Investigation of Interfacial and Bulk Dissociation of HBr, HCl, and HNO3 Using Density Functional Theory-Based Molecular Dynamics Simulations

In this study, we calculate free-energy profiles for the dissociation of the strong acids XH (HBr, HCl, and HNO3), specifically, for the deprotonation and subsequent formation of ion pairs, X...H3O+, in the vicinity of the airwater interface. We show that the free-energy profiles for the dissociation of HCl and HNO3 are significantly different at the airwater interface compared to in the bulk, and we predict that the undissociated, molecular form of these two acids is stable at the interface. For the strongest acid we consider, HBr the traditional picture of completely dissociated strong acids is preserved in the vicinity of the airwater interface. Our results have implications for our understanding of the interfacial chemistry of acids, as well as the effects of acids on the surface tensions of aqueous solutions.