What anion is that

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In aqueous solutions, charged particles are always surrounded by a shell of water molecules. However, much about the nature of this so-called hydrate shell is still not understood. With terahertz spectroscopy, chemists from Bochum have gained new insights into how an ion influences the water molecules in its environment. An overview of the findings from the experiments is given by Prof. Dr. Martina Havenith, Dr. Gerhard Schwaab and Dr. Federico Sebastiani from the Chair of Physical Chemistry II at the Ruhr University Bochum (RUB) in the journal Angewandte Chemie on July 18, 2018.

"The hydration shell of ions is extremely important for understanding fundamental processes such as the transport of ions through membranes or batteries," says Martina Havenith, spokeswoman for the Ruhr Explores Solvation Cluster of Excellence. "But apparently simple questions like the size of the hydration shell or why there are ion pairs are still unanswered."

New spectroscopic methods developed

At the Ruhr University in Bochum, Martina Havenith's team is tackling this question with spectroscopic methods developed in-house. The researchers send short pulses of radiation in the terahertz range - with a wavelength of just under a millimeter - through the sample. The mixture absorbs the radiation to different degrees in different frequency ranges, which is made visible in the form of a spectrum. The spectrum, i.e. the absorption pattern, reveals something about the movement of certain bonds in the molecules examined, for example about the hydrogen bonds in a water network.

The Bochum group developed special methods with low-frequency terahertz radiation in order to be able to determine the size of the hydration shell, i.e. the number of water molecules that are influenced by an ion. The researchers mathematically break down the recorded absorption pattern into its components and can thus identify the components in the spectrum that reveal something about individual ions or pairs of ions.

Counting water molecules in hydration shell

The result: In more than 37 investigated salts, hydration shells with a size between 2 and 21 water molecules were determined. The number depends, for example, on the size of the ion and its number of charges. Singly charged ions generally affect fewer water molecules than multiply charged ions. "However, this is not entirely systematic, but also depends on which cation or anion is present," explains Martina Havenith.

The researchers use their method to determine the so-called effective number of water molecules; this is the minimum number of water that can be influenced by an ion and thus cannot move as freely as the unaffected surrounding water. Due to the positive or negative charge of an ion, the water molecules align themselves with their partially positively charged hydrogen atoms or their partially negatively charged oxygen atom towards the ion. “The influence of the ion on the water molecules gradually decreases with distance,” explains Havenith. “There is not always a clear boundary between influenced and unaffected water molecules.” The team therefore specifies a minimum number for the size of the hydrate shell.

Ion pairs studied

The Bochum group has not only dealt with individual ions, but also with pairs of cations and anions. The water molecules influence the formation of the ion pair. They can either form a common hydration shell around the two partners or separate shells around the cation and anion. The team can quantify how many water molecules these shells consist of. “It is not enough to know how many water molecules are influenced by a single chloride ion and how many are influenced by a single iron ion to know how many water molecules surround an iron chloride,” explains Havenith. That is not a simple additive process.

"In general, our results clearly show that it is not individual ion properties but rather cooperative effects that are decisive," the researcher sums up. It is not enough to know a single ion property in order to be able to predict how an ion will affect the water molecules in its environment. Instead, the various parameters - such as the ion radius or which cation-anion pair is involved - are related and together determine how a hydration shell or ion pair is formed.

Simulation results confirmed

The experimental data match the theoretical simulations of other groups and can serve as input parameters for chemical process engineering.

The German Research Foundation supported the work as part of the Resolv Cluster of Excellence (EXC 1069).

Martina Havenith, Gerhard Schwaab, Federico Sebastiani: Ion hydration and ion pairing as probed by THz spectroscopy, in: Angewandte Chemie, 2018, DOI: 10.1002 / anie.201805261

Martina Havenith, Gerhard Schwaab, Federico Sebastiani: Ion hydration and ion pairing as probed by THz spectroscopy, in: Angewandte Chemie International Edition, 2018, DOI: 10.1002 / anie.201805261

Prof. Dr. Martina Havenith
Physical chemistry II
Faculty of Chemistry and Biochemistry
Ruhr-University Bochum
Tel .: 0234 32 28249
Email: [email protected]

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