Phase transitions of interfacial water at 165 and 240 K. Connections to bulk water physics and protein dynamics

We are considering water adsorbed as a monolayer on Vycor, a porous silica glass. The interfacial water molecules interact with the substrate through hydrogen bonding with the numerous silanol (Si-OH) groups present all over the surface. This special form of water exhibits peculiar dynamical properties. A combined calorimetric, diffraction, high resolution quasi-elastic and inelastic neutron scattering study shows that interfacial water experiences a glass transition at 165 K and a liquid-liquid transition at 240 K from a low-density to a high density-liquid. We show that this unusual behaviour, compared to the bulk, is due to a strong weakening of the hydrogen-bond strength, possibly due to the reduced number of hydrogen-bonds engaged by water molecules when they are in an interfacial two dimensional situation. The connections of these findings to the physics of bulk water and protein dynamics are discussed.     

Structure and dynamics of ordered water in a thick nanofilm on ionic surfaces

The structure and dynamics of water in a thick film on an ionic surface are studied by molecular dynamic simulations. We find that there is a dense monolayer of water molecules in the vicinity of the surface. Water molecules within this layer not only show an upright hydrogen-down orientation, but also an upright hydrogen-up orientation. Thus, water molecules in this layer can form hydrogen bonds with water molecules in the next layer. Therefore, the two-dimensional hydrogen bond network of the first layer is disrupted, mainly due to the O atoms in this layer, which are affected by the next layer and are unstable. Moreover, these water molecules exhibit delayed dynamic behavior with relatively long residence time compared with those bulk-like molecules in the other layers. Our study should be helpful to further understand the influence of water film thickness on the interfacial water at the solid-liquid interface.     

Molecular Dynamics Simulation of 4-N-Hexyl-4'-Cyanobiphenyl Adsorbed at the Air-Water Interface

The interfacial behavior of 4-n-hexyl-4'-cyanobiphenyl(6CB) molecules at the air-water interface is investigated by full atomistic molecular dynamics simulations. To understand the morphology and the structure of adsorbed 6CB molecules in detail, the snapshots and mass density profiles of the simulation system are generated. The average tilt angles between the interface normal and various vectors defined in the rigid and alkyl parts of 6CB are in good agreement with the experimental data available. The interfacial thickness and monolayer width are obtained from the mass density profiles of water and 6CB phase, respectively. The second and fourth rank orientational order parameters of cyanobiphenyl core are found to be larger than those of an elastic alkyl chain.Bond order parameters for 6CB are also calculated. The calculated oxygen-oxygen radial distribution function and hydrogen bonding statistics for bulk water are compared with those for the interfacial region. The surface tensions of the systems are calculated. All simulation results are compared with the available literature data.     

Molecular simulation of sodium butyl benzene sulfonate at air–water interface and in aqueous solution

Molecular mechanics(MM) calculations for interfacial behaviour of sodium n-butyl benzene sulfonate (NaNBBS), sodium iso-butyl benzene sulfonate (NaIBBS) and sodium tert-butyl benzene sulfonate (NaTBBS) show a significant effect of the butyl group geometry on the surface area occupied by these molecules at the air–water interface. NaNBBS, in comparison with NaIBBS and NaTBBS, shows a closer molecular packing at the interface. The simulation predicts minimum hydrotrope concentration of each hydrotrope to reach surface saturation and molecular surface area at the interface match with good accuracy. The shape, size and charge of the hydrotrope aggregates obtained by molecular dynamics simulation also match well with the results of small angle neutron scattering experiments on the same hydrotrope. The simulation shows non-regular and ellipsoidal hydrotropes aggregates with substantial charge on the surface. The aggregates are also more open structures as compared to surfactant micelles. The water accessible surface area of a NaNBBS aggregate was 25% lower in comparison to that of NaTBBS aggregate, indicating closer packing of NaNBBS molecules. The fractional charge on the NaNBBS aggregate decreases with the increase in the number of NaNBBS molecules in the aggregate indicating more counter-ion association.     

