Molecular dynamics simulations of zinc oxide solubility: From bulk down to nanoparticles

The solubility of metal oxides is one of the key descriptors for the evaluation of their potential toxic effects, both in the bulk form and in nanoparticulated aggregates. Current work presents a new methodology for the in silico assessment of the solubility of metal oxides, which is demonstrated using a well-studied system, ZnO. The calculation of the solubility is based on statistical thermodynamics tools combined with Density Functional Tight Binding theory for the evaluation of the free energy exchange during the dissolution process. Models of small ZnO clusters are used for describing the final dissolved material, since the complete ionic dissolution of ZnO is hindered by the formation of O^2- anions in solution, which are highly unstable. Results show very good agreement between the computed solubility values and experimental data for ZnO bulk, up to 0.5 mg L^-1 and equivalents of 50 μg L^-1 for the free Zn²⁺ cation in solution. However, the reference model for solid nanoparticles formed by free space nanoparticles can only give a limited quantitative solubility evaluation for ZnO nanoparticles.