Ellipsometric Search for Vapor Layers at Liquid-hydrophobic Solid Surfaces

Takata, Yasumitsu; Cho, J.-H. J.; Law, Bruce M.; Aratono, Makoto

doi:10.1021/la052599s

Cited by 38 publications

(29 citation statements)

References 32 publications

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“…We also note that extended depletion layers as extracted from the neutron data should also show up in optical ellipsometry where no such profiles were found (22,23). Furthermore, no indication for the influence of gases dissolved into the water on the hydrophobic gap (d Ͻ 2.5 Å) was found in heat conductance measurements (24).…”

Section: Resultsmentioning

confidence: 76%

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 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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Section: Resultsmentioning

confidence: 76%

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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“…This agreed with the literature in that few nanobubbles can be detected simply by immersing a hydrophobic substrate in water. 20,57,58 Zhang et al 59 reported that interfacial nanobubbles were not observed when a smooth OTS-Si wafer was placed in a CO 2 -saturated water solution, and the interfacial bubbles are formed only through a fast solvent exchange treatment. It is widely accepted that the fast variation of gas solubility in water solution is key to inducing the nucleation of nanobubbles.…”

Section: Discussionmentioning

confidence: 99%

Microwave-Induced Interfacial Nanobubbles

2016

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A new method for generating nanobubbles via microwave irradiation was verified and quantified. AFM measurement showed that nanobubbles with diameters ranging from 200 to 600 nm were generated at a water-HOPG surface by applying microwave radiation to aqueous solutions with 9.0−30.0 mg/L dissolved oxygen. Graphite displays strong microwave absorption and transmits high thermal energy to the surface. Because of the high dielectric constant (20°C, 80 F/m) and dielectric loss factor, water molecules have a strong ability to absorb microwave radiation. The thermal and nonthermal effects of microwave radiation made contributions to decreasing the gas solubility, thus facilitating nanobubble nucleation. The yield of nanobubbles increased about 10-fold when the irradiation time increased from 60 to 120 s at 200 W of microwave radiation. The nanobubble density increased from 0.8 to 15 μm −2 by improving the working power from 200 to 600 W. An apparent improvement in nanobubbles yield was obtained between 300 and 400 W, and the resulting temperature was 34−52°C. When the initial dissolved oxygen increased from 11.3 to 30.0 mg/L, the density of nanobubbles increased from 1.2 to 13 μm . The generation of nanobubbles could be well controlled by adjusting the gas concentration, microwave power, or irradiation time. The method may be valuable in preparing surface nanobubbles quickly and conveniently for various applications, such as catalysis, hypoxia/anoxia remediation, and templates for preparing nanoscale materials.

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“…The lack of a correlation between water density and hydrophobicity reflects the complexities present in realistic systems arising from different molecular topologies and different head group water interactions. It also highlights and probably rationalizes the difficulty in obtaining unambiguous conclusions from experiments regarding wetting/dewetting of realistic hydrophobic interfaces (27)(28)(29)(30)(31)(32)(33)(34)(35).…”

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confidence: 81%

“…Although a number of experimental tools including X-ray (27)(28)(29)(30)(31) and neutron reflectivity (32,33), ellipsometry (34), and thermal conductivity (35) have been used to probe the width of the ''depletion layer'' at a hydrophobic surface, a clear picture has not yet emerged from these measurements (42). Fig.…”

mentioning

confidence: 99%

Characterizing hydrophobicity of interfaces by using cavity formation, solute binding, and water correlations

Godawat

Jamadagni

Garde

2009

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

333

396

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Hydrophobicity is often characterized macroscopically by the droplet contact angle. Molecular signatures of hydrophobicity have, however, remained elusive. Successful theories predict a drying transition leading to a vapor-like region near large hard-sphere solutes and interfaces. Adding attractions wets the interface with local density increasing with attractions. Here we present extensive molecular simulation studies of hydration of realistic surfaces with a wide range of chemistries from hydrophobic (؊CF 3, ؊CH3) to hydrophilic (؊OH, ؊CONH 2). We show that the water density near weakly attractive hydrophobic surfaces (e.g., ؊CF 3) can be bulk-like or larger, and provides a poor quantification of surface hydrophobicity. In contrast, the probability of cavity formation or the free energy of binding of hydrophobic solutes to interfaces correlates quantitatively with the macroscopic wetting properties and serves as an excellent signature of hydrophobicity. Specifically, the probability of cavity formation is enhanced in the vicinity of hydrophobic surfaces, and water-water correlations correspondingly display characteristics similar to those near a vapor-liquid interface. Hydrophilic surfaces suppress cavity formation and reduce the water-water correlation length. Our results suggest a potentially robust approach for characterizing hydrophobicity of more complex and heterogeneous surfaces of proteins and biomolecules, and other nanoscopic objects.hydration ͉ hydrophilic ͉ hydrophobic ͉ wetting ͉ fluctuations H ydrophobicity, reflected in the low solubility of nonpolar solutes or in their tendency to aggregate in water, is known to play an important role in many biological and colloidal self-assembly processes (1-4). Yet defining it precisely is challenging, and its molecular signatures remain unclear. Macroscopically, hydrophobicity is often characterized by measuring the droplet contact angle, with surfaces showing angles greater than 90°termed hydrophobic. Water beads up into droplets on hydrophobic surfaces and spreads on hydrophilic ones. Translating these ideas into the molecular domain presents special challenges. In a recent perspective, Granick and Bae (5) highlight the ambiguity in defining hydrophobicity at molecular length scales, such as near proteins or nanotubes, where droplet contact angle measurements are not possible.At the molecular level, hard-sphere solutes have served as excellent models for studies of hydrophobicity, with their hydration thermodynamics capturing the solubility of noble gases as a function of temperature (6, 7), pressure (8), and salt addition (9, 10). With increasing solute length scale, the elegant theory by Lum, Chandler, and Weeks (11) as well as computer simulations (12, 13) predict a gradual dewetting of the solute. Near large solutes or a hard-wall, water density is small and vapor-like, and the wall-water interface resembles a water vapor-liquid interface (14).Realistic solutes exert van der Waals and/or electrostatic interactions and pull the water interface closer, ...

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Ellipsometric Search for Vapor Layers at Liquid-hydrophobic Solid Surfaces

Cited by 38 publications

References 32 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

Microwave-Induced Interfacial Nanobubbles

Characterizing hydrophobicity of interfaces by using cavity formation, solute binding, and water correlations

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