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Salt Cluster With Surface Defect Shows Anomalous Acid-Base Chemistry
Jessica C Hartmann1, Jia Yang Lim2, Yiqi Sheng2
1Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, Innsbruck, 6020, Austria.
Angewandte Chemie (International Ed. in English)
|January 8, 2026
Summary
The reaction of formic acid with sea-salt aerosols, previously an atmospheric chemistry enigma, is explained by surface defects on sodium chloride clusters. These defects facilitate hydrogen chloride release, reversing typical acid-base behavior.
Area of Science:
- Atmospheric Chemistry
- Physical Chemistry
- Surface Science
Background:
- The reaction of formic acid with sea-salt aerosols to release hydrogen chloride challenges conventional acid-base principles.
- This process is crucial for understanding atmospheric chemistry and aerosol behavior.
Purpose of the Study:
- To elucidate the mechanism behind the acid displacement reaction of formic acid with sea-salt aerosols.
- To investigate the role of cluster structure and defects in this reaction.
Main Methods:
- Comparison of formic acid reactivity with Na13Cl12+ and Na14Cl13+ clusters using mass spectrometry under ultra-high vacuum conditions.
- Quantum chemical calculations to determine reaction energetics.
Main Results:
- Na14Cl13+ (magic cluster) showed no reaction with formic acid.
- Na13Cl12+ (defect cluster) readily reacted, releasing hydrogen chloride and incorporating formate.
- Quantum calculations confirmed the reaction is exothermic for the defect cluster due to formate ion "induced fit" and strong electrostatic interactions.
Conclusions:
- Surface defects on sea-salt clusters are key to enabling the acid displacement reaction.
- The "induced fit" of formate ions in defect sites drives the reaction, overcoming gas-phase acidity differences.
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