Bare interfaces between water and hydrophobic media like air or oil are of
fundamental scientific interest and of great relevance for numerous applications. A number of
observations involving water/hydrophobic interfaces have, however, eluded a consensus mechanistic
interpretation so far. Recent theoretical studies ascribe these phenomena to an interfacial accumulation
of charged surfactant impurities in water. In the present work, we show that identifying surfactant
accumulation with X-ray reflectometry (XRR) or neutron reflectometry (NR) is challenging under
conventional contrast configurations because interfacial surfactant layers are then hardly visible.
On the other hand, both XRR and NR become more sensitive to surfactant accumulation when
a suitable scattering length contrast is generated by using fluorinated oil. With this approach,
significant interfacial accumulation of surfactant impurities at the bare oil/water interface is observed
in experiments involving standard cleaning procedures. These results suggest that surfactant
impurities may be a limiting factor for the investigation of fundamental phenomena involving
water/hydrophobic interfaces
