Revisiting the Thickness of the Air-Water Interface from Two Extremes of Interface Hydrogen Bond Dynamics

TL;DR

This study introduces a novel simulation-based method to estimate the air-water interface thickness, revealing convergence at approximately 4 Angstroms based on hydrogen bond dynamics, validated by multiple spectroscopic analyses.

Contribution

The paper presents a new approach using density functional theory and deep potential simulations to determine the interface thickness, considering extreme hydrogen bond dynamics scenarios.

Findings

Interface hydrogen bond dynamics converge at about 4 Angstroms thickness.

The convergence point is validated by anisotropic OH stretch decay and free OH dynamics.

The method offers a different perspective from previous estimates of 3-10 Angstroms.

Abstract

The air-water interface plays a crucial role in many aspects of science, because of its unique properties, such as a two-dimensional hydrogen bond (HB) network and completely different HB dynamics compared to bulk water. However, accurately determining the boundary of interfacial and bulk water, that is, the thickness of the air-water interface, still challenges experimentalists. Various simulation-based methods have been developed to estimate the thickness, converging on a range of approximately 3--10 (Angstrom). In this study, we introduce a novel approach, grounded in density functional theory-based molecular dynamics and deep potential molecular dynamics simulations, to measure the air-water interface thickness, offering a different perspective based on prior research. To capture realistic HB dynamics in the air-water interface, two extreme scenarios of the interface HB dynamics are obtained: one underestimates the interface HB dynamics, while the other overestimates it. Surprisingly, our results suggest that the interface HB dynamics in both scenarios converges as the thickness of the air-water interface increases to 4 (Angstrom). This convergence point, indicative of the realistic interface thickness, is also validated by our calculation of anisotropic decay of OH stretch and the free OH dynamics at the air-water interface.