A molecular dynamics study of water near silicate surfaces

We have conducted a molecular dynamics simulation of water between uncharged silicate surfaces in order to understand, on a molecular basis, the structural and dynamical properties of vicinal water (i.e., the water between the surfaces). The results of our preliminary simulation indicate that the vicinal water differs substantially from pure bulk water in the transient orientation of molecular dipole moments and rate of relaxation of these moments. In contrast, no significant long-range differences between the radial distribution functions or hydrogen bonding patterns of vicinal and bulk water are evident.     

Nanoscale water condensation on click-functionalized self-assembled monolayers

We have examined the nanoscale adsorption of molecular water under ambient conditions onto a series of well-characterized functionalized surfaces produced by Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC or "click") reactions on alkyne-terminated self-assembled monolayers on silicon. Water contact angle (CA) measurements reveal a range of macroscopic hydrophilicity that does not correlate with the tendency of these surfaces to adsorb water at the molecular level. X-ray reflectometry has been used to follow the kinetics of water adsorption on these "click"-functionalized surfaces, and also shows that dense continuous molecular water layers are formed over 30 h. For example, a highly hydrophilic surface, functionalized by an oligo(ethylene glycol) moiety (with a CA = 34°) showed 2.9 ? of adsorbed water after 30 h, while the almost hydrophobic underlying alkyne-terminated monolayer (CA = 84°) showed 5.6 ? of adsorbed water over the same period. While this study highlights the capacity of X-ray reflectometry to study the structure of adsorbed water on these surfaces, it should also serve as a warning for those intending to characterize self-assembled monolayers and functionalized surfaces to avoid contamination by even trace amounts of water vapor. Moreover, contact angle measurements alone cannot be relied upon to predict the likely degree of moisture uptake on such surfaces.     

Surface restructuring behavior of various types of poly(dimethylsiloxane) in water detected by SFG

Surface structures of several different poly(dimethylsiloxane) (PDMS) materials, tetraethoxysilane-cured hydroxy-terminated PDMS (TEOS-PDMS), platinum-cured vinyl-terminated PDMS (Pt-PDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer in air and water have been investigated by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra collected from all PDMS surfaces in both air and water are dominated by methyl group stretches, indicating that all the surfaces are mainly covered by methyl groups. Other than surface-dominating methyl groups, some -Si-CH2-CH2- moieties on the Pt-PDMS surface have also been detected in air, which are present at cross-linking points. Information about the average orientation angle and angle distribution of the methyl groups on the PDMS surface has been evaluated. Surface restructuring of the methyl groups has been observed for all PDMS surfaces in water. Upon contacting water, the methyl groups on all PDMS surfaces tilt more toward the surface. The detailed restructuring behaviors of several PDMS surfaces in water and the effects of molecular weight on restructuring behaviors have been investigated. For comparison, in addition to air and water, surface structures of PDMS materials mentioned above in a nonpolar solvent, FC-75, have also been studied. By comparing the different response of phenyl groups to water on both PDPS-co-PDMS and PS-co-PDMS surfaces, we have demonstrated how the restructuring behaviors of surface phenyl groups are affected by the structural flexibility of the molecular chains where they are attached.     

Roughness and ordering at the interface of oxidized polystyrene and water

For the first time, atomistically detailed molecular dynamics calculations revealed molecular ordering of the water-oxidized atactic polystyrene (aPS) interface. Both ordering of the water molecules and the phenyl rings occur. In addition, the natural roughness of the surface has been simulated and compared to experimental values. The composition of the simulated aPS films is based on spin-coated aPS films that have been oxidized and characterized experimentally. The aPS surfaces are oxidized with ultraviolet-ozone radiation and have been characterized by XPS, AFM, and water contact angle measurements. XPS measurements show that the oxygen content in the sample increases rapidly with exposure and reaches saturation near 24 at. % of oxygen. The molecular dynamics simulations show smoothening of an hydrophobic aPS surface upon transition from vacuum to water. The smoothening decreases with increasing hydrophilicity. The calculations reveal ordering of oxidized phenyl rings for aPS surfaces in water. The order increases with increasing hydrophilicity. Additionally, we investigated the water structure near the aPS-water interface as a function of the surface hydrophilicity. With increasing hydrophilicity, the density of water at the aPS-water interface increases. The water density profile is steeper in the presence of hydrophobic aPS. The water shows an ordered layer near both the hydrophobic and hydrophilic surfaces; the position of this layer shifts toward the interface with increasing hydrophilicity.     

