Hydrated Excess Proton at Water-Hydrophobic Interfaces

The behavior of the hydrated excess proton is investigated at the water-vapor, water-hydrophobic wall, and water-carbon tetrachloride interfaces through molecular dynamics simulations using the third-generation multistate empirical valence bond model (MS-EVB3). The MS-EVB3 simulations show a surface preference of the excess proton at the water-vapor interface, consistent with the discovery of this effect using an earlier version of the MS-EVB model (Petersen et al. J. Phys. Chem. B 2004, 108, 14804) and with the experimental results. The preference of the hydrated excess proton for other water-hydrophobic interfaces is also analyzed for the first time. The hydrated proton structures and charge defect delocalization effects at these interfaces are discussed in detail. By decomposing the free energy profiles into the internal energy and entropic contributions, the thermodynamic (free energy) driving forces for the surface preference of the excess proton are also elaborated. These results indicate that the rigid hydrated proton structures at all the interfaces are energetically (as opposed to entropically) stabilized due to the amphiphilic nature of the hydrated excess proton, resulting in its overall interfacial concentration enhancement. The effects of acid concentration and nuclear quantization are also explored.