Properties Of Interfacial Water Research Articles

Thermal unfolding of barstar and the properties of interfacial water around the unfolded forms

Identification of the intermediates along the folding-unfolding pathways and probing their interactions with surrounding solvent are two important but relatively unexplored issues in protein folding. In this work, we have carried out atomistic molecular dynamics simulations to study the thermal unfolding of barstar in aqueous solution from its folded native form at two different temperatures (400 K and 450 K). The calculations at 400 K reveal partial unfolding of two α-helices (helix-1 and helix-2) and their interconnecting loop. At 450 K, on the other hand, the entire protein attains an expanded flexible conformation due to disruption of a large fraction of tertiary contacts and breaking of almost all the secondary structures. These two disordered structures obtained at such high temperatures are then studied around room temperature to probe their influence on the properties of surrounding solvent. It is found that though the unfolding of the protein in general leads to increasingly hydrated interface, but new structural motifs with locally dehydrated interface may also form during the structural transition. Additionally, independent of the conformational state of the protein, its influence on surrounding solvent has been found to be restricted to the first hydration layer.

Wetting Behavior of Water near Nonpolar Surfaces

We use molecular simulation to study the wetting behavior of water near flat nonpolar surfaces. The interface potential approach is used to capture the evolution of various interfacial properties over a broad range of temperature and substrate strength. Three model substrates are considered: an atomistically detailed face-centered cubic (FCC) lattice, a graphite lattice, and a structureless wall. We first examine the evolution of the contact angle with substrate strength for conditions ranging from near drying to complete wetting at select temperatures. A notable characteristic of all systems is the presence of a surface strength at which the contact angle is independent of temperature. We also identify an analogous point in which the quantity γlv cos θ is temperature invariant. We discuss the relationship between this point and the excess entropy of the solid–liquid interface. We next consider the temperature dependence of interfacial properties. We find the contact angle to decrease with temperature at relatively strong surfaces and increase with the temperature at relatively weak surfaces. The work of adhesion is found to be a useful quantity for describing the interfacial properties of water. The enthalpic and the entropic contributions to the work of adhesion are obtained from its temperature dependence. These properties are found to be directly related to the affinity of water for a solid surface and therefore serve as useful measures of the hydrophobicity of a surface. Finally, we examine the effect of surface strength and temperature on the density depletion associated with water at hydrophobic surfaces. We study various metrics that quantify the density and compressibility of water in the vicinity of hydrophobic surfaces. Comparisons are drawn between the behavior of water and simple nonpolar fluids at solvophobic surfaces.

Reprint of “Role of the fluidity of a liquid phase in determining the surface properties of the opposite phase”

Molecular dynamics simulations of the water–CCl4 interface have been done in two different ways. In the first simulation the CCl4 phase has been frozen in an equilibrium configuration, and only the water molecules have been allowed to move, whilst in the other one no such artificial freezing has been done. This way the effect of the fluid-like structure and fluid-like dynamics of the CCl4 phase on the surface properties of the aqueous phase could be investigated separately. Due to the separate thermostatting of the two types of molecules in the simulations all the differences seen between the interfacial properties of water in the two systems can indeed be attributed to the rigid vs. fluid nature of the organic phase, and not to the thermal contact with a phase of zero temperature. The obtained results reveal that the rigidity of the opposite phase introduces an ordering both in the layering structure and orientation of the surface water molecules. The enhanced orientational ordering leads to a stronger lateral hydrogen bonding structure of the molecules within the subsequent molecular layers beneath the surface, and hence also to a slower exchange of the water molecules between the surface and the bulk aqueous phase.

Wetting and Interfacial Properties of Water on the Defective Graphene

The wettability of defective graphene and its relation with the interfacial property of water have not been well tackled. By conducting molecular dynamics simulations on the interactions between water and graphene with some vacancy defects, we show that the contact angle is more sensitive to the Stone–Wales (SW) and single-vacancy (SV) defects than the double-vacancy (DV) defects. Density profiles indicate that the graphene sheet induces the interfacial water to change from a liquid into an ordered three-layer structure, which restricts the self-diffusion of water molecules seriously. Moreover, the effect of defects on the ordered liquid layer has also been discussed. Our results are helpful for controlling the wettability of graphene and developing the applications of graphene in nanodevices.

