Accelerated Chlorination at the Air-Organic Interface Revealed by Molecular Simulations and Kinetic Modeling

An interface is a distinct chemical environment where reactivity can proceed differently than the bulk. Chemical reactions can speed up at air-water interfaces due to the rapidly varying electrostatic environment between highly polar liquid and nonpolar gas phases. Here, we show that reaction acceleration can also occur at nonpolar air-organic interfaces, which are prototypical chemical environments important for understanding atmospheric aerosols, industrial membranes, and biological cells. We examine the uptake dynamics of chlorine gas at the air/squalene interface to better interpret the anomalous reaction kinetics observed in previous aerosol experiments. Utilizing molecular dynamics simulations and coarse-grained kinetic models, we find evidence that chlorine addition to squalene carbon double bonds at the interface occurs over an order of magnitude faster than in the bulk. This acceleration is due to two mechanisms: faster diffusion of chlorine on the surface, and a collective thermodynamic driving force termed "tail spearfishing" that increases the probability of encounter between chlorine and squalene tails that extend into the gas phase.