PHOTO LÉO RAMOS ILLUSTRATION ABIUROImagine a liquid that moves faster when confined to a small environment than when held in a larger one or a compound that flows through a nanotube hundreds of times faster than would be expected, if the mechanism involved were identical to water flowing through a faucet. This strange substance is none other than water, the ubiquitous H2O that covers 70% of our planet, accounts for over half of our own bodies, and participates in the production and maintenance of all forms of life. The reasons for these and other strange behaviors of water are a topic of scientific debate, and researchers have often proposed complex explanations for these phenomena, such as the idea that the liquid shows quantum properties when exposed to certain conditions. Over the past ten years, theoretical physicist Márcia Barbosa from the Federal University of Rio Grande do Sul (UFRGS) has been fine-tuning a computational model that attempts to produce a simpler explanation for the central mechanism behind some of the eccentricities of water, such as those mentioned above.
We know that the hydrogen bonds between molecules of water favor the formation of four-molecule clusters called tetramers. The manner in which these molecule quartets interact is what will explain many of water’s anomalies, according to the computational simulations run by Barbosa’s group. It’s as if the basic unit to describe the behavior of water is not the molecule itself, but rather these tetramers. The bonds among water tetramers tend to alternate between two preferred configurations: one is closed, more stable and has less energy, in which the molecules of one cluster are farther apart from those of the other; and the other is an open arrangement where the molecules of two tetramers are closer together (see infographic). According to the virtual physics experiments, this alternation from one configuration to the other – or, in the jargon used by the researchers, from a larger scale of spatial distance between molecular clusters to a smaller one – is enough to explain certain strange behaviors of water. “In our simulations, we only saw the anomalies when we introduced these two scales of potential interaction,” says Barbosa. “We believe that water is a mix of tetramers that make and break connections with each other.”
If it is true that these tetramers constantly shift from one scale to the other in certain situations, then these frequent structural rearrangements will change the density of the water. When the molecular clusters are closer together, the liquid becomes denser. When they spread out, the structure becomes less dense. “In our potential interactions that attempt to mimic the behavior of water, there is competition between scales. This creates competition between environments, or between local arrangements of varying densities,” affirms chemist Paulo Netz from UFRGS, who has co-authored several studies with Barbosa. “This enables us to explain many of the anomalies presented by water.”
Ana Paula CamposMost recently, the researchers’ simulations have focused on the unusual behaviors of water in tiny, confined environments. The virtual model of water developed by the Brazilian scientists emulates things like the anomalous diffusion or flow properties of H2O molecules inside a nanotube. In a paper published May 21, 2013 in the Journal of Physical Chemistry B, Barbosa and collaborators presented a model of her concept of virtual water traveling through nanotubes of fixed length, but different diameters. The purpose of this simulation was to observe what would happen to the liquid’s flow as it moved along nanotubes of different thicknesses. As a rule, the flow of a specific quantity of a liquid inside a tube-like structure will always increase as diameter diminishes. We need only remember how much “stronger” the flow of water gets when we obstruct a garden hose partially with our thumb. In nanotubes, the simulations indicate a much higher increase in flow than expected. “In some experiments, the flow was 2,000 times stronger than expected,” says Barbosa. “In our simulations, the resulting number was 200 times above normal.”
The proposed mechanism to explain this phenomenon has to do with the way that the H2O tetramers rearrange themselves inside the nanotubes. Given the infinitesimal diameter of the channel through which they are traveling, the water molecules might be arranging themselves in two distinct configurations: one of them, more dense, remains in direct contact with the nanotube walls, coating the less dense water that makes up the central portion of the liquid. “It’s as if a ‘frosty’ layer were forming on the walls of the nanotube, making it easier for the water in the center to flow faster,” notes Barbosa, who is one of the five winners of the L’Oréal-Unesco Awards for Women in Science – 2013.
Whatever the mechanism behind this property might truly be, water superflow does not appear to be a deviation caused by the simulations. Real experiments using real water flowing in nanotubes have also produced similar results. One possible application of this property is the development of nanofilters for seawater desalination. Because the water would flow faster than the salt, this approach could be commercially viable.