Photochemistry of adsorbed nitrate

In the atmosphere, gas-phase nitrogen oxides including nitric acid react with particle surfaces (e.g., mineral dust and sea salt aerosol) to yield adsorbed nitrate, yet little is known about the photochemistry of nitrate on the surface of these particles. In this study, nitrate adsorbed on alumina surfaces, a surrogate for mineral dust aerosol, is irradiated with broadband light (lambda > 300 nm) in the absence and presence of coadsorbed water, at <1% and 45 +/- 2% relative humidity (%RH), respectively, and molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products, NO2, NO, and N2O. Although NO2, NO, and N2O form under the different conditions investigated, both coadsorbed water and molecular oxygen change the gas-phase product distribution, with NO being the major product under dry and humid conditions in the absence of molecular oxygen and NO2 the major product in the presence of molecular oxygen. To the best of our knowledge, this is the first study to investigate the role of solvation by coadsorbed water in the photochemistry of adsorbates at solid interfaces and the roles that molecular oxygen, adsorbed water, and relative humidity may have in photochemical processes on aerosol surfaces that have the potential to alter the chemical balance of the atmosphere.     

Theoretical Description of Water from Single-Molecule to Condensed Phase: Recent Progress on Potential Energy Surfaces and Molecular Dynamics

In this work, we review recent progress on the view of potential energy surfaces and molecular dynamics study of water and its related reactions in the last decade or so. Some important gas-phase reactions of water with radicals, chemisorbed dissociative dynamics of water on solid surfaces, and statistical mechanics and vibrational spectrum simulations of water from clusters to the condensed phase have been introduced. The recently developed machine learning techniques, such as the neural networks in a combination of permutational invariant polynomials or fundamental invariants, the atomic neural networks framework, the gaussian approximation potentials with the smooth overlap of atomic position kernel, as well as the many-body expansion framework for the construction of highly accurate potential energy surfaces, have also been discussed. Finally, some suggestions have been provided for further improvement of the potential energy surfaces and dynamics methods of water-related systems.     

Molecular dynamics investigation of the intrinsic structure of water-fluid interfaces via the intrinsic sampling method

Capillary wave fluctuations smooth out the structure of fluid interfaces, making difficult the detailed analysis of the interfacial structure. Most computer simulation investigations performed to date have focused on the computation of average density profiles, ignoring the characterization of the intrinsic structure of the interface. Recent theoretical developments have reversed this situation, making possible the detailed investigation of the interfacial intrinsic structure at an unprecedented level of detail. In this article we investigate via molecular dynamics simulations the intrinsic structure of water-alkane (hexane and dodecane) interfaces. The implementation of the recently introduced, intrinsic sampling method to compute the intrinsic surface of water-fluid interfaces is discussed. We provide quantitative molecular information on the structure, corrugation, and stiffness of the liquid surfaces. The intrinsic structure of water at alkane interfaces is shown to be insensitive to the alkane-chain length, and can be very accurately described by the intrinsic structure of the water free surface.     

