Adsorption layers at the air-water interface formed from the soluble zwitterionic surfactant dodecylphosphorylcholine (DPC) serving as a soluble model substance for phospho-lipids were characterized with respect to their equilibrium and dynamic surface properties. To clarify the effect of ionic interactions with electrolyte present in the bulk phase, surface rheological properties upon addition of Ce(NO3)3 and Yb(NO3)3 were determined. In order to account for the surface activity of the nitrate ion, comparative measurements using NaNO3 were carried out additionally. Further experimental information on the bulk hydration characteristics of the aqueous solutions was obtained using dielectric relaxation spectroscopy (DRS). A possible mechanism causing the deviating surface dilatational modulus E in terms of ion specificity is suggested.