On supersaturated hydrogen in water: Emphasis on homogeneous nucleation and stability of bulk nanobubbles

Recently, nanobubbles (NBs) have garnered significant attention due to their unique properties. Surface-attached NBs are thermodynamically favored, and their stability is widely accepted. However, the mechanisms underlying the homogeneous nucleation and stability of bulk NBs remain not fully understood. In this work, we have investigated the dynamic evolution of supersaturated hydrogen in water using all-atom molecular dynamics (MD) simulations across varying hydrogen-to-water ratios (Nhydrogen/Nwater) and ambient temperature, focusing on the homogeneous nucleation and stability of bulk NBs. The results demonstrate that as the initial Nhydrogen/Nwater increases, distinct evolutionary states can be identified, including sustained supersaturation, stable nanobubbles, and gas–liquid phase separation. The correlations between Nhydrogen/Nwater, temperature, and dynamic evolution were quantified, showing that stable NBs appear only within an intermediate range. The nucleation was well explained theoretically by free energy analyses. These results show that the Nhydrogen/Nwater significantly affects driving energy, while ambient temperature impacts the energy requirements for bubble nucleation. Additionally, classical nucleation theory was applied to the nanobubbles but was found to overestimate the energy barrier, resulting in predicted nucleation rates several orders of magnitude lower than those obtained from MD simulations. Finally, the mechanisms governing the stability of bulk hydrogen NBs were explored. Both surface effects and supersaturation play critical roles in maintaining the mechanical equilibrium. During the nucleation stage, the diffusion rate of hydrogen molecules is reduced in NBs, yet the diffusion of hydrogen is dominated by Brownian motion, featuring unrestricted behaviors.