Structures and Acidity Constants of Silver-Sulfide Complexes in Hydrothermal Fluids: A First-Principles Molecular...

In order to quantify the speciation and structures of silver-sulfide complexes in aqueous solutions, we have carried out systematic first-principles molecular dynamics (FPMD) simulations at three temperatures (25, 200, and 300 °C). It is found that monosulfide (i.e., Ag(HS)) and disulfide species (i.e., Ag(HS)₂^-) are the major silver-sulfide species over a wide T-P range, while Ag(HS)₃^2- can hold stably only at ambient temperatures, and Ag(HS)₄^3- does not exist even at the ambient conditions. Ag(H₂S)⁺ has a tetrahedral structure up to 300 °C (i.e., Ag(H₂S)(H₂O)₃⁺). Ag(H₂S)₂⁺ remains 4-coordinated to 200 °C (i.e., Ag(H₂S)₂(H₂O)₂⁺), but it transforms to 3-coordinated at 300 °C (i.e., Ag(H₂S)₂(H₂O)⁺). All of the other mono- and disulfide species (Ag(HS)(H₂O)⁰, Ag(HS)(OH)^-, Ag(HS)(H₂S)⁰, Ag(HS)₂^-, and AgS(HS)^2-) have 2-fold linear structures. For their solvation structures, the H₂S ligands donate weak H-bonds to water O; the HS^- ligands accept weak H-bonds from water H; the dangling S^2- form strong H-bonds with H of water molecules, and the OH^- ligands can form strong H-bonds as donors and weak H-bonds as acceptors. We further calculated the acidity constants (i.e., pK_as) of Ag(H₂S)⁺ and Ag(H₂S)₂⁺ complexes using FPMD based vertical energy gap method. Based on the calculated pK_as, the mono- and disulfide species distributions versus pH have been derived. We found that for monosulfide species, Ag(HS)(H₂O)⁰, is the major species in near neutral pH, while Ag(H₂S)(H₂O)₃⁺ and Ag(HS)(OH)^- exist in the acid and alkaline pH range at T ≤ 200 °C, respectively. At 300 °C, both Ag(HS)(OH)^- and Ag(HS)(H₂O)⁰ are dominant in the neutral pH range, and Ag(H₂S)(H₂O)₂⁺ only exists in acidic solutions. For disulfide species, Ag(HS)₂^- is dominative in near neutral pH condition at the three temperatures; Ag(HS)(H₂S)⁰ stays in mild acidic pH range only at 25 °C; AgS(HS)^2- and Ag(H₂S)₂(H₂O)₂⁺ (Ag(H₂S)₂(H₂O)⁺ at 300 °C) are trivial at the three conditions. The results of structures and acidity constants provide quantitative and microscopic basis for understanding the behavior of silver complexes in hydrothermal fluids.