X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers

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...

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“…2 Inset). These values are in very good agreement with the work of Tidswell et al (19,20), showing that the OTS layer is homogenous.…”

supporting

confidence: 82%

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

Mezger

Reichert

Schöder

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

263

231

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“…The presence of too much water causes polymerization of OTS molecules and hence leads to poor quality of monolayers. 5 Since silica is hydrophilic a thin layer of water exists on its surface, which helps physisorption of the OTS molecule. At this stage OTS molecules are mobile.…”

Section: Introductionmentioning

confidence: 99%

Octadecyltrichlorosilane adsorption kinetics on Si(100)/SiO2 surface: contact angle, AFM, FTIR and XPS analysis

Mirji

2006

Surf. Interface Anal.

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“…Under submonolayer conditions the surface is often described as either being uniformly covered with an expanded monolayer of decreased height or intermittently covered with condensed "islands" of monolayer (2)(3)(4)(5)(6). Thus a microheterogeneous surface of mixed surface chemistry can be created.…”

Section: Introductionmentioning

confidence: 99%

An AFM Study of the Effects of Silanization Temperature, Hydration, and Annealing on the Nucleation and Aggregation of Condensed OTS Domains on Mica

Britt

Hlady

1996

Journal of Colloid and Interface Science

122

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Partial monolayers of octadecyltrichlorosilane (OTS) were formed on mica under different reaction conditions in which the silanization temperature, time, and amount of water adsorbed on the mica substrates were varied. OTS surface coverage increased with silanization time for all samples; however, the amount and distribution of adsorbed OTS varied greatly under these different reaction conditions. AFM analysis showed that OTS formed two phases on mica silanized at 25°C: condensed "island-like" domains and expanded "liquid-like" domains. Partially dehydrated mica silanized at 9°C, however, displayed only condensed domains which were of smaller size compared to those on the 25°C samples. The lateral diffusion and aggregation of small condensed OTS domains to form larger aggregates was evident on all surfaces except the 25°C partially dehydrated mica. A uniform distribution of many small condensed domains surrounded by expanded OTS phases was seen instead. Extended annealing resulted in surface diffusion and aggregation of these domains and nucleation of new condensed domains from the surrounding expanded OTS phases. These observations are consistent with a deposition, diffusion, and aggregation model (DDA) which allows for activated diffusion; however, rigorous modeling is not presented here.

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X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers

Cited by 136 publications

References 30 publications

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

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

Octadecyltrichlorosilane adsorption kinetics on Si(100)/SiO2 surface: contact angle, AFM, FTIR and XPS analysis

An AFM Study of the Effects of Silanization Temperature, Hydration, and Annealing on the Nucleation and Aggregation of Condensed OTS Domains on Mica

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