How water meets a very hydrophobic surface

Recent X-ray and neutron scattering experiments [1] have provided evidence both for and against the existence of low-density regions between water and hydrophobic surfaces. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using synchrotron X-ray reflectivity. These surfaces are significantly more hydrophobic than methyl-terminated monolayers. We find that the width of the depleted region increases with aqueous contact angle and becomes larger than the experimental resolution. When the advancing contact angle is 120$^{\circ}$, the effective depletion layer thickness is $\sim $5{\AA}. We interpret the apparent contradictions in previous studies as scatter in measurements of sub-resolution effects. The water-gap interface is expected to undergo large fluctuations [2]; our data place an upper limit of 3.5{\AA} on the RMS width caused by fluctuations. \newline [Supported by NSF grant no. DMR-0705137] \newline \newline [1] e.g., A. Poynor et al., \textit{PRL}\textbf{ 97}, 266101, 2006; M. Mezger et al., \textit{PNAS }\textbf{103}, 18401, 2006; M. Maccarini et al., \textit{Langmuir} \textbf{23}, 598, 2007 \newline [2] e.g. X. Zhang et al., \textit{Science} \textbf{295}, 663 (2002); D. Chandler, \textit{Nature} 445, 831 (2007)