The chloroform / TBP / aqueous nitric acid interfacial system: a molecular dynamics investigation

We report MD studies on a chloroform / nitric acid water interface, either neat, or saturated by TBP molecules. Two extreme models are compared, where the acid is either neutral HNO₃ or dissociated to NO₃⁻ and H₃O⁺. The latter species are found to be “repelled” by the neat interface, while the neutral HNO₃ molecules are surface active. When the neat interface is saturated by TBP molecules, the latter form highly disordered arrangements instead of a regular monolayer, and water is dragged to the organic phase as 1:1, 1:2 and 2:2 hydrates of TBP. Simulations with the neutral HNO₃ model lead to extraction of acid to the organic phase, hydrogen bonded to the phosphoryl oxygen of TBP, forming HNO₃:TBP adducts of 1:1 and 2:1 types. Simulations with the ionic model lead to H₃O⁺:TBP adducts of 1:1, 1:2 and 1:3 types in the organic phase and significant mixing of the chloroform, TBP and water liquids in the interfacial region.     

How polar hydroxyl groups affect surface hydrophobicity on model talc surfaces

Based on molecular dynamics simulations, we have studied the wetting behaviors of water on the talc-like surface with different surface polarity by modifying the charge distribution of surface hydroxyl (–OH) groups. With the change of the charge of the hydrogen atom (denoted as δ_q) in –OH group, the contact angle decreases from 91° to 50° and then remains constant. On the surfaces with the larger charge of hydrogen atoms (δ_q ≥ 0.2 e), a water droplet is formed above a water monolayer, which is exactly contacted on the surface. Each water molecule in the monolayer forms one hydrogen bond (H bond) with surface –OH groups, without participating in any H bond with the water molecules within the monolayer or with the water molecules above the monolayer. The polarity of the –OH group also has a great influence on the dynamic behaviors of the interface water, such as residence time, hydrogen bond lifetime and self-diffusion coefficient. The diffusion of water molecules in the water monolayer near the highly polar surface is greatly suppressed, and the residence time of water molecules in the water monolayer even exceeds 12 ns.     

Hydrophobicity-induced drying transition in alkanethiol self-assembled monolayer—water interface

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 non-aqueous 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.     

Role of oxygen functional groups for structure and dynamics of interfacial water on low rank coal surface: a molecular dynamics simulation

Molecular dynamics simulations were employed to study the effects of oxygen functional groups for structure and dynamics properties of interfacial water molecules on the subbituminous coal surface. Because of complex composition and structure, the graphite surface modified by hydroxyl, carboxyl and carbonyl groups was used to represent the surface model of subbituminous coal according to XPS results, and the composing proportion for hydroxyl, carbonyl and carboxyl is 25:3:5. The hydration energy with ?386.28 kJ/mol means that the adsorption process between water and coal surface is spontaneous. Density profiles for oxygen atoms and hydrogen atoms indicate that the coal surface properties affect the structural and dynamic characteristics of the interfacial water molecules. The interfacial water exhibits much more ordering than bulk water. The results of radial distribution functions, mean square displacement and local self-diffusion coefficient for water molecule related to three oxygen moieties confirmed that the water molecules prefer to absorb with carboxylic groups, and adsorption of water molecules at the hydroxyl and carbonyl is similar.     

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.     

Recent advances on “ordered water monolayer that does not completely wet water” at room temperature

The molecular scales behavior of interfacial water at the solid/liquid interfaces is of a fundamental significance in a diverse set of technical and scientific contexts,ranging from the efficiency of oil mining to the activity of biological molecules.Recently,it has become recognized that,both the physical interactions and the surface morphology have significant impact on the behavior of interfacial water,including the water structures as well as the wetting properties of the surface.In this review,we summarize some of recent advances in the atom-level pictures of the interfacial water,which exhibits the ordered character on various solid surfaces at room or cryogenic temperature.Special focus has been devoted to the wetting phenomenon of"ordered water monolayer that does not completely wet water"and the underlying mechanism on model and some real solid surfaces at room temperature.The possible applications of this phenomenon are also discussed.     

Analysis of methanol extraction from aqueous solution by n-hexane: Equilibrium diagrams as a function of temperature

This paper contains the results of a new experimental study of liquid–liquid equilibrium (LLE) as a function of temperature for the ternary mixture n-hexane + methanol + water under atmospheric conditions. The computed coexistence curves were very asymmetrical with respect to n-alkane + water composition at low operation temperature. A comparative analysis was performed by application of group contribution methods to predict the experimental equilibria behaviour of this ternary mixture. The experimental tie lines data were correlated to test consistency with the Othmer–Tobias equation. The obtained results point out the strong interaction by hydrogen bonds forming the so-called iceberg clusters of hydroxylated molecules at low concentration of n-hexane, a slight influence of temperature being observed. As a result of this intense interaction, sharp separations of n-hexane + methanol mixtures by extraction are possible at low temperature and small quantities of solvent. Extraction process simulation confirms n-hexane as an adequate medium for diluted methanol concentration.     

