NMR investigation of the structure and single-particle dynamics of inorganic salt solutions in a protic ionic liquid

Abstract

With the aim to analyze the microstructure of the polar nanoregions of mixtures of ionic liquids and inorganic salts and the single particle dynamics of metal cations in these densely ionic environments, solutions of inorganic nitrate salts (lithium, magnesium, calcium and aluminium nitrates) in the protic ionic liquid ethylammonium nitrate have been characterized by means of nuclear magnetic resonance. The measured spectra reveal that, despite being dried under vacuum, all the studied mixtures contained variable amounts of water, but, while in those with the salt cations of lower ionic potentials (Li+, Ca2+) only bulk free water that can be almost completely removed from the mixture is found, mixtures with the cations of higher ionic potentials (Mg2+, Al3+) contain only hydration water structurally anchored in the primary solvation shells of the metal cations forming coordination complexes inside the polar nanoregions of the densely ionic solvent and, thus, non-removable under vacuum. This hydration water is not properly detected by common Karl Fisher methods and these solutions are sometimes mistakenly considered anhydrous. Moreover, our NMR measurements confirm the nanostructured solvation paradigm, since they show that the influence of the addition of salt on the chemical shifts of the signals associated to hydrogen atoms in the apolar regions of the protic ionic liquid is negligible. Single-particle dynamics of cations of low and high ionic potentials was also studied, and our observations for the diffusion coefficients of lithium and aluminium cations support the conclusion that Li+ diffusion takes place inside anhydrous [Li(NO3)4]3− complexes, while kinetic complexes with Al3+ gather around [Al(H2O)6]3+) aqua ions.