Soil water repellency: Dynamics of heterogeneous surfaces

Processes controlling the dynamics of the soil–water interfacial properties have a high impact on habitat, filter, buffer, storage, and transformation functions of a soil. Besides surface roughness and chemical heterogeneity, also the dynamics of surfaces properties in soils limit the application of well established methods of wettability determination for solid materials. Numerous studies investigated the influence of changing environmental conditions, like water content, pH and drying and wetting temperature on the repellency of soils. The present paper presents an integral approach linking some of the individual results of several studies. Two hypothetical models are suggested in order to explain differences in the nature of repellency between two types of sites and between wettable and repellent samples within each type of site. The chemical nature dominating the soil water repellency at the one type of sites can be best explained by hydrolysis–condensation reactions. The physico-chemical nature dominating the repellency on the other type of site is probably controlled by micelles- or reverse-micelles-like arrangement of amphiphilic molecules during drying. Wetting properties of the surface layers of organic coating on the soil particles then depend on number and size of amphiphilic molecules, pH and ionic strength in the soil solution. It is concluded that local site properties, e.g., soil-type, climate, or land-use, determine which mechanism controls the dynamics of repellency. Future research has to verify the suggested mechanisms before critical environmental conditions can be identified in order to prevent the negative impacts of soil water repellency.

Effects of protein conformational motions in the native form and non-uniform distribution of electrostatic interaction sites on interfacial water

Protein–water interactions and their influence on surrounding water is a long-standing problem. Despite its importance, the origin of differential water behavior at the protein surface is still elusive. We have performed molecular simulations of the protein barstar in aqueous medium. Efforts have been made to explore how the conformational motions of the protein segments in the native form and the heterogeneous electrostatic interactions with the polar and charged groups of the protein affect the interfacial water properties. The calculations reveal that reduced dimension of the hydration layer on freezing the protein’s degrees of freedom does not modify the heterogeneous water distributions around the protein. However, turning off the protein–water electrostatic contribution leads to non-preferential near-uniform water arrangements at the surface. It is further shown that with protein–water electrostatic interactions turned on, the local structuring of water molecules around the segments are correlated with their degree of exposure to the solvent.

Dynamics at a Janus Interface

Electric field effects on water interfacial properties abound, ranging from electrochemical cells to nanofluidic devices to membrane ion channels. On the nanoscale, spontaneous orientational polarization of water couples with field alignment, resulting in an asymmetric wetting behavior of opposing surfaces—a field-induced analogue of a chemically generated Janus interface. Using atomistic simulations, we uncover a new and significant field polarity (sign) dependence of the dipolar-orientation polarization dynamics in the hydration layer. Applying electric fields across a nanoparticle, or a nanopore, can lead to close to 2 orders of magnitude difference in response times of water polarization at opposite surfaces. Typical time scales are within the O(10–1) to O(10) picosecond regime. Temporal response to the field change also reveals strong coupling between local polarization and interfacial density relaxations, leading to a nonexponential and in some cases, nonmonotonic response. This work highlights the ...

Polarizability effects in molecular dynamics simulations of the graphene-water interface

The importance of including the polarizability of both water and graphene in molecular dynamics simulations of the water/graphene system was quantified. A thin film of either rigid single point charge extended (SPC/E) water or polarizable simple 4-site water model with Drude polarizability (SWM4_DP) water on non-polarizable and polarizable graphene surfaces was simulated. The graphene surface was either maintained neutral or charged, positively and negatively. The results suggest that SPC∕E and SWM4_DP water models yield very similar predictions for the water structural properties on neutral non-polarizable graphene, although they yield slightly different dynamical properties of interfacial water on neutral non-polarizable graphene. More pronounced were the differences obtained when graphene was modeled with a polarizable force field. In particular, the polarizability of graphene was found to enhance the number of interfacial SWM4_DP water molecules pointing one of their OH bonds towards the neutral surface. Despite this structural difference, the dynamical properties predicted for the interfacial SWM4_DP water were found to be independent on polarizability as long as the polarizability of a carbon atom is smaller than α = 0.878 A. On charged graphene surfaces, the effect of polarizability of graphene on structural properties and some dynamical properties of SWM4_DP water is negligible because electrostatic forces due to surface charge dominate polarization forces, as expected. For all cases, our results suggest that the hydrogen bond network is insensitive to the polarizability of both water and graphene. Understanding how these effects will determine the accumulation of ions near neutral or charged graphene could have important implications for applications in the fields of energy storage and water desalination.

