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.