Accelerated OH Oxidation of Methyl Chloride via Surface Accumulation at the Air-Water Interface

Chemical reactions often proceed differently at the air-water interface than in bulk solution. In particular, "on-water" catalysis refers to cases where reactions appear to be accelerated at aqueous surfaces. A growing body of work shows that rate acceleration can arise from intertwined effects, including modified solvation and acid-base behavior, as well as enhanced reactant concentrations. Despite this progress, it remains difficult to quantify, for specific atmospheric reactions, how much interfacial rate enhancement arises from (i) intrinsic changes in the free energy barrier versus (ii) concentration-driven effects, because fully converged interfacial free energy profiles are costly to obtain with ab initio molecular simulations. Here, we address this challenge for the reaction of methyl chloride CH₃Cl with hydroxyl radicals (OH), an important atmospheric sink of CH₃Cl, by developing a reaction-specific empirical valence bond (EVB) model parametrized against high-level ab initio data. The EVB model, combined with a flexible polarizable water force field, enables efficient condensed-phase simulations and systematic comparison of gas phase, bulk aqueous, and interfacial reactivity. Our results show that the reaction free energy barrier is only weakly affected by proximity to the air-water interface, whereas thermodynamic enrichment of the surface-active reactants increases the effective interfacial rate by more than 2 orders of magnitude relative to the aqueous bulk and by over 6 orders of magnitude relative to the gas phase.