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 spectroscopic methods developed in-house, chemists from the Excellence Cluster Ruhr Explores Solvation have gained new insights into how an ion affects the water molecules in its environment. The team succeeded in determining the number of water molecules in different hydration shells.
Prof Dr Martina Havenith, Dr Gerhard Schwaab and Dr Federico Sebastiani provide an overview of the results of the experiments in the journal Angewandte Chemie in July 2018.
Resolving the water shell
Using terahertz spectroscopy, the team investigated 37 different salts consisting of positively and negatively charged ions in aqueous solution. The result: the hydration shells of the different ions contain between two and 21 water molecules. The exact number depends, for instance, on the size of the ion and its valency.
However, the Bochum group not only dealt with individual ions, but also with ion pairs. These can either have a common hydration shell or separate shells around the positively and negatively charged ion. “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. How an ion affects the water molecules in its environment depends on several ion properties.
Original Publication: G. Schwaab, F. Sebastiani, M. Havenith: Ion hydration and ion pairing as probed by THz spectroscopy, in: Angewandte Chemie, 2018, DOI: 10.1002/ange.201805261