X-ray reflectivity of the OTS layer immersed in water, which was saturated with a variety of gases [light green (bottom), degassed; blue, CO; purple, CO2; red, Ar; dark green (top), 0.5 M HCl]. The curves are shifted vertically for clarity. (Inset) Magnification of the high q-range, where the measurement is most sensitive. All measured reflectivity curves are identical up to the maximum momentum transfer achieved in the experiments.