Stabilization of Nanobubbles under Hydrophobic Confinement

The lifetime of nanobubbles can exceed by more than 10 orders of magnitude the theoretical expectation, which predicts an almost immediate dissolution due to their very high Laplace internal pressure. This makes nanobubbles promising candidates for energy applications, as high-pressure nanoreactors in fuel cells, and for gas delivery in biological systems. Here, we use molecular simulation to shed light on the formation and stabilization of nanobubbles in carbon nanotubes. Using an entropic order parameter, we elucidate the nucleation pathway and determine its free energy. We identify a critical volume for which the existence of nanobubbles is thermodynamically favored, with a flat free energy profile around this critical volume, and mechanically favored, since the fluid pressure along the nanotube axis is positive at this juncture. The stabilization process is assisted by the hydrophobic nature of the nanotube and by the formation of strong hydrogen bonds at the interface.