We investigate various mechanisms contributing to the surface ion distributions in simple and mixed
aqueous alkali-halide solutions depending on the total salt concentration, using a combination of
photoelectron spectroscopy and molecular dynamics simulations. In simple solutions, the surface
enhancement of large polarizable anions is reduced with increasing concentration. In the case of a
NaBr/NaCl mixed aqueous solution, with bromide as the minority component, the situation is more
complex. While the total anion/cation charge separation is similarly reduced with increasing salt content,
this alone does not uniquely determine the ion distribution due to the co-existence of two different
anions; Br- and Cl-. We show that bromide is selectively surface enhanced at higher concentrations, despite
the fact that the total anion surface enhancement is reduced. This phenomenon, which can be viewed as
.salting out. of bromide by NaCl might have consequences for our understanding of the surface structure
of mixed aqueous solutions subjected to concentration increase due to dehydration, such as seawater-born
aerosols.