  We extend our previous study of surface tension of ionic solutions and apply
it to the case of acids (and salts) with strong ion-surface interactions. These
ion-surface interactions yield a non-linear boundary condition with an
effective surface charge due to adsorption of ions from the bulk onto the
interface. The calculation is done using the loop-expansion technique, where
the zero-loop (mean field) corresponds of the non-linear Poisson-Boltzmann
equation. The surface tension is obtained analytically to one-loop order, where
the mean-field contribution is a modification of the Poisson-Boltzmann surface
tension, and the one-loop contribution gives a generalization of the
Onsager-Samaras result. Our theory fits well a wide range of different acids
and salts, and is in accord with the reverse Hofmeister series for acids.
