High-resolution in situ x-ray study of the hydrophobic gap at the water–octadecyl-trichlorosilane interface

Abstract

The knowledge of the microscopic structure of water at interfaces is essential for the understanding of interfacial phenomena in numerous natural and technological environments. To study deeply buried liquid water-solid interfaces, high-energy x-ray reflectivity measurements have been performed. Silicon wafers, functionalized by a self-assembled monolayer of octadecyltrichlorosilane, provide strongly hydrophobic substrates. We show interfacial density profiles with angstrom resolution near the solid-liquid interface of water in contact with an octadecyltrichlorosilane layer. The experimental data provide clear evidence for the existence of a hydrophobic gap on the molecular scale with an integrated density deficit d ‫؍‬ 1.1 Å g cm ؊3 at the solid-water interface. In addition, measurements on the influence of gases (Ar, Xe, Kr, N2, O2, CO, and CO2) and HCl, dissolved in the water, have been performed. No effect on the hydrophobic water gap was found.hydrophobicity ͉ interfacial water ͉ x-ray reflectivity H ydrophobicity, i.e., the repulsion of water, is a well known phenomenon in our environment (1). The generic hydrophobic interaction occurs between a nonpolar molecule and the water molecule. In bulk water, the hydrophobic interaction leads to the so-called hydrophobic hydration of unpolar solvents which generically results in a reduced density and an increased heat capacity. The seminal study on the thermodynamics of nonpolar solvation goes back to Frank and Evans in the mid-1940s (2). While of course the details of structural ordering remained unclear, it became evident that nonpolar solvation is a negentropic process which appeared later to become a key element to understand protein folding and stability (3). The microscopic details of how the nonpolar molecules interact with each other in water is a key information to understand how proteins and biological membranes maintain their structural integrity. Today we know that hydrophobic bonds are a major force driving proper protein folding, and that the interplay between hydrophobic and hydrophilic interactions is important to stabilize the shape of biological structures, such as proteins and cell membranes (4).Hydrophobic surfaces are of particular interest, since they control many interfacial phenomena in biology and technology. However, the microscopic details of how water meets a hydrophobic interface are still not settled and in fact rather controversial. A basic missing piece of information is the size of the hydrophobic gap between the water phase and the hydrophobic surface. Wetting studies on mesoporous silica (5) indicate that water is separated from the hydrophobic walls by a vapor gap of thickness 3-4 Å. Molecular dynamics simulations carried out for liquid water between f lat hydrophobic surfaces predict density oscillations extending up to 10 Å into the adjacent water accompanied by a molecular orientational order affecting a water layer of 7 Å. The simulations, as well as the results from surface vibrational spectroscopy, show that the water mole...

Exposing nanobubble-like objects to a degassed environment

“…For example, the contact angle depends in some studies on the radius of curvature 27,28 , whereas in other experiments the contact angle is found to be constant 10,19 . The presence of a gas layer at the solid-liquid interface is observed in several experimental studies 29,30 , where in other studies such a gaseous phase is not found 31,32 . Also, nanobubbles are sometimes found in ethanol 6,33 , while others observe pristine surfaces if immersed in ethanol 23 .…”

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

“…Early neutron reflectivity studies [31][32][33] of hydrocarbon-water interfaces reported gap widths larger than 10Å, while later x-ray reflectivity measurements on similar systems reported gap widths of less than 5Å. 35, 36 Yet other X-ray 30 and neutron 34 studies reported that there is no gap. In retrospect, several major experimental obstacles can be identified:…”

“…In reality, although the water-hydrophobic interface has been studied by a number of groups, [30][31][32][33][34][35][36][37] the results are inconsistent and have not led to a consensus. Early neutron reflectivity studies [31][32][33] of hydrocarbon-water interfaces reported gap widths larger than 10Å, while later x-ray reflectivity measurements on similar systems reported gap widths of less than 5Å.…”

“…Thus silicon-supported SAMs provide excellent, well-defined hydrophobic surfaces that have been used in many experimental studies. [31][32][33][35][36][37] However, there is a high price: the system now contains at least three interfaces, namely silicon-oxide, oxide-SAM, and SAM-water. Only the last of these is of interest here, but all interfaces reflect X-rays or neutrons and must be accounted for during data analysis.…”

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

“…23 Ambiguity also remains as to how dry contact is established underwater. The entropy gained by releasing interstitial water between hydrophobic surfaces prior to contact could be facilitated by a depleted density profile at the hydrophobic water interface, 24,25 the presence of nanobubbles, 26 or the concept of increased fluctuations in interfacial water. 27 To understand the role of water in adhesion and contact angles, we have used surface sensitive sum frequency generation spectroscopy (SFG) to directly study the contact interface between two hydrophobic surfaces underwater.…”