Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2019, 10, 3744−3749
Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance Qin-Yi Li,†,‡ Ryo Matsushita,† Yoko Tomo,† Tatsuya Ikuta,†,‡ and Koji Takahashi*,†,‡ †
Department of Aeronautics and Astronautics, Kyushu University, Fukuoka 819-0395, Japan International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan
‡
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S Supporting Information *
ABSTRACT: 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.
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nonvacuum environment inside the CNTs. Conventionally, researchers believe that TEM can only be applied in observing fluid in CNTs with two “closed ends” since the liquid should be quickly evaporated when exposed to the high vacuum environment of TEM.8−11 However, our group12 has recently discovered superstable ultrathin water films in “open-ended” hydrophilized CNTs exposed to high vacuum (1.4 nm diameter should exhibit a bulk-water phase. Meanwhile, another MD simulations work26 reported that water confined in CNTs with 1.49−4.20 nm diameters produced little vapor pressure even at temperatures much higher than the boiling point of bulk water. An experimental study5 showed that water inside single-walled CNTs with 1.05−1.52 nm diameters exhibits ∼100 °C elevation of the freezing point, which can be explained by the rigid hydrogen-bonded network. In our cupstacked CNTs with ∼50 nm diameter, the central part of the observed water film with a thickness of a few to tens of molecules can also possibly form a strong hydrogen-bonded 2D membrane under confinement, which results in an intermediate phase between liquid and solid. Furthermore, the inflected meniscus can play an important role in the water film stability. As illustrated in Figure 2(e), the water molecules in the extended meniscus can be captured onto the zigzag inner-wall surface and symmetrically provide attraction to the central thin film, which can strengthen the film stability.
experiments. In addition, the interaction between hydrophobic surfaces and water molecules should be relatively weak. MD simulations showed that there is a depletion layer near the hydrophobic surface inside single-wall CNTs (diameter ≈ 7 nm) due to the lack of hydrogen bond, which results in ultrahigh water transport rate in the CNTs.22 Theoretical calculations also predicted that water films with sub-100 nm sizes between two flat, hydrophobic surfaces will form depletion layers near the surfaces and soon dry up.8,23 Thus, the stability of our observed water films under high vacuum is beyond the explanation of conventional theoretical studies. We attribute the stability of the ultrathin water films to (1) the strong interactions among water molecules and (2) the adsorption of water molecules on the zigzag inner-wall surfaces. Previous MD simulations have shown that water confined in CNTs with ∼1 nm diameter exhibits an intermediate stable state with both solid- and fluid-like properties.24,25 Nevertheless, Pascal and co-workers25 found from MD simulations that a critical CNT diameter exists for 3746
DOI: 10.1021/acs.jpclett.9b00718 J. Phys. Chem. Lett. 2019, 10, 3744−3749
Letter
The Journal of Physical Chemistry Letters
Figure 3. Filled water with nanoscale bubbles. (a−d) TEM images of large amount of filled water with bubbles; (b) magnified image of the red square area in panel (a), where the contact angles are measured; (e) schematic illustration of the water-bubble structure.
Figure 4. TEM images of the zigzag-shaped interface observed in two samples. (a) One sample showing the open end and water meniscus, with the inset illustrating the interface structure; (b) another sample showing the zigzag interface.
When we filled water into the CNTs by the vacuumchamber-assisted condensation method under very high chamber humidity (see the Supporting Information for details), we can observe several CNTs filled with large amounts of water, as shown in Figures 3 and 4. In some
CNT samples, nanoscale bubbles were observed inside water or at the liquid−solid interface, as shown in Figure 3. These bubbles should have been generated from the dissolved air in water under the TEM high-vacuum environment. Besides, the hydrophobic inner wall should be advantageous for the 3747
DOI: 10.1021/acs.jpclett.9b00718 J. Phys. Chem. Lett. 2019, 10, 3744−3749
The Journal of Physical Chemistry Letters
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ACKNOWLEDGMENTS This work was partially supported by the JST CREST Grant Number JPMJCR18I1, Japan, and JSPS KAKENHI (Grant Nos. JP18K13704, JP17H03186 and JP16H04280). The Ultramicroscopy Research Center, Kyushu University, is highly appreciated for TEM observations.
generation of interfacial nanobubbles. In the sample shown in Figure 3(a,b), a ∼3 nm-thin water film is sandwiched between the high-vacuum environment and an interfacial nanobubble, where we measured two contact angles to be 68° and 97°. Besides the stability of the nanometer thin water film, it has been reported that nanoscale interfacial bubbles also have very long lifetime due to the pinning effect.27−29 Hence, the meniscus of the filled water in Figure 3(a,b) does not move or deform due to the block from the stable water film and nanobubble. In the samples in Figures 3(c,d), we observed many bulk bubbles inside water as well as interfacial bubbles. The stable nanometer thin water films between these nanobubbles hindered the bubbles from deforming, coalescing, or escaping to the vacuum, and thus, these water-bubble structures as illustrated in Figure 3(e) remained superstable during the observation. In Figure 4, the liquid−gas interface of the confined water film clearly exhibited zigzag edges. This phenomenon is beyond surface-tension dominance since the surface tension tends to pull the water surface to the lowest energy with smoothly rounded edges. We attribute this deviation from surface-tension dominance to the pinning effect27 at the inhomogeneous steps of the inner wall, which is also a contributing factor to the water film stability. In conclusion, we observed stable water phases confined in hydrophobic CNTs exposed to high vacuum condition using TEM and discovered water morphologies beyond surfacetension dominance, including the superstability of nanometer thin free water films and zigzag-shaped water−air interface. The water film stability is attributed to the strong molecular interactions among the water molecules and water capture on the solid surface, while the zigzag water−air interface is mainly attributed to the pinning effect. 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°. Moreover, the stability of free water films as well as interfacial nanobubbles results in superstable water-bubble structures observed in large amount of filled water. These experimental findings will refresh our knowledge on the fascinating fluid behaviors under nanoscale confinement.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00718. Vacuum-chamber-assisted condensation method to fill water into CNTs; electron energy loss spectroscopy of the CNTs with and without water inside; tube diameterdependent vapor pressure predicted by the Kelvin equation (PDF)
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qin-Yi Li: 0000-0003-1388-7686 Koji Takahashi: 0000-0002-3552-9292 Notes
The authors declare no competing financial interest. 3748
DOI: 10.1021/acs.jpclett.9b00718 J. Phys. Chem. Lett. 2019, 10, 3744−3749
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