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Physics

Below zero

Computer simulations explain the electrical properties of ice.

MIGUEL BOYAYANNot only is spare time creative. Fear can also generate good ideas, even in an indirect mode. During 2000 the physicists Adalberto Fazzio and Antonio José Roque da Silva participated in a marathon of scientific conferences that obliged them to cross-cross the globe by air. As he did not feel happy with his feet far from the ground, physicist Fazzio sometimes looked for tranquility with a shot of whisky on the rocks before getting into the aircraft. Whilst he had been waiting in one of the embarkation lounges, the tinkle of the ice cubes in his glass set off a doubt: “From the physics point of view, what do we know about ice?” physicist Fazzio had asked himself.

Six years and a few airports later, the physicist twosome from the University of Sao Paulo (USP) can respond to the question with firmness. In a partnership with collaborators from the State University of Campinas (Unicamp), namely the Paulista physicist Alex Antonelli and the Dutch physicist Maurice de Koning, they presented, in two scientific articles, their first contributions towards maintaining the planet’s climate agreeable and to allow for the existence of life.

Published in February of 2006 in Physical Review Letters, the first study helps to better understand the electrical properties of ice, identified around 70 years ago, that had still not been well explained. In the decade of the 1930’s, chemical and physical studies had shown that ice was much more complex than it had appeared, although it was formed by one of the most simple molecules of nature – water, resulting from the bonding of two atoms of hydrogen with one of oxygen (H2O), in a spatial structure that reminds one of the letter V.

During that era, at the University of Cambridge in England, the physicists John Bernal and Ralph Fowler verified during 1933 that ice was, in reality, a crystal. Below zero degrees Celsius the water molecules form themselves into groups of six, forming hexagons that repeat themselves always at the same distance and with the same orientation. Using a technique that allows for the identification of the position of each atom in the interior of a molecule, they noted that in this crystalline structure a standard exists: there is one atom of hydrogen between two of oxygen, whilst one atom of oxygen always intercalates two of hydrogen of distinct molecules. A very intense force – hydrogen bonding – maintains the water molecules firmly associated one to another, impeding their free movement as in liquid water.

A little before the experiments at Cambridge, the Dutch physicist Peter Debye encountered an unexpected effect on submitting a piece of ice to an electric field. The water molecules, with their negative charges concentrated around the oxygen atom and positive charges close to the hydrogen, aligned themselves with the electric field. It would be natural that this would happen in water, because the molecules are freer. But not in ice, in which they are found fixed in the hexagonal rings by hydrogen bonding. The explanation would only come much later.

As he knew of the difficulty of breaking hydrogen bonds in a perfect crystal, in which the molecules adjust themselves each to the other, in 1952 the Danish physicist Niels Bjerrum proposed the idea that the ice must present faults that will allow the water molecules to align themselves with the electrical field. Formed by chance or through the growth of acids in the ice, this molecular defect took on the name of Bjerrum and did not go beyond an exchange of position of a hydrogen atom in the hexagonal ring. This subtle change creates, at one fell swoop, two unstable bonds: one between two hydrogen atoms of distinct molecules, which repel each other as they have positive charge; and another between two oxygen atoms, of negative charge.

Imperfect crystal
The consequence is a cascading effect. “After they happen”, explained physicist Silva, “these effects move on as if they were running through the ice and facilitate the rotation of other water molecules”. For this reason, the greater the number of defects in the ice the easier its molecules align themselves to the electric field. These defects contribute even for the molecules to organize themselves in ten other ways, more stable than the hexagons, as one reduces the temperature or increases the pressure. Another consequence is the transportation of electrically charged particles: in the case of ice, hydrogen ions, of positive charge, differently from what occurs in copper wires, in which it is negatively charged particles (electrons) that move.

Although the Bjerrum effect is well known, precise measurements of the energy necessary to generate the effect and to allow it to propagate from one molecule to another were missing. With the help of computer programs that simulate the behavior of atomic particles, physicist De Koning obtained a faithful estimate of these values. He created a crystalline network in which the hexagonal rings repeated themselves 16 times and artificially spun the position of a hydrogen atom, creating a Bjerrum effect. Next he awaited the result.

The energy needed to twist a hydrogen bond and to create the Bjerrum effect is up to 73% greater than that estimated. However, the energy for this twist to pass from one molecule to another is some 63% lower. “These effects must displace themselves easier than in fact moving forward”, affirmed physicist Silva. “This difference in energy suggests that there are traps that imprison the defects and impede their progress”, he explained.

In another virtual experiment, physicist De Koning decided to investigate the question of how the frequency brings up in the ice two other types of defects, common in silicon crystals. These are the so called point defects: the lack or excess of an element in the crystalline network – in the ice’s case, a molecule of water. Tests carried out during 1982 in Japan suggested that, depending upon the temperature, there would be more point defects of one type or another.

Physicists De Koning, Antonelli, Silva and Fazzio verified that the closer to the fusion point of ice (zero degrees Celsius) the greater the quantity of water molecules that intrude into the hexagonal network, according to the article published last October in Physical Review Letters. On the other hand, below negative 43 degrees Celsius the most common defect becomes the lack of a water molecule. “Neither of these defects alters the orientation of the water molecules in ice”, explained physicist De Koning, “but it’s believed that they alter its electrical properties”. Even if this in fact occurs, it does not impede one from adding a few ice cubes to whisky.

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