Formation of the smallest droplet of acid

June 19, 2009
Tiny droplets of acid.

Exactly four water molecules and one hydrogen chloride molecule are necessary to form the smallest droplet of acid. This was the result of work by the groups of Prof. Dr. Martina Havenith (physical chemistry) and Prof. Dr. Dominik Marx (theoretical chemistry) within the research group at the Ruhr-Universitaet-Bochum, Germany.

They have carried out experiments at ultracold temperatures close to absolute zero temperature using spectroscopy to monitor the molecules. This has been accompanied by theoretical ab initio simulations. According to their calculations, the reaction at these extremely cold temperatures is only possible if the molecules are aggregating one after the other. Their results will be published in the newest issue of Science.

Chemistry at ultracold temperatures in space

If you put a classical , for example hydrogen chloride in water, the acid molecules will preferentially lose a proton (H+). Thereby the pH-value of the solution is decreased and the solution becomes acidic. In particular, so-called hydronium ions (H3O+), are formed by protonated water molecules. This hydronium ion is an important ingredient in many chemical reactions. Despite of the fact that this is one of the most fundamental reactions, it was not clear until now how many water molecules are actually required in order to form a charge separated negative Cl- ion and a positive H3O+ ion. "Whereas we all know acids from our daily life, we have now been able to observe for the first time acid formation on a molecular level."

"We will need this knowledge in order to understant chemical processes on nanoscopic structures, on small particles and on surfaces” explains Prof. Havenith-Newen. This indicates that there is a rich chemistry even at very low temperatures; a fundamental basis for reactions within stratospheric clouds or in interstellar media. Previously, it had been unclear whether reactions with only a few water molecules can take place at theses ultracold temperatures.

Ultracold trap

For their experiments, the researchers have successively embedded hydrogen chloride as well as single water molecules in a special ultracold trap. They used nanodroplets of suprafluid helium which have a temperature of less than -272,8 °C. Molecules will first be cooled down before they have a chance to aggregate. “Suprafluid” is a special property of the helium which implies that the embedded molecules are still free to rotate before they are frozen, thereby allowing monitoring with unsurpassed precision. Captured in such a way, it is possible to obtain the chemical fingerprint of the acid - its infrared spectrum. By combining trapping with high resolution IR laser spectroscopy and theoretical calculations, the chemists demonstrated that exactly four water molecules are required to form the smallest droplet of acid: (H3O)+(H2O)3Cl-.

Important: One molecule after the other

After these results, the researchers were left with the question of how this reaction can take place at ultracold temperatures near absolute zero. “Usually, activation of chemical reactions requires the input of energy, just like for cooking at home you need a cooking plate or a gas flame” explains Prof. Marx. “However, how should this be possible at a few Kelvin (close to absolute zero)?”

The calculations, in combination with experiment, showed that the reaction is only possible by a successive aggregation process. Instead of putting together 4 water molecules and an HCl molecule simultanesously at the beginning and the waiting for a dissociation process to occur, they found in their simulations that when adding the water molecules step by step, a proton is transferred exactly when adding the fourth water molecule. Then, a hydronium ion will immediately form with one of the four added . This unusual mechanism is called “aggregation induced dissociation”. “We suspect that such aggregation induced reactions, can explain chemical transformations at ultracold conditions, such as can be found at small ice particles in clouds and in interstellar media”, explains Prof. Marx.

Source: Ruhr-Universitaet-Bochum (news : web)

Explore further: Dynamic Ice: Surface Physics Technique Reveals Complex Chemical Reactions on Icy Surfaces and Atmospheric Ice Crystals

Related Stories

Hydrogen ions caught in the act of wandering

October 5, 2005

Erik T.J. Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) and colleagues report for the first time experimental evidence of the motions of hydrogen ions (protons, H+) from acids ...

Scattering of hydrogen makes calculation easier

February 23, 2006

The chemical reaction of hydrogen molecules (H2) with a platinum surface can be calculated much more straightforwardly than many researchers to date had thought. This is encouraging for research into hydrogen as a clean fuel ...

Recommended for you

A new form of real gold, almost as light as air

November 25, 2015

Researchers at ETH Zurich have created a new type of foam made of real gold. It is the lightest form ever produced of the precious metal: a thousand times lighter than its conventional form and yet it is nearly impossible ...

New 'self-healing' gel makes electronics more flexible

November 25, 2015

Researchers in the Cockrell School of Engineering at The University of Texas at Austin have developed a first-of-its-kind self-healing gel that repairs and connects electronic circuits, creating opportunities to advance the ...

Getting under the skin of a medieval mystery

November 23, 2015

A simple PVC eraser has helped an international team of scientists led by bioarchaeologists at the University of York to resolve the mystery surrounding the tissue-thin parchment used by medieval scribes to produce the first ...

Atom-sized craters make a catalyst much more active

November 24, 2015

Bombarding and stretching an important industrial catalyst opens up tiny holes on its surface where atoms can attach and react, greatly increasing its activity as a promoter of chemical reactions, according to a study by ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jun 20, 2009
Can one speak about the pH of this water?

Is this then the most acid solution that exist?

Are those droplets aggressive?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.