Vibrational spectrum at a water surface: a hybrid quantum mechanics/molecular mechanics molecular dynamics approach

A hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulation is applied to the calculation of surface orientational structure and vibrational spectrum (second-order nonlinear susceptibility) at the vapor/water interface for the first time. The surface orientational structure of the QM water molecules is consistent with the previous MD studies, and the calculated susceptibility reproduces the experimentally reported one, supporting the previous results using the classical force field MD simulation. The present QM/MM MD simulation also demonstrates that the positive sign of the imaginary part of the second-order nonlinear susceptibility at the lower hydrogen bonding OH frequency region originates not from individual molecular orientational structure, but from cooperative electronic structure through the hydrogen bonding network.     

Surface vibrational spectroscopic study of surface melting of ice

Surface melting on the (0001) face of hexagonal ice ( I(h)) was studied by sum-frequency vibrational spectroscopy in the OH stretch frequency range. The degree of orientational order of the dangling OH bonds at the surface was measured as a function of temperature. Disordering sets in around 200 K and increases dramatically with temperature. The results show that the disordered (quasiliquid) layer on ice is structurally different from normal liquid water.     

界面氢键对受限水结构和动态特性的影响

应用反应力场分子动力学方法, 模拟了水限制在全羟基化二氧化硅晶体表面间的弛豫过程, 研究了基底表面与水形成的界面氢键, 及其对受限水结构和动态特性行为的影响. 当基底表面硅醇固定时, 靠近基底表面水分子中的氧原子与基底表面的氢原子形成强氢键, 这使得靠近表面水分子中的氧原子比对应的氢原子更靠近基底表面, 从而水分子的偶极矩远离表面. 当基底表面硅醇可动时, 靠近基底表面水分子与基底表面原子形成两种强氢键, 一种是水分子中的氧原子与表面的氢原子形成的强氢键, 数量较少, 另一种是水分子中的氢原子与表面的氧原子形成的强氢键, 数量较多, 这使得靠近表面水分子中的氢原子比对应的氧原子更靠近表面, 从而水分子的偶极矩指向表面. 在相同几何间距下, 当基底表面硅醇可动时, 表面的活动性使得几何限制作用减弱, 导致了受限水分层现象没有固定表面限制下的明显. 此外, 固定表面比可动表面与水形成的界面氢键作用较强, 数量较多, 导致了可动表面限制下水的运动更为剧烈.     

Ice tessellation on a hydroxylated silica surface

The adsorption of water on a fully hydroxylated silica surface is studied by using density-functional total-energy and molecular dynamics calculations. The (100) surface of beta(alpha)-cristobalite covered by geminal hydroxyls has been taken as the substrate. A well-ordered and stable two-dimensional ice with quadrangular and octagonal patterns of hydrogen bond (H-bond) networks-an ice tessellation-is found on the surface for the first time. With the vibrational recognition, the four water molecules in the quadrangle are found to be bonded by strong H bonds while the quadrangles are connected to each other by weak H bonds. This configuration is the most stable, because all the water molecules are fully saturated with H bonds either to each other or to the surface hydroxyl groups.     

Hydrogen bonds at silica–CO2 saturated water interface under geologic sequestration conditions

To investigate the effects of sequestration condition on hydrogen bonds between mineral and water, molecular dynamics simulations have been performed. The simulations were conducted at conditions related with geologic sequestration sites: pressure (3.1–32.6 MPa), temperature (318 and 383 K), salinity (0–3 M), salt (NaCl and CaCl₂) and silica surface models Q² (geminal), Q³ (isolated) and amorphous Q³. The hydrogen bonds were classified into four types: silica–silica, silica–dissolved CO₂, silica–water as donors and silica–water as acceptors. The mean numbers of hydrogen bonds for each type were analysed. The results show that: (1) silica surface silanol groups do not form H-bonds with dissolved CO₂ molecules in water (brine); (2) The mean number of hydrogen bonds between silanol groups follows the order: Q² > amorphous Q³ > Q³; (3) The mean number of hydrogen bonds between silanol and water molecules follows the order: Q³ > amorphous Q³ > Q².     

Orientational correlations in liquid hydrogen sulphide

Neutron diffraction data on liquid H₂S, D₂S and an equimolar mixture of the two are presented for two thermodynamic states along the liquid—vapour coexistence curve. Comparison of the site-site radial distribution functions obtained with those of a model fluid where orientational correlations are absent and with those of water demonstrates that, although hydrogen sulphide does not form hydrogen bonds, nevertheless weak orientational correlations between first neighbouring molecules are detectable in both thermodynamic states. An analysis of the molecular configurations compatible with the experimental data, as calculated by applying the empirical potential structural refinement, is presented also.     

