Water at Hydrophobic Substrates:  Curvature, Pressure, and Temperature Effects

Mamatkulov, Shavkat; Khabibullaev, P. K.; Netz, Roland R.

doi:10.1021/la036036x

Cited by 123 publications

(121 citation statements)

References 29 publications

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“…In contrast, molecular dynamics computer simulations (22)(23)(24) indicate a depletion region extending at most 2.5 Å from the hydrophobic surface, whereas recent Monte Carlo simulations show that dissolved gases such as N 2 adsorb as a monolayer at the hydrophobic-water interface with a density that is Ϸ30 times higher than in the bulk solution (25). However, no stable bubbles are predicted by these simulations, and they are not expected thermodynamically.…”

mentioning

confidence: 64%

Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Doshi

Watkins

Israelachvili

et al. 2005

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

253

264

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Here, direct noninvasive neutron reflectivity measurements reveal the presence of a reduced (deuterated) water density region, with a sigmoidal density profile at the hydrophobic silane-water interface that depends on the type and concentration of dissolved gases in the water. Removal of dissolved gases decreases the width of the reduced water density region, and their reintroduction leads to its increase. When compared with recent computer simulations, a locally fluctuating density profile is proposed, whereas preexisting nanobubbles are excluded. The presence of a fluctuating reduced water density region between two hydrophobic surfaces and the attractive ''depletion force'' to which it leads may help explain the hydrophobic force and its reported diminution in deaerated water. Our results are also quantitatively consistent with recent dynamic surface force apparatus results that drastically revise previous estimates of the slip length of water flowing past hydrophobic surfaces from microns to Ϸ20 nm. Our observations, therefore, go a long way toward reconciling three quite different types of experiments and phenomena: water depletion at hydrophobic surfaces, water slip at hydrophobic surfaces, and the hydrophobic interaction.interfacial water ͉ neutron reflectivity ͉ slip conditions

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mentioning

confidence: 64%

Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Doshi

Watkins

Israelachvili

et al. 2005

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

253

264

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“…Another issue that may be relevant to the charging of small objects or convexities of a surface is the dependency of the adsorbed water density profile and the surface potential on the curvature radius of the object. 95 In regard to these points, it is also reasonable to expect that the shape of the apex of the rubber, being either concave or convex, will have an impact on the charging characteristics, a trend already observed by Shaw. 9 The release of the hydronium ions at the front of the water meniscus during rubbing may also be triggered by self-repulsion of the positive ions due to their own increasing space charge field.…”

Section: Water Dissociation and Ion Separationmentioning

confidence: 76%

Squeezing out hydrated protons: low-frictional-energy triboelectric insulator charging on a microscopic scale

Knorr¹

2011

AIP Advances

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Though triboelectric charging of insulators is common, neither its mechanism nor the nature of the charge is well known. Most research has focused on the integral amount of charge transferred between two materials upon contact, establishing, e.g., a triboelectric series. Here, the charge distribution of tracks on insulating polymer films rubbed by polymer-covered pointed swabs is investigated in high resolution by Kelvin probe force microscopy. Pronounced bipolar charging was observed for all nine rubbing combinations of three different polymers, with absolute surface potentials of up to several volts distributed in streaks along the rubbing direction and varying in polarity on μm-length scales perpendicular to the rubbing direction. Charge densities increased considerably for rubbing in higher relative humidity, for higher rubbing loads, and for more hydrophilic polymers. The ends of rubbed tracks had positively charged rims. Surface potential decay with time was strongly accelerated in increased humidity, particularly for polymers with high water permeability. Based on these observations, a mechanism is proposed of triboelectrification by extrusions of prevalently hydrated protons, stemming from adsorbed and dissociated water, along pressure gradients on the surface by the mechanical action of the swab. The validity of this mechanism is supported by explanations given recently in the literature for positive streaming currents of water at polymer surfaces and by reports of negative charging of insulators tapped by accelerated water droplets and of potential built up between the front and the back of a rubbing piece, observations already made in the 19th century. For more brittle polymers, strongly negatively charged microscopic abrasive particles were frequently observed on the rubbed tracks. The negative charge of those particles is presumably due in part to triboemission of electrons by polymer chain scission, forming radicals and negatively charged ions

