Once upon a time, there was a cation and an anion in solution. Under the effect of electrostatic forces, they were attracted to each other, but the solvent molecules surrounding them (solvation) kept them apart. This fairy tale isn't just fiction—in fact, the pairing is most successful in ion-rich environments such as seawater, atmospheric aerosols, and biological fluids and tissues. In these environments, the pair concentration may even exceed that of single ions.

Ion pairs play an essential role in countless processes: crystal growth, release of proteins in living organisms, priming of chemical reactions, dissolution in ionic liquids, electrolysis of water, battery charge or discharge, etc.

Almost inexistent up to that point, the experimental study of ion pairs is now possible with laser-induced desorption vaporization, which allows for the transfer of fragile molecules to the gas phase. Researchers at IRAMIS applied this to saline samples to produce isolated ion pairs and characterize their structure by UV and IR spectroscopy. They showed that this method can now be used to characterize a large range of ion pairs.

This study is now followed by research of the microsolvation of paired ions and their supramolecular organization. 

Spectroscopie optique de paires d'ions: de la caractérisation des modèles en phase gazeuse à l'identification des paires d'ions en solution (Optical spectroscopy of ion pairs: From the characterization of gas phase models to identifying ion pairs in solution), thesis by Sana Habka, Université Paris-Saclay.