How ions gather water molecules around them

August 9, 2018, Ruhr-Universitaet-Bochum
The Bochum research team: Gerhard Schwaab, Martina Havenith and Federico Sebastiani (from the left). Credit: RUB, Marquard

Charged particles in aqueous solutions are always surrounded by a shell of water molecules. However, much is still unknown about the nature of this so-called hydration shell. Using terahertz spectroscopy, chemists from Bochum have gained new insights into how an ion affects the water molecules in its environment. Prof Dr. Martina Havenith, Dr. Gerhard Schwaab and Dr. Federico Sebastiani from the Chair of Physical Chemistry II of Ruhr-Universität Bochum (RUB) provide an overview of the results of the experiments in the journal Angewandte Chemie in July 2018.

"The hydration of ions is extremely important for understanding fundamental processes such as the transport of ions through membranes or batteries," says Martina Havenith, spokesperson of the Cluster of Excellence Ruhr Explores Solvation. "However, seemingly simple questions, like the size of the hydration shell or the occurrence of ion pair formation, still remain unanswered."

New spectroscopic methods developed

At the Ruhr-Universität Bochum, Martina Havenith's team approaches this question with spectroscopic methods developed in-house. The researchers send short pulses of radiation in the terahertz range, i.e. with a wavelength just under one millimetre, 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 investigated molecules, for example about hydrogen bonds in a water network.

The Bochum group developed special techniques using low-frequency terahertz radiation to determine the size of the hydration shell, i.e. the number of that are affected by an ion. They mathematically break down the recorded absorption pattern into its components and can thus identify the parts in the spectrum that reveal something about individual ions or pairs of ions.

Resolving water molecules in hydration shell

The result: Hydration shells with a size between two and 21 water molecules were determined for more than 37 salts investigated. The number depends for instance on the size of the ion and its valency. Single-charged ions usually affect fewer water molecules than multiple-charged ions. "However, this is not entirely systematic, but also depends on the cation or anion present," explains Martina Havenith.

The researchers use their method to determine the so-called effective number of water molecules, which is the minimum number of water molecules that is affected by an ion, i.e. that cannot move as freely as the unaffected surrounding water. Due to the positive or negative charge of an ion, the water molecules with their partially positively charged hydrogen atoms or their partially negatively charged oxygen atom align themselves with the ion. "The effect of the ion on the water molecules gradually decreases with distance," Havenith explains. "Thus there is not always a clear boundary between affected and unaffected water molecules." The team therefore specifies a minimum number for the size of the hydration shell.

Ion pairs studied

However, the Bochum group dealt not only with individual ions, but also with pairs of cations and anions. The water molecules affect the formation of the ion pair. They can either form a joint hydration shell around the two partners or separate shells around cation and anion. The team is able to estimate how many water molecules these shells each consist of. "In order to know how many water molecules surround an iron chloride, it is not enough to know how many water molecules are affected by a single chloride ion and how many by a single iron ion," explains Havenith. This is not a simple additive process.

"In general, our results clearly show that cooperative effects rather than individual ion properties are decisive," sums up the researcher. It is therefore not enough to know a single ion property in order to predict how a salt will affect the in its environment. Instead, various parameters, such as the charge density or the combination of the cation-anion will determine whether an ion pair is formed.

Explore further: Mapping changes in the dynamics and structure of water molecules in the vicinity of solutes

More information: Martina Havenith-Newen et al. Ion hydration and ion pairing as probed by THz spectroscopy, Angewandte Chemie International Edition (2018). DOI: 10.1002/anie.201805261

Related Stories

Tracking a solvation process step by step

December 21, 2017

Chemists of Ruhr-Universität Bochum have tracked with unprecedented spatial resolution how individual water molecules attach to an organic molecule. They used low-temperature scanning tunneling microscopy to visualize the ...

Building bridges with water molecules

June 28, 2018

A team at TU Wien now has the proof behind the speculations that water molecules can form complex bridge-like structures when they accumulate on mineral surfaces.

Finding where the missing proton goes in water

August 8, 2016

Researchers at Yale University traced how a cluster of water molecules adapts to incorporate an extra proton, the positively charged subatomic particle, in the formation of an aqueous acid. Their spectroscopic "snapshot" ...

Recommended for you

Transforming carbon dioxide

August 21, 2018

A team of researchers at the University of Delaware's Center for Catalytic Science and Technology (CCST) has discovered a novel two-step process to increase the efficiency of carbon dioxide (CO2) electrolysis, a chemical ...

Gut bacteria provide key to making universal blood

August 21, 2018

In January, raging storms caused medical emergencies along the U.S. East Coast, prompting the Red Cross to issue an urgent call for blood donations. The nation's blood supply was especially in need of O-type blood that can ...


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.