Ultra-high Density of Gas Molecules Confined in Surface Nanobubbles in Ambient Water.

To understand the unexpected and puzzling long-term stability of nanoscale gas bubbles, it is crucial to probe their nature and intrinsic properties. We report synchrotron-based scanning transmission X-ray microscopy (STXM) evidences of highly condensed oxygen gas molecules trapped as surface nanobubbles. Remarkably, the absorption spectra from individual nanobubble revealed that the oxygen density inside was 1-2 orders of magnitude higher than atmospheric air. In addition, the oxygen concentration in the surrounding water was estimated to be hundreds of times higher than the oxygen solubility. Molecular dynamics simulations further confirmed that surface nanobubbles with a high internal gas density would remain stable in an oversaturated liquid. Our observations help explain well the surprisingly long lifetime of the nanobubbles and shed light on nanoscale gas aggregation behaviors.