Structure of water–sulfuric acid clusters from molecular dynamics simulations

Molecular dynamics method is applied to study the structure of stable sulfuric acid–water clusters at various compositions. Also a planar liquid–vapor interface is studied. Two different potential models were used. In the simpler model, sulfuric acid (H 2 SO 4 ) remains in an undissociated state. A more realistic scheme requires that in the presence of water (H 2 O) H 2 SO 4 protonates to form bisulfate (HSO 4 ) and hydronium (H 3 O) ions. This effect is described by considering a system consisting of HSO 4 – and H 3 O ions, and water. The main focus is on the structure of clusters of hundred molecules at different compositions. The results are compared with those for the planar liquid–vapor interface. In the unprotonated system sulfuric acid lies on the cluster surface, if the total mole fraction of H 2 SO 4 is smaller than 0.1, whereas at a planar interface such enhanced surface activity is not seen. In the protonated system the bisulfates are at the center of the cluster and the hydronium ions on the surface when the sulfuric acid concentration is small. The presence of ions is found to destabilize the clusters at higher compositions and the planar interfaces at all compositions.