Deconstructing the Origins of Interfacial Catalysis: Why Electric Fields are Inseparable from Solvation

TL;DR

This study critically examines the role of electric fields and solvation at the air-water interface in catalysis, using simulations and machine learning to challenge assumptions about their significance in reaction enhancement.

Contribution

It demonstrates that electric fields at the water surface are similar to bulk conditions and are primarily influenced by solvation, questioning their presumed unique role in interfacial catalysis.

Findings

Electric field fluctuations at the surface are not anomalous compared to bulk water.

Electric field fluctuations de-correlate within ~10 ps, making their role in slow reactions uncertain.

Electric fields on phenol are mainly determined by hydration and water molecule orientation.

Abstract

In the last decade, there has been a surge of experiments showing that certain chemical reactions undergo an enormous boost when taken 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 are 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, the 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 of the role that electric fields have garnered in enhancing chemical reactivity at the air-water interface, may not reflect their actual importance.