Effects of surface concentration on the porphine monolayers: Molecular simulations at the nanoscale water-gas interface

The effect of surface concentration on the structure and stability of porphine (PH₂) monolayers at the water-gas interface was studied by using molecular dynamics simulation. Five monolayer systems having different surface concentrations were investigated in order to cover a full range of the experimental π-A isotherm. The simulation results show that increment of a number of the PH₂ molecules not only affects the significantly decreasing water density at the interface but also the monolayer surface tensions. The calculated surface tensions of the five systems indicate that the monolayer phase transfer corresponding to gaseous, expanded, condensed, and collapsed phases are observed. The hydrogen bonding between water and the PH₂ molecules at the interface plays an important role on the monolayer film formation, especially at the lower surface concentrations. The PH₂ orientations for all surface concentrations, except the highest one, are favored to be the β-structure as observed in the copper porphyrazine (CuPz) monolayer.     

Investigation of vapour--liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation （WDA） for short-range interactions and a Weaks Chandler Andersen （WCA） approximation and a Barker Henderson （BH） theory for long-range attraction. Within the framework of density functional theory （DFT）, vapour liquid interracial properties including density profile and surface tension, and vapour-liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chain- like molecules. The obtained vapour liquid surface tension and the number of particles in critical nucleus for Lennard- Jones （L J） fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour liquid nucleation properties are discussed.[第一段]     

Liquid–vapour equilibrium of n -alkanes using interface simulations

Molecular Dynamics simulations were performed to calculate liquid–vapour coexisting properties of n-alkane chains up to 16 carbon atoms using interface simulations. The lattice sum or Ewald method on the dispersion forces of the Lennard–Jones potential was applied to calculate the full interaction. The liquid and vapour coexisting densities were obtained for two flexible force field models, NERD and TraPPE-UA, where the intermolecular interactions are of the Lennard–Jones type. We have recently shown [P. Orea, J. López-Lemus, and J. Alejandre, J. Chem. Phys. 123, 114702 (2005)] that the liquid–vapour densities for simple fluids do not depend on interfacial area and therefore it is possible to use a small number of molecules in a simulation. We show that the same trend is found on the simulation of these hydrocarbon molecules. The phase diagram of ethane/n-decane binary mixtures is also obtained at 410.95 K for the NERD model. The simulation results from this work were compared with those obtained using methods with interfaces using large cut-off distances and with methods without interfaces for the same potential model. In both comparisons, excellent agreement was found. The results of liquid density from the TraPPE-UA model are in good agreement with experimental data while those from the NERD model are underestimated at low temperatures. Our findings are consistent with results published by other authors for small hydrocarbons.     

Thermodynamic properties of gold–water nanolayer mixtures using molecular dynamics

The physical behavior of a fluid in contact with solid layers is still not fully understood. The present work focuses on the study and understanding of thermodynamic and structural properties of gold–water nanolayer mixtures using molecular dynamics simulations. Two different systems are considered, where approximately 1,700 water molecules are confined between gold nanolayers with separations of 7.4 and 6.2 nm, respectively. Novelties of the present work are in the use of accurate force fields for modeling the inter- and intra-molecular interactions of the components, and providing comprehensive thermodynamic properties of the mixtures. The results are validated by examination of the pure fluid and pure solid properties. Results indicate that the thermodynamics of the system does not behave as an ideal mixture. The structure of the pure fluid is also analyzed and compared against the structure of the confined fluid in the mixture. Anisotropicity is observed in the fluid structure close to the surface of the nanolayer. Higher ordering and higher flux are detected in the fluid molecules close to the fluid–solid interface. Unusual thermodynamic behavior, anisotropicity, liquid layering, and higher interfacial fluid flux could be just some of the factors leading to the enhanced energy transport observed in mixtures involving at least one nanoscale component, such as nanofluids.     