Competitive adsorption on wollastonite: an atomistic simulation approach

Atomistic simulation techniques are used to simulate surface structure and adsorption behavior of scarcely floatable wollastonite mineral in the presence of molecular and dissociated water, methanoic acid, and methylamine. The latter two additives represent the two widely used collector head-group molecules. The static energy minimization code METADISE was used to perform the simulation to obtain pure surface energy and adsorption energy in the presence of added molecule. The hydroxylation was performed on those surfaces where low-coordinated silicon was made to saturate by bonding with hydroxyl group, and the subsequent charge neutralization was maintained by adding proton on single-coordinated surface oxygen. A comparison of surface energies revealed that all the surfaces become stabilized in the presence of added molecules; however, the presence of methylamine decreased the surface energy to lower values. Adsorption of dissociated water is preferred by the {100} and {102} surfaces, whereas the {001} surface preferred methylamine adsorption, because these show highly negative adsorption energies. In terms of molecular adsorption, the preferred adsorption sequence for all the surfaces is methylamine > methanoic acid > water without considering coadsorption. For the {100} and {102} surfaces, the adsorption energy values of carboxylic acid and amine are more negative than that of water and therefore we conclude that both carboxyl and amine head-group molecules adsorb preferably on wollastonite. Our simulation verify usability of carboxylic acid head group as widely used collectors for wollastonite flotation and, at the same time, it predicts the use of amine head-group collectors as possible modifiers, which corresponds well with our experimental findings.     

On the fragmentation pathway of the ionized enol of glycine in the gas phase

Density functional and second-order many body perturbation approaches were used to compute the potential energy surface for the fragmentation of the ionized enol of glycine [H2NCH = C(OH)2]+* into water and aminoketene radical cation [H2N-HC = CO]+*. Two possible pathways were considered. The potential energy surfaces obtained are very similar and both predict the existence of a molecular complex in which the water is coordinated to the aminoketene moiety in two different fashions with a noticeable binding energy. The fragmentation is kinetically controlled by the step in which the molecular complex is formed from the most stable cation enol of glycine. Our quantum-mechanical data confirm the hypothesis that the ylide ion [H3NCHCOOH]+* is an intermediate in the water loss.     

Reactivity of V2O3(0001) surfaces: molecular vs dissociative adsorption of water

The adsorption of water on V2O3(0001) surfaces has been investigated by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy with use of synchrotron radiation. The V2O3(0001) surfaces have been generated in epitaxial thin film form on a Rh(111) substrate with three different surface terminations according to the particular preparation conditions. The stable surface in thermodynamic equilibrium with the bulk is formed by a vanadyl (VO) (1x1) surface layer, but an oxygen-rich (radical3xradical3)R30 degrees reconstruction can be prepared under a higher chemical potential of oxygen (microO), whereas a V-terminated surface consisting of a vanadium surface layer requires a low microO, which can be achieved experimentally by the deposition of V atoms onto the (1x1) VO surface. The latter two surfaces have been used to model, in a controlled way, oxygen and vanadium containing defect centres on V2O3. On the (1x1) V=O and (radical3xradical3)R30 degrees surfaces, which expose only oxygen surface sites, the experimental results indicate consistently that the molecular adsorption of water provides the predominant adsorption channel. In contrast, on the V-terminated (1/radical3x1/radical3)R30 degrees surface the dissociation of water and the formation of surface hydroxyl species at 100 K is readily observed. Besides the dissociative adsorption a molecular adsorption channel exists also on the V-terminated V2O3(0001) surface, so that the water monolayer consists of both OH and molecular H2O species. The V surface layer on V2O3 is very reactive and is reoxidised by adsorbed water at 250 K, yielding surface vanadyl species. The results of this study indicate that V surface centres are necessary for the dissociation of water on V2O3 surfaces.     

Electronic,mechanistic, and structural factors that influence the performance of molecular water oxidation catalysts anchored on electrode surfaces

To harness solar energy to generate fuels powerful anodes for the water oxidation reaction need to be developed. During the last decade, an extensive number of molecular water oxidation catalysts based on transition metals have been reported, and in some cases, these molecular catalysts have been anchored on conductive surfaces generating molecular anodes. In this review, we analyze the factors that influence the performance of these molecular anodes, which are largely related to their mechanism of O–O bond formation and the nature of the anchoring functionality.     

