High-resolution in situ x-ray study of the hydrophobic gap at the water-octadecyl-trichlorosilane interface

The knowledge of the microscopic structure of water at interfaces is essential for the understanding of interfacial phenomena in numerous natural and technological environments. To study deeply buried liquid water-solid interfaces, high-energy x-ray reflectivity measurements have been performed. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane, provide strongly hydrophobic substrates. We show interfacial density profiles with angstrom resolution near the solid-liquid interface of water in contact with an octadecyl-trichlorosilane layer. The experimental data provide clear evidence for the existence of a hydrophobic gap on the molecular scale with an integrated density deficit pd = 1.1 Å g cm^-3 at the solid-water interface. In addition, measurements on the influence of gases (Ar, Xe, Kr, N₂, O₂, CO, and CO ₂) and HCl, dissolved in the water, have been performed. No effect on the hydrophobic water gap was found.