Distinguishing the gas and water sides of the gas–water interface: Where do amines capture CO2 in microdroplets?

Direct capture of carbon dioxide (CO 2 ) by organic amines is often inefficient in bulk aqueous solutions under mild conditions, whereas microdroplets provide a unique gas-water interfacial environment that enables the transformation. However, whether CO 2 capture occurs from the gas side or the water side of the microdroplet interface remains unclear. Here we show that amines capture CO 2 predominantly on water microdroplets to form carbamates, suggesting that the gas–water interface plays a key role in this process. In the presence of dipropylamine vapor, spraying water microdroplets generates a carbamate anion which is directly detected by mass spectrometry (MS) and tandem mass spectrometry (MS 2 ). Control experiments show that removing dissolved CO 2 from water has little effect on product formation, whereas replacing the surrounding gaseous CO 2 with N 2 strongly suppresses the carbamate signal. These results indicate that CO 2 capture occurs predominantly from the gas side rather than from the water side of the gas–water interface. Comparison of vapor-phase amine and dissolved amine conditions shows that the carbamate formation occurs more readily when the amine is introduced from the gas side of the gas–water interface. Water microdroplets are also found to be more effective than other organic solvents, demonstrating that water provides a unique interfacial environment for efficient CO 2 -capture and stabilization of the product. Primary and secondary amines give analogous products, whereas a tertiary amine does not, further supporting carbamate formation. This work suggests a directional role of the gas–water interface in amine-mediated CO 2 capture and provides preliminary mechanistic insight into gas uptake at microscopic aqueous interfaces.