CO hydration at the air-water interface: A surface-mediated "in-and-out" mechanism.

An understanding of the CO + HO 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 CO 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 CO hydration at the air-water interface. We show that reaction at the interface follows a surface-mediated "in-and-out" mechanism: CO 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 CO hydration is equally as feasible under these two solvation environments and that acidification rates are likely enhanced by this additional surface contribution.