羟基磷灰石界面水行为的分子模拟

本文采用分子动力学模拟(MD)方法研究了羟基磷灰石(HAP)(001)和(100)晶面上的水分子行为，发现HAP晶面间的水是处在高电场和高内压的环境下，并可在晶面处形成2～3层高度结构化的水层，这些水具有有序结构和类冰固化特征。其中在HAP晶体的[001]方向具有较强的极性，相对于[100]方向能诱导产生更多的有序结构化水层。研究发现HAP-水界面处钙和磷酸根位点分布和水分子的吸附位点相关，并且水在HAP界面上的吸附形式具有多样性。该工作揭示了HAP界面结构化水层的形成及其结构细节特征。HAP晶面附近的结构化水层可阻止溶液离子自由出入晶面，对HAP颗粒在水溶液中的动力学稳定性具有重要的影响。     

Adsorption of sulfur dioxide on hematite and goethite particle surfaces

The adsorption of sulfur dioxide (SO(2)) on iron oxide particle surfaces at 296 K has been investigated using X-ray photoelectron spectroscopy (XPS). A custom-designed XPS ultra-high vacuum chamber was coupled to an environmental reaction chamber so that the effects of adsorbed water and molecular oxygen on the reaction of SO(2) with iron oxide surfaces could be followed at atmospherically relevant pressures. In the absence of H(2)O and O(2), exposure of hematite (alpha-Fe(2)O(3)) and goethite (alpha-FeOOH) to SO(2) resulted predominantly in the formation of adsorbed sulfite (SO(3)(2-)), although evidence for adsorbed sulfate (SO(4)(2-)) was also found. At saturation, the coverage of adsorbed sulfur species was the same on both alpha-Fe(2)O(3) and alpha-FeOOH as determined from the S2p : Fe2p ratio. Equivalent saturation coverages and product ratios of sulfite to sulfate were observed on these oxide surfaces in the presence of water vapor at pressures between 6 and 18 Torr, corresponding to 28 to 85% relative humidity (RH), suggesting that water had no effect on the adsorption of SO(2). In contrast, molecular oxygen substantially influenced the interactions of SO(2) with iron oxide surfaces, albeit to a much larger extent on alpha-Fe(2)O(3) relative to alpha-FeOOH. For alpha-Fe(2)O(3), adsorption of SO(2) in the presence of molecular oxygen resulted in the quantitative formation of SO(4)(2-) with no detectable SO(3)(2-). Furthermore, molecular oxygen significantly enhanced the extent of SO(2) uptake on alpha-Fe(2)O(3), as indicated by the greater than two-fold increase in the S2p : Fe2p ratio. Although SO(2) uptake is still enhanced on alpha-Fe(2)O(3) in the presence of molecular oxygen and water, the enhancement factor decreases with increasing RH. In the case of alpha-FeOOH, there is an increase in the amount of SO(4)(2-) in the presence of molecular oxygen, however, the predominant surface species remained SO(3)(2-) and there is no enhancement in SO(2) uptake as measured by the S2p : Fe2p ratio. A mechanism involving molecular oxygen activation on oxygen vacancy sites is proposed as a possible explanation for the non-photochemical oxidation of sulfur dioxide on iron oxide surfaces. The concentration of these sites depends on the exact environmental conditions of RH.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Interfacial water on hydrophobic surfaces recognized by ions and molecules

Recent spectrophotometric and molecular dynamics simulation studies have shown that the physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ from those of the bulk water. However, the interfacial water acting as a separation medium on hydrophobic surfaces has never been detected and quantified experimentally. In this study, we show that small inorganic ions and organic molecules differentiate the interfacial water formed on the surfaces of octadecyl-bonded (C(18)) silica particles from the bulk water and the chemical separation of these solutes in aqueous media with hydrophobic materials can be interpreted with a consistent mechanism, partition between the bulk water phase and the interfacial water formed on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in the nanopores of the C(18) silica materials and the solubility parameter of the water calculated from the distribution coefficients of organic compounds have indicated that the interfacial water may have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit of distance from the hydrophobic surface at which molecules and ions can sense the surface was estimated to be 1.25 ± 0.13 nm from the volume of the interfacial water obtained by a liquid chromatographic method and the surface area, suggesting that the hydrophobic effect may extend beyond the first solvation shell of water molecules directly surrounding the surfaces.     

