Are Surface Nanobubbles Stabilized by Hydrophobic Attraction? Insights from Molecular Dynamics and Potential of Mean Force Simulations

Surface nanobubbles tend to remain stable due to hydrophobic attraction, as long as the three-phase contact line stays pinned. However, if the binding energy between nitrogen gas molecules and graphene atoms is reduced in the presence of a solvent, the energy becomes insufficient to prevent the escape of gas molecules into the atmosphere through the solvent. We performed molecular dynamics simulations of the nitrogen–water–graphene system to estimate the bubble characteristics up to a simulation time of 0.5 μs. To restrict contact line motion, we introduced surface irregularity and chemical functional groups. Chemical functional groups on the surface proved more effective in pinning the three-phase contact line than geometric heterogeneity. Our findings indicate that with the conventional nitrogen–graphene interaction energy of 3.39kBT gas molecules escape from the surface after 250 ns, and the bubble dissolves within 300 ns, leaving behind individual segregated molecules on the surface. Results from the potential of mean force simulations suggest an interaction energy of 4.05kBT or higher than the conventional value needed for gas molecules to remain adsorbed on the surface. Therefore, the hypothesis that the hydrophobic interaction makes the nanobubble stable is incorrect. Long-time stability requires either a higher interaction energy between the gas molecules and the solid surface or the presence of a layer that serves as a barrier to diffusion.