Study of the Air–Water Interfacial Properties of Biodegradable Polyesters and Their Block Copolymers with Poly(ethylene glycol)

It has been reported that the surface pressure-area isotherm of poly(D,L-lactic acid-ran-glycolic acid) (PLGA) at the air-water interface exhibits several interesting features: (1) a plateau at intermediate compression levels, (2) a sharp rise in surface pressure upon further compression, and (3) marked surface pressure-area hysteresis during compression-expansion cycles. To investigate the molecular origin of this behavior, we conducted an extensive set of surface pressure and AFM imaging measurements with PLGA materials having several different molecular weights and also a poly(D,L-lactic acid-ran-glycolic acid-ran-caprolactone) (PLGACL) material in which the caprolactone monomers were incorporated as a plasticizing component. The results suggest that (i) the plateau in the surface pressure-area isotherm of PLGA (or PLGACL) occurs because of the formation (and collapse) of a continuous monolayer of the polymer under continuous compression; (ii) the PLGA monolayer becomes significantly resistant to compression at high compression because under that condition the collapsed domains become large enough to become glassy (such behavior was not observed in the nonglassy PLGACL sample); and (iii) the isotherm hysteresis is due to a coarsening of the collapsed domains that occurs under high-compression conditions. We also investigated the monolayer properties of PEG-PLGA and PEG-PLGACL diblock copolymers. The results demonstrate that the tendency of PLGA (or PLGACL) to spread on water allows the polymer to be used as an anchoring block to form a smooth biodegradable monolayer of block copolymers at the air-water interface. These diblock copolymer monolayers exhibit protein resistance.

Phase equilibrium and interfacial properties of water + CO2 mixtures

Carbon dioxide (CO2) has a high industrial relevance. From all these, the mixtures of CO2 with water show great importance due to their potential in carbon capture and storage systems. It is therefore important to be able to model both phase equilibrium and interfacial properties of this system in a proper manner. In order to do this, the density gradient theory can be combined with a suitable equation of state. This leads to calculations of both phase equilibrium and interfacial properties of the mixture. In this particular case, the polar perturbed chain statistical associating fluid theory (PCP-SAFT) was used for this work. The proposed framework allows the inclusion of some relevant properties of the compounds, such as association of water and the orientation effects of molecules due to the quadrupole moment of CO2. Phase and interfacial properties (e.g. interfacial tension) are then calculated and compared with data from the literature, in vapor–liquid (VLE) and liquid–liquid (LLE) equilibrium systems. The calculations show good agreement with the experimental data from other authors.

Molecular Structure and Dynamics in Thin Water Films at Metal Oxide Surfaces: Magnesium, Aluminum, and Silicon Oxide Surfaces

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, SiO2, Al2O3) 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 contac...

Molecular Dynamics Simulations of Water Structure and Diffusion in Silica Nanopores

We present molecular dynamics (MD) simulations of water-filled silica nanopores such as those that occur in ordered oxide ceramics (MCM-41, SBA-15), controlled pore glasses (such as Vycor glass), mesoporous silica, bioglasses, and hydrous silica gel coatings of weathered minerals and glasses. Our simulations overlap the range of pore diameters (1–4 nm) where confinement causes the disappearance of bulk-liquid-like water. In ≥2 nm diameter pores, the silica surface carries three statistical monolayers of density-layered water, interfacial water structure is independent of confinement or surface curvature, and bulk-liquid-like water exists at the center of the pore (this last finding contradicts assumptions used in most previous neutron diffraction studies and in several MD simulation studies of silica nanopores). In 1 nm diameter pores, bulk-liquid-like water does not exist and the structural properties of interfacial water are influenced by confinement. Predicted water diffusion coefficients in 1–4 nm diameter pores agree with quasi-elastic neutron scattering (QENS) data and are roughly consistent with a very simple “core–shell” conceptual model whereupon the first statistical water monolayer is immobile and the rest of the pore water diffuses as rapidly as bulk liquid water.

