Nanotubes ‘freeze’ water no matter the temperature

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Researchers have discovered a way to “freeze” water molecules in place with nanotubes.

Researchers used molecular models to demonstrate their theory that weak van der Waals forces between the inner surface of the nanotube and the water molecules are strong enough to snap the oxygen and hydrogen atoms into place.

Rouzbeh Shahsavari, a materials scientist at Rice University, refers to the contents as two-dimensional “ice,” because the molecules freeze regardless of the temperature. He says the findings provide valuable insight on ways to leverage atomic interactions between nanotubes and water molecules to fabricate nanochannels and energy-storing nanocapacitors.

Shahsavari and his colleagues built molecular models of carbon and boron nitride nanotubes with adjustable widths. They discovered boron nitride is best at constraining the shape of water when the nanotubes are 10.5 angstroms wide. (One angstrom is one hundred-millionth of a centimeter.)

The researchers already knew that hydrogen atoms in tightly confined water take on interesting structural properties. Recent experiments by other labs showed strong evidence for the formation of nanotube ice and prompted the researchers to build density functional theory models to analyze the forces responsible.

Shahsavari’s team modeled water molecules, which are about 3 angstroms wide, inside carbon and boron nitride nanotubes of various chiralities (the angles of their atomic lattices), and between 8 and 12 angstroms in diameter. They discovered that nanotubes in the middle diameters had the most impact on the balance between molecular interactions and van der Waals pressure that prompted the transition from a square water tube to ice.

“If the nanotube is too small and you can only fit one water molecule, you can’t judge much,” Shahsavari says. “If it’s too large, the water keeps its amorphous shape. But at about 8 angstroms, the nanotubes’ van der Waals force starts to push water molecules into organized square shapes.”

The researchers found the strongest interactions in boron nitride nanotubes due to the particular polarization of their atoms, he says.

Nanotube ice could be used in molecular machines or as nanoscale capillaries, or foster ways to deliver a few molecules of water or sequestered drugs to targeted cells, like a nanoscale syringe, Shahsavari says.

A paper on the research appears in the journal Langmuir.

Additional coauthors are from Shahid Rajaee Teacher Training University and Amirkabir University in Tehran, Iran. The National Institutes of Health supported the work. An IBM Shared Rice University Research grant provided computing resources.

Source: Rice University