The guest-induced oscillation of a monolayer composed of polypeptide containing beta-cyclodextrin at the terminal

We prepared a rod-like amphiphile with a molecular recognition end group, alpha-helical and hydrophobic poly(gamma-methyl L-glutamate) (PMG) containing hydrophilic beta-cyclodextrin (CyD) as an active end group (PMG-CyD), and formed its monolayer at the n-hexane/water interface. The interfacial pressure (pi)-area (A) isotherms of the monolayer showed that alpha-helix rod of PMG-CyD could be vertically oriented at the oil/water interface, facing the hydrophilic terminal CyD group to the water phase, by increasing the interfacial concentration of the polypeptide. Under the condition 2-p-toludinyl-naphthalene-6-sulfonate (TNS), an intimate guest molecule for the CyD in water was introduced into the water phase beneath the monolayer. Within a minute the monolayer began to oscillate which could be monitored by the rhythmic response of the interfacial pressure of the monolayer. The oscillation continued over ten minutes and then terminated. The mode of the oscillation was found to change with time, i.e., the initial stage showing a periodic sharp reduction in the interfacial pressure (period I), the second stage having sharp increase in the pi value (period II), and the last stage of irregular oscillations (period III). The Fourier analysis of each period also supported the three stages during the oscillatory process. It was also found that when the alpha-helix rod of PMG-CyD lay down in the monolayer, the guest TNS did not induce any changes in the interfacial tension. This nonlinear rhythmic interfacial phenomenon was explained in terms of the periodic movement of the PMG-CyD monolayer resulting from the binding and releasing of the guest TNS across the oil/water interface. (c) 1999 American Institute of Physics.     

Electronic origin of disorder and diffusion at a molecule-metal interface: self-assembled monolayers of CH3S on Cu111

The relationship between structure, interfacial electrostatics, bonding, and dynamics of organic molecules on metals is studied using a self-assembled monolayer of methylthiolate, CH3S, on Cu(111). The flat adsorption energy landscape of CH(3)S/Cu(111) results from metal-to-molecule charge redistribution which allows for a high mobility of CH3S. This contributes a nonuniform diffuse background to Bragg scattering, which needs to be considered in diffraction analyses. Ramifications on the interpretation of experimental data and the potential impact on the design of metal-organic interfaces are discussed.     

Crystalline ice growth on PT(111): observation of a hydrophobic water monolayer

The growth of crystalline water films on Pt(111) is investigated using rare gas physisorption. The water monolayer wets Pt(111) at all temperatures investigated (20-155 K). At low temperatures (T< or =120 K), additional water layers kinetically wet the monolayer surface. However, crystalline ice films grown at higher temperatures (T > 135 K) do not wet the water monolayer. These results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on Pt(111).     

Molecular dynamics simulation study of the nano-wear characteristics of alkanethiol self-assembled monolayers

A molecular dynamics (MD) simulation study of the probe-based nano-lithography of an alkanethiol self-assembled monolayer (SAM) on a metal surface was performed. The motivation of this work was to understand the nano-tribological phenomena of the nano-metric scribing process of alkanethiol molecules and gain insight into the interaction between the probe tip and the SAM-coated surface during the scribing process. The simulation results revealed that the organothiol molecules were displaced and dragged by the probe tip during scribing due to the strong interchain interactions. It was also found that the scribed pattern width was largely dependent on the tip–surface interaction induced by the probe shape rather than the tip–surface contact size. Also, the minimum load for tip–substrate contact changed with the number of molecules that interact with the probe tip. Furthermore, from the investigation of the effect of the scribing speed on the surface-damage characteristics of the chain molecules, it was found that relatively high-speed scribing could induce excessive removal of the SAM molecules from the surface. PACS 02.70.Ns; 31.15.Qg; 81.16.Nd; 68.35.Af     

The effect of surface and Coulomb interaction on the liquid–gas phase transition of finite nuclei

By means of the Furnstahl, Serot and Tang's model, the effects of surface tension and Coulomb interaction on the liquid–gas phase transition for finite nuclei are investigated. A limit pressure p_lim above which the liquid–gas phase transition cannot take place has been found. It is found that comparing to the Coulomb interaction, the contribution of surface tension is dominate in low temperature regions. The binodal surface is also addressed.