PNNL

Abstract

The free energy profile for transfer of OH across the air/water interface at 300 K was calculated using classical molecular dynamics computer simulation with polarizable potential. The experimental hydration free energy (?Gs ) is satisfactorily reproduced by the present force field. The free energy profile exhibits a minimum at the air/water interface, with the free energy of adsobtion (?Gs ) being about 1 kcal/mol larger than the hydration free energy. The propensity of the OH radical for the air/water interface was further explored in simulation, in which OH radicals were originally placed inside the bulk liquid region of an infinite water slab about 30 Å thick. During 1 ns propagation, the OH radicals were observed to diffuse through the interior of the water slab, however, they were predominantly located in either of the two interfacial regions. Collisions of OH radical with the water surface were also investigated in a series of scattering simulations. From 250 initial conditions of a gas phase OH radical approaching the surface of liquid water with a thermal impact velocity, the thermal and mass accommodation coefficients at 300 K were determined to be 0.95 and 0.90, respectively. The partitioning of OH radicals between the bulk water and the interface was observed. The enhancement in the surface concentration of OH relative to the concentration in the aqueous phase, resulting from the surface activity of the hydroxyl radical, suggests that important OH chemistry may be occurring in the interfacial water layer of the water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modelling heterogeneous atmospheric chemical processes.