Diffusion at the liquid-vapor interface of an aqueous ionic solution utilizing a dual simulation technique

The recently proposed dual simulation technique [J. Phys. Chem. B 2004, 108, 6595.], with slight modification, was used to determine the diffusion coefficients for a variety of regions of a 2.2 M sodium chloride aqueous solution with a vapor-liquid interface. The diffusion of all species was shown to be isotropic far away from the interface, but at different regions in the interface, the diffusion coefficients parallel and perpendicular to the interface did not agree for water and chloride. Specifically, interfacial water diffusion parallel to the interface was significantly higher than diffusion perpendicular to the interface. Chloride ions showed even larger anisotropicity in its diffusion coefficient at the interface, with its perpendicular diffusion being similar to its bulk value, but parallel diffusion being much higher, corresponding to the region of highest chloride ion concentration. The origin for this was found to be hydrogen bonds with waters which are highly oriented perpendicular to the interface, somewhat impeding chloride ion diffusion perpendicular to the interface. While sodium ion diffusion increased at the interface, its interfacial concentration is low in that region, and its diffusion was fairly isotropic throughout all regions.