Investigating States of Gas in Water Encapsulated between Graphene Layers
Ing-Shouh Hwang1*, Wei-Hao Hsu1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* Presenter:Ing-Shouh Hwang,
Dissolution of gases in liquid water is a general, fundamental, and critical phenomenon across living and nonliving things. Despite numerous studies over the last century, there remain many mysteries about gas dissolved in water, such as the nucleation mechanism of gas bubbles in water, whether nanobubbles exist in bulk water, the abnormal thermodynamic properties of gas-saturated water, and more. Conventionally, only two states are assumed to exist in water: well-dispersed gas monomers and gas bubbles. Rarely is this paradigm explored experimentally. To close this gap, we use transmission electron microscopy (TEM) to study degassed water, deionized water, and water supersaturated with pure gas (N2, O2, Ar, Xe, CO2, and SF6) at room temperature; we encapsulate water between two laminated graphene layers spanning the holes in TEM grids. While neither degassed water nor deionized water yields specific features, two major microscopic structures are evident in gas-supersaturated water: (1) individual polycrystalline nanoparticles formed of gas molecules (typically several nanometers in diameter) in liquid water, and (2) clathrate hydrate structures (often ~100 nm or larger in lateral size). In the clathrate structures, water molecules form a polycrystalline matrix hosting a high density of gas-containing cells (cell separation of 4-8 nm). Dark-field TEM imaging reveals that water molecules surrounding each cell form crystalline structures—a surprising discovery of a clathrate hydrate state in gas-supersaturated water that may help resolve several long-standing puzzles. Together, these observations challenge the conventional view of gas in water and reveal intricate and surprising interactions among water molecules and gas molecules [1].

[1] W.-H. Hsu and I.-S. Hwang, Chem. Sci. 12, 2635 (2021).

Keywords: gas, water, transmission electron microscopy, clathrate hydrate