Liquid-vapor interfaces of simple electrolyte solutions: Molecular dynamics results for ions in Stockmayer fluids

The equilibrium and dynamical properties of liquid-vapor interfaces of electrolyte solutions are investigated by employing a simple model where the ions are represented by charged Lennard-Jones particles and the dipolar solvent molecules are characterized by the so-called Stockmayer potential. The technique of molecular dynamics simulation is employed to calculate the density profiles and the orientational structure of the interfaces, the surface tension, the translational and rotational diffusion coefficients, and also the dipole orientational relaxation times of both interfacial and bulk molecules. It is found that the ions prefer to stay in the interior of the solutions and tend to avoid the surfaces. The dipole vectors of the interfacial solvent molecules tend to align parallel to the surfaces. The surface tension shows an increasing trend with increase of ion concentration, and this increase of the surface tension is found to be the net outcome of a decrease of the solvent-solvent contribution and an increase of the ion-solvent and ion-ion contributions. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. The solvent molecules at the interfaces rotate and translate in the parallel direction at a faster rate than that of bulk molecules. The perpendicular diffusion, however, occurs at a slower rate for the interfacial molecules. Also, on increase of ion concentration of the solutions, the dynamical properties of the interfaces are found to change differently from those of the bulk solutions. The equilibrium and dynamical results of the present interfacial solutions are also compared with the results of liquid-vapor interfaces of aqueous NaCl solutions that were reported in an earlier study (Chem. Phys. Lett. 2003, 373, 87).