SCIENCE & TECHNOLOGY
Researchers probe origin of PUZZLING PHENOMENON and discover emptying, too POSSIBLE APPLICATIONS of carbon nanotubes (CNTs) for nanofluidics and nanofiltration depend on an important but perplexing phenomenon: Water spontaneously enters CNTs, despite the fact that such nanotubes are made from hydrophobic graphene sheets. Two recent papers explore the effect. One finds that the answer to the mystery lies in water’s unique hydrogen-bonding interactions, while the other shows that at low temperatures CNTs containing water also unexpectedly empty. In the first paper, researchers report molecular dynamics simulations of water confined to single-walled CNTs of various sizes to compare their entropy, enthalpy,
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and free energy and pin down the driving force behind spontaneous filling at 300 K (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/ pnas.1108073108). Postdoctoral researcher Tod A. Pascal and professor Yousung Jung of Korea Advanced Institute of Science & Technology’s (KAIST’s) Graduate School of Energy, Environment, Water & Sustainability, along with California Institute of Technology chemistry professor William A. Goddard, found that the driving force behind water spontaneously filling CNTs depends on tube diameter. For tubes smaller than 1.1 nm and larger than 1.2 nm in diameter, Pascal and colleagues calculated that entropy is the key to filling those nanotubes with water. Inside the CNTs, water molecules near the walls are constrained by few hydrogen bonds. ConDISORDER WITHIN sequently, water in the 1.1 to 2.4 nm (J. Chem. Phys., DOI: A simulation shows nanotubes is disordered 10.1063/1.3593064). water molecules inside and has higher entropy Echoing the results of Pascal and a 1.5-nm-wide CNT. compared with the bulk coworkers, Maniwa and colleagues fluid. “This entropic gain found that the melting point of wadominates any enthalpic ter in CNTs decreases as diameter loss due to broken hydrogen bonding,” Pasincreases—they see ice at 300 K for tubes cal says. 1.2 nm in diameter but liquid water for In tubes smaller than 1.1 nm, the phase CNTs of larger diameter. of the water is gaslike; for those larger than Maniwa and colleagues also observed 1.2 nm, the water looks more like a liquid. for the first time the reverse of the filling But when it comes to filling CNTs with phenomenon: Nanotubes wider than 1.5 diameters from 1.1 to 1.2 nm, ice reigns: The nm all spontaneously emptied water at nanotube diameter is the same as those of temperatures below 240 K, for reasons that common pentagon and hexagon ice strucare not yet understood. tures, giving rise to what Pascal and colReflecting on the results of the Japanese leagues call a “highly diffusing solid.” The team, KAIST’s Pascal says, “One could enthalpy of hydrogen bond formation to imagine a water nanopump controlled by create the ice structures drives water to fill temperature.” He adds that his group had CNTs in this size range. simulated the same result but hadn’t been “The results provide a fundamental confident that their work was accurate. He grounding to something that had been anticipates a host of future studies to exa puzzle,” says Lydéric Bocquet, a physplore the effect further.—JYLLIAN KEMSLEY WWW.CEN-ONLINE.ORG
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TOD A. PASCAL/KAIST
WHY WATER FILLS NANOTUBES
ics professor of the University of Lyon, in France. “It’s very interesting and very counterintuitive.” Pascal emphasizes that the modeled behavior is entirely dependent on the unique hydrogen-bonding nature of liquid water. “Were water not to have such tetrahedral ordering, the filling of CNTs would be a thermodynamically unfavorable process,” he says. The results are therefore unlikely to hold for other gases or liquids. In a separate paper, a group led by physics professor Yutaka Maniwa of Tokyo Metropolitan University and Japan Science & Technology Agency combined X-ray diffraction, nuclear magnetic resonance spectroscopy, and modeling studies to put together a phase diagram of water in CNTs with diameters ranging from
