Breaking the Air-Water Paradigm: Ion Behavior at Hydrophobic Solid-Water Interfaces

TL;DR

This study challenges the traditional view of ion behavior at hydrophobic interfaces by combining advanced spectroscopy and simulations, revealing ions densely adsorb at the surface without disrupting water structure, which impacts energy-related applications.

Contribution

It introduces a new understanding of ion adsorption at hydrophobic solid-water interfaces, showing ions accumulate without altering interfacial water structure, unlike at air-water interfaces.

Findings

Ions densely accumulate at the hydrophobic interface.

Interfacial water structure remains largely unchanged by ions.

Ion populations influence the long-range connectivity of interfacial water.

Abstract

Hydrophobic solid-water interfaces underpin processes in nanofluidics, electrochemistry, and energy technologies. Microscopic insights into these systems are often inferred from our understanding of the air-water interface, which is assumed to exhibit similar behavior. Here, we challenge this paradigm by combining heterodyne-detected vibrational sum-frequency generation spectroscopy with machine-learning molecular dynamics simulations at first-principles accuracy to investigate the graphene-NaCl(aq) interface as a prototypical hydrophobic solid-water system. Spectroscopic results suggest that ions have a minimal effect on the structure of the interfacial water, while simulations reveal that Na$^{+}$ and Cl$^{-}$ accumulate densely at the surface. Together, these findings reveal a new adsorption mechanism that departs from the established air-water interface paradigm, where interfacial ion adsorption is typically associated with, and often detected through, pronounced alteration of the interfacial water alignment and orientation. This difference arises because ions cannot penetrate the solid boundary and reside at a similar depth as the interfacial water molecules. As a consequence, large ion populations can be accommodated within the extended two-dimensional hydrogen-bond network at the interface, causing only minor local distortions but significant changes to its longer-range connectivity. These results reveal a distinct mechanism of electrolyte organization at aqueous-carbon interfaces, relevant to energy applications, where performance is highly sensitive to the local organization of interfacial water.