Orienting water molecules? maybe it’s true. What about experiments?

Simulations Help Explain Fast Water Transport In Nanotubes

ScienceDaily (Sep. 22, 2008) β€” By
discovering the physical mechanism behind the rapid transport of water
in carbon nanotubes, scientists at the University of Illinois have
moved a step closer to ultra-efficient, next-generation nanofluidic
devices for drug delivery, water purification and nano-manufacturing.

“Extraordinarily fast transport of water in carbon nanotubes has
generally been attributed to the smoothness of the nanotube walls and
their hydrophobic, or water-hating surfaces,” said Narayana R. Aluru, a
Willett Faculty Scholar and a professor of mechanical science and
engineering at the U. of I.

“We can now show that the fast transport can be enhanced by
orienting water molecules in a nanotube,” Aluru said. “Orientation can
give rise to a coupling between the water molecules’ rotational and
translational motions, resulting in a helical, screw-type motion
through the nanotube,” Aluru said.

Using molecular dynamics simulations, Aluru and graduate student
Sony Joseph examined the physical mechanism behind orientation-driven
rapid transport. For the simulations, the system consisted of water
molecules in a 9.83 nanometer long nanotube, connected to a bath at
each end. Nanotubes of two diameters (0.78 nanometers and 1.25
nanometers) were used. Aluru and Joseph reported their findings in the
journal Physical Review Letters.

For very small nanotubes, water molecules fill the nanotube in
single-file fashion, and orient in one direction as a result of
confinement effects. This orientation produces water transport in one
direction. However, the water molecules can flip their orientations
collectively at intervals, reversing the flow and resulting in no net
transport.

In bigger nanotubes, water molecules are not oriented in any particular direction, again resulting in no transport.

Water is a polar molecule consisting of two hydrogen atoms and one
oxygen atom. Although its net charge is zero, the molecule has a
positive side (hydrogen) and a negative side (oxygen). This polarity
causes the molecule to orient in a particular direction when in the
presence of an electric field.

Creating and maintaining that orientation, either by directly
applying an electric field or by attaching chemical functional groups
at the ends of the nanotubes, produces rapid transport, the researchers
report.

“The molecular mechanism governing the relationship between
orientation and flow had not been known,” Aluru said. “The coupling
occurs between the rotation of one molecule and the translation of its
neighboring molecules. This coupling moves water through the nanotube
in a helical, screw-like fashion.”

In addition to explaining recent experimental results obtained by
other groups, the researchers’ findings also describe a physical
mechanism that could be used to pump water through nanotube membranes
in next-generation nanofluidic devices.

Funding was provided by the National Science Foundation and the National Institutes of Health.

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