Contact angle of water on a model heterogeneous surface. A density functional approach (original) (raw)
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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.
Molecular dynamics simulations of the contact angle between water droplets and graphite surfaces
Fluid Phase Equilibria, 2012
Wetting is a widespread phenomenon, most prominent in a number of cases, both in nature and technology. Droplets of pure water with initial radius ranging from 20 to 80 [Å] spreading on graphitic surfaces are studied by molecular dynamics simulations. The equilibrium contact angle is determined and the transition to the macroscopic limit is discussed using Young equation in its modified form. While the largest droplets are almost perfectly spherical, the profiles of the smallest ones are no more properly described by a circle. For the sake of accuracy, we employ a more general fitting procedure based on local averages. Furthermore, our results reveal that there is a possible transition to the macroscopic limit. The modified Young equation is particularly precise for characteristic lengths (radii and contact-line curvatures) around 40 [Å].
Molecular Dynamics Simulation Study of Water Adsorption on Hydroxylated Graphite Surfaces
The Journal of Physical Chemistry B, 2006
In this paper, we present results from molecular dynamic simulations devoted to the characterization of the interaction between water molecules and hydroxylated graphite surfaces considered as models for surfaces of soot emitted by aircraft. The hydroxylated graphite surfaces are modeled by anchoring several OH groups on an infinite graphite plane. The molecular dynamics simulations are based on a classical potential issued from quantum chemical calculations. They are performed at three temperatures (100, 200, and 250 K) to provide a view of the structure and dynamics of water clusters on the model soot surface. These simulations show that the water-OH sites interaction is quite weak compared to the water-water interaction. This leads to the clustering of the water molecules above the surface, and the corresponding water aggregate can only be trapped by the OH sites when the temperature is sufficiently low, or when the density of OH sites is sufficiently high.
The Journal of Physical Chemistry B, 1999
Adsorption of associating hard spheres and particles interacting via the Lennard-Jones potential at a Lennard-Jones adsorbing wall is studied using the density functional theory. A model of association with one site per particle is considered. To test the theory we have also performed Monte Carlo simulation in the canonical ensemble for a Lennard-Jones associating fluid at high temperatures and for hard associating spheres. Next, the influence of association on the wetting behavior of the fluid-solid interface is investigated. It is shown that the prewetting transition in associating systems is considerably influenced by the association processes.
Interaction between a Water Molecule and a Graphite Surface
Arxiv preprint arXiv: …, 2008
The interaction energy between a water molecule and graphitic structured clusters terminated by hydrogen atoms is analyzed by ab initio methods and decomposed into electrostatic, induction, Pauli repulsion, and correlation energy contributions. Contributions to the energy which are due solely to the perimeter of the clusters are identified. These can be isolated and discarded which greatly simplifies the problem of extrapolation to the large cluster limit. The remaining terms are intrinsic to the interaction of a water molecule with real graphitic layers and an explicit analytical form is given for the potential energy surface. The minimum energy configuration is found to have both hydrogen atoms of the water molecule pointing symmetrically away from the graphitic plane. The electronic interaction in this mode is-16.8 ±1.7 kJ/mol for water-graphite and the zero point energy is estimated as 1.3 kJ/mol.
