Disorder is key to nanotube mystery

Scientists often find strange and unexpected things when they look at materials at the nanoscale — the level of single atoms and molecules. This holds true even for the most common materials, such as water.

Case in point: In the last couple of years, researchers have observed that water spontaneously flows into extremely small tubes of graphite or graphene, called carbon nanotubes. This unexpected observation is intriguing because carbon nanotubes hold promise in the emerging fields of nanofluidics and nanofiltration, where nanotubes might be able to help maintain tiny flows or separate impurities from water. However, no one has managed to explain why, at the molecular level, a stable liquid would want to confine itself to such a small area.

Now, using a novel method to calculate the dynamics of water molecules, Caltech researchers believe they have solved the mystery. It turns out that entropy, a measurement of disorder, has been the missing key.

“It’s a pretty surprising result,” says William Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech and director of the Materials and Process Simulation Center. “People normally focus on energy in this problem, not entropy.”

That’s because water forms an extensive network of hydrogen bonds, which makes it very stable. Breaking those strong interactions requires energy. And since some bonds have to be broken in order for water to flow into small nanotubes, it would seem unlikely that water would do so freely.

“What we found is that it’s actually a trade off,” Goddard says. “You lose some of that good energy stabilization from the bonding, but in the process you gain in entropy.”

Read the whole of this very interesting article here Disorder is key to nanotube mystery.

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