Adsorbed Water Promotes Chemically Active Environments on the Surface of Sodium Chloride

Gas-particle interfaces are chemically active environments. This study investigates the reactivity of SO₂ on NaCl surfaces using advanced experimental and theoretical methods with a NH₄Cl substrate also examined for cation effects. Results show that NaCl surfaces rapidly convert to Na₂SO₄ with a new chlorine component when exposed to SO₂ under low humidity. In contrast, NH₄Cl surfaces have limited SO₂ uptake and do not change significantly. Depth profiles reveal transformed layers and elemental ratios at the crystal surfaces. The chlorine species detected originates from Cl^- expelled from the NaCl crystal structure, as determined by atomistic density functional theory calculations. Molecular dynamics simulations highlight the chemically active NaCl surface environment, driven by a strong interfacial electric field and the presence of sub-monolayer water coverage. These findings underscore the chemical activity of salt surfaces and the unexpected chemistry that arises from their interaction with interfacial water, even under very dry conditions.