Hydrogen ions caught in the act of wandering

Oct 05, 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 via water to bases. Until now this has only been estimated as a possible reaction mechanism with theoretical calculations. With this study, the international research team provides insight into fundamental processes in nature (acid-base neutralization, proton transmission through water and through biomembranes), that may well become relevant for technological applications, e.g. in fuel cells.

The scientists report on these findings in Science (Vol. 310, pp. 83 – 86) Nibbering’s team consisted of his colleagues from the MBI, Omar F. Mohammed (a Ph. D. student from Egypt) and the theoretician Jens Dreyer, and the group of Ehud Pines at Ben Gurion University of the Negev (Israel).

For a long time, it was not clear how the transfer of protons in aqueous solutions occurs. This is because protons do not move freely in water, but form complexes with water molecules (H2O) through hydrogen bonds. Hydronium (H3O+) is formed, but this ion will not stay alone, because it forms complexes with nearby water molecules in continuously exchanging configurations, e.g. in the form of the so-called Zundel (H5O2+) and Eigen (H9O4+) cations. Erik Nibbering and colleagues succeeded to make snapshots of the proton motions with ultrashort laser flashes. It turned out that hydrogen ions are transmitted from acid to base by water molecules.

Hydrogen ions caught in the act of wandering 1


Fig. 1: Grotthuss mechanism for proton conductivity in water. Protons are shown in grey

Hydrogen ions are transmitted very efficiently through water. First theoretical considerations on this were made exactly 200 years ago by the german-baltic scientist Theodor von Grotthuss, and since exactly 100 years scientists use the phrase “Grotthuss mechanism” to indicate the jump-like transmission of protons to neighbouring water molecules (see Fig. 1). “One can use the picture of the improving a dike with sandbags”, says Nibbering. A chain of people will transport the sandbags more efficiently and faster towards the dike than everybody on his own. “You could speak of proton hopping”, explains Nibbering. Only recently, numerous theoretical refinements have become available. Detailed calculations, for example, made clear that proton transmission becomes possible when the surrounding water rearranges at particular points in time to enable the Zundel-cation and at other times the Eigen-cation configuration.

Furthermore, theoreticians have derived that the exchange of protons between acids and bases in aqueous solution should occur in a similar fashion. Now, the recent report in Science confirms the hopping model (Fig. 2).

Hydrogen ions caught in the act of wandering 2


Fig. 2: Here the proton transfer reaction proceeds from an organic acid towards a carboxylic base via an intermediate hydronium stage with an Eigen-cation configuration.

The experimental study has become possible by a technique, that enables the determination of the reaction progress in time steps of 150 femtoseconds. This is extremely fast. For comparison: A laser beam will reach the moon in one second. In 100 femtoseconds on the other hand a laser beam will only have reached a distance equivalent to the diameter of a human hair. The scientists have used in their experiments an aqueous acid-base mixture, with which they already have been performing proton transfer studies since two years. “Two years ago, we were not able to observe the intermediate steps. We could only see the beginning and the end of the proton transfer reaction”, says Nibbering. By a change of the components of the acid-base mixture the reaction has been slowed down so that now the sequential proton hopping via water molecules can be recorded.

Source: Forschungsverbund Berlin e.V. (FVB)

Explore further: Beam on target: CEBAF accelerator achieves 12 GeV commissioning milestone

add to favorites email to friend print save as pdf

Related Stories

Detecting diamonds with X-ray technology

Apr 02, 2014

X-rays penetrate objects and reveal information about its contents. Using two X-ray spectra, you can identify different materials. And now, a new algorithm is making it possible to find diamonds in the rock.

Photosynthesis reimagined

Mar 28, 2014

(Phys.org) —Using water as fuel has been a recurrent theme of science fiction since the days of Jules Verne. A recent discovery, however, may bring it one step closer to science fact by mimicking the very ...

Rocket launches into an aurora to study auroral swirls

Mar 07, 2014

If you've ever wondered what makes the aurora take on the amazing forms it does you've got company. Marilia Samara and the crew of aurora researchers at Alaska's Poker Flat Range head up the NASA-funded Ground-to-Rocket Electrodynamics-Electrons Correlative Experiment, ...

Recommended for you

CERN: World-record current in a superconductor

17 hours ago

In the framework of the High-Luminosity LHC project, experts from the CERN Superconductors team recently obtained a world-record current of 20 kA at 24 K in an electrical transmission line consisting of two ...

High power laser sources at exotic wavelengths

Apr 14, 2014

High power laser sources at exotic wavelengths may be a step closer as researchers in China report a fibre optic parametric oscillator with record breaking efficiency. The research team believe this could ...

Novel technique opens door to better solar cells

Apr 14, 2014

A team of scientists, led by Assistant Professor Andrivo Rusydi from the Department of Physics at the National University of Singapore's (NUS) Faculty of Science, has successfully developed a technique to ...

User comments : 0

More news stories

CERN: World-record current in a superconductor

In the framework of the High-Luminosity LHC project, experts from the CERN Superconductors team recently obtained a world-record current of 20 kA at 24 K in an electrical transmission line consisting of two ...

Glasses strong as steel: A fast way to find the best

Scientists at Yale University have devised a dramatically faster way of identifying and characterizing complex alloys known as bulk metallic glasses (BMGs), a versatile type of pliable glass that's stronger than steel.

Low Vitamin D may not be a culprit in menopause symptoms

A new study from the Women's Health Initiative (WHI) shows no significant connection between vitamin D levels and menopause symptoms. The study was published online today in Menopause, the journal of The North American Menopa ...

Astronomers: 'Tilt-a-worlds' could harbor life

A fluctuating tilt in a planet's orbit does not preclude the possibility of life, according to new research by astronomers at the University of Washington, Utah's Weber State University and NASA. In fact, ...