Wetting Transparency of Graphene: A macroscopic Window but Nanoscopic Mirror

TL;DR

The study addresses whether graphene truly modulates interfacial water when placed on charged substrates. By combining heterodyne-detected SFG spectroscopy with atomistic simulations, it shows that CaF$_2$ surface electrostatics govern water orientation at the CaF$_2$/water and CaF$_2$-Gr/water interfaces, accounting for graphene’s macroscopic wetting transparency; however, graphene's polarizability induces a nanoscale mirror-like charge that locally alters the incipient water layer, with these effects averaging out in bulk spectra. This duality clarifies the molecular origin of graphene's apparent transparency and highlights nanoscale deviations relevant for nanofluidic, sensing, and energy applications. The work provides a molecular-level mechanism for tuning interfacial phenomena via substrate charge, graphene polarizability, and nanoscale field distributions, suggesting new routes to control water flow, friction, and reactivity at graphene-based interfaces.

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.

Figures (3)

• Figure 1: Interfacial water structure at CaF$_2$/water and CaF$_2$–Gr/water interfaces under varying pH conditions is dominated by electrostatic effects and remains largely unaffected by graphene at the macroscopic scale. a) Schematic representation of the charged water interfaces. b) The O-H stretching Im$\chi^{(2)}$ spectra were measured at varying pH conditions. c) Net surface charge density inferred from the bulk contributions by comparing SFG signals at different NaCl concentrations, at varying pH conditions for the two interfaces.
• Figure 2: Graphene-induced polarization reshapes the initial interfacial water layer but vanishes within the first nanometer. Water structure and orientation at the CaF$_2$–graphene interface ($-19.0$ mC/m$^2$) (a) Oxygen density profile ($\rho$) as a function of distance from graphene, highlighting the incipient and first interfacial layers by blue and blue+red shaded areas, respectively. The incipient layer is defined as the region within 3 Å of the graphene (see main text). (b) Angular distribution $P(\theta$) of water molecules in the incipient layer for polarizable (red) and non-polarizable (blue) graphene models. Vertical dashed lines represent the average value, while horizontal dashed lines represent the uniform distribution present in bulk. (c) Same as b) for the whole first interfacial layer, where the impact of graphene polarization becomes negligible.
• Figure 3: Graphene polarizability inverts the effective sign of the localized charge, inducing an inversion in the orientation of water molecules. a) Spatial orientational average of the incipient interfacial water layer for non-polarizable graphene with a surface charge of $\approx$ +20 mC/m$^2$. b) Same as a), for a surface charge of $\approx -20$ mC/m$^2$. Black circles denote the positions of the localized charges. c-d) show cartoon representations of the interface, the charges, and water orientations corresponding to a-b), respectively. Blue and red dots represent negative and positive charges, respectively. Panel e-h) show the corresponding results for polarizable graphene.