The surface affinity of cations depends on both the cations and the nature of the surface.
Specific ion effects at interfaces are important for a variety of thermodynamic properties of electrolyte solutions, like surface tension and the phase behavior of surfactants. We report the relative surface affinity of Na+ and D3O+ at both the D2O-air and the sodium dodecyl sulfate (surfactant)-covered D2O surface by studying the alignment of interfacial D2O, using vibrational sum frequency generation spectroscopy. The surface propensity of ions is found to be a function of both the nature of the ion and the nature of the surface. Specifically, for the charged, surfactant-covered interface, Na+ has a higher affinity than D3O+. In contrast, D3O+ has a higher affinity than Na+ at the air-D2O interface. The relative surface affinity of cations thus depends on both details of the cation and the type of interface.