Identification and properties of ion-pairs in the aqueous solutions of LiI and NaI by FTIR and quantum chemical calculations

Existence and identification of ion pairing in aqueous electrolyte solutions are open questions to unveil the hydration structures of ions in water. In this work, we used a combination of infrared spectroscopy and quantum chemical calculations to investigate the ion pairing and hydrating properties of the aqueous solutions of lithium and sodium iodides by taking water molecules as probes. Spectral information of water molecules staying between the cations and anions in each case, as well as those contacting with the free ions of lithium and sodium, were extracted by using the strategy of equi-molar difference spectra. Peak positions of the solvent-shared ion-pair (SIP) water molecules were found to be 3340 cm−1 and 3580 cm−1 for LiI and NaI, respectively. With increasing concentration of the salts, the SIPs of NaI appear earlier than the SIPs of LiI, in accordance with the law of matching water affinities. This work opens a window for the identification of ion pairs using FTIR and may shed light on the separation of the precious lithium from sodium and the investigation of the hydration properties of other inorganic salts.