Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water

It
has been widely recognized that the pH environment influences
the nanobubble dynamics and hydroxide ions adsorbed on the surface
may be responsible for the long-term survival of the nanobubbles.
However, understanding the distribution of hydronium and hydroxide
ions in the vicinity of a bulk nanobubble surface at a microscopic
scale and the consequent impact of these ions on the nanobubble behavior
remains a challenging endeavor. In this study, we carried out deep
potential molecular dynamics simulations to explore the behavior of
a nitrogen nanobubble under neutral, acidic, and alkaline conditions
and the inherent mechanism, and we also conducted a theoretical thermodynamic
and dynamic analysis to address constraints related to simulation
duration. Our simulations and theoretical analyses demonstrate a trend
of nanobubble dissolution similar to that observed experimentally,
emphasizing the limited dissolution of bulk nanobubbles in alkaline
conditions, where hydroxide ions tend to reside slightly farther from
the nanobubble surface than hydronium ions, forming more stable hydrogen
bond networks that shield the nanobubble from dissolution. In acidic
conditions, the hydronium ions preferentially accumulating at the
nanobubble surface in an orderly manner drive nanobubble dissolution
to increase the entropy of the system, and the dissolved nitrogen
molecules further strengthen the hydrogen bond networks of systems
by providing a hydrophobic environment for hydronium ions, suggesting
both entropy and enthalpy effects contribute to the instability of
nanobubbles under acidic conditions. These results offer fresh insights
into the double-layer distribution of hydroxide and hydronium near
the nitrogen–water interface that influences the dynamic behavior
of bulk nanobubbles.