Less space, more movement
The anomalies successfully simulated by Barbosa’s team through their computational model also include the unusual diffusion of water in confined environments. Barbosa was actually the discoverer of anomalous diffusion, in a theoretical project conducted jointly with Netz in 2001. In simple terms, diffusion is a molecule’s ability to move and spread out in a given space. “The diffusion of molecules in a liquid can be viewed as (something) similar to people moving in a crowd,” Netz observes. “Let’s imagine that a crowd is assembled in a city square, and suddenly it needs to move into a smaller square, where its mobility will be reduced. This is what happens to most liquids.” When you increase the pressure of a liquid – i.e., when you reduce its volume and increase its density —, its diffusion coefficient goes down. Its molecules start moving more slowly. But with water, exactly the opposite happens. Higher pressure increases its diffusion coefficient. Under these conditions, water molecules speed up instead of slowing down like “normal” ones do.
This behavior was reported in a simulation published by Barbosa and collaborators in the Journal of Chemical Physics of August 23, 2012. In this experiment, the water tetramers are kept in two reservoirs connected by a nanotube. When the sluices that close off the two extremities of the nanotube are opened up, the water molecules start flowing into the nanotube. In nanotubes with diameters of about one nanometer, water behaves in the conventional manner. Less space means less molecular diffusion. But when the one nanometer threshold is crossed, instead of the molecules slowing down, they begin “running” faster. This is water’s so-called diffusion anomaly. According to Barbosa, water molecules are more mobile in narrower nanotubes because a confined environment of such small size will produce competition between the tetramer scales mentioned earlier. Each cluster of water molecules will alternate between the two scales of potential interaction, one with the tetramers closer together and the other with them farther apart. This internal dance of the molecule quartets will constantly rearrange the internal structure of the water. In wider nanotubes, this effect is not observed and the H2O tetramers tend to stay in the lower energy, stabler arrangement.
Dozens of theoretical models attempt to explain and reproduce, through computational simulations, some of the 69 known thermal, structural or dynamic anomalies of water. Not all the strange behaviors of H2O are limited to specific situations like its diffusion and flow anomalies in nanotubes. Some of water’s eccentricities are so commonplace that they go virtually unnoticed. Most liquids contract and become denser when cooled. Water does the opposite. At 0ºC, ice is 9% less dense than liquid water. That is why it floats. To see another bizarre behavior of water, one need only take a dip in the ocean. Anyone who has been to the beach on a scalding hot day may have noticed that the water is always cooler than the sand. Both are exposed to the same solar rays, but the silica in the sand heats up more than the water in the ocean. The reason for this is that water has a much higher specific heat than sand. The tiniest increase in temperature requires exposure to huge quantities of heat. “We are largely made up of water, so its high specific heat is beneficial to life,” Barbosa affirms.
According to physical chemist Munir Salomão Skaf from the Institute of Chemistry at the University of Campinas (Unicamp), it’s “amazing” that a model as economical as the one used by his colleague at UFRGS can explain so much, including the behavior of confined water. “Unlike the atomistic approaches widely used in chemistry and physics to describe water as a solvent medium, the model developed by Barbosa can be classified as ‘minimalistic’,” says Skaf. “It tries to capture the most essential part of the problem’s physics in a simplified manner. In the case of water, the whole issue apparently boils down to the existence of two distinct spatial scales of interaction in the liquid.” Theoretical physicist Giancarlo Franzese from the University of Barcelona has a similar opinion. “The approximations that are the basis of Barbosa’s model partially limit its ability to describe water, but [this model] can be seen as an interesting option for describing systems with the same anomalous properties as water.”
A highly productive researcher in this field of study, Franzese published a simulation in 2011 showing the unique properties of liquid water confined in nanochannels and cooled to about -100°C (yes, water can be liquid at such low temperatures). Although he supports the contributions of simpler theoretical models, the Italian researcher believes that some of water’s anomalies can only be explained by quantum effects.
BORDIN, J. R. et al. Relation between flow enhancement factor and structure for core-softened fluids inside nanotubes. Journal of Physical Chemistry B. v.117, n.23, p. 7,047-56. May 21, 2013.
BORDIN, J. R. et al. Diffusion enhancement in core-softened fluid confined in nanotubes. Journal of Chemical Physics. v. 137, n. 8. 23 ago. 2012.