Interfacial and emulsifying behaviour of rice protein concentrate

Interfacial and emulsifying properties of rice protein concentrate (RPC) have been studied in order to evaluate its potential application to stabilize O–W emulsions. The interfacial behaviour of adsorbed proteins films constituted with RPC has been studied at the air–water and oil–water interfaces at two pH values (2 and 8). The type and the amount of soluble proteins have been determined in aqueous dispersions and results put forward the presence of most frequent rice protein profile and a significant degree of protein denaturation with a very low solubility. Air–water and oil–water interfacial properties have been determined as a function of time, concentration and pH: air–water by surface pressure under compression–expansion cycles and oil–water by interfacial tension. Interfacial rheology has been studied under dilatational deformations, either at the initial step of film formation or once the interfacial tension was at equilibrium (the film was completely formed). RPC-stabilised O–W emulsions has been also analysed by Droplet Size Distribution (DSD) measurements and interfacial protein concentration. Both interfacial and bulk emulsion properties reveal that RPC showed an enhanced potential as emulsifier at low pH. Globally; results indicate clearly important differences in the structural characteristics of rice protein films between pH 2 and 8 that impact on emulsifying properties.

Nonlinear Light Scattering and Spectroscopy of Particles and Droplets in Liquids

Nano- and microparticles have optical, structural, and chemical properties that differ from both their building blocks and the bulk materials themselves. These different physical and chemical properties are induced by the high surface-to-volume ratio. As a logical consequence, to understand the properties of nano- and microparticles, it is of fundamental importance to characterize the particle surfaces and their interactions with the surrounding medium. Recent developments of nonlinear light scattering techniques have resulted in a deeper insight of the underlying light-matter interactions. They have shed new light on the molecular mechanism of surface kinetics in solution, properties of interfacial water in contact with hydrophilic and hydrophobic particles and droplets, molecular orientation distribution of molecules at particle surfaces in solution, interfacial structure of surfactants at droplet interfaces, acid-base chemistry on particles in solution, and vesicle structure and transport properties.

Mesoscale modeling of the water liquid-vapor interface: A surface tension calculation

We report a mesoscale modeling of the liquid-vapor interface of water. A mesoscopic model of water has been established in dissipative particle dynamics (DPD) to reproduce the interfacial properties of water. The surface tension and coexisting densities are compared between atomistic and mesoscopic simulations. Simple scaling relations have been established to link the atomistic and mesoscopic length and time scales. Our study demonstrates the capability of the DPD method to explore the interfacial properties of a planar water liquid-vapor interface and a water nanodroplet. This constitutes an important step toward the calculation of the surface tension of larger and more complex interfacial systems.

Structure and Orientation of Interfacial Water Determine Atomic Force Microscopy Results: Insights from Molecular Dynamics Simulations

Massive all-atom molecular dynamics simulations were employed to study hydration forces near α-Al2O3 (0001) surfaces as sampled during a hypothetical AFM force spectroscopy experiment conducted using a (28,0) single-walled carbon nanotube as the tip at ambient conditions. The results provide the force acting on the carbon nanotube tip, as well as detailed properties of interfacial water, as a function of the nanotube-surface distance. As the tip approaches the solid substrate, interfacial water undergoes conformational and structural changes. These changes are responsible for the features observed in the force profiles, including the range at which forces can be measured (up to two hydration shells), the intensity of the forces experienced by the AFM tip, and their oscillatory character. Our detailed analysis shows that heterogeneous surface chemical composition results in appreciably different force profiles. This observation may explain the variability of AFM data sampling hydration forces even on atomically smooth substrates. In addition, our results suggest that sufficiently accurate AFM force spectroscopy could be used to study how hydration forces depend on surface heterogeneous properties and on the orientation and local density of interfacial water, which could aid our understanding of interfacial phenomena and lead to significant scientific breakthroughs.

