Molecular dynamics simulations of small halogenated organics at the air-water interface: implications in water treatment and atmospheric chemistry.

Free energy profiles associated with transfer of chlorinated and brominated halomethane molecules from the gas phase across the water-vapor interface to the aqueous phase were calculated using classical molecular dynamics simulations. The investigated species include chloromethane (CH3Cl), bromomethane (CH3Br), dichloromethane (CH2Cl2), dibromomethane (CH2Br2), chloroform (CHCl3), and bromoform (CHBr3). The employed halomethane force field was tuned by scaling up the atomic charges to reproduce the experimental hydration free energies. The computed free energy profiles have a minimum at the water-vapor interface of about 12-15 kJ·mol(-1) relative to full hydration in the bulk liquid. This implies that the halomethanes exhibit enhanced interfacial concentrations in systems with large surface area per unit volume, such as air bubbles dispersed in water or water droplets dispersed in air. Implications for water treatment as well as for atmospheric chemistry are discussed.