How Water Meets a Very Hydrophobic Surface

Abstract

Is there a low-density region ("gap") between water and a hydrophobic surface? Previous x-ray and neutron reflectivity results have been inconsistent because the effect (if any) is subresolution for the surfaces studied. We have used x-ray reflectivity to probe the interface between water and more hydrophobic smooth surfaces. The depleted region width increases with contact angle and becomes larger than the resolution, allowing definitive measurements. Large fluctuations are predicted at this interface; however, we find that their contribution to the interface roughness is too small to measure.

Understanding controls on interfacial wetting at epitaxial graphene: Experiment and theory

“…A positive definite integrated density difference indicates the existence of an intrinsic gap between two phases. This generic estimation is equivalent with the product of the depletion region width and the density deficit (δD·δρ), a more robust parameter to describe the net depletion of the density at a hydrophobic interface since the details of a hydrophobic gap are often not intrinsically resolved by most low angle reflectivity measurements [56][57][58]. From Fig.…”

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…39 In the present study, all four types of molecules were self-assembled on thick oxide surfaces using the following procedure. 41 The substrates were cleaned by sonication in acetone and ethanol baths and were treated with oxygen plasma for 1 min.…”

“…The thin film cell used to hold SAM substrates and water was described in our previous Letter. 39 The X-ray energy was 19 KeV (λ = 0.65Å) and the beam was focused to 0.5mm vertically and 2mm horizontally. The data were collected with a Pilatus area detector.…”

“…39 Second, X-ray reflectivity is not chemically sensitive, and so the interfacial density profile of the hydrophobic substrate, which is not of specific interest here, affects the reflectivity and obscures the profile of interest (that of interfacial water). This experimental difficulty does not exist at the air-water interface to which the water-hydrophobic interface is theoretically compared, since air has negligible electron density.…”

“…First, we have previously shown 39 that increasing the hydrophobicity by using fluorocarbon-based SAMs is a good way to enhance the gap relative to the resolution limit. That Letter was limited to two different SAMs; in the paper we have added two more types of fluorocarbon SAMs to our study to see more clearly whether the interfacial gap width depends on monolayer properties.…”