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 SO 2 on NaCl surfaces using advanced experimental and theoretical methods with a NH 4 Cl substrate also examined for cation effects. Results show that NaCl surfaces rapidly convert to Na 2 SO 4 with a new chlorine component when exposed to SO 2 under low humidity. In contrast, NH 4 Cl surfaces have limited SO 2 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.