STEPHEN MORRIS/FLICKR.COMBelousovZhabotinski: classical demonstration of an oscillatory reaction STEPHEN MORRIS/FLICKR.COM
A chemist observes a glass vial connected to several rubber tubes. The yellow liquid inside the device is constantly shaken. At first glance, it resembles a chemical experiment of the kind performed by children, were it not for the computer next to the vial. In reality, the device is an electrochemical cell in which electricity activates chemical reactions. The objective is to record countless graphs that quantify the substances that appear and disappear in the reaction. The group headed by chemical engineer Hamilton Varela, from the Chemistry Institute of the University of São Paulo in São Carlos (IQSC-USP), is immersed in chemical reactions that can be seen as an analogy to understand how live systems maintain their stability even when they are constantly subjected to environmental variations.
“Chemical reactions can oscillate in states that are far from being thermodynamic states,” explains Varela. In his opinion, this is what defines live entities: they oscillate, or vary, between one state and another. This is a feature of the heart, of the circadian rhythm and of the brain, among other elements of life and it is precisely this flexibility that makes these systems resistant to the environment’s instability. “Of course, a chemical reaction is a very elementary representation of the complexity of life, but so far no one has come up with a better analogy,” he explains.
Until the mid twentieth century, the general belief was that chemical reactions went in only one direction: reagents were the origin of products. Today, however, it is widely known that a reaction can come and go, while intermediary substances appear and disappear as time goes by. The Belousov-Zhabotinski reaction is a classical example of this. On a glass plate, one can see that the reaction propagates in waves, creating concentric patterns. “This was the first oscillatory reaction to be taken seriously,” says Varela. Sometime in the 1950’s, Russia’s Boris Belousov noticed that a mixture that included potassium bromate and some other reagents generated a reaction with the intermediaries, whose concentration fluctuated, causing the color to oscillate between a yellow and a colorless solution. The notion that a reaction could oscillate was viewed as heresy and took a long time to be accepted. A few years later, another Russian, Anatol Zhabotinski, came to the same conclusion. The color of the mixture – bromate and malonic acid – pictured here varies from red to blue, depending on the solution’s pH.
Varela is investigating the oscillatory behavior of reactions commonly studied in electrochemistry because of its simplicity and practical interest. The team places a five-millimeter platinum plate in a solution of formic acid inside the electrochemical cell. The formic acid molecules contain a single carbon atom, two oxygen atoms and two hydrogen atoms (HCOOH). During the reaction, the molecules bond temporarily with the platinum and, after some intermediary steps, release carbon gas (CO2) or carbon monoxide (CO), which covers the platinum electrode.
Varela discovered that this platinum and formic acid system has a unique property. Unlike the typical outcome of a chemical reaction, the process does not speed up when the temperature rises. The intermediary stages of the formic acid’s reaction with the platinum bond in such a way that the frequency of the oscillations remains constant when the temperature rises, as the group described last year in an article published in the Journal of Physical Chemistry A. This is another parallel with living systems, which remain stable even when the environment’s temperature fluctuates within a certain range. The researcher explains that this biochemical stability, or homeostasis, is responsible for the constant body temperature of warm-blooded living beings, such as mammals and birds.
In depth
Varela is now studying the system in detail to understand where these particularities stem from. The group tested different experimental parameters and found that under certain conditions the formic acid requires very little activation energy to lose a water molecule and produce carbon monoxide, which is rather uncommon in reactions of this kind and which require energy to occur. The findings were published in this year’s October issue of the Journal of Physical Chemistry C and suggest that perhaps this characteristic underlies the behavior of the formic acid with the platinum electrode. Varela also found that the oscillations that can offset temperature changes are not general features of simple molecules. Methanol, which also has a single carbon atom (plus one oxygen atom and four hydrogen atoms), behaves in a “totally trivial” manner, in the words of the researcher in an article published this year in Physical Chemistry Chemical Physics.
Varela is one of the 20 members of the Ertl Center for Electrochemistry and Catalysis, a research center in South Korea headed by Gerhard Ertl, the 2007 Chemistry Nobel Prize laureate. Varela intends to continue his in-depth investigations on how chemical reactions can help one to understand life. The next step will be to assemble platinum electrodes in series and observe the properties that may emerge from this, whereby functioning of the set is different from the functioning of its parts. This is what happens in the brain or in an anthill, he exemplifies. The brain as a whole performs functions that a single neuron does not. Likewise, the behavior of a solitary ant does not make any sense; the complex organization only becomes apparent when one looks at the entire anthill. The engineer from USP wants to assemble a structure with 80 electrodes in series to study the system’s emerging properties. Raphael Nagao, a PhD candidate at his laboratory, is working on this technical endeavor and says that at present he can test 32 electrodes in series. Once the technical hurdles have been overcome, Varela intends to help expand the integration of disciplines such as chemistry, physics and biology, an integration sometimes restricted by the specific jargon of each of these fields of knowledge.
The project
Dynamic self-organization in a solid-liquid interface (nº 04/04528-0); Type Young Researcher Program; Coordinator Hamilton Varela – IQSC-USP; Investment R$ 371,700.56
Scientific articles
NAGAO, R. et al. Temperature (over)compensation in an oscillatory surface reaction. Journal of Physical Chemistry. v. 112, n. 20, p. 4,617-24. Apr. 2008.
ANGELUCCI, C. A. et al. Activation energies of the electrooxidation of formic acid on Pt(100). Journal of Physical Chemistry. Sep. 2009.
