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

Abstract

The free surface of water, and the interface between water and a hydrophobic surface, both have positive interface energies. The water density near a free surface drops below the bulk density, and thus it is expected that water near a hydrophobic surface will also show a density depletion.However, efforts by multiple groups to detect and characterize the predicted 'gap' at waterhydrophobic interfaces have produced contradictory results. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using specular X-ray reflectivity, and analyzed the parameter-space landscapes of the merit functions being minimized by data fitting. This analysis yields a better understanding of confidence intervals than the customary process of reporting a unique 'best' fit. We conclude that there are unambiguous gaps at water-hydrophobic interfaces when the hydrophobic monolayer is more densely packed.

Functionalities of Non-Bonding Electrons: Size Emergence

“…This finding indicates that hydrophilic interface produces no surface effect and the interface between the untreated silica and water is indeed hydrophobic. Using specular X-ray reflectivity analysis, Uysal et al [103] confirmed that gap exists between the water the hydrophobic substrate. Therefore, the repelling of the like charges at both the channel and the solid-like drop surfaces governs the observed superfluidity.…”

Molecular Clusters, Skins, and Ultrathin Films

“…The thinner of the channel diameter is, the faster the flow rate of the fluid will be [58,59]. An air gap of 0.5-1.0 nm thick exists between the water droplet and the hydrophobic substrates [60]. The gap increases with the contact angle of the droplet.…”

Skin Supersolidity of Water and Ice

“…The air gap between the water and the hydrophobic substrate [16] makes the contacting skin more like free. However, water confined in hydrophilic nanopores [50,51] or wetting with hydrophilic topologic configurations [52] exhibits supercooling effect, or melts at lower temperature than usual because of the lacking of broken bonds.…”

“…Furthermore, the volume of water molecules confined in 5.1 and 2.8 nm TiO 2 pores increase by 4 and 7.5 %, respectively, with respect to that in the bulk [15]. A 5-10 Å thick air gap between water molecules and the hydrophobic surface [16] make water molecules at the interface like free to exhibit skin vibration attributes [17] Stiopkin et al [18] found that the strength of interactions of surface water molecules is distinctly different in structure from the rest of the liquid, and that the water surface has the thickness of just one layer of molecules, which is about 0.3 nm. The surface does not have a long-range impact on water.…”