Interaction of H2O with (CuS)n, (Cu2S)n, and (ZnS)n small clusters (n = 1-4, 6): relation to the aggregation characteristics of metal sulfides at aqueous solutions.

The interaction of H2O onto small CuS, Cu2S, and ZnS clusters was theoretically studied by Density Functional Theory computations to get insights into the aggregation characteristics of metal sulfides at aqueous solutions. The results show the charge-controlled interactions with polarized solvent molecules are favored on the ZnS clusters compared with CuS and Cu2S clusters. Moreover, the chemical adsorption of H2O molecules is energetically favored onto ZnS clusters with higher interaction energies of up to 35.4 kcal/mol compared with CuS and Cu2S clusters (up to 31.3 kcal/mol), where the stability of H2O adsorption decreases as the size of the clusters increases. However, thermochemical analysis shows that the adsorption of H2O on copper sulfides is not a spontaneous process at room temperature. Additionally, the electrostatic energy of H2O onto the Cu2S and CuS clusters is lower than that associated with the H2O-H2O interactions, suggesting that copper precipitates prefer to bind between them at early stages of the precipitation process due to an unfavorable solvent-solute interaction. Dispersion forces play a relative key role in the interaction of water on copper sulfides, while for zinc sulfide clusters, the adsorption energy is slightly influenced by dispersion contributions. Accordingly, the aggregation of zinc sulfides in a water environment is expected to be lower compared with copper sulfides, and where the aggregation characteristics are not determined by the binding energy of the sulfides, but of the ability to interact with the solvent molecules. These statements were confirmed by experimental optical microscopy analysis and settling tests during precipitation processes in water. Therefore, this work allows proposing a simple strategy to study the aggregation characteristics of metal sulfides, which turns useful for use in hydrometallurgical applications.