Ag⁺ coordination species as functions of temperature and ammonia concentration. Moreover, we compare the diffusion constants of the [Ag(H₂O)₆]⁺ to [Ag(H₂O)₃(NH₃)₃]⁺ coordination complexes, based on the solutions of the corresponding ammonia content.