  It has been recently shown that nanobubbles exhibit a remarkable and
unexpected stability. The lifetime of nanobubbles, formed either within liquids
or on hydrophobic surfaces, can exceed by more than 10 orders of magnitude the
theoretical expectation, which predicts an almost immediate dissolution due to
the very high Laplace internal pressure in such small bubbles. This unexpected
property of nanobubbles has made them leading candidates for energy
applications, e.g. as high-pressure nanoreactors in fuel cells, and for
biological systems, as transport systems for gas delivery to membranes and
cells. Here we use molecular simulation to shed light on the molecular
mechanisms accounting for the formation and stabilization of nanobubbles under
an hydrophobic nanoconfinement. Using an entropic reaction coordinate, we
elucidate the nucleation pathway and determine the formation free energy of
nanobubbles in water confined in carbon nanotubes. We identify a critical
volume for which the existence of nanobubbles is thermodynamically favored, as
the free energy profile flattens around this critical volume, and mechanically
favored, since the nanoconfined fluid pressure, along the nanotube axis, is
positive at this juncture. We also show that the stabilization process is
assisted by the hydrophobic nature of the carbon nanotube and by the formation
of strong hydrogen bonds at the interface. Caroline Desgranges and Jerome
Delhommelle
