The unexpected size-dependence results from two phenomena, Strano suggests. According to a theory developed by the team, there is first an attractive force, in which ions' electrical charge causes them to be pulled by an electric field through the pore. Since the ions and the tubes are all submerged in water, some water gets pulled along as well.
Up to a certain diameter, those water molecules form a layer, or a few layers, around the ion and are pulled along with it, the team theorizes. But as the opening gets bigger, the water behaves as a bulk material, slowing the ions' passage. "This explanation is consistent with our experimental observations and molecular simulations of water inside of nanotubes of this type," Strano says though he stresses that while the data on the ion flow is clear-cut, additional theoretical work is needed to fully understand this process.
The finding may help in designing better membranes for desalination of water. The biggest problem with today's membranes is the tradeoff between selectivity versus flow rates: Bigger pores let the water flow through faster, but are less selective. Nanotubes' nonlinear response may provide a way around that.
"The results suggest that by using nanopores of a specific diameter, it may be possible to achieve maximum selectivity with maximum throughput" by optimizing the pore size, Strano says.
The work could also lead to new sensors capable of detecting specific contaminants in water, the team says. For example, arsenic contamination of groundwater is a serious health concern in some regions, but there is no reliable way of testing arsenic concentrat
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology