Solution structures in alkali nitrates and nitrites at high concentrations

In highly concentrated electrolyte solutions, where classical models often fail, specific ion-solvent interactions dictate bulk properties. Here, we combine small-angle X-ray scattering (SAXS) and Raman spectroscopy to link molecular coordination to mesoscale structure across alkali nitrate and nitrite solutions. Our results reveal a structural hierarchy driven by cation identity in that smaller cations (Li⁺, Na⁺) form discrete contact ion pairs, while larger cations (K⁺, Rb⁺) promote increasingly disordered local coordination and extended solute-solvent networks. Using nitrite salts as a structural control, we confirm this organizing principle across different anion geometries. Cs⁺ undergoes a concentration-induced transition to a uniquely ordered state, resulting in a highly structured solution. This work provides a direct, mechanistic explanation for how cation choice dictates the 'rules' of solution architecture, offering a predictive basis for understanding phase behavior in complex industrial and environmental systems.