Mechanism Of Water Permeation Research Articles

Molecular dynamics simulation of water permeation mechanism in polymer grouting material

Polyurethane (PU) grouting materials are commonly used in infrastructure trenchless rehabilitation projects and are frequently exposed to water environments. However, the underlying microscopic mechanism of polymer damage caused by water infiltration has not been systematically investigated. In this paper, we employed molecular dynamics (MD) simulation to explore the wetting behavior of water on the PU surface and the permeation behavior of PU under pressure. The results showed that when sufficiently high water pressure is applied, PU deforms and creates voids. Subsequently, water molecules begin to penetrate the interior of the PU. For PU models with thicknesses of 10 Å, 15 Å, and 20 Å, penetration behavior was observed at water pressures of 189 MPa, 256 MPa and 314 MPa, respectively. Moreover, with PU models of identical thickness, the depth of water molecules penetration and the volumetric water content of the PU increased with rising water pressure. Furthermore, it was observed that PU has many small interatomic pores in its free state. Under the influence of water, distribution of these pores deviates from a normal distribution and becomes irregular. These findings provide valuable insights into the microscopic mechanism underlying infiltration damage of PU grouting materials at the molecular level.

SWEET family transporters act as water conducting carrier proteins in plants.

Dedicated water channels are involved in the facilitated diffusion of water molecules across the cell membrane in plants. Transporter proteins are also known to transport water molecules along with substrates, however the molecular mechanism of water permeation is not well understood in plant transporters. Here, we show plant sugar transporters from the SWEET (Sugar Will Eventually be Exported Transporter) family act as water-conducting carrier proteins via a variety of passive and active mechanisms that allow diffusion of water molecules from one side of the membrane to the other. This study provides a molecular perspective on how plant membrane transporters act as water carrier proteins, a topic that has not been extensively explored in literature. Water permeation in membrane transporters could occur via four distinct mechanisms which form our hypothesis for water transport in SWEETs. These hypothesis are tested using molecular dynamics simulations of the outward-facing, occluded, and inward-facing state of AtSWEET1 to identify the water permeation pathways and the flux associated with them. The hydrophobic gates at the center of the transport tunnel act as a barrier that restricts water permeation. We have performed in silico single and double mutations of the hydrophobic gate residues to examine the changes in the water conductivity. Surprisingly, the double mutant allows the water permeation to the intracellular half of the membrane and forms a continuous water channel. These computational results are validated by experimentally examining the transport of hydrogen peroxide molecules by the AtSWEET family of transporters. We have also shown that the transport of hydrogen peroxide follows the similar mechanism as water transport in AtSWEET1. Finally, we conclude that similar water-conduction states are also present in other SWEET transporters due to the high sequence and structure conservation exhibited by this transporter family.

Understanding interfacial polymerization in the formation of polyamide RO membranes by molecular simulations

Since the water permeation mechanisms of polyamide membranes are highly dependent on the micro-structure of polyamide membranes, a deeper understanding of the formation process of them is necessary for the optimization of the membrane performance. As most simulation works construct the polyamide membranes in an ideal way, the interfacial diffusion of monomers, which is crucial for the formation of polyamide membranes, is usually ignored. To address this issue, we mimic the experimental conditions to develop an atomic model of a highly crosslinked polyamide membrane by conducting molecular dynamics simulations. Via tuning the concentration and molar ratio of monomers, the diffusion of monomers and its influence on the subsequent reaction are altered, and the final membrane structure consequently changed. Simulation results reveal that increasing the trimesoyl chloride concentration results in thicker membranes with a reduced specific surface area and consequently decreased water permeance. On the other hand, increasing the m-phenylenediamine concentration will accelerate the reaction rate and reduce the final crosslinking degree. A deeper understanding of the mechanism of polyamide-membrane formation is unveiled in this work, which can aid in the design of high-performance polyamide membranes in the future.

