  We demonstrate that the driving forces for ion adsorption to the air-water
interface for point charge models results from both cavitation and a term that
is of the form of a negative electrochemical surface potential. We carefully
characterize the role of the free energy due to the electrochemical surface
potential computed from simple empirical models and its role in ionic
adsorption within the context of dielectric continuum theory. Our research
suggests that the electrochemical surface potential due to point charge models
provides anions with a significant driving force to the air-water interface.
This is contrary to the results of ab initio simulations that indicate that the
average electrostatic surface potential should favor the desorption of anions
at the air-water interface. The results have profound implications for the
studies of ionic distributions in the vicinity of hydrophobic surfaces and
proteins.
