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

TL;DR

The study interrogates whether interfacial electric fields at the air–water boundary are distinct drivers of accelerated interfacial chemistry or whether solvation governs the field that couples to reactivity. Using phenol as a model, it combines classical MD with Information Imbalance analyses (including the differentiable extension, DII) to quantify how solvation coordinates relate to the field along the phenol O–H bond. The results show that interfacial Fields are not inherently stronger or slower than bulk fields; fluctuations occur on a timescale of about $10$ ps and the field is mainly determined by proximal water molecules, especially their orientation and distance to phenol. The II/DII analyses reveal a strong coupling between solvation structure and the electric field, suggesting that solvation coordinates may be the primary drivers of interfacial reactivity rather than intrinsic interfacial fields alone. These findings urge caution when attributing enhanced interfacial chemistry to static surface fields and highlight solvation as a central, inseparable factor in interfacial catalysis.

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.

Figures (19)

• Figure 1: Snapshots of the three systems studied in this work: a) phenol in the bulk, b) one phenol molecule at the air/water interface (1PHX) and c) 25 phenol molecules at both interfaces of a water slab (25PHX).
• Figure 2: a) Probability density distribution of the magnitude of the electric field at the midpoint of the O-H bond of randomly selected water molecules (dotted curve) and of phenol molecules in all the three systems studied in this work (solid lines). b) Magnitude of the electric field generated by the water molecules at the midpoint of the O-H bond of the phenol molecules for the three systems studied as a function of the distance to this bond. As expected the decay goes as $1/r^2$.
• Figure 3: Autocorrelation function of the $\bar{E}$ decomposed in the three cartesian components and the total $|\bar{E}|$, for the three systems studied: a. bulk, b. 1PHX and c. 25PHX.
• Figure 4: Density distributions showing the correlation between the $|proj\bar{E}|$ and the OO distance between the phenol ($O_{PHX}$) and the four nearest water molecules ($O_w$) for the a. bulk, b. 1PHX and c. 25PHX. The Pearson correlation coefficient (r) is showed in each panel.
• Figure 5: Density distributions showing the correlation between each cartesian component of the projected electric field ($projE_i$) and the corresponding component of the dipole vector of the four nearest water molecules ($\mu_i$) in the 1PHX system.