Stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions

In our previous work [Phys. Chem. Chem. Phys. 2022, 24, 9685], we show with molecular dynamics simulations that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces. This observation closely resembles the stability origin of microemulsions and inspires us to propose here that stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions: the gas-in-water nanobubble phase coexisting with the external gas phase. The stability mechanism for bulk nanobubbles is then given: The formation of compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension, and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles. With the monolayer model, we further interpret several strange behaviors of bulk nanobubbles: the gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension, and the typical nanobubble size of 100nm is due to the small bending constant of the monolayer. Finally, through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can ubiquitously exist in aqueous solutions.