  Anions generally associate more favorably with the air-water interface than
cations. In addition to solute size and polarizability, the intrinsic structure
of the unperturbed interface has been discussed as an important contributor to
this bias. Here we assess quantitatively the role that intrinsic charge
asymmetry of water's surface plays in ion adsorption, using computer
simulations to compare model solutes of various size and charge. In doing so,
we also evaluate the degree to which linear response theory for solvent
polarization is a reasonable approach for comparing the thermodynamics of bulk
and interfacial ion solvation. Consistent with previous works on bulk ion
solvation, we find that the average electrostatic potential at the center of a
neutral, sub-nanometer solute at the air-water interface depends sensitively on
its radius, and that this potential changes quite nonlinearly as the solute's
charge is introduced. The nonlinear response closely resembles that of the
bulk. As a result, the net nonlinearity of ion adsorption is weaker than in
bulk, but still substantial, comparable to the apparent magnitude of
macroscopically nonlocal contributions from the undisturbed interface. For the
simple-point-charge model of water we study, these results argue distinctly
against rationalizing ion adsorption in terms of surface potentials inherent to
molecular structure of the liquid's boundary.
