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Instant insight: Cosmic dust as chemical factories
01 June 2010
Daren Caruana and Katherine Holt discuss how electrochemistry could be the missing link to understanding chemistry in space
The search of the origin of life's molecular building blocks has extended into interstellar space to find evidence of life forming chemicals. As yet no positive identification of molecules that can be considered as precursors to living systems on earth has been made. But, a rich mixture of complex chemicals has been discovered in the spaces between stars in our galaxy, which brings the question of how are these chemicals synthesised?
Mostly, the space between the stars is a vacuum with a light peppering of atoms and molecules. However, high concentrations of chemical species exist in some areas forming dense clouds also containing dust. These dusty clouds appear to be an incubation centre for the synthesis of quite a large variety of molecular species. More than 140 molecular species have been identified in dust clouds, using a variety of spectroscopic techniques. Sophisticated computer models that describe the abundance of chemical species present in clouds have been developed to investigate the formation of these species. Yet, these models fail to describe the formation of many polyatomic molecules, and their origin remains one of the longstanding enigmas of interstellar chemistry.
The involvement of dust grains in the synthesis of complex polyatomic molecules is generally accepted to be important in astrochemistry. For example, molecular hydrogen - the most abundant molecule in the interstellar medium (ISM) - is formed on the surface of grains. So far computer models assume dust grains act only as benign surfaces, where reactants are adsorbed on the surface before reacting. But it is likely that dust grains could play a more active role in chemical synthesis.
Recently, a new hypothesis has been proposed that connects chemical synthesis on the surface of dust grains with electrical charging of the grains by known physical phenomena in the ISM. Individual grains can be negatively charged by attaching to electrons in areas of the gas that is ionised, or positively charged in the periphery of dust clouds where electromagnetic radiation is powerful enough to eject electrons from grains.
The heart of this hypothesis hinges on the fact that electrically charged surfaces can be reducing when negative and oxidising when positive. So depending on the charging processes the balance of electrons in grains can drive surface reduction or oxidation reactions, which may otherwise be thermodynamically unfavourable.
At the moment there is little understanding of redox reactions at the solid-gas interface, but by combining knowledge of redox process at the solid-liquid interface, this hypothesis can be developed into a possible processing mechanism involving solid dust grains found in the ISM. Here the individual grains act as 'single electrode' electrochemical reactors in the gas phase. The number of electrons in the grain changes the overall potential (or Fermi energy) - for a 10 nm diameter dust grain attachment of 32 electrons changes the potential difference of the particle by 1 eV. A voltage of this magnitude applied to a solid conducting surface is known to be able to drive many chemical reactions in liquid phase electrochemistry. Therefore, it is likely that a polarised interface in the gas phase can drive similar chemical reactions.
There is speculation that the formation of some larger molecules (> 6 atoms) may be synthesised through mechanisms that involve coupled surface electrochemical reduction or oxidation reactions, which could occur at spatially distinct areas of a dust cloud. Once developed this surface electrochemical mechanism in conjunction with other gas phase based chemical models may account for the unexpected abundances of certain polyatomic chemical species. First, the understanding of redox reactions at the solid/gas interface needs to be furthered. But in the future, this electrochemical mechanism may also be applied to other environments such as chemistry in the upper atmosphere.
Read more in the perspective 'Astroelectrochemistry: the role of redox reactions in cosmic dust chemistry' in Physical Chemistry Chemical Physics.
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Link to journal article
Astroelectrochemistry: the role of redox reactions in cosmic dust chemistry
Daren J. Caruana and Katherine B. Holt, Phys. Chem. Chem. Phys., 2010, 12, 3072
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