Effects of NaCl on the morphology and interfacial tension of preformed interfacial nanobubbles: an in-situ AFM study.

HYPOTHESIS: Salt ions may accumulate near nanobubble surfaces, forming an ion-enrichment layer that enhances nanobubble stability. It is hypothesized that this layer can markedly increase the stiffness and gas-liquid interfacial tension of nanobubbles. EXPERIMENTS: Interfacial nanobubbles (INB) were generated on a highly oriented pyrolytic graphite (HOPG) by warming chilled water. An air-saturated NaCl stock solution was then introduced and diluted to obtain a concentration series. The evolution of the morphology, stiffness, and gas-liquid interfacial tension of individual INB was determined in-situ by atomic force microscopy (AFM). Changes in macroscopic gas-liquid interfacial tension and air oversaturation were also measured to aid the interpretation of the results. FINDINGS: INB volume increased with increasing NaCl concentration because of the salting-out effect, as confirmed by air-oversaturation measurements. After 12 h in NaCl solution, the INB no longer retained their spherical-cap shape and instead exhibited irregular morphologies. INB stiffness increased sharply upon NaCl addition and then increased more gradually with further increases in NaCl concentration. The INB interfacial tension showed a similar trend, rising sharply after NaCl addition and then increasing gradually with concentration. In contrast, NaCl induced only negligible changes in macroscopic interfacial tension, suggesting a distinct response at INB interfaces.