Dynamics and vibrational spectroscopy of water at hydroxylated silica surfaces (original) (raw)
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Sum-frequency vibrational spectroscopic studies of water/vapor interfaces
Chemical Physics Letters, 2009
ABSTRACT Interactions of water and aqueous solutions with mineral surfaces play an important role in a variety of environmental processes. It is affected by solution pH, presence of dissolved ions and the surface structure of the solid and adsorption species. To study the structure of water at a mineral surface, we use sum-frequency vibrational spectroscopy -- a surface-specific technique with monolayer sensitivity. In the past, it has been used to observe ice-like ordering of water molecules on a vitreous silica surface. Here we extend the study to the interface of water and a well-characterized (0001) surface of alpha-crystalline quartz. Comparison with the case of vitreous silica shows that crystallinity of the surface results in a higher degree of ordering in the interfacial layer of water at a given pH and changes in the relative proportions and frequencies of the vibrational bands. The data suggest that different crystallographic surfaces may induce different interfacial water structures as well as modification of response to pH and adsorbing species. The study also provides additional constraints for MD simulations of water on mineral surfaces and enhanced interpretation of the surface vibrational spectra.
Journal of Chemical Physics, 2018
Using gradient-and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated α-Al 2 O 3 (0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm −1 as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces.
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
Molecular Structure and Dynamics in Thin Water Films at the Silica and Graphite Surfaces
The Journal of Physical Chemistry C, 2008
All-atom equilibrium molecular dynamics simulations were employed to investigate the structural and dynamical properties of interfacial water on the magnesium oxide surface. The solid support was modeled utilizing two different formalisms, both based on the CLAYFF force field. In one case, the atoms in the MgO substrate are allowed to vibrate, whereas in the other they are maintained rigid. The properties of water within the thin film are assessed in terms of density profiles in the direction perpendicular to the substrate as well as along planes parallel to the substrate, in-plane radial distribution functions, density of hydrogen bonds, residence times in contact with the substrate, and orientation distribution of interfacial water molecules. The contact angle for a small droplet on various substrates (MgO, SiO 2 , Al 2 O 3) was also calculated and compared with experimental observations. On MgO, the substrate in which the atoms are maintained fixed is the one that most closely reproduces experimental contact angles. This contrasts with results on other substrates, for example, silicon dioxide, on which the vibrations of the solid atoms were found to be useful for better predicting experimental observations. These differences suggest that proper force-field validation is necessary before investigating the structure of interfacial water on solid substrates. In the case of MgO, our analysis suggests that the vibrations of the solid atoms yield atomic-scale roughness. This, in turn, causes water molecules to spread on the substrate. A brief comparison of water properties on MgO, alumina, and silica is provided.
Sum frequency vibrationnal spectroscopy of water molecules at interfaces
Annales de Physique
We present the surface vibrationnal sum frequency spectroscopy technique and we illustrate the ability of this method to analyze the properties of water at different interfaces. We stress the originality of this spectroscopy to evidence-water structure at hydrophobic interfaces and discuss the extension of the sum frequency generation spectroscopy.
The Journal of Physical Chemistry Letters, 2013
We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water−vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at ∼1650 cm −1 and a positive peak at ∼1730 cm −1. Our results reveal that these widely (∼80 cm −1) separated peaks result from the interference of two closely spaced (∼29 cm −1) peaks of opposite sign. The positive peak at ∼1689 cm −1 originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at ∼1660 cm −1 from water with free O−H groups, pointing up. The small frequency difference of 29 cm −1 indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds.
Molecular features of water films created with bubbles at silica surfaces
Surface Innovations, 2015
Sum frequency vibrational spectroscopy (SFVS) spectra indicate that a very ordered water structure exists in the stable water film at a hydrophilic silica surface during contact with a bubble and that the extent of hydrogen bonding increases with an increase in contact pressure. In contrast, the SFVS spectra of water at a hydrophobic silica surface show a lack of hydrogen bonding and are characterized by a distinct absorption at about 3700 cm −1 , similar to the spectrum of the air/water interface. These results suggest the presence of a water exclusion zone at the hydrophobic surface, as supported by X-ray reflectivity measurements reported in the literature, by AFM images, and by results from molecular dynamics simulations. Of course, the water film at the hydrophobic surface is unstable with film thinning and rupture upon bubble contact. Under these circumstances, it is shown that an attractive van der Waals force between a bubble and the hydrophobic surface can be expected when the water exclusion zone is taken into consideration. As the thickness of the water exclusion zone increases to a thickness corresponding to the size of nanobubbles, the calculated attractive van der Waals force increases. This analysis may help to explain the so-called 'short-range' and 'long-range' hydrophobic forces.
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.
Physical Chemistry Chemical Physics, 2012
Silica and silica based materials are widely used in chemistry and material science due to their importance in many technological fields. The properties of these materials, which are crucial for their applications, are mainly determined by the presence of hydrogen bonding between surface silanols. Here, we present ab initio molecular dynamics simulations (AIMD) on different surfaces derived from the crystallographic α-quartz (100) and the α-cristobalite (001) and (101) faces, both free and at the interface with liquid water. The focus was on studying whether water adsorption can disrupt the H-bond pattern at the pristine free silica surface and how deep the perturbation due to the contact with the surface affects the structure of the water multilayer. Results highlight that the water phase is over structured at the interface with silica, as compared to water bulk. Furthermore, an apparent counterintuitive behavior has been observed for quartz (100) and cristobalite (001) surfaces: the interaction with water does not cleave the pre-existent H-bonds between the surface silanol groups. On the contrary, in several cases, it is observed that SiOH•••OHSi H-bonds are even strengthened, as the result of a mutual cooperative H-donor/H-acceptor enhancement between silanols and water molecules, which may alter the adsorption capability of these silica surfaces.