Crystalline Water Structure in Room-Temperature Clathrate State: Hydrogen-Bonded Pentagonal Rings

Ing-Shouh Hwang^1*, Ching-Hsiu Chen¹, Wei-Hao Hsu¹, Ryoko Oishi-Tomiyasu², Chi-Cheng Lee³, Ming-Wen Chu⁴

¹Institute of Physics, Academia Sinica, Taipei, Taiwan
²Instituteof Mathematics for Industry, Kyushu University, Fukuoka, Japan
³Department of Physics, Tamkang University, New Taipei, Taiwan
⁴Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan

* Presenter:Ing-Shouh Hwang, email:ishwang@phys.sinica.edu.tw

Water hydrogen bonding is extremely versatile; approximately 20 ice structures and several types of clathrate hydrate structures have been identified. These crystalline water structures form at temperatures below room temperature and/or at high pressure. We used transmission electron microscopy to study a new crystalline water structure in a clathrate state that is prepared by sandwiching gas-supersaturated water between graphene layers under ambient conditions [1-3]. In this clathrate state, water molecules form a three-dimensional hydrogen bonding network that encloses gas-filled cages 2–4 nm in size. We derived the crystalline water structure by recording and analyzing electron diffraction patterns and performing first-principles calculations. The structure consists purely of pentagonal rings and has a topology similar to that of water ice XVII. The study proposed a mechanism for the formation of the clathrate state. Determining the water structure is essential for understanding the stability and formation mechanism of this clathrate state as well as the interactions between water and small non-polar molecules. It also helps further theoretical and experimental characterization of various properties of this new clathrate hydrate state. The present results offer novel insights into the local structures of ambient liquid water.

[1] W.-H. Hsu, I.-S. Hwang, Investigating States of Gas in Water Encapsulated between Graphene Layers, Chem. Sci. 12, 2635 (2021).
[2] W.-H. Hsu, T.-C. Yen, C.-C. Chen, C.-W. Yang, C.-K. Fang, and I.-S. Hwang, Observation of mesoscopic clathrate structures in ethanol-water mixtures, J. Mol. Liq. 366, 120299 (2022).
[3] C.-H. Chen, W.-H. Hsu, R. Oishi-Tomiyasu, C.-C. Lee, M.-W. Chu, and I.-S. Hwang, Crystalline Water Structure in Room-Temperature Clathrate State: Hydrogen-Bonded Pentagonal Rings, arXiv:2409.00415.

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