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Enigmatic attraction

New theory helps explain the behavior of ions diluted in water

Eduardo CesarIons: unexpected concentration at the interface between water and airEduardo Cesar

When Isaac Newton was inspired by a falling apple to draw up his Law of Gravity, the English physicist and mathematician was fairly confident that this phenomenon always happened in the same way: the apple would fall downward, toward the center of the Earth. How surprised would Newton be if, now and again, the fruit were to fall upwards, for example?

The surprise he would probably express would perhaps be similar to that of researchers who are trying to understand the behavior of electrically charged particles (ions) diluted in water in the microscopic world. The almost 100 year-old theory on the interactions of these particles suggests that they should always remain distant from the region where the water meets the air, the so-called water-air interface. However, laboratory experiments and computer-simulations have shown that this does happen with certain ions, especially those with a negative electrical charge, which seem attracted to this region.

After a century, the mystery is finally beginning to be cleared up, thanks to a new theory developed under the guidance of the American-Brazilian physicist Yan Levin, a researcher at the Federal University of Rio Grande do Sul (UFRGS). In work published at the end of 2009 in the prestigious scientific journal Physical Review Letters, Levin and his group showed that it is possible to re-establish the theoretical forecast capacity of the behavior of these particles if researchers stop treating them in a simplistic and practical way as spheres with a positive or negative charge located at their center. By considering them as small spheres with a central electrical charge, the theoreticians managed to predict a varied range of behaviors of these particles. However, certain enigmas remained, such as the selective attraction to the water’s surface.

In nature, however, the ions are not rigid spheres like billiard balls. What transforms electrically neutral atoms or molecules into ions is the loss or gain of particles with a negative electrical charge (electrons); overall, when there are more positive charges than negative ones the ion has a positive charge; conversely, when there are more negative charges than positive ones, the ion has a negative electrical charge.

What complicates the story is that electrons in general do not behave like one-off particles. They obey the quantum theory rules, the physics of the sub-microscopic world that often contradicts intuition.

What does this mean? It means that they behave as if they were a diffuse cloud around the atom or the molecule. Anyone who tries to measure the position of an electron will have more chance of finding it in a certain region in the cloud. However, it is only possible to define precisely where it is when it is, in fact, observed. Before measurement it is as if  it were in every possible place at the same time – that is why physicists say that the electron is a wave of probability.

However, these details should not bother us, since so far the researchers who work with quantum physics do not know how to interpret what the theory really means concerning the nature of particles and the world at its smallest scale. Despite being difficult to understand, quantum physics represents what happens in the world of particles with a fair degree of precision. Using quantum physics, one can calculate waves of probability that adjust perfectly to the results obtained in experiments.

The interpretation of what happens with ions at the water-air interface began to change when Yan Levin decided to check what would happen if he considered that, instead of being located in the center of the ion as was imagined, the charge was distributed in an unequal way on the surface of the ion – an effect known as polarizability. This unequal distribution is produced by the electrical field generated by the molecules of water: each hydrogen atom of a molecule has a positive charge and is connected to the oxygen of another by a negative charge, creating chemical bonds (hydrogen bridges). Ions disturb these bonds, generating competition between the two effects.

Compatible numbers
In the case of large and highly polarizable ions, the hydrogen bridges prevail and push these ions to the water-air interface, the opposite of what the former theories forecast, explains Levin. It was precisely this that he observed in the laboratory experiments, particularly with negative ions produced by dissolving salts containing chemical halogen elements (chlorine, bromine, iodine and fluorine).

The theoretical calculations produced by Levin and his colleagues correspond perfectly to the experimental observations. They have already counted the ions produced by these salts at the water-air interface and now intend working with acids to see if the effect is similar. Over the next few months, the group also plans to look into the effect of salt solutions in their interaction with proteins. It is known that in the case of proteins diluted in water, the behavior of molecules at the water-air interface may be very similar to what happens at the interface between water and oil or water and air. This is important in order to understand why certain salts cause the precipitation (and others the stability) of proteins, the molecules responsible for practically everything that happens in the metabolism of living beings.

In Levin’s opinion, if his equations prove to be correct, they may be applied in very different situations. It is possible, for example, that they may further the understanding of certain nuances of life?s evolution on Earth. “There were moments in the history of the planet when mass extinction occurred in the oceans, which may have happened because of the decrease of ions in the seas,” he recalls. “With the new theory, we shall also be able to investigate the limit before proteins are precipitated because of the salt.”

Another phenomenon that can be explained by this new theory is the degradation of ozone in the lower atmosphere. It is believed that close to the surface of the oceans, droplets (aerosols) of water help to destroy this gas –  and therefore to diminish its concentration. According to the previous theory on the behavior of ions, ozone degradation in these regions occurs at much lower rates than observed in reality. Levin believes that in this case the theory proposed by his group also produces results that are closer to those obtained experimentally.

Scientific Articles
LEVIN, Y. Polarizable ions at interfacesPhysical Review Letters. v. 102. p. 1478031-34. 10 Apr. 2009.
LEVIN, Y. et al. Ions at the air-water interface: an end to a hundred-year-old mysteryPhysical Review Letters. v. 103,  p. 2578021-24. 18 Dec. 2009.