Theoretical studies of strong acid dissociation at the surface and in the bulk of aqueous solutions

Acid dissociation of sulfuric acid (H2SO4) and nitric acid (HNO3) at the surface of atmospheric aerosols plays a key role in various heterogeneous processes, related to e.g. stratospheric ozone depletion and tropospheric NOx removal. The molecular features of these interfaces are still unknown, and only recently surface-sensitive experiments have highlighted some limited aspects. In this thesis, acid dissociation of H2SO4 and HNO3 both at the surface and in the bulk of aqueous solutions is studied via theoretical methods.;First, reaction paths and free energies for the reactions H2SO 4 + H2O → HS O-4 + H3O+ and HNO3 + H2O → N O-3 + H3O+ at a water surface are calculated by electronic structure methods. For HNO3 and N O-3 at this model aqueous surface, infrared signatures are calculated and compared to experiment. The results in the temperature range of 0-300 K indicate that undissociated molecular HNO3 is always favored, whereas H2SO4 dissociation can be thermodynamically either favored or disfavored depending both on the degree of solvation of the reactant and product species and the temperature.;Next, the HNO3 depth-dependent acid dissociation at an air/water interface is investigated via Car-Parrinello molecular dynamics at ∼240 K. The influence of the structural evolution of the solvation environment is characterized and discussed. HNO3 is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociate — although not always — when embedded at various depths within the aqueous layer.;Finally, the equilibrium ionic compositions in the bulk and at the surface of concentrated H2SO4 and HNO3 aqueous solutions is examined via the Reactive Monte Carlo method, here extended to model proton transfer reactions in condensed phase. The interaction potentials used in the simulations are parameterized via a novel, iterative procedure. Several adaptations of this methodology are designed to improve sampling of ionic compositions. Preliminary results show reasonable agreement with experiments.