Molecular dynamics investigation on seawater desalination mechanism driven by external pressure through porous graphene membranes

The seawater desalination through porous graphene membrane is considered as promising candidate technology for water purification, but the desalination mechanism is seldom concerned at nanoscale. In this study, the microstructures of pressure driven seawater desalination based on porous graphene membrane was established, and the desalination performance was studied by means of molecular dynamics simulations. Based on the observed dynamic evolution of the water molecules transportation behaviors and microstructures, the salt rejection and water permeation mechanism were proposed. The effects of graphene pore diameter, external pressure and temperature on the desalination performance were investigated. The results indicate that the water flux can be increased by 67.20% when the external pressure increases from 100 MPa to 200 MPa. Under the external pressure of 300 MPa, when the diameter of graphene pore increased from 1 nm to 2 nm and 3 nm, the number of filtered water molecule is increased by 48.83% and 67.33%, respectively. The filtered water molecules can be increased by 4.06% with the increase of temperature from 300 K to 360 K. The possible desalination mechanism was explored and revealed by us, which is expected to be helpful for efficient seawater desalination.

Investigation of the influence of the location of oxygenated functional groups in graphene nanostructures on water permeation via molecular dynamics simulations

The oxygenated functional groups are typically distributed on the basal plane in graphene oxide (GO) but only exist at the plane edges in graphene nanoplatelets (GNPs). Experimentally, the water permeability of GNPs outperforms that of GO despite the larger interlayer spacing and higher oxidation level of the latter. Here, we employ molecular dynamics (MD) simulations to investigate this contradiction. The MD results align with the experimental results and they can be explained by scrutinizing the trajectories of water molecules and the characteristics of hydrogen bonds. It shows that water molecules encounter a much lower energy barrier in the GNPs during the permeation process where the translation paths and wandering time of water molecules through the GNPs nanostructures are shorter. There exists a threshold amount of the hydroxyls in both GO and GNPs, retarding the water transport. For GO, exceeding the threshold amount induces the pinning effect where the water molecules are pinned by the hydroxyls while for GNPs, the isolation effect which deteriorates the hydrogen bonding strength between the hydroxyls and the water molecules is induced. Therefore, two distinctive water permeation mechanisms prevail due to the location of the oxygenated functional groups in the graphene nanostructures.

Performance enhancement of subcooled flow boiling on graphene nanostructured surfaces with tunable wettability

Modifying the surface wettability at the solid-liquid interface can significantly improve flow boiling heat transfer. The graphene nanoplatelets (GNPs) and epoxy additives form nanocomposites coatings with tunable wettability (superhydrophilic, superhydrophobic and hydrophobic) via a thermal curing process. They are applied for enhancing the subcooled flow boiling heat transfer in a minichannel. By comparing with the uncoated surface, the superhydrophilic GNPs coating shows the highest boiling heat transfer coefficient with a maximum enhancement of 64.9 %. The nanocapillary-networks of the superhydrophilic GNPs coatings facilitate the fast water permeation effect and induce an effective filmwise boiling heat transfer. Moreover, the superhydrophilic GNPs coatings facilitate bubble detachment process and suppress the formation of dry spot during the boiling process. On the contrary, the hydrophobic and superhydrophobic GNPs coatings act as impervious blankets for the intercalation of water and deteriorate the flow boiling rates. The effects of the subcooled degree and the mass flux of the bulk flow on the performance enhancement are also investigated. This study aims to investigate the flow boiling enhancement attributed to the graphene functionalized coatings and provides interesting insights of the ultrafast water permeation mechanism of graphene into the subcooled flow boiling heat transfer.

Vapor phase esterification of acetic acid with ethanol in a CHA zeolite membrane reactor: A CFD analysis

The combination of new technologies for achieving more efficient and sustainable processes due to their mutual integration is gaining particular interest in industry. In this study, the role of vapor membrane permeation technology in enhancing the catalytic performance of esterification of acetic acid with ethanol was considered. A computational fluid dynamic model was developed using COMSOL Multiphysics 5.4 as a software to predict the behavior of a zeolite membrane reactor used for carrying out the aforementioned esterification reaction. Reliable esterification reaction kinetics and water permeation mechanism through a zeolite membrane were used in the CFD model, comparing from a theoretical point of view the performance of a zeolite membrane reactor with an equivalent traditional reactor. After model validation, this theoretical study allowed to evaluate the effect of different operating parameters (operating pressure between 1 and 5 bar, feed molar ratio between 1 and 10, feed flow rate between 1.0·10−6 and 4.0·10−5 mol/s) on the performance of both the reactors. The theoretical predictions demonstrated the superiority of the zeolite membrane reactor over the traditional reactor during the acetic acid esterification with ethanol, reaching as best simulation results 99% AcOH conversion and 92% water vapor removal.

