Significance
Reactions at interfaces are an important and ubiquitous type of process. Despite their prevalence in nature, obtaining a molecular-level understanding of these processes remains challenging due to difficulties associated with probing the interfacial regime. Using machine learning simulations trained to various levels of theory, we uncover insights into how reactions proceed at the air–water interface. Specifically, we uncover a reaction mechanism for CO2 hydration in which the position of the reaction site is intimately coupled with the extent of reaction at the interface. This mechanism likely underpins a number of important surface reactions and forms an integral component of ocean acidification. Our work places a heightened importance on the contribution of surface-adsorbed CO2 to the overall acidification rate.
Abstract
An understanding of the CO2 + H2O hydration reaction is crucial for modeling the effects of ocean acidification, for enabling novel carbon storage solutions, and as a model process in the geosciences. While the mechanism of this reaction has been investigated extensively in the condensed phase, its mechanism at the air–water interface remains elusive, leaving uncertain the contribution that surface-adsorbed CO2 makes to the overall acidification reaction. In this study, we employ machine-learned potentials trained to various levels of theory to provide a molecular-level understanding of CO2 hydration at the air–water interface. We show that reaction at the interface follows a surface-mediated “in-and-out” mechanism: CO2 diffuses into the aqueous surface layer, reacts to form carbonic acid, and is subsequently expelled from solution. We show that this surface layer provides a bulk-like solvation environment, engendering similar modes of reactivity and near-identical free energy profiles for the bulk and interfacial processes. Our study unveils an unconventional reaction mechanism that underscores the dynamic nature of the molecular reaction site at the air–water interface. The similarity between bulk and interfacial profiles shows that CO2 hydration is equally as feasible under these two solvation environments and that acidification rates are likely enhanced by this additional surface contribution.
Brookes S. G. H., Kapil V., Michaelides A. & Schran C., 2025. CO2 hydration at the air–water interface: a surface-mediated “in-and-out” mechanism. PNAS 122(34): e2502684122. doi: 10.1073/pnas.2502684122. Article.