The dynamics of water in nanoporous silica studied by dielectric spectroscopy

Broad-band dielectric spectroscopy is used to investigate the dynamics of hydration water on the surface of the cylindrical pores of a nanostructured silica material (MCM-41, with pore diameter of 3.2 nm) at various hydrations, in the temperature range 250-150 K. We focus our attention on orientational relaxations that shift from 0.5 MHz at 250 K to less than 1 Hz at 150 K. The measurements distinguish the relaxation of the hydroxyl groups at the surface of silica from the orientational dynamics of hydration water which strongly depends on the degree of hydration. Although it is significantly faster than the dynamics of water in ice, the orientational relaxation of non-freezing water has an activation energy comparable to that in ice when the hydration layer is complete and approximately two-molecule thick.     

Hydrogen-bond dynamics of interior and surface molecules in a water nanocluster: temperature and size effects

Molecular dynamics simulations are employed to investigate the effects of temperature and size on the hydrogen-bond dynamics of interior molecules and surface molecules in a water nanocluster. The flexible three-centred (F3C) water model is invoked in the simulations. To inspect the dynamics of the interior hydrogen bonds and the surface hydrogen bonds, a spherical water nanocluster is modelled and then divided into interior molecules and surface molecules according to the density profile of the water nanocluster. It is observed that at higher temperatures the average number of hydrogen bonds decreases and yields faster hydrogen-bond relaxation for both interior molecules and surface molecules of the water nanocluster. Furthermore, the surface molecules have a lower average number of hydrogen bonds than the interior molecules. The lifetime of the surface hydrogen bonds is slightly longer than that of the interior hydrogen bonds, whereas the hydrogen-bond structural relaxation time of the surface molecules is more obviously slower than that of the interior molecules. Regarding the size effect, a larger water nanocluster is seen to have a larger average number of hydrogen bonds and a longer hydrogen-bond structural relaxation time.     

Structure and dynamics of water: from ambient to supercritical

Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm³. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D₂O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.     

Peptide friction in water nanofilm on mica surface

Peptide frictions in water nanofilms of various thicknesses on a mica surface are studied via molecular dynamics simulations. We find that the forced lateral motion of the peptide exhibits stick-slip behaviour at low water coverage; in contrast, the smooth gliding motion is observed at higher water coverage. The adsorbed peptide can form direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds with the substrate. We propose that the stick-slip phenomenon is attributed to the overall effects of direct and indirect hydrogen bonds formed between the surface and the peptide.     

Density profile of water confined in cylindrical pores in MCM-41 silica

Recently, water absorbed in the porous silica material MCM-41-S15 has been used to demonstrate an apparent fragile to strong dynamical crossover on cooling below ～220 K, and also to claim that the density of confined water reaches a minimum at a temperature around 200 K. Both of these behaviours are purported to arise from the crossing of a Widom line above a conjectured liquid-liquid critical point in bulk water. Here it is shown that traditional estimates of the pore diameter in this porous silica material (of order 15 ?) are too small to allow the amount of water that is observed to be absorbed by these materials (around 0.5 g H(2)O/g substrate) to be absorbed only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25 ? (radius 12.5 ?). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300 to 210 K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25 ? or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic crossover in supercooled confined water could in fact be a crystallization transition in the larger pore or surface water.     

Enhancement of water self-diffusion at super-hydrophilic surface with ordered water

It has been well acknowledged that molecular water structures at the interface play an important role in the surface properties, such as wetting behavior or surface frictions. Using molecular dynamics simulation, we show that the water self-diffusion on the top of the first ordered water layer can be enhanced near a super-hydrophilic solid surface. This is attributed to the fewer number of hydrogen bonds between the first ordered water layer and water molecules above this layer, where the ordered water structures induce much slower relaxation behavior of water dipole and longer lifetime of hydrogen bonds formed within the first layer.     

Hydrophobic hydration at the level of primitive models. II: Large solutes and water restructuring

Details of structural changes that take place in water near an apolar solute have been studied by Monte Carlo simulations for hard sphere solutes of increasing size, including the limiting case of water at a hard structureless wall. Water has been modelled by two different types of extended primitive model, the four-site EPM4 model and five-site EPM5 model. Two different patterns of the orientational ordering of the water molecules around the solute as a function of its size have been found. For the EPM5 model, the structure of water and the orientation of its molecules near an apolar solute of finite diameter do not seem to be sensitive to the size of the solute, and only become more pronounced when the solute becomes a hard wall. On the other hand, the orientation ordering of the EPM4 molecules gradually changes with increasing size of the solute, and for solutes larger than approximately five times the size of the water molecule it is opposite to that near a small solute. A novel method to evaluate the excess chemical potential of large solutes has been implemented, and some thermodynamic quantities for water (distribution of hydrogen bonds and the excess chemical potential) have been computed as a function of the distance from the solute.     

