Structure and dynamics of sulfur dioxide in water

Sulfur dioxide (SO2) aqueous solutions are pivotal in atmospheric chemistry and geochemistry and have industrial relevance. However, full characterization of the microscopic behavior of SO2-H2O mixtures is elusive since heterogeneous length- and timescales enter in structural and diffusion phenomena. By exploiting classical molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations, we investigate SO2 aqueous solutions under different regimes of temperature and SO2 concentrations χ. A fairly good agreement between MD and AIMD simulations is found in reproducing the short-range molecular structure of the liquid, although classical MD severely underestimates the strength of hydrogen bonds (H-bonds) between H2O and SO2 molecules. An unexpected behavior of the SO2 diffusion coefficient DSO2 is observed: DSO2 is always largest for χ = 2%, independently from the specific temperature regime. Cluster analysis provides evidence that such a diffusion maximum is rooted in the increase in the number of SO2-SO2 and SO2-H2O aggregates, along with a peak of the mean residence time of H2O species in the SO2 solvation shell, found for 2% ≤ χ ≤ 3%. Although the mixture is globally homogeneous, the occurrence of local small aggregates—in which water molecules arrange themselves in a quite stable fashion around SO2 molecules via interactions other than H-bonding—promotes SO2 diffusion in water. This scenario indicates that steric effects ascribed to the large number of water molecules around SO2 molecules/small aggregates may be more effective than H-bonds in shaping dynamical properties of the mixture.