Findings on how electrons are solvated in water widen the range of potential influences on chemical reactions
Chemistry can be quite confusing. In order for the desired substance to be produced in a reaction or for two substances to interact - or not - chemists have to take into account a number of factors. Researchers at the Fritz Haber Institute of the Max Planck Society in Berlin can now offer them additional access. They have measured how strongly electrons are bound when they are put into water – namely right at the start as soon as the negative charge carriers are released by, for instance, a potential reaction partner. Electrons are the actual players in chemical reactions, as they are moved between different atoms during this process. Whether that happens depends on their binding energy to the different components. And in reactions in aqueous solutions, the binding energy of an electron at the beginning of the solvation process is a key factor. Now that it is known, chemists can consider it when planning or preventing reactions.
An electron that floats unattached from an atom or molecule in water behaves rather like a hermit crab without a shell: Just as the crab quickly looks for a new home and, as it does so, carefully avoids having anything to with its fellow species, so an electron, too, attempts to take shelter again as quickly as possible with an atom and presses into the chemical compounds it finds in the water. Naked electrons such as this become involved in a number of chemical reactions if water is present: in the chemical processes in biological cells, for example, or in ozone layer depletion and other reactions in the atmosphere that take place in tiny droplets of water.
Before an electron in the water takes up residence in its new home in an atom or molecule, however, it firstly attempts to achieve at least some compensation for its negative charge in order to cover its electronic nakedness. To this end, it surrounds itself with water molecules that have positive electrical poles and direct them to the negative charge of the electron. When that happens, the electron is solvated in the water. A team headed by Julia Stähler, Leader of a Research Group at the Fritz Haber Institute of the Max Planck Society, has now obtained detailed information about the solvation process.
Chemists need the binding energy of the diving electron
The researchers have, on the one hand, determined the energy with which the naked electron is bound directly after it is immersed in water and its charge is not yet buffered off from water molecules with the positive poles. Consequently, such an electron is clearly more weakly bound than even the outer electrons of the alkali metals such as sodium or potassium, which are also extremely reactive because of the low binding energy of their electrons. On the other hand, they have found that it only takes 22 femtoseconds until the electron begins to gather water molecules around it – a femtosecond is a millionth of a billionth of a second.
The tremendous speed of the process explains why scientists have not, until now, been able to measure the binding energy of the electron directly after it has been immersed in water. "This information is important for chemists, however, if they wish to promote or prevent reactions with solvated electrons", explains Julia Stähler. It is this energy that decides whether or not the first step in a reaction with solvated electrons takes place. When a substance releases an electron into water, it must have the binding energy of an electron that is just immersed in the water.
The team of physicists has obtained the findings on the immersion process of the electron using so-called time-resolved two-photon photoelectron spectroscopy (2PPE) in which light catapults electrons from their atomic environment. Their binding energy can be determined from the energy of the light and the kinetic energy of the electron that is flying away. States in which electrons are not normally found can only be filled with an energy boost from, for example, a laser. A second light pulse allows the binding energy of such states.
A model made of ice with the structure of liquid water
To determine the binding energy of the immersing electron, the team headed by Julia Stähler uses a model of water - a thin layer of ice in which the water molecules are not in fact arranged as in conventional ice but similar to liquid water, in other words amorphously. The researchers produce this thin layer of ice on a copper plate. They use a first very short laser flash to initially eject an electron from the copper that subsequently penetrates the amorphous layer of ice. They then direct a second laser pulse into their sample that throws the electron out of the layer of ice. They tailor the time gap between the two laser flashes to an accuracy of femtoseconds. Thanks to this time resolution, they can track how long the electron remains in the state in which it is not yet solvated by the water molecules.
In this way, the team found, too, that an electron can also land directly – in other words without the 22 femtoseconds time delay – in the solvated state in which the water molecule compensates its charge. "This enables us to answer the question of whether an electron digs its own potential hole in the water or falls into an at least small hole that already exists", says Julia Stähler. A potential hole is produced when the dipoles of the water molecule are arranged so that their positive poles envelop the negatively charged electron. "If the electron can land directly in such a potential hole, it must naturally be there in advance", says the physicist. If the electron created its potential hole first, the water dipoles turned their positive poles towards it only once they perceived it. And that would take at least a little while. In contrast, an electron abruptly jumps into an existing hole and then deepens it.
Julia Stähler is convinced that the results from the study on the model system can be transferred to liquid water to a large degree. "The absolute value of the binding energy can of course differ", she says. "However, the solvation process in water can take place just as rapidly and in the same way as in our model system." The researchers at the Max Planck Institute in Berlin are therefore conducting further research on how electrons can be immersed in amorphous ice. Currently, for example, they work on the determination of how far the electron penetrates the water before it is influenced by the water molecules. This will also give chemists indications on how they can influence reactions in water.