How Water Meets a Hydrophobic Surface

Electron density at the interface between water and a hydrophobic surface (an octadecylsilane monolayer on oxidized silicon surface). The decrease in density near the water-organic interface (region “X”) indicates the effect of hydrophobicity. The other regions indicated are (A) water, (B) alkyl chain, (C) silane headgroup, and (D) oxidized silicon surface.

Interactions at hydrophobic interfaces are important in many areas, ranging from soil interactions to protein folding. Past experiments on the structure of hydrophobic interfaces have revealed conflicting results on the nature of this interface: some results found a nanometers thick depletion region while other results saw no depletion region; some experiments saw evidence of "nano-bubbles”.

Synchrotron x-ray reflectivity measurements of the water-hydrophobic interface conclusively show a depletion layer with a 2- to 4-Å thickness (approximately one water molecule in size) with an electron density <40% that of bulk water, whether or not the water is degassed.

The interfacial density depletion cannot be explained by the possibility of “nanobubbles”.

Reference

A. Poynor, L. Hong, I. K. Robinson, S. Granick, Z. Zhang and P. A. Fenter, “How Water Meets a Hydrophobic Surface,” Physical Review Letters 97, 266101 (2006)