Deciphering the Structure and Association Behavior in Aqueous Lithium Nitrate Solution

Abstract

Among all associative species, lithium ions predominantly form a four-coordinated tetrahedral structure, with exceptions occurring only rarely in the case of five-coordinated structures, which exhibit higher energy. As the concentration of the lithium nitrate solution increases, an ion association process may occur, accompanied by multiple ion pair transformations. This study employs micro-Raman spectroscopy and component analysis methods to investigate lithium nitrate hexahydrate aqueous solutions with water-to-salt molar ratios (WSR) ranging from 1 to 22. Within the examined concentration range, as the solution concentration increases, the characteristic vibration wavenumber of nitrate ions shifts from 1047 to 1062 cm⁻¹, while the half-peak width broadens from 14.5 to 25 cm⁻¹. In dilute lithium nitrate solutions, lithium ions and nitrate ions adopt tetrahedral (tetrahydrate) and bowl-shaped (hexahydrate) structures, respectively. Depending on the WSR: At WSR ≥ 5, the primary species include free hydrated ions, solvent-shared ion pairs (SIP), and contact ion pairs (CIP). When 12.5 ≤ WSR < 20, the main species are solvent-shared ion pairs. For WSR < 5, the predominant species consist of two types of ion pairs: CIP and complex ion pairs, with the amounts of free ions and CIP rapidly decreasing to zero. As WSR decreases from 5 to 1, the proportion of CIP diminishes from a maximum of 35% to 0%, while the percentage of Complex species increases significantly. Notably, when the WSR approaches 1, the percentage content of Complex species reaches approximately 95%.