Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance

• Published in: The journal of physical chemistry letters Vol. 10; no. 13; pp. 3744 - 3749
• Main Authors: Li, Qin-Yi, Matsushita, Ryo, Tomo, Yoko, Ikuta, Tatsuya, Takahashi, Koji
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.07.2019
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.9b00718

Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid–gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68–104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid–gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement.