How water meets a hydrophobic surface (original) (raw)
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X-ray studies of the density depletion at hydrophobic water-solid interfaces
2008
Die Analyse der Reflektivitätsmessungen an hydrophoben Wassergrenzflächen ergibt ein integrales Dichtedefizit von d w (ρ H 2 O − ρ w) = 1.1 Å g cm −3 , welches weniger als einer halben Monolage von Volumen Wasser entspricht und sich über einen Bereich von bis zu zwei molekularen Wasserdurchmessern [23, 24] erstreckt (siehe Abb. 1.1). Aufgrund der begrenzten Auflösung der verwendeten Methode lassen sich aus diesen Messungen alleine jedoch keine eindeutigen Aussagen zur molekularen Wasserstruktur und deren Ursprungs treffen. Zusammen mit anderen experimentellen Methoden, numerischen Simulationen und theoretischen Modellen, liefern sie jedoch wichtige Informationen zum tieferen Verständnis von Wasser an hydrophoben Grenzflächen. Die Existenz einer derartigen Grenzschicht erniedrigter Dichte und vergleichbarer Dicke im Bereich einiger Å wurde sowohl in Molekulardynamiksimulationen [25-30] vorhergesagt, als auch durch indirekte Methoden [31] experimentell bestätigt. Spektroskopische Messungen (SFG) zeigen eine deutliche Ausrichtung der Wassermoleküle an hydrophilen [32, 33] und hydrophoben [34, 35] Grenzflächen. Im Gegensatz dazu lieferten frühere Neutronenstreuexperimente [36-38] an Wassergrenzflächen auflösungsbedingt weitaus größere Schichtdicken.
Langmuir, 1998
The interfacial structure and fluctuations of a L-R-dipalmytoilphosphatidylcholine monolayer at the hexadecane/water interface were investigated using grazing-incidence surface scattering of X-ray synchrotron radiation. The interfacial structure of the densely packed monolayer is consistent with that of a similar monolayer at the air/water interface. The fluctuations are shown to be limited by interfacial tension on length-scales down to in-plane wavelengths of a few tens of nanometers. This result at the alkane/water interface demonstrates the power of the X-ray grazing-incidence technique applied to the study of liquid-liquid interfaces.
Structural and dynamical aspects of water in contact with a hydrophobic surface
European Physical Journal E, 2010
By means of molecular dynamics simulations we study the structure and dynamics of water molecules in contact with a model hydrophobic surface: a planar graphene-like layer. The analysis of the distributions of a local structural index indicates that the water molecules proximal to the graphene layer are considerably more structured than the rest and, thus, than the bulk. This structuring effect is lost in a few angstroms and is basically independent of temperature for a range studied comprising parts of both the normal liquid and supercooled states (240K to 320K). In turn, such structured water molecules present a dynamics that is slower than the bulk, as a consequence of their improved interactions with their first neighbors.
Water in Contact with Extended Hydrophobic Surfaces: Direct Evidence of Weak Dewetting
Physical Review Letters, 2003
X-ray reflectivity measurements reveal a significant dewetting of a large hydrophobic paraffin surface floating on water. The dewetting phenomenon extends less than 15 Å into the bulk water phase and results in an integrated density deficit of about one water molecule per 25-30 A 2 of water in contact with the paraffin surface. The results are supported by molecular dynamics simulations and related to the hydrophobic effect.
Langmuir, 2012
Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or "ionic" bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including "dynamic" contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant−solid surface interactions, the organization of surfactant-coated nanoparticles, etc.
Nanoscale condensation of water on self-assembled monolayers
Soft Matter, 2011
We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10-8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry. 2011 The Royal Society of Chemistry.
Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces
European Physical Journal E, 2002
It has been shown that the flow of a simple liquid over a solid surface can violate the so-called noslip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers. PACS. 47.15.Gf Low-Reynolds-number (creeping) flows -68.35.-p Solid surfaces and solid-solid interfaces: structures and energetics
Characterisation and stability of hydrophobic surfaces in water
Applied Surface Science, 2005
The stability of four different hydrophobic surfaces in contact with water is assessed and discussed: H-terminated silicon, hexamethyldisilazane (HMDS) coated silicon, silicon surfaces covered with self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and gold surfaces modified with SAMs of alkanethiols. Changes in hydrophobicity and surface oxidation were determined by contact angle measurements, X-ray photoelectron spectroscopy and AFM. #