The hydrated proton was studied at the water liquid/vapor interface using the multistate empirical valence bond (MS-EVB) methodology, which enables the migration of the excess proton to and about the interface through the fluctuating bond topology described by the Grotthuss shuttle mechanism. It was found in our model that the hydrated excess proton displays a marked preference for water liquid/vapor interfaces. The resulting stable surface structures can be explained through an examination of the bond network formed between the water/proton moiety and solvating water. These results suggest the excess proton can effectively behave as an amphiphile, displaying both hydrophobic and hydrophilic character.In this communication, interesting new results are reported for the excess proton at the water liquid/vapor interface. These results are at odds with the conventional concepts of ionic solvation and have their origin in the strong solvation asymmetry of the hydronium cation. In particular, the water liquid/vapor interface for the H + (H 2 O) 1000 Cl -system, constituting a "slab" 1,2 geometry for the water molecules and the two ions, was studied via molecular dynamics (MD) simulations using rectangular periodic boundaries at 300 K and a constant volume of dimensions 31.2 × 31.2 × 75.0 Å 3 . Starting configurations were generated from a constant temperature trajectory after an initial equilibration of 500 ps with the Nose-Hoover thermostat. Ten independent microcanonical trajectories were then collected, for a total of 2.5 ns of simulation time, using MD simulations performed with the MS-EVB2 model, 3,4 and the Ewald summation method was implemented for all electrostatic interactions. The MS-EVB2 model has been successfully used to treat proton transport in bulk liquids and several biological systems. [5][6][7][8] The important distinction in this approach is that the definition of the protonated species can change during the dynamical process; that is, the proton can hop along an optimal conformation of water molecules consistent with the Grotthuss mechanism of proton transfer. 9,10 As a result of the present simulation, it was found that the proton was preferentially distributed on the surface of the water/vacuum interface. This surface localization of the hydronium was also evident for a simple "classical" model of the cation, that is, one which is unable to participate in Grotthuss hopping (see Figure 1). The orientation of the hydrated proton is such that the lone-pair side was directed away from the aqueous portion of the interface. A representative structure from an MS-EVB2 trajectory is shown in Figure 2 with the hydronium (orange) located at the interface and the counterion, in this case chloride (green), visible several molecules below the surface. Although not discussed here in detail, it is worth noting that the simulated chloride ion's radial distribution, diffusion, and coordination are in agreement with previously published values. 11,12 While the phenomenon of the "surface" excess proton observed in ...