Wetting Transparency of Graphene: A macroscopic Window but Nanoscopic Mirror

TL;DR

This study combines spectroscopy and simulations to reveal that graphene's wetting transparency is macroscopic, but at the nanoscale, its polarizability causes local water orientation inversion influenced by substrate charges.

Contribution

It demonstrates how substrate charges and graphene's polarizability influence water structure at the interface, combining experimental and computational approaches.

Findings

Graphene's macroscopic wetting transparency confirmed.

Nanoscale water orientation can invert due to graphene's polarizability.

Substrate pH-dependent electrostatics govern interfacial water orientation.

Abstract

Graphene supported on a substrate in contact with water underpins a wide range of processes and technologies, yet its wettability remains controversial. Understanding how substrate charges and graphene's properties influence water organization is crucial. Here, we combine heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy with molecular dynamics simulations to investigate CaF$_2$-supported graphene interfaces in contact with water. We find that interfacial water orientation is primarily governed by the CaF$_2$ substrate's pH-dependent local electrostatics, confirming graphene's macroscopic wetting transparency. However, at the nanoscale, graphene's polarizability induces a local inversion of water's molecular orientation above substrate charges, revealing subtle structural ordering that is masked in spatially averaged measurements. These insights elucidate the molecular origins of graphene's wetting behavior and suggest new avenues to tailor interfacial phenomena in graphene-based nanofluidic, sensing, and energy applications.