Oxide/water interfaces: how the surface chemistry modifies interfacial water properties

The organization of water at the interface with silica and alumina oxides is analysed using density functional theory-based molecular dynamics simulation (DFT-MD). The interfacial hydrogen bonding is investigated in detail and related to the chemistry of the oxide surfaces by computing the surface charge density and acidity. We find that water molecules hydrogen-bonded to the surface have different orientations depending on the strength of the hydrogen bonds and use this observation to explain the features in the surface vibrational spectra measured by sum frequency generation spectroscopy. In particular, 'ice-like' and 'liquid-like' features in these spectra are interpreted as the result of hydrogen bonds of different strengths between surface silanols/aluminols and water.     

The structure and dynamics of water inside armchair carbon nanotube

In this paper we present some simulation results about the behaviour of water molecules inside a single wall carbon nanotube (SWNT). We find that the confinement of water in an SWNT can induce a wave-like pattern distribution along the channel axis, similar phenomena are also observed in biological water channels. Carbon nanotubes(CNTs) can serve as simple nonpolar water channels. Molecular transport through narrow CNTs is highly collective because of tight hydrogen bonds in the protective environment of the pore. The hydrogen bond net is important for proton and other signal transports. The average dipoles of water molecules inside CNTs (7,7), (8,8) and (9,9) are discussed in detail. Simulation results indicate that the states of dipole are affected by the diameter of SWNT. The number of hydrogen bonds, the water--water interaction and water--CNT interaction are also studied in this paper.     

Molecular investigation of water adsorption on graphene and graphyne surfaces

In this paper the adsorption action of a water droplet on the graphene and graphyne externals has been examined. Conclusions received from the calculation of the water contact angle on the graphene and the graphynes surfaces have demonstrated that graphyne is more hydrophobic than graphene. Sketching the contour maps of the water interaction showed different behaviors of water droplet on these surfaces. The results show that water molecules, form a sub_layer of water on the graphyne substrate while this sub_layer does not exist on the graphene. Molecular investigations of the water on the surfaces show that the attendance of a sub_layer of water on the substrate can cause changes, such as the number of hydrogen bonds per water molecule in the water droplet, the order of molecules in different layers of water droplet, and parallel forces to the surface between surface water molecules and substrate, in the structural properties of water droplet. In this study the interaction between first layer and sub-layer of water was investigated. Water drops on surface can affect on the behavior of water sub-layer.     

Simulation of the interface of (1 0 0) rutile with aqueous ionic solution

The interface of the rutile (1 0 0) surface with NaCl solutions has been simulated by classical molecular dynamics. In contrast to earlier simulations the protonation and hydroxylation equilibriums have been adjusted for different pH values (4, 7.4, and 9). The short range order close to the surface is described by two water layers with some orientational order and intermediate layers of positive or negative ions depending on the surface charge. A Stern model is confirmed with a dense layer of counterions on the charged TiO₂ surface and a diffuse layer, which only consists of few ions in our system. The increase of orientational order of the water molecules close to the surface is described by an exponential function with a decay parameter of 1.9 Å, superposed by a damped oscillation which is independent of the pH value. The diffusion is significantly slower than in the bulk within a range of 13 Å from the surface. We propose a common approach for describing the different z-dependences of orientational order and of the diffusion coefficients.     

Motional effect in surface sum-frequency vibrational spectroscopy

Fast orientational motion of molecules at a surface can affect the sum-frequency vibrational spectra of the surface. Calculation shows that the effect is significant if the molecular orientation varies over a broad range within the vibrational relaxation time. The stretch vibration of the free OH bonds at the vapor/water interface is used to illustrate the importance of the effect.     

Predictions of dynamic behavior under pressure for two scenarios to explain water anomalies

Using Monte Carlo simulations and mean field calculations for a cell model of water we find a dynamic crossover in the orientational correlation time tau from non-Arrhenius behavior at high temperatures to Arrhenius behavior at low temperatures. This dynamic crossover is independent of whether water at very low temperature is characterized by a "liquid-liquid critical point" or by the "singularity-free" scenario. We relate tau to fluctuations of hydrogen bond network and show that the crossover found for tau for both scenarios is a consequence of the sharp change in the average number of hydrogen bonds at the temperature of the specific heat maximum. We find that the effect of pressure on the dynamics is strikingly different in the two scenarios, offering means to distinguish between them.     

Water in porous glasses. A computer simulation study

We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pores in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydration of the pore. We have performed five simulations of systems between 19 and 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two layers of water molecules are affected strongly by the interactions with the glass surface. With decreasing degree of hydration an increasing volume in the center of the pore is devoid of water molecules. At 96 % hydration the center is a continuous and homogeneous region that has, however, a lower density than bulk water at ambient conditions. A well-pronounced mobility profile exists, where molecules in the center of the pores have substantially higher self diffusion coefficients than molecules on the pore surface. The spectral densities of center of mass and hydrogen atom motion show the signature of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk water. The molecular dynamics results are in good agreement with recent experiments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative contribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages.