Hydroxyl Radical at the Air−Water Interface

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air−water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (Δ<i>G</i><sub>a</sub>) being about 1 kcal/mol more negative than the hydration free energy (Δ<i>G</i><sub>s</sub>). The propensity of the hydroxyl radical for the air−water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.