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.

Molecular Dynamics Studies of Interfacial Water at the Alumina Surface

Interfacial water properties at the alumina surface were investigated via all-atom equilibrium molecular dynamics simulations at ambient temperature. Al-terminated and OH-terminated alumina surfaces were considered to assess the structural and dynamic behavior of the first few hydration layers in contact with the substrates. Density profiles suggest water layering up to ∼10 A from the solid substrate. Planar density distribution data indicate that water molecules in the first interfacial layer are organized in well-defined patterns dictated by the atomic terminations of the alumina surface. Interfacial water exhibits preferential orientation and delayed dynamics compared to bulk water. Water exhibits bulk-like behavior at distances greater than ∼10 A from the substrate. The formation of an extended hydrogen bond network within the first few hydration layers illustrates the significance of water−water interactions on the structural properties at the interface.

Calculation of Local Water Densities in Biological Systems: A Comparison of Molecular Dynamics Simulations and the 3D-RISM-KH Molecular Theory of Solvation

Water plays a unique role in all living organisms. Not only is it nature's ubiquitous solvent, but it also actively takes part in many cellular processes. In particular, the structure and properties of interfacial water near biomolecules such as proteins are often related to the function of the respective molecule. It can therefore be highly instructive to study the local water density around solutes in cellular systems, particularly when solvent-mediated forces such as the hydrophobic effect are relevant. Computational methods such as molecular dynamics (MD) simulations seem well suited to study these systems at the atomic level. However, due to sampling requirements, it is not clear that MD simulations are, indeed, the method of choice to obtain converged densities at a given level of precision. We here compare the calculation of local water densities with two different methods: MD simulations and the three-dimensional reference interaction site model with the Kovalenko-Hirata closure (3D-RISM-KH). In particular, we investigate the convergence of the local water density to assess the required simulation times for different levels of resolution. Moreover, we provide a quantitative comparison of the densities calculated with MD and with 3D-RISM-KH and investigate the effect of the choice of the water model for both methods. Our results show that 3D-RISM-KH yields density distributions that are very similar to those from MD up to a 0.5 Å resolution, but for significantly reduced computational cost. The combined use of MD and 3D-RISM-KH emerges as an auspicious perspective for efficient solvent sampling in dynamical systems.

The effects of oil displacement agents on the stability of water produced from ASP (alkaline/surfactant/polymer) flooding

Alkaline/surfactant/polymer (ASP) flooding technology has been successfully used in Chinese oil fields, such as Daqing and Shengli. However, water produced from ASP flooding contains large quantities of residual chemicals (alkali, surfactant and polymer) making it a complex and stable emulsion system which is difficult to treat. The emulsion stability of water produced from ASP flooding was investigated by conducting settling experiments and measuring the oil–water interfacial properties. The experimental results showed that the addition of polymer (HPAM, hydrolyzed polyacrylamide) degrades the emulsion stability when its concentration is below 300 mg/L for the 1.2 × 10 7 MW polymer, and 800 mg/L for the 3.0 × 10 6 MW polymer. But it enhances the emulsion stability when polymer concentrations are above those levels. At low polymer concentrations, flocculation induced by the polymer on oil droplets in the produced water is the dominant factor, while at high polymer concentrations the produced water viscosity plays an important role in the emulsion stability. The adsorption of surfactant on the oil–water interface increases the zeta potentials and decreases interfacial tension, and thus remarkably enhances the emulsion stability. Furthermore, the emulsion stability is enhanced gradually with the increase of NaOH concentration up to 300 mg/L due to the increase of zeta potentials and decrease of interfacial tension, and then weakened with the further increase of NaOH concentration, which is attributed to the decreased strength of the interfacial film. A pilot experiment for the treatment of simulated water was done, and the result showed that the simulated produced water from ASP can be successfully treated by using a leaching solution of alkaline white mud.