Observation of Gas Structures at the Interface between Hydrophobic Graphite and Water Using Atomic Force Microscopy
Yi-Hsien Lu1*, Chih-Wen Yang1, Chung-Kai Fang1, Hsien-Chen Ko1, Ing-Shouh Hwang1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* presenting author:Yi-Hsien Lu, email:zonslan@gmail.com
The microscopic picture at the vicinity of the water-solid interface is of fundamental and extreme importance to a great number of physical, chemical, biological, industrial, and environmental processes. The dissolved gases, with only ~ 0.8 mM equilibrium concentration in water (standard conditions), have been attracted only limited attention. However, experimental evidence and simulation studies is growing in support of the critical role of dissolved gases at water interfaces, and particularly for hydrophobic/water interfaces.
In this work, we use high sensitivity atomic force microscopy (AFM) modes, including the frequency modulation (FM) and the PeakForce (PF) modes, to investigate the interface between water and a common hydrophobic surface, highly ordered pyrolytic graphite (HOPG). We show that dissolved gas can accumulate and form an ordered layer at HOPG/water interface, even when the gas concentration is well below the saturation level. When the gas concentration is high, various gas structures can be observed, including ordered epitaxial adlayer(s), disordered pancake-like layer(s), and cap-shaped structures (nanobubbles). We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. These interfacial gas structures may possess novel properties that can explain several scientific problems. Understanding the underlying mechanisms may enable numerous technological applications.

Keywords: Atomic Force Microscopy (AFM), nanobubbles, hydrophobic, dissolved gas