Aquaporins are remarkable molecules. Frequently they are called the plumbing System of the cell, because they allow the rapid flow of water across cell membranes, and of water alone. Ions and other compounds stay where they are. There is nothing that matches their selectivity. Or is there? According to an article in the somewhat oddly named Journal Small (Aquaporins are fairly large, as proteins go), there is an unlikely competitor: Boron nitride, rolled into nanotubes.
Nanotubes, albeit of the carbon sort, have long been the focus of similar research. With growing worldwide demand for drinking water and up to one in four people already threatened by water scarcity, desalination of sea water looks like the way to go. But current techniques are ineffective and prohibitively expensive. A possible way forward is to use hollow nanotubes of the right diameter and charge distribution as a highly selective filter.
Experiments with single-walled carbon nanotubes looked promising, but their ability to keep out ions breaks down at salt concentrations above 250 millimoles, about half the salinity of sea water. Obviously that’s not enough. But there are other compounds that form Nanotubes sufficiently thin to exclude anything but water. Silicon, for example, and boron nitride. Such nanotubes are analogous to carbon nanotubes in that they consist of hexagon lattices and come in several structural variants like the “armchair” and the “zigzag”-types. Crucial for the nanotubes properties is the behavior of water and ions within the tubes. It’s not just a question of diameter, electrostatic interactions play a significant role too.
Hilder, Gordon and Chung performed molecular dynamics simulations of water molecules flowing through (5,5), (6,6), (7,7) and (8,8)-boron nitride nanotubes embedded in a mebrane. They found that water within such tubes forms distinct structures that themself influences the permeability for solvated ions. One of the more surprising results is that, while the (5,5)-Nanotube effectively excludes Sodium due to the high energy barrier of de-solvatation, the only slightly larger (6,6)-Nanotube transports ions, apparently due to a special three-ligand configuration that only forms in such tubes. However, chloride doesn’t form such a serendipitous configuration and is thus excluded. (8,8)-tubes, on the other hand, show exactly the opposite behavior – boron nitride nanotubes don’t just act as molecular sieve, but also as ion-selective channel.
For complete desalination the (5,5)-nanotube appears most useful. Water molecules immediately fill the tube and march through in single file, while ions are effectively excluded thanks to a high energy barrier – no hydratation is possible within the channel. Significantly, the channel is also too small to permit chloride ions, which prevents ions from forming pairs and passing the channel together (‘ion chaperoning’), as it is observed in carbon nanotubes at higher concentrations.
The authors claim that a hypothetical membrane based on boron nitride nanotubes could be four to five times as efficient as current reverse osmosis membranes. However, the assumption rests entirely on the hope that nanotube-containing membranes will withstand pressures of over 5 MPa, which at least the material simulated here clearly wouldn’t. Much work remains to be done.