Water flies through fluorine-lined channels

Inspired by the water-repelling properties of the fluoropolymer PTFE (better known by its trade name, Teflon), chemists have created nanochannels lined with fluorine atoms that transport water at ultrafast speeds. These channels, which also exclude negatively-charged chloride ions, could guide strategies for improving desalination materials.

Anyone who has ridden down a waterslide will tell you that aqueous modes of transport are fast. But on the molecular scale, water’s tendency to form clusters and hydrogen bonds can make it travel sluggishly through tiny channels. Cells speed up water’s transport using aquaporins—protein channels with hydrophobic interiors that repel water and break up those clusters. Similarly, water can travel quickly through carbon nanotubes.

A team led by the University of Tokyo’s Yoshimitsu Itoh, Kohei Sato, and Takuzo Aida, wondered what would happen if they created nanochannels with a lining that resembled Teflon’s fluorinated surface. The chemists synthesized a series of oligoamide nanorings with fluorine atoms lining the rings’ interior. These rings stack to make supramolecular channels in phospholipid bilayer membranes. When the chemists tested nanochannels with the smallest diameter—just 0.9 nm—they found that water travels through the channels at speeds that are hundreds of times faster than water transport through either aquaporins or carbon nanotubes (Science 2022, DOI: 10.1126/science.abd0966). Like the aquaporins’ hydrophobic interiors, the nanochannels’ fluorinated lining breaks up water clusters. The electrostatically negative surface also repels chloride ions, preventing salt from moving through the channels.

This result is startling, says Aleksandr Noy, a scientist at the University of California, Merced, who studies water transport through carbon nanotubes. “Because these data smash our understanding of fast water transport in artificial water channels, the mechanism of this effect needs further explanation,” he says in an email. To date, most artificial water channels have forced water to travel through single-file, but Noy points out that the wider fluorine-lined channels, where water could still cluster, also have fast water permeability. “Overall, these results could suggest ways to engineer membrane pores that can enable fast flow and enhanced selectivity.”

The fluorous nanochannels described in the paper will probably not find practical applications because they’re difficult to make. Still, the researchers think that their findings hint at ways to improve existing filters used in desalination technologies. Adding fluorine, they say, could improve these materials.