Exploring the behaviour of the hydrated excess proton at hydrophobic interfaces

The affinity of the excess proton for the aqueous solution-hydrophobic interface was examined for two specific examples, the air-water and hydrophobic wall-water cases, using a multiconfigurational molecular dynamics algorithm. The use of a reactive simulation method is important as it allows for a realistic description of the excess proton, namely, its propensity to hop between water molecules via the Grotthuss mechanism. The free energy profile reveals a minimum at these interfaces due to a favourable enthalpic term that outweighs the entropic penalty. The key factors that contribute to this enthalpic minimum were examined using a generalization of a scheme that decomposes the interaction energy into separate terms arising from various local environments [Otten et al., Proc. Natl. Acad. Sci. USA, 109, 701 (2012)] (coordination shell, bulk, and interface) and the delocalization energy (which allows the proton to hop). For both systems, it was observed that the energetic penalty for loss of coordinating water molecules as the excess proton moves toward the hydrophobic interface is more than compensated by the displacement of unfavourable interfacial water molecules. In addition, the ion becomes more delocalized, more Zundel-like, and therefore possesses a larger effective radius as it moves to the interface. The fluctuations of the instantaneous interface were reduced near the vicinity of the ion, thereby giving rise to an entropic penalty. This paper will discuss the application of energy decomposition schemes to multiconfigurational simulations and the resulting consequences realized for the excess proton at hydrophobic interfaces.