Hydrated Protons at the Water–Air and Water–Oil Interfaces: Structure and Dynamics

The hydrated excess proton is central to many chemical and biological processes, yet its interfacial behavior remains largely unknown. Here, we employ Multistate Empirical Valence Bond (MS-EVB) simulations to investigate the structure and dynamics of the hydrated excess proton at the water-air and water-oil (water-cyclohexane) interfaces. Free energy profiles reveal a modest affinity (around 0.3 kcal/mol) of the proton for hydrophobic-hydrophilic interfaces. Radial distribution functions and joint probability distributions show that presolvation by a fourth water molecule, essential for proton transfer, diminishes as the proton moves into the vacuum or cyclohexane phase. The excess proton complex adopts a more "Zundel-like" structure near interfaces and into the hydrophobic phase. Proton dynamics, assessed via hydronium identity correlation and decomposed lateral diffusion coefficients, demonstrate that interfacial environments strongly modulate proton mobility and hopping kinetics. A strong anticorrelation between the discrete (hop) and continuous (vehicular) diffusion is observed when the excess proton is near the interfaces and into the cyclohexane phase. These findings provide molecular-level insight into how interfaces influence thermodynamics, solvation, and transport of the hydrated excess protons and clarify key factors that govern proton mobility in heterogeneous environments. This work furthermore offers a foundation for understanding hydrated proton behavior in more complex interfacial systems and has broad implications for various related applications.