Where does acid hydrolysis take place?

We present the results of computations on the dissociation of HCl and HNO3 at the air-water interface. Molecular dynamics simulations of the acid molecule and 200 water molecules were propagated for several nanoseconds, and the resulting structures were used as input to QM/MM geometry optimization runs. Approximately 20-30 water molecules were included along with the acid in the QM portion of the calculation, which was carried out at the B3LYP/6-31+G(d) level. Whereas dissociation to ions is always spontaneous in the bulk, we find that acid molecules confined to the water surface dissociate only with the participation of two additional water molecules, forming a "critical cluster'' about the solute. Thus acid dissociation may occur in the near-surface zone at a dynamic air-water interface, in agreement with our earlier experimental conclusions [Clifford et al., Phys. Chem. Chem. Phys.. 2007, 9, 1362].