Effect of Organic Compounds and Alkalinity on the Stability of Bulk Nanobubbles: A Molecular Dynamics Study

Bulk nanobubbles (NBs) are remarkably long-lived in liquids, yet the molecular mechanisms underpinning their stability remain unresolved. In this work, 50 ns all-atom molecular dynamics simulations were performed to investigate how gas identity (O₂, N₂, and air with N₂:O₂ = 4:1), initial gas loading, alkalinity (pH 7 and 13), and organic additives (acetic acid/acetate, ethanol/ethoxide, and hexane) influence the stability of 5 nm NBs in water. Stability was evaluated by the percentage of gas atoms retained in the bubble, density profiles, hydrogen-bond statistics, and radial distribution functions. Higher initial gas density markedly enhanced stability, and N₂-NBs consistently outperformed O₂-NBs, consistent with the lower solubility of N₂. Alkaline conditions exerted only a minor stabilizing effect, most pronounced for air-NBs. Organic additives affected stability according to their hydrophobicity: hydrophobic hexane substantially increased gas retention, especially at low gas loading, by promoting gas clustering and re-adsorption at the NB interface, whereas hydrophilic solutes had negligible influence. RDF analyses revealed that this stabilization correlates with weakened gas-water hydrogen bonding and enhanced gas-gas and gas-hexane interactions. These results elucidate the molecular determinants of NB persistence and offer design guidelines for tuning bubble longevity in environmental and industrial systems.