For many years, scientists have argued about the existence of a depletion gap between water and hydrophobic surfaces. Several recent reports based on high-resolution synchrotron X-ray reflectivity seemed to give a positive conclusion, but they were not in good agreement quantitatively, mainly because the amount being discussed was at experimental resolution. A research group led by Professor P. Dutta (Northwestern University, Illinois, USA) has recently reported some synchrotron X-ray reflectivity results on the interface between water and self-assembled monolayers. To enlarge the depletion gap (if any) as much as possible, they chose hydrophobic fluoroalkylsilane, CF3(CF2)5(CH2)2SiCl3 and CF3(CF2)11(CH2)2SiCl3, of which the contact angles were 111 deg and 120 deg, respectively. It was found that the depleted region width increased with contact angle and exceeded the resolution. They also concluded that the contribution of its fluctuation to the interface roughness was substantially smaller than has been considered so far. For more information, see the paper, "How Water Meets a Very Hydrophobic Surface", S. Chattopadhyay et al., Phys. Rev. Lett. 105, 037803 (2010).
