Solution structures in alkali nitrates and nitrites at high concentrations

TL;DR

This study explores how different cations influence the structure of concentrated alkali nitrate and nitrite solutions, revealing how cation size affects solution architecture.

Contribution

The work provides a mechanistic link between cation identity and mesoscale structure in concentrated electrolytes using SAXS and Raman.

Findings

Smaller cations like Li+ and Na+ form discrete contact ion pairs.

Larger cations like K+ and Rb+ lead to disordered local coordination and extended solute–solvent networks.

Cs+ forms a uniquely ordered state at high concentrations.

Abstract

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.

SAXS and Raman reveal smaller cations favor localized ion pairs, while larger cations induce cooperative mesoscale structuring, resolving molecular–mesoscale interactions in electrolytes relevant to industrial, energy & waste management applications.

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