New Study Might Explain Crystalline Ice’s Anomalous Behavior

Ice, water's solid form, has always been a curious case from the early humans to modern-day physicists. However, a new study observing an obscure property of ice might explain its seemingly anomalous behavior.

Researchers from the Scuola Internazionale Superiore di Studi Avanzati (SISSA), Abdus Salam International Centre for Theoretical Physics (ICTP), and the Institute of Physics Rosario (IFIR-UNR) collaborated in observing ice and why its molecules at very low temperatures are not as organized as other liquids tend to be.

In their study, researchers described a "relatively obscure" yet fundamental property found in ice: ferroelectricity. The results of their findings are published in the Proceedings of the National Academy of Sciences (PNAS).

Water Molecules in Ice and Its Anomalous Behavior

"In an ideally ordered piece of ice the hydrogen atoms of each water molecule should point in the same direction, like soldiers in a platoon looking in front of them," said Alessandro Laio, physicists from both SISSA and ICTP, in a press release. He explains that if this was the case, ice should have exhibited a phenomenon called "macroscopic electric polarization," or that the substance would be "ferroelectric."

Instead, water molecules, even in low temperatures, behave like "unruly soldiers," according to Laio, with the molecules looking in different directions.

This "anomalous" behavior of water molecules in ice was first discovered experimentally in the 1930s, brought to the attention of the scientific community by American chemist Linus Pauling. He noted the disorder in the molecules as an effect of what was termed as the "ice rule" constraint. At any moment, a single oxygen atom should possess two and only two protons to allow it to form H2O - water.

The rather unique and difficult kinetics imposed by the constraint results in an extremely slow ordering process. In the SISSA press release, it was likened to a platoon having each soldier with four neighbors, with soldiers having to keep two hands on the shoulders of two other soldiers.

Effects of Impurities on Water Ordering Process

"Were it not for impurities or defects, which turned out to play a revealing role, one would still today not know whether proton order and ferroelectricity of bulk crystalline ice is a real possibility or a figment of the imagination," noted Erio Tosatti, a physicist with SISSA, ICTP, and CNR-IOM Democritos, adding that experiments or even simulations could overcome the ice rule slowdown.

The impurities noted by Tosatti includes one KOH molecule replacing H2O have been observed to encourage nucleation in the ordering process, creating an ordered arrangement for ice and, therefore, make it ferroelectric.

Researchers then turned to design a theoretical model to illustrate the behaviors of pure and impure ice. According to the model, once an impurity has been introduced into an "initial non-equilibrium low-temperature disordered state," the doped substance acts as a "seed" for an ordered process. However, it is only limited to those near the impurity.

The team concluded that while their work is currently limited to bulk ice, they believed that the same mechanism also applies to ice surfaces.