Evaporation Kinetics and Vibrational Sum Frequency Generation Spectroscopy of Water Layers on Graphene and Graphene Oxide Surfaces

Graphene and graphene oxide (GO) sheets have different surface characters with respect to their hydration properties, hence can influence the kinetics of evaporation of thin water layers on them in a different manner. In the current study, we have explored the evaporation dynamics of water layers from graphene and graphene oxide surfaces by means of molecular dynamics simulations and also performed the associated free energy calculations using an enhanced sampling technique. In particular, we have focused on the effects of surface hydrophobicity/hydrophilicity and the thickness of water layers on the evaporation behavior, and our findings are further corroborated by vibrational sum frequency generation spectroscopic calculations. The current study shows that graphene promotes higher evaporation rates than GO, especially for thinner water layers. This is likely due to the strengthening of hydrogen bonds as evident from the relatively increased intensity and red-shifted O-H peak at ∼3350 cm^-1 in the VSFG spectrum of the water layer on GO compared to that on graphene surfaces. The potential of mean force (PMF) profiles reveal a lower free energy barrier for water evaporation for graphene than for GO, suggesting that the hydrophobic nature of graphene enhances evaporation of water. Kinetic energy analysis shows significant momentum transfer between interacting molecules, while hydrogen bond analysis further emphasizes the role of molecular interactions in evaporation dynamics. These results provide key insights into the evaporation kinetics of water layers on hydrophobic and hydrophilic surfaces.