Part of one long-term project undertaken by the MPI-MF group has been the investigation of the interfacial structure of water in contact with hydrophobic materials. The high-energy x-rays at ID15A were required because 20mm of water were penetrated. This fundamental concept is relevant in nature for such examples as cell membranes and protein interaction with water. The understanding of the interfacial structure was not settled. Some points of contention were: the size of the gap between water and hydrophobic surface, the presence of nano-bubbles, and the effect of gas dissolved in the water.

X-ray reflectivity was used to probe the electron density normal to the surface of the interface of water in contact with of a hydrophobic octadecyl-trichlororsilane (OTS) film grown on a silicon wafer. Measurements were first made to determine the structure of the film on the silicon in air and also the time before the film suffered from beam damage effects. Once these conditions were established, the interfacial structure with degassed-water was measured. In addition, water with different dissolved gases (N2, O2, Ar, Xe, CO, or CO2) were measured. From the data, a hydrophobic gap was found and that this gap was not influenced by gas dissolved in the water. Within the experimental resolution (4Å), the integrated density deficit of the gap was found to be 1.1Åg/cm3 with the best fit of the data to give a depletion layer of 3.8Å thick. This can be seen in Figure 29, where the fit is compared to the experimental data and a structural model is shown. Additionally, nano-bubbles reported by AFM experiments were not observed.

XRR of the OTS film in degassed-water and corresponding fits to data. The black, dashed line represent no depletion layer, the red line is the best fit to the data, and the blue and green line represent the lower (2.0Å) and upper (8.0Å) bounds for the depletion layer thickness.	Schematic of the interfacial structural of the self-assembled OTS film on silicon in “contact” with water.

The structure of the interface of ice at temperatures close to the melting point remains even more elusive. Michael Faraday predicted 1860 the presence of a film of liquid water at the interface, a phenomena known today as interface melting. Since then many experiments have been conducted at the free surface of ice, but it remained impossible to investigate the phenomenon at buried interfaces. Recent experiments done at the ice-SiO2 interface have been able to measure the thickness of the pre-melted region depending on the temperature. They show that the melting behaviour is comparable to the observations done at the free surface of ice. This could help in the understanding of adhesion of ice, permafrost and the flow behaviour of some glacier types.

Mezger M., Reichert H., Schöder S., Okasinski J., Schröder H., Dosch H., Palms D., Ralston J., Honkimäki V. - High-resolution in situ x-ray study of the hydrophobic gap at the wateroctadecyl-trichlorosilane interface - Proc. Natl. Acad. Sci. U.S.A. 103, 18401-18404 (2006).
S. Engemann, H. Reichert, H. Dosch, J. Bilgram, V. Honkimäki, and A. Snigirev - Interfacial melting of ice in contact with SiO₂ - Physical Review Letters 92, 205701 (2004).