X-ray studies of the Density Depletion at Hydrophobic Water-Solid Interfaces

Deeply buried hydrophobic solid-water interfaces were probed with high-energy x-ray reflectivity. The experimental data provide clear evidence for a thin density depletion with an integrated deficit corresponding to approximately 40\% of a water monolayer extending over a maximum of two molecular layers. In addition, measurements on the influence of gases (Ar, Xe, Kr, N$_2$, O$_2$, CO, CO$_2$) dissolved in the water have been performed. No effect on the hydrophobic water gap was found. The presence of nanobubbles at the interface could also be excluded. By comparing the experimental results with an generic DFT model we can give a quantitative estimation for different contributions to the observed gap size. \newline [1] M. Mezger {\em et al.}, {\em Proc. Natl. Acad. Sci. USA} {$\mathbf{103}$}, 18401 (2006). \newline [2] M. Mezger {\em et al.}, {\em J. Chem. Phys.} {$\mathbf{128}$}, 244705 (2008).