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“…As the main result of those experiments, it was shown that the effective depletion thickness (defined as the thickness of a step-like depletion layer consisting of vacuum with the same integrated depleted amount as the rounded and smeared-out depletion profiles found in experiments) is roughly 2.5 Angstroms on hydrophobic poly-styrene substrates [273] and 5 Angstroms on hydrophobic self-assembled monolayers [274] using neutron reflectivity methods, and about 1 Angstrom on paraffin substrates using X-ray reflectivity measurements [275]. The reason for the discrepancies among different experiments is not well understood, but since the strength of water depletion is reduced with decreasing radius of curvature of the hydrophobic solutes [276], it is clear that surface roughness is one important factor (among many others, as e.g. small traces of attractive interactions between wall and water molecules) and will, if present, reduce the depleted amount.…”

Section: Water At Hydrophobic Substratesmentioning

confidence: 92%

“…In Figure 28a) a snapshot of the MD simulation is shown, which serves to illustrate the geometry of the system [276]. The alkane molecules form a compact slab in the middle of the simulation box.…”

Section: Water At Hydrophobic Substratesmentioning

confidence: 99%

Statics and dynamics of strongly charged soft matter

Boroudjerdi¹,

Kim²,

Naji³

et al. 2005

Physics Reports

347

523

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Soft matter materials, such as polymers, membranes, proteins, are often electrically charged. This makes them water soluble, which is of great importance in technological application and a prerequisite for biological function. We discuss a few static and dynamic systems that are dominated by charge effects. One class comprises complexation between oppositely charged objects, for example the adsorption of charged ions or charged polymers on oppositely charged substrates of different geometry. Here the main questions are whether adsorption occurs and what the effective charge of the resulting complex is. We explicitly discuss the adsorption behavior of polyelectrolytes on substrates of planar, cylindrical and spherical geometry with specific reference to DNA adsorption on supported charged lipid layers, DNA adsorption on oppositely charged cylindrical dendro-polymers, and DNA binding on globular histone proteins, respectively. In all these systems salt plays an important role, and some of the important features can already be obtained on the linear Debye-Hückel level. The second class comprises effective interactions between similarly charged objects. Here the main theme is to understand the experimental finding that similarly and highly charged bodies attract each other in the presence of multi-valent counterions. This is demonstrated using field-theoretic arguments as well as Monte-Carlo simulations for the case of two homogeneously charged bodies. Realistic surfaces, on the other hand, are corrugated and also exhibit modulated charge distributions, which is important for static properties such as the counterion-density distribution, but has even more pronounced consequences for dynamic properties such as the counterion mobility. More pronounced dynamic effects are obtained with highly condensed charged systems in strong electric fields. Likewise, an electrostatically collapsed highly charged polymer is unfolded and oriented in strong electric fields. All charged systems occur in water, and water by itself is not a very well understood material. At the end of this review, we give a very brief and incomplete account of the behavior of water at planar surfaces. The coupling between water structure and charge effects is largely unexplored, and a few directions for future research are sketched. On an even more nanoscopic level, we demonstrate using ab-initio methods that specific interactions between oppositely charged groups (which occur when their electron orbitals start to overlap) are important and cause ion-specific effects that have recently moved into the focus of interest.

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Water at Hydrophobic Substrates: Curvature, Pressure, and Temperature Effects

Cited by 123 publications

References 29 publications

Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Squeezing out hydrated protons: low-frictional-energy triboelectric insulator charging on a microscopic scale

Statics and dynamics of strongly charged soft matter

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