Comment on “How Water Meets a Hydrophobic Surface”

Ocko, B. M.; Dhinojwala, Ali; Daillant, Jean

doi:10.1103/physrevlett.101.039601

Cited by 47 publications

(84 citation statements)

References 10 publications

(14 reference statements)

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“…40 The position of the CH 3,Fermi peak is slightly red-shifted (Δ 7 cm −1 ), consistent with the hypothesis that these chemical groups are in direct contact with water and that there is no depletion layer between the water and the hydrophobic OTS surface. 24 The spectral signature in the water region for OTS−water interface shows a strong, hydrogen-bonded water peak (3100 cm −1 ) 41 (Figure 3b). In comparison, the OTS−air interface is relatively dry where a small signature in the OH region could be due to the interface between OTS and the underlying OH groups on the sapphire substrate.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…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.…”

Section: ■ Introductionmentioning

confidence: 99%

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Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

et al. 2015

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The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the waterfree contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly-(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a selfassembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS−PDMS contact. The Young−Dupréequation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting. ■ INTRODUCTIONHydrophobic interactions are used to explain many phenomena prevalent in physical and biological sciences, such as protein folding, 1 self-assembly, 2−6 dewetting, 7 adhesion, 8 friction, 9 adsorption, 10 water transport, 11,12 and chemical reactions. 13 The hydrophobic adhesion is defined as the difference in interfacial energy between two hydrophobic surfaces before and after contact underwater. 14 Experimentally, direct force measurements 15−17 or contact angle measurements 18 have been used to measure adhesion energy where the contact between two hydrophobic surfaces is assumed to be dry. This drying phenomenon has been supported by molecular simulations between hydrophobic surfaces 6,19,20 but never experimentally verified.Recent findings are challenging the concept of dry hydrophobic contact. X-ray crystallography has observed the presence of water within protein cavities of varying hydrophobicity which can affect the strength of protein−ligand binding. 21,22 Simulations have also shown that water can be sequestered between hydrophobic plates with a relatively small centralized hydrophilic patch. 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....

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

et al. 2015

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“…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.…”

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confidence: 99%

“…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: 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.

328

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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“…[3,4] On the other hand, incompatibility between a liquid and an adjacent solid-water against a hydrophobic surface, say-can lead to a depletion in the density of the fluid phase at the interface. [5][6][7][8][9][10][11][12][13] These deviations from bulk properties can exacerbate the chemical inhomogeneities of interfaces, altering for example the dielectric or solvation characteristics there. [14] Furthermore, ions may be depleted or enriched at the solidwater or air-water interfaces relative to the bulk, [15][16][17][18][19][20][21] an effect that involves subtle balances of enthalpic and entropic factors.…”

Section: Introductionmentioning

confidence: 99%

Nanobubbles are not a Superficial Matter

Ball¹

2012

ChemPhysChem

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Comment on “How Water Meets a Hydrophobic Surface”

Cited by 47 publications

References 10 publications

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

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

Nanobubbles are not a Superficial Matter

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