Study of the dynamics of surface molecules by time-resolved sum-frequency generation spectroscopy

Sum-frequency generation (SFG) is a nonlinear laser-spectroscopy technique suitable for analysis of adsorbed molecules. The sub-monolayer sensitivity of SFG spectroscopy enables vibrational spectra to be obtained with high specificity for a variety of molecules on a range of surfaces, including metals, oxides, and semiconductors. The use of ultra-short laser pulses on time-scales of picoseconds also makes time-resolved measurements possible; this can reveal ultrafast transient changes in molecular arrangements. This article reviews recent time-resolved SFG spectroscopy studies revealing site-hopping of adsorbed CO on metal surfaces and the dynamics of energy relaxation at water/metal interfaces. Time-resolved sum frequency generation spectroscopy at surfaces with non-resonant laser pulse irradiation     

Glycine at the pyrite-water interface: the role of surface defects

Ab initio molecular dynamics simulations were performed in order to study chemisorption, electronic properties, and desorption of glycine at wet pyrite surfaces focusing on the role of surface point defects. The change in the electronic structure and its influence on the chemical reactivity of the free FeS(2)(100) surface due to sulfur vacancies was studied in detail yielding several adsorption modes of glycine and water molecules. Energetically preferred adsorption modes were furthermore investigated in the presence of hot pressurized water mimicking "Iron Sulfur World" prebiotic conditions. The metadynamics Car-Parrinello technique was employed to map the free energy landscape including paths and barriers for desorption of glycine from such wet defective surfaces. The ubiquitous sulfur vacancies are found to increase the retention time of the adsorbed amino acid by many orders of magnitudes in comparison to the ideal pyrite-water interface. The importance of these findings in terms of a possible two-dimensional primordial chemistry on mineral surfaces is discussed.     

Solvation of excess electrons trapped in charge pockets on hydrated molecular surfaces

We have previously computed a set of hypothetical molecular surfaces, which formed charge pockets that were capable of excess electron entrapment. These charge pockets arose due to the fact that the molecular surfaces possessed an extended network of OH groups on one side of the surface and hydrogen atoms on the opposite side. The uneven distribution of the OH groups coupled to the partial positive charge of the hydrogen atoms caused electrons to be attracted to the surface. In the present investigation we will consider the ability of the hydrogen cyanide (HCN)‐water complex in stabilizing excess electrons on molecular surfaces. The computed vertical detachment energy (VDE) values are high, suggesting anion stability. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

A molecular beam study of the evaporation of water from a liquid jet

A method to maintain a clean surface of a liquid in a high vacuum is described. Using a very thin and fast liquid jet it is not only possible to prevent freezing of the liquid but also to reduce the number of collisions between evaporating molecules to negligibly small values. Thus many of the standard, vacuum dependent, particle probing techniques for solid surfaces can be used for studies of rapidly vaporizing, high vapor pressure liquids. In a first molecular beam investigation we have used time-of-flight analysis to measure the velocity distribution of H₂O molecules vaporizing from thin jets of pure liquid water. The experiments were carried out for liquid jet diameters between 50 and 5 µm. In this range the expanding vapor is observed to undergo the transition to the collision-free molecular flow regime. From the measured velocity distributions the local surface temperature is determined to be less than 210 K. This appears to be the lowest temperature ever reported for supercooled liquid water.