Ionic Effects on Bulk Nanobubble Formation and Persistence in Aqueous Media

Bulk nanobubbles are nanoscale gas cavities dispersed in the liquid phase, and their experimental persistence contrasts with the classical predictions of rapid dissolution associated with Laplace pressure and diffusive transport. This discrepancy has given rise to a stability paradox between classical thermodynamic/diffusive predictions and experimentally observed kinetic persistence, which is exacerbated by the broad use of the term “stability” to describe distinct phenomena such as initial formation, colloidal stability, diffusive persistence, population persistence, and operational performance. This review analyzes the physicochemical mechanisms governing the formation and persistence of bulk nanobubbles in aqueous media, emphasizing the role of ionic composition. It discusses the limitations of classical models, nonlinear effects of ionic strength, ion specificity, preparation pathways, collective dynamics, and limitations associated with experimental characterization metrics. The analysis shows that the ionic strength does not act unidirectionally: it can favor initial formation by reducing gas solubility and promoting local supersaturation, but it can also accelerate the loss of colloidal stability through compression of the electric double layer. Furthermore, ionic identity, the timing of electrolyte incorporation, and bubble–bubble interactions condition the temporal evolution of the population. Based on this interpretation, bulk nanobubble stability cannot be evaluated as a single property or solely based on the initial concentration, average size, or zeta potential. It must be interpreted according to the stage of the system, the aqueous matrix, the generation pathway, and the desired operational function.