Ionic Distribution at the Air–Water Interface of Sodium Carbonate and Bicarbonate Solutions

The microphysics of carbonate ions near the air/water surface plays an important role in CO₂ absorption into water bodies. The sea-air flux model ignores surface hydration, which is different from bulk chemical reactions. This study aims to address the challenge by examining the interfacial electrostatic properties of Na₂CO₃ and NaHCO₃ solutions through experimental measurements of surface potential and molecular simulations. The results indicated that both ionic pairs generated similar surface capacitance, at 17.71 and 18.15 μF/cm² for Na₂CO₃ and NaHCO₃, respectively. The capacitances were much lower than those of sodium halides due to strong H-bonds between carbonates and the water surface molecules. These findings explain the role of interfacial electrostatic properties in determining CO₂ interaction with air-water interfaces. The determined surface potential can also help to predict the adsorption of other ions. The results can be used to model CO₂ transportation within the Earth's water bodies and atmospheric aerosols.