Wetting of chemically heterogeneous striped surfaces - Molecular dynamics simulations.pdf
Using molecular dynamics simulations, we thoroughly investigated the wetting behaviors of a chemically heterogeneous striped substrate patterned with two different wetting materials, face-centered cubic gold and face-centered cubic silver. We analyzed the density distributions, normal stress distributions, surface tensions, and contact angles of a water droplet placed on the substrates at different heterogeneities. We found that the density and stress profile of the water droplet near the substratewater interface were noticeably affected by altering the gold and silver contents in the substrate. Specifically, a greater portion of gold (more wetting) or smaller portion of silver (less wetting) in the substrate composition induced higher densities and higher normal stresses in the vicinity of the substrate surface. Also, it was observed that the surface tensions at liquid-vapor interface and solid-vapor interface were not largely impacted by the change of the substrate composition while the solid-liquid surface tension decreased exponentially with increasing fraction of gold. Most importantly, we found that contact angle of a nanometer-sized water droplet resting on the chemically heterogeneous striped substrate does not show linear dependence on corresponding surface fractions like that predicted by Cassie-Baxter model at the macro-scale. Consequently, we proposed a method for successfully predicting the contact angle by including the critical effects of the substrate heterogeneity on both surface tensions and line tension at the three-phase contact line of the water droplet and the chemically striped substrate. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
Water Structure at Solid Surfaces of Varying Hydrophobicity
The Journal of Physical Chemistry C, 2009
The structure of liquid water at solid surfaces with tunable hydrophobicity has been examined by molecular dynamics simulation. Methods of analysis include water density profiles, angular distributions, tilt and twist order parameters, and hydrogen-bonding coordination. It was found that interfacial water structures could be classified according to two hydrophobic regimes, a nonwetting structure and a semi-wetting structure. A smooth transition between the two occurs at surfaces with a contact angle around 130°. The nonwetting regime is characterized by water immediately adjacent to the interface oriented such that hydrogens are directed toward the surface. The semiwetting regime has water oriented in the plane of the interface. We propose that the emergence of the wetting-type order is strongly dependent on the density profile across the interfacial region. Regions of low density, flanked by high-density areas, present fewer hydrogen bonding opportunities than are found in more dense regions. Our findings are able to provide an explanation for experimental observations that, in surface-sensitive nonlinear vibrational spectroscopy, solid surfaces must be extremely hydrophobic to display spectroscopic signatures of uncoupled OH stretching modes.
Molecular dynamics simulation of the contact angle of liquids on solid surfaces
The Journal of Chemical Physics, 2009
In this work, Lennard-Jones liquid and water droplets are simulated adjacent to a solid surface using molecular dynamics. The contact angle is obtained by using direct simulation. The particle-particle particle-mesh method ͓B. Shi, S. Sinha, and V. K. Dhir, J. Phys. Chem. 124, 204715 ͑2006͔͒ for long range force correction is used in the simulation. The contact angle is studied as a function of system temperature and the solid/fluid interaction potential. It is shown that the contact angle decreases with increasing system temperature and increases when the potential decreases. At high system temperature ͑pressure͒, the contact angle drops to zero. The predictions are compared with data from experiments and a reasonable agreement is found.
Effects of heterogeneous surface geometry on adsorption
Langmuir, 1993
We demonstrate through simulations that surface geometry heterogeneity is sufficient to induce an adsorption/desorption hysteresis loop. The simulations show that the hysteresis originates from the quasiequilibrium nature of the adsorption branch. Different shapes of hystereses were obtained (parallel, widening, and unclosed at low relative pressure), according to the geometry of the surface (low microporosity, high microporosity, and mesoporosity). A unique population of permanently trapped gas phase molecules was identified and the dynamic nature of this population, termed "latent adsorbed" molecules, was demonstrated. The behavior of the BET equation in the presence of geometrical surface heterogeneity and lateral interactions was examined. It was found that the error in calculating the BET surface area, for the same surface-adsorbate and adsorbate-adsorbate interactions, is greater for irregular surfaces than for smooth surfaces. This observation is attributed to the effect of surface heterogeneity on higher adsorbed layers, which in turn increases the influence of adsorbate-adsorbate interactions. For weak lateral interactions the BET surface area estimation was within 10% of the theoretical prediction. The effects of surface heterogeneity on the thermodynamic functions of the adsorption/desorption process were calculated. The important role of the entropy of adsorption in the generation of the hysteresis loop was identified.
The Journal of Physical Chemistry C, 2014
First principles molecular dynamics simulations are used to gain an atomistic-level insight into how the molecular behavior of interfacial water is influenced by specific surface adsorbates. Although the overall hydrophobic versus hydrophilic character of a given surface is widely recognized to be important in determining the behavior of interfacial water molecules, we show that subtle molecular details may also play a role in determining the dynamical behavior of water. By comparing water diffusivity at three different non-polar surfaces, we find that specific surface features can lead to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distributions of water molecules near the surface. Such a distinct molecular dependent behavior of the interfacial water was found to persist well into the liquid, while the most structural properties are noticeably influenced in only the first water layer. Fig 8. Enumeration of rings in (a) region I and (b) region II at H-, CH3and CF3-Si surfaces