The influence of water interfacial potentials on ion hydration free   energies and density profiles near the surface
  The surface or contact potential at the water liquid-vapor interface is discussed in relation to determinations of absolute ion hydration free energies and distributions of ions near the interface. It is shown that, rather than the surface potential itself, the net electrostatic potential at the center of an uncharged solute can aid both in relating differences between tabulations of hydration free energies and in explaining differing classical and quantum surface potential estimates. Quantum mechanical results are consistent with conclusions from classical simulations that there is a net driving force that enhances anion density at the surface relative to cations. 
Thomas L. Beck
cond-mat.soft physics.chem-ph