Deconstructing the origins of interfacial catalysis: Why electric fields are inseparable from solvation

Over the past decade, a surge of experiments have shown that certain chemical reactions undergo an enormous boost when transferred from bulk aqueous conditions to microdroplet environments. The microscopic basis of this phenomenon remains elusive and continues to be widely debated. One of the key driving forces invoked is the specific properties of the air-water interface, including the presence of large electric fields and distinct solvation at the surface. Here, using a combination of classical molecular dynamics simulations, chemical physics of solvation, and unsupervised learning approaches, we place these assumptions under close scrutiny. Using phenol as a model system, we demonstrate that the electric field at the surface of water is not anomalous or unique compared to bulk water conditions. Furthermore, the electric field fluctuations de-correlate on a timescale of ∼10 ps, implying that their role in activating much slower chemical reactions remains inconclusive. We deploy a recently developed unsupervised learning approach, dubbed information balance, which detects in an agnostic fashion the relationship between the electric field and solvation collective variables. It turns out that the electric field on the hydroxyl group of the phenol is mostly determined by phenol hydration, including the proximity and orientation of nearby water molecules. We caution that the growing attention to the role that electric fields play in enhancing chemical reactivity at the air-water interface may not reflect their actual importance.