Ab Initio Molecular Dynamics Studies of the Liquid-Vapor Interface of an HCl Solution

Ab initio molecular dynamics is used to investigate the propensity of the hydronium ion for the interface of an HCl solution containing 1 HCl and 96 water molecules in a slab geometry. Unconstrained trajectories in the NVT and NVE ensemble reveal a clear preference of the hydronium ion for the interfacial region and several qualitative spectral features of interfacial hydronium ions. Orientational distribution functions indicate that the C-3 axis of the hydronium is tilted with respect to the surface normal, thereby allowing surface proton transfer reactions to occur. Finally, constrained simulations combined with thermodynamic integration are used to compute the potential of mean force for the transfer of the hydronium from the bulk to the interface and into the gas phase as a pure H3O+. The potential of mean force is found to exhibit a shallow free energy minimum of roughly 1.3 kcal/mol with respect to the bulk, in good agreement with very recent calculations based on polarizable force fields and empirical valence bond potentials.