Hydrophobic/hydrophilic Interactions of Water with Alkanethiolate Chains from First Principles Calculations (original) (raw)
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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
Studies of Interaction of Small Molecules with Water Condensed Media
2006
STUDIES OF INTERACTION OF SMALL MOLECULES WITH WATER CONDENSED MEDIA The present work reports experimental and theoretical studies of the intermolecular interactions in condensed water media. The chemical objects comprise pristine ice and polar organic substances: acetone, acetaldehyde, methanol and chloroform and bi-component waterorganic deposits. The experimental part of the studies includes the Fourier Transform Infrared Reflection Absorption spectral (FTIR RAS) examination of the processes of film growth by vapor deposition on cold metal substrate and subsequent annealing. The theoretical studies include ab initio (MP2) and semi-empirical (B3LYP) calculations on the small water and water-organic clusters and classical molecular dynamics simulations of the adsorption of inert guests (Xe/Rn) on the ice surface. The FTIR RA spectral studies reveal that depending on the deposition conditions condensed water media exist in two principal structural forms: noncrystalline and polycrystalline. The former is characterized by porous structure while the latter exists as a non-porous medium with smooth external interface. On annealing, characteristic spectral changes indicate on a rapid crystallization occurring at a certain temperature range. The initial adsorption of organic molecules is accompanied by the hydrogen-bonded coordination between the functional group of organic species and noncoordinated hydroxyl group of the ice surface, the topology of which depends on the electronic properties of the functional group. The computational studies of small water-organic clusters reveal, in particular, two major coordination minima for carbonyl group: a single hydrogenbonded in-plane complex and a double hydrogen-bonded in-plane complex. The classical molecular dynamics of Xe/Rn species on the ice interface is consistent with two distinctly different surface adsorption sites: one that delocalized over the entire surface and one that confined to small opening in the top ice layer, disrupted by the thermal molecular motion. The penetration barrier is associated with van der Walls repulsion of guest species from the ordered water hexagonal arrangement. A thermo-disruption of latter leads to a rapid diffusion of guest species inside ice medium. iv ACKNOWLEGMENTS At the top of the list let me express my sincere thanks to the members of advisory committee Professors Tong Leung, James Sloan, Bruce Torrie and Dan Thomas for many helpful comments and suggestions during the long period of my studies. I am indebted to Professor Tong Leung who has supervised these studies for providing a rare opportunity to explore different scientific aspects of the water studies. Special thanks go to Professor Bruce Torrie for many hours of insightful discussions and advices.
Hydrophobicity-induced drying transition in alkanethiol self-assembled monolayer—water interface
Pramana, 2003
During the course of our investigation of the electron transfer properties of some redox species through highly hydrophobic long chain alkanethiol molecules on gold in aqueous and nonaqueous solvents, we obtained some intriguing results such as unusually low interfacial capacitance, very high values of impedance and film resistance, all of which pointed to the possible existence of a nanometer size interfacial gap between the hydrophobic monolayer and aqueous electrolyte. We explain this phenomenon by a model for the alkanethiol monolayer-aqueous electrolyte interface, in which the extremely hydrophobic alkanethiol film repels water molecules adjacent to it and in the process creates a shield between the monolayer film and water. This effectively increases the overall thickness of the dielectric layer that is manifested as an abnormally low value of interfacial capacitance. This behaviour is very much akin to the 'drying transition' proposed by Lum, Chandler and Weeks in their theory of length scale dependent hydrophobicity. For small hydrophobic units consisting of apolar solutes, the water molecules can reorganize around them without sacrificing their hydrogen bonds. Since for an extended hydrophobic unit, the existence of hydrogen bonded water structure close to it is geometrically unfavourable, there is a net depletion of water molecules in the vicinity leading to the possible creation of a hydrophobic interfacial gap.
Journal of Molecular Structure: THEOCHEM, 2010
Molecular dynamics simulations of self-assembled monolayers (SAMs) of alkanethiol derivatives interfaced with water reveal the structure of the interface and show how it influences the properties of water. Three SAMs of different character (neutral, anionic and cationic) are compared: 6-hexanethiol, 11-mercaptoundecanoic acid and 11-amino-1-undecanethiol. The simulation captures phenomena such as the hydrophobic gap, local increase of the density of water near the interface and ordering of water into layers.
Molecular Dynamics Simulations of Water Condensation on Surfaces with Tunable Wettability
Langmuir, 2020
Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.
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.
Concentration-driven surface transition in the wetting of mixed alkanethiol monolayers on gold
Journal of The American Chemical Society, 1991
The construction of mixed monolayers containing hydrophobic and hydrophilic components for which the contact angles for three different liquids vary as a highly nonlinear function of the monolayer composition is reported. It is suggested that a prewetting, crystalline-like layer of water, possibly formed from bulk vapor, is present near the hydrophilic surface, because of an enhanced surface chemical potential ("surface field"). As the concentration of the hydrophilic component is lowered, increasing "quenched randomness" in the distribution of surface fields destroys the surface condensed water phase, thus triggering the observed nonlinearity in the contact angles. The microscopic structure of the water molecules adsorbed on an OH surface is revealed by continuum Monte Carlo simulations, with realistic force fields, and the scenario is supported by mean-field calculations on a simplified lattice-gas model. The observed wetting behavior at 30% relative humidity was altered for a relative humidity 52%, as well as when the surface of the monolayer was molecularly roughened by the addition of two CH2 groups to the hydrophobic (CH,-terminated) component of the mixed monolayers. It is suggested that this transitional phenomenon is due to a possible (true or rounded) surface phase transition, due to the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields.
Molecular dynamics simulation of water condensation on surfaces with tunable wettability
2020
Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.
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.
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.