  The interface between the vapor and liquid phase of quadrupolar-dipolar
fluids is the seat of an electric interfacial potential whose influence on ion
solvation and distribution is not yet fully understood. To obtain further
microscopic insight into water specificity we first present extensive classical
molecular dynamics simulations of a series of model liquids with variable
molecular quadrupole moments that interpolates between SPC/E water and a purely
dipolar liquid. We then pinpoint the essential role played by the competing
multipolar contributions to the vapor-liquid and the solute-liquid interface
potentials in determining an important ion-specific direct electrostatic
contribution to the ionic solvation free energy for SPC/E water---dominated by
the quadrupolar and dipolar contributions---beyond the dominant polarization
one. Our results show that the influence of the vapor-liquid interfacial
potential on ion solvation is strongly reduced due to the strong partial
cancellation brought about by the competing solute-liquid interface potential.
