The structure of liquid alkali nitrates and nitrites

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Understanding the structure of liquid nitrates and nitrites is crucial for predicting their chemical behavior.
• Previous assumptions about ammonium cation size may not accurately reflect its interactions in these liquids.

Purpose of the Study:

• To determine the liquid-state structure of alkali and ammonium nitrates and nitrites.
• To model these systems using molecular dynamics simulations, accounting for anion flexibility and charge movement.
• To investigate emergent length scales in alkali nitrites and nitrates and their relation to cation identity and temperature.

Main Methods:

• High-energy X-ray diffraction combined with containerless techniques.
• Molecular dynamics simulations incorporating flexible molecular anions and charge delocalization.
• Analysis of scattering functions across low- and high-Q regimes to probe inter- and intra-molecular interactions.

Main Results:

• The molecular dynamics model successfully reproduces experimental scattering functions for both nitrates and nitrites.
• For ammonium nitrate, diffraction data best fits an ammonium cation radius similar to Cesium (Cs+).
• Alkali nitrites exhibit an emergent length scale in nitrogen-nitrogen correlations dependent on temperature and cation type.
• Alkali nitrates show a less pronounced effect on nitrogen-nitrogen correlations compared to nitrites.

Conclusions:

• The study refines understanding of cation-anion and anion-anion interactions in liquid nitrates and nitrites.
• The size of the ammonium cation is better represented by a larger ionic radius.
• An emergent length scale in alkali nitrites, linked to N-N correlations, differs significantly from that in alkali nitrates.