  We argue that the kosmotropes remain strongly hydrated in the vicinity of a
hydrophobic surface, while the chaotropes lose their hydration shell and can
become adsorbed to the interface. The mechanism of adsorption is still a
subject of debate. We argue that there are two driving forces for anionic
adsorption: the hydrophobic cavitational energy and the interfacial
electrostatic surface potential of water. While the cavitational contribution
to ionic adsorption is now well accepted, the role of the electrostatic surface
potential is much less clear. The difficulty is that even the sign of this
potential is a subject of debate, with the ab initio and the classical force
field simulations predicting electrostatic surface potentials of opposite sign.
In this paper, we will argue that the strong anionic adsorption found in the
polarizable force field simulations is the result of the artificial
electrostatic surface potential present in the classical water models. We will
show that if the adsorption of anions were as large as predicted by the
polarizable force field simulations, the excess surface tension of the NaI
solution would be strongly negative, contrary to the experimental measurements.
While the large polarizability of heavy halides is a fundamental property and
must be included in realistic modeling of the electrolyte solutions, we argue
that the point charge water models, studied so far, are incompatible with the
polarizable ionic force fields when the translational symmetry is broken. The
goal for the future should be the development of water models with very low
electrostatic surface potential. We believe that such water models will be
compatible with the polarizable force fields, which can then be used to study
the interaction of ions with hydrophobic surfaces and proteins.
