  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.
