Oxoanion-Mediated Stabilization of Carbonate Species at the Air/Water Interface

The surface chemistry and reactivity of carbon dioxide at air/liquid interfaces, including inorganic carbonates, play a central role in various environmental processes. While carbonate/bicarbonate interfacial activity has been studied in simple solutions, the influence of technologically relevant oxoanions remains underexplored. Here, we report that orthovanadate induces strong sum frequency generation (SFG) activity of carbonate species through interfacial oxoanion pairing, enabling direct spectroscopic observation under ambient conditions. Concentration-dependent SFG measurements reveal two distinct interfacial species, characterized by vibrational features near 1650 and 1400 cm-1, corresponding to bicarbonate and carbonate adducts, respectively. In contrast, bulk spectroscopic measurements (attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and 51V NMR) show no evidence of vanadate-carbonate interactions. Density functional theory (DFT) calculations suggest hydrogen-bonded adducts consistent with the observed spectral features. We attribute this interfacial stabilization to reduced dielectric screening and enhanced anion-anion correlations at the interface, enabled by protonation and hydrogen bonding between anions. These results establish oxoanion pairing as a mechanism to control interfacial speciation and provide a pathway for tuning surface reactivity and transport in bulk aqueous carbon dioxide removal (CDR) systems.