Role of Air Gas at the Interface Between Water and Graphite Surfaces
Ing-Shouh Hwang1*, Chih-Wen Yang1, Chung-Kai Fang1, Yi-Hsien Lu1, Hsien-Chen Ko1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* Presenter:Ing-Shouh Hwang,
The saturation concentrations of nitrogen and oxygen in water under ambient conditions are very small (~10 ppm), thus their roles have been largely ignored. Using advanced atomic force microscopy, we study the evolution of gas-containing structures at graphite/water interfaces at room temperature. Our study indicates that gas (mainly nitrogen and oxygen) molecules dissolved in water tend to adsorb onto hydrophobic/water interfaces [1]. In gas-undersaturated water, we observe gradual nucleation and growth of small two-dimensional (2D) ordered domains over time on graphite surfaces [2]. The ordered structures may eventually cover the entire interface. When water is gas-supersaturated or when fresh DI water is briefly heated, we observe cap-shaped fluid nanostructures in addition to the ordered domains [3]. The cap-shaped nanostructures are the so-called interfacial nanobubbles (INBs) or surface nanobubbles, whose nature, stability, and formation remain controversial. When water is slightly gas-supersaturated, we see evolution of the fluid-like structures. The fluid phase first appears as a circular wetting layer ~0.3 nm in thickness and is later transformed into a cap-shaped INB [4]. 2D ordered domains are nucleated and grow over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that INBs and the fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature.

Our study suggests that, in gas-undersaturated water, dissolved gas molecules may mainly be in the dispersed monomer form. Their rearrangement with water molecules at hydrophobic/water interface may lead to gradual nucleation and growth of the ordered domains. In gas-supersaturated water, some dissolved gas molecules are well dispersed in water, but others may aggregate into clusters. Adsorption of gas clusters leads to the formation of circular fluid layers at the graphite/water interface. The work clearly shows the crucial role of gas molecules at hydrophobic/water interfaces and has broad implications in diverse research fields.

[1] H.-C. Ko, W.-H. Hsu, C.-W. Yang, C.-K. Fang, Y.-H. Lu, I.-S. Hwang, Langmuir 32, 11164 (2016)
[2] Y.-H. Lu, C.-W. Yang, and I.-S. Hwang, Langmuir 28, 12691 (2012).
[3] Y.-H. Lu, C.-W. Yang , C.-K. Fang, H.-C. Ko, I.-S. Hwang, Sci. Rep. 4, 7189 (2014).
[4] C.-K. Fang, H.-C. Ko, C.-W. Yang , Y.-H. Lu, I.-S. Hwang, Sci. Rep. 6, 24651 (2016).

Keywords: water, hydrophobic/water interface, nanobubbles, atomic force microscopy