Two-dimensional water-molecule-cluster layers at nanobubble interfaces

Hypothesis: Bulk nanobubbles (NBs) have high surface charge densities and long lifetimes. Despite several attempts to understand the lifetime of NBs, their interfacial layer structure remains unknown. It is hypothesized that a specific interfacial layer exists with a hydrogen bond network that stabilizes NBs.Experiments: In situ infrared reflectance-absorption spectroscopy and density functional theory were used to determine the interfacial layer structure of NBs. Furthermore, nuclear magnetic resonance spectroscopy was used to examine the interfacial layer hardness of bubbles filled with N-2, O-2, and CO2, which was expected to depend on the encapsulated gas species.Findings: The interfacial layer was composed of three-, four-, and five-membered ring clusters of water molecules. An interface model was proposed in which a two-dimensional layer of clusters with large electric dipole moments is oriented toward the endohedral gas, and the hydrophobic surface is adjacent to the free water. The interfacial layer hardness was dependent on the interaction with the gas (N-2 > O-2 > CO2), which supports the proposed interface model. These findings can be generalized to the structure of water at gas-water interfaces.

Automated summary

The interfacial layer structure of bulk nanobubbles was determined, revealing the presence of three-, four-, and five-membered ring clusters of water molecules. It was hypothesized that a specific interfacial layer with a hydrogen bond network stabilizes the nanobubbles. The hardness of the interfacial layer was found to be dependent on the encapsulated gas species.