Decoding Oxyanion Aqueous Solvation Structure: A Potassium Nitrate Example at Saturation.

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Materials Science

Background:

• Understanding salt solutions at the atomic scale is crucial for chemical reactions.
• Neutron diffraction excels at locating light atoms like hydrogen and deuterium in electrolytes.
• Accurate structural data is needed to refine computational models of solutions.

Purpose of the Study:

• To precisely determine the water coordination number around nitrate ions in aqueous solutions.
• To compare experimental findings with existing methods and computational simulations.
• To validate and improve atomistic computer simulations of electrolytes.

Main Methods:

• Utilized neutron diffraction with oxygen isotope labeling on aqueous nitrate solutions.
• Measured the hydrogen-bonded water coordination number around the nitrate oxyanion.
• Compared experimental results with nitrogen isotope substitution methods and molecular dynamics simulations.

Main Results:

• Revealed a small hydrogen-bonded water coordination number of 3.9 ± 1.2 around nitrate.
• Observed a discrepancy compared to computational simulations predicting >5-6 water molecules.
• Indicated significant ion-pair formation in aqueous nitrate solutions.

Conclusions:

• Oxygen isotope labeling in neutron diffraction provides accurate structural insights into electrolytes.
• The low coordination number suggests strong ion-pairing, challenging previous estimates.
• Experimental diffraction data is vital for benchmarking and enhancing atomistic simulations and intermolecular potentials.