Structural Stabilization Mechanisms and Energy Distribution Patterns of Nanobubbles with Different Sizes

Traditional macroscopic interfacial theory cannot fully explain the anomalous stability of nanobubbles, which limits their use in micro/nano interfacial engineering. In this study, molecular dynamics simulations were used to systematically investigate the evolution of oxygen nanobubbles with diameters of 3, 5, and 8 nm in water. The results show a clear size effect in bubble structural evolution: the 3 nm bubble had the largest motion radius, about 3.5 nm, which was 40% and 133% larger than those of the 5 and 8 nm bubbles, respectively. At the nanoscale, surface tension still provides a basic constraint, but its value is lower than the classical theoretical prediction, with a deviation of 35.7% for the 3 nm bubble, indicating that it is not the sole dominant stabilizing force. Water molecules at the gas–liquid interface experienced a net force of 371.92–421.56 pN, with 60–65% directed toward the bubble interior, forming an asymmetric force field that cooperates with surface tension to maintain bubble stability. This force field further drives directional polarization of water molecules and induces a dense hydrogen-bond network: stronger charge parameters correspond to denser hydrogen bonding, with the peak net charge of Charge4 being 37.5 times that of Charge1 and the hydrogen-bond number being 11.6 times that of Charge2. The dense hydrogen-bond network is quantitatively associated with restricted interfacial water mobility. The interfacial potential energy decreases, with an 8 nm bubble showing a reduction of 0.31 kcal/mol, and the U–T coupling energy exhibits a clear interfacial peak of 36–66 kcal/mol. Further analysis shows that the apparent diffusion coefficient decreases nearly exponentially with the maximum hydrogen-bond number, and the water MSD decreases markedly as hydrogen bonding increases, with the MSD of Charge1 being 1303 times that of Charge4. These results indicate that enhanced interfacial electrostatic force, densification of the hydrogen-bond network, and restricted interfacial water mobility jointly increase structural constraints at the gas–liquid interface and suggest a possible increase in gas-transfer resistance across the interface. This study clarifies, at the molecular scale, a physical picture in which surface tension, electrostatic force, hydrogen-bond densification, energy stabilization, and restricted mobility jointly maintain nanobubble stability.