The concentration dependence of the behavior of the Na+ and Cl-ions solvated in a water slab has been investigated using molecular dynamics simulations employing a polarizable potential model. At all concentrations, the sodium ions are almost exclusively located in the interior, while the chloride anions occupy also a significant portion of the surface of the slab. This difference, which has important consequences for the atmospheric reactivity of aqueous sea-salt microparticles (microbrines), is discussed in terms of the relative sizes and polarizabilities of the two ions. The salt concentration, which has been varied from infinite dilution to saturation, has a strong effect on many physical properties, such as the ion solvation numbers and the degree of ion pairing, which are quantified separately for the interior and surface regions of the water slab.