What Is the Contact Angle of Water on Graphene?

TLDR

Experimental and theoretical work has examined graphene wetting, but the contact angle of water on a single graphene layer remains unknown, with graphite reported at 90–95° and graphene speculated to be as high as 127° or only slightly higher. The study uses classical MD simulations to argue that the realistic water contact angle on graphene is 95–100°, not 127°. The authors employ classical MD with empirical force fields, linking the work of adhesion to the water–graphene interaction potential and attributing the remaining contribution to entropic fluctuations. They find that a 90°–127° angle difference would require equal interaction energy from the first two graphite layers, contradicting the short‑range water–graphene interaction, and that the adhesion is dominated by a layer‑independent potential energy term with an entropic component.

Abstract

Although experimental and theoretical studies have addressed the question of the wetting properties of graphene, the actual value of the contact angle of water on an isolated graphene monolayer remains unknown. While recent experimental literature indicates that the contact angle of water on graphite is in the range 90–95°, it has been suggested that the contact angle on graphene may either be as high as 127° or moderately enhanced in comparison with graphite. With the support of classical molecular dynamics simulations using empirical force-fields, we develop an argumentation to show that the value of 127° is an unrealistic estimate and that a value of the order of 95–100° should be expected. Our study establishes a connection between the variation of the work of adhesion of water on graphene-based surfaces and the interaction potential between individual water molecules and these surfaces. We show that a variation of the contact angle from 90° on graphite to 127° on graphene would imply that both of the first two carbon layers of graphite contribute approximately the same interaction energy with water. Such a situation is incompatible with the short-range nature of the interaction between water and this substrate. We also show that the interaction potential energy between water and the graphene-based substrates is the main contribution to the work of adhesion of water with a relative magnitude that is independent of the number of graphene layers. We introduce the idea that the remaining contribution is entropic in nature and is connected to the fluctuations in the water–substrate interaction energy.