Partial hydration of n-alkyl halides at the water–vapor interface: a molecular simulation study with atmospheric implications

Abstract

Classical molecular dynamics simulations with a polarizable force field were used to study adsorption of gas-phase alkyl halides to the surface of liquid water and their hydration properties in the interfacial environment. A systematic investigation has been performed for a set of monosubstituted alkyl chlorides, bromides and iodides of the alkyl chain length from one to five carbon atoms (C n H2n+1X, n = 1–5, X = Cl, Br, or I). All alkyl halides readily adsorb to the water surface and exhibit a strong preference for interfacial (partial) hydration. When adsorbed, the alkyl halide molecules reside primarily in the outermost region of the water–vapor interface. The (incomplete) hydration shell of the surface-adsorbed methyl halide species is centered on the methyl end of the molecule, with the halogen atom largely exposed and facing away from water into the gas phase. The maximum hydration of the longer-chain alkyl halides is localized around the α-CH2 group next to the halogen. With an increasing chain length, the alkyl halide molecules align more parallel to the surface. However, ethyl and propyl halides still have the halogen atom rather exposed, pointing almost freely into the gas phase. The behavior of butyl and pentyl halides on the water surface resembles that of alcohols, with the polar region of the CH2X group interacting with water and the rest of the increasingly nonpolar hydrocarbon chain pointing on average away from water. Consequently, the halogen atom becomes more, albeit not fully, hydrated. The propensity of alkyl halides for the water–vapor interface along with the specific character of the partial hydration of the surface-adsorbed alkyl halides and their preferred interfacial orientation is likely to be of importance for heterogeneous chemical processes, involving alkyl halides adsorbed on the surface of aqueous aerosol droplets or ice particles in the atmosphere.