Nanotubes Change the Shape of Water

Nanotubes can freeze water regardless of temperature

Everybody knows that ice floats in water. Some may also know that this is due to differences in the structure of water molecules in ice versus in water. In ice, the molecules are frozen in place in rigid lattices with spaces between them, but in water, they flow around amorphously, making ice less dense than water. In a recent development, however, researchers have found a different way to align these water molecules in an ice-like structure without even reducing the temperature.

Earlier this year, a team of three engineers from Rice University in Houston, Texas, led by Rouzbeh Shahsavari, completed an in-depth study about how water forms ice-like structures when placed in nanotubes. These nanotubes are extremely small hollow tubes that have a diameter less than a thousandth that of human hair—not dissimilar from a regular drinking straw, except shrunken by a factor greater than a million. In their research, the engineers were able to calculate the theoretical optimal size of the nanotube for these ice-like structures, and to experimentally verify these predictions.

Since 2006, it has been known that water takes on unique structural properties when placed in confined spaces. For example, studies have shown that water confined between two hydrophobic surfaces can form low-density water. Its unique structural properties are primarily due to strong hydrogen bonds that water molecules are able to form with each other, and possibly with the material that is used to enclose them. As such, the structure of water molecules is complex because water molecules, with structure H2O, each contain two hydrogens and one oxygen atom. They can be thought of as special magnets that each have two north poles and one south pole—we can try to arrange all of them together in a stable configuration, ignoring the fact that such magnets do not exist in reality. Forming any such structure already seems complicated. Now, we can imagine the case in which the water molecules are confined externally, analogous to having a boundary of specific magnetic poles placed around this group of special magnets. We quickly see how this becomes a complex computational problem of trying to arrange the water molecules to best fit their constrained space and how this can give rise to the unique structural properties of water.

Even before this recent study by Shahsavari’s team, the use of nanotubes to confine water had already been briefly studied. Past research has shown that water molecules can form pentagonal, hexagonal, and even heptagonal structures when placed in these nanotubes. However, the lack of a complete and systematic study of this phenomenon intrigued Shahsavari, who then decided to study it in more depth. In particular, Shahsavari’s team aimed to develop a more comprehensive theoretical model of the structure of water molecules in nanotubes and the conditions necessary for the ice-like structure to form.

To develop their theory, the engineers first used molecular models to compute the strength of the force between water molecules and the molecules on the inner surface of the nanotube. They found that the primary force between them is the weak van der Waals force, a distance-dependent force that occurs because of random but correlated fluctuations in the polarizations of both molecules. To understand this, we can imagine each molecule as a regular bar magnet that is spinning rapidly and randomly. If we place two such bar magnets close together, they will sometimes attract each other when opposite poles happen to face each other, but sometimes repel each other when the same poles happen to face each other. On average, we will expect a net force of zero if the spinning is truly random. However, in the case of molecules, due to a consequence of quantum dynamics, these random fluctuations—or spinnings—are not truly random, and are instead correlated with each other, giving rise to an overall average attractive force between the molecules. By computing the magnitude of this force between water molecules and the molecules on the inner surface of the nanotube, Shahsavari’s team showed that it was strong enough to freeze water molecules in rigid lattices in the nanotube, forming the desired ice-like structures.

Having understood the mechanism behind the formation of these ice-like structures, the engineers proceeded to investigate the physical conditions required for these structures to form. In theoretical simulations, as well as in lab experiments, Shahsavari’s team altered various parameters of the nanotube, including its diameter and its material—carbon and boron nitride. “The dependence [of structure formation] on the tube diameter was surprising for boron nitride nanotubes,” Shahsavari said. On second thought, however, this dependence makes perfect sense. Imagine the cross section of the nanotube—a circle—as a ring of parents trying to manage some young kids, who are running around haphazardly inside this ring—like the water molecules flowing amorphously in the nanotube. “If the nanotube is too small and you can only fit one water molecule, you can’t judge much, but if the nanotube is too large, then the water keeps its amorphous shape,” Shahsavari said. Analogously, if the ring is too small and only has one kid in it, no structure can form. If the ring is too large, only kids along the edge of the ring will be held in place by parents, leaving most of the kids still running around haphazardly within the ring. It is only when the ring is of the optimal size, when there is approximately the same number of parents as there are kids, that each parent can hold on to one kid, and form a stable structure within the ring. “At a diameter of about 8 Angstroms [8 hundred-millionth of a centimeter], the nanotubes’ van der Waals force starts to push water molecules into organized square shapes,” Shahsavari said.

According to Shahsavari, the most challenging part of their research was the conducting of experiments at such small scales. Indeed, when operating with individual molecules of water and nanotubes that are a millionth the size of a regular drinking straw, simple actions like adding water into the tube requires extreme precision and care. In addition, examining the arrangement of water molecules in the nanotube is certainly not as simple as looking at them; the engineers utilized Raman spectroscopy—a method to observe the vibrational and rotational frequency modes of molecules in a system—to deduce the arrangement of water molecules in the nanotube.

Looking to the future, researchers can now capitalize on this ability to control water at the molecular level. A broad variety of applications are possible: for example, nanotubes could be used as energy-storing nanocapacitors. Alternatively, these nanotubes could be used as nanochannels to transport water—or potentially other material—at extremely small scales, particularly because water flows through nanochannels much faster than typical channels, due to reduced friction with the surface. Shahsavari is hopeful that nanotubes will eventually be made into precise nanoscale syringes, which can be used to deliver specific drugs to targeted body cells. All in all, this comprehensive study by Shahsavari’s team might just revolutionize the way we manipulate liquids at a molecular level.