How water meets a hydrophobic surface

Synchrotron x-ray reflectivity measurements of the interface between water and methyl-terminated octadecylsilane monolayers with stable contact angle > 100 degrees conclusively show a depletion layer, whether or not the water is degassed. The thickness is of order one water molecule: 2-4 A with electron density < 40% that of bulk water. Considerations of coherent and incoherent averaging of lateral inhomogeneities show that the data cannot be explained by "nanobubbles." When the contact angle is lower, unstable in time, or when monolayers fail to be sufficiently smooth over the footprint of the x-ray beam, there is no recognizable depletion.