Computational Study of the Ion and Water Permeation and Transport Mechanisms of the SARS-CoV-2 Pentameric E Protein Channel.

Coronavirus disease 2019 (COVID-19) is caused by a novel coronavirus (SARS-CoV-2) and represents the causative agent of a potentially fatal disease that is a public health emergency of international concern. Coronaviruses, including SARS-CoV-2, encode an envelope (E) protein, which is a small, hydrophobic membrane protein; the E protein of SARS-CoV-2 shares a high level of homology with severe acute respiratory syndrome coronavirus (SARS-CoV). In this study, we provide insights into the function of the SARS-CoV-2 E protein channel and the ion and water permeation mechanisms using a combination of in silico methods. Based on our results, the pentameric E protein promotes the penetration of cation ions through the channel. An analysis of the potential mean force (PMF), pore radius and diffusion coefficient reveals that Leu10 and Phe19 are the hydrophobic gates of the channel. In addition, the pore exhibits a clear wetting/dewetting transition with cation selectivity under transmembrane voltage, indicating that it is a hydrophobic voltage-dependent channel. Overall, these results provide structure-based insights and molecular dynamic information that are needed to understand the regulatory mechanisms of ion permeability in the pentameric SARS-CoV-2 E protein channel.

Simultaneous Measurements of Structure and Water Permeability in an Isolated Human Skin Stratum Corneum Sheet

Stratum corneum (SC), the outermost layer of human skin, acts as an intelligent physicochemical interface between the inside and the outside of our body. To make clear the relationship between structure and physical barrier properties of SC, we developed a method that enables us to simultaneously acquire X-ray diffraction (XD) patterns and transepidermal water loss (TEWL) values using a spread SC sheet isolated from human skin. The synchrotron X-ray was incident on the SC sheet surface at an angle of 45° to avoid interference between the two kinds of measurements. Detailed comparison between XD and TEWL data suggested that the thermal behavior of water permeability is closely related to the thermal expansion of the lattice spacings of the hexagonal phases above 40 °C and to the existence ratio of the orthorhombic phase below 40 °C. Thus, the new method we developed can give useful information on the mechanism of water permeation in SC without ambiguity caused by separate measurements of structure and water permeability with different samples.

Humidity-induced significant microstructural reordering in partially reduced graphene oxide: Insights on water permeation mechanism

Interaction of water and water-based solvents with graphene oxide (GO) has attracted much attention, due to the ability of GO to serve as a highly effective water filtration membrane. In this work, we study the evolution of the structure of GO in a partially reduced form, before and after being hydrated in high humidity conditions. X-ray diffraction (XRD) studies reveal that progressive thermal reduction leads to the increase in the microstructural disorder in the stacking of GO flakes. However, upon hydration of partially reduced GO, microstructural ordering is revealed. This ordered state is characterized by two XRD peaks with substantially smaller full-width-at-half-maximum (FWHM), when compared to the pre-hydration state. The peak corresponding to the sp3 regions has larger d-spacing of ∼9.7 Å and an FWHM ∼6 times smaller compared to pre-hydration state, while the other peak corresponds to the ordered sp2 regions with a d-spacing of ∼3.3 Å, observed at the characteristic graphitic peak position. Gravimetry studies on suspended films reveal both accelerated and diminished water permeation rates upon annealing when compared to unreduced GO films, which can be attributed to void-assisted permeation in the microstructurally disordered films. The hydrated films in a similar way show a permeation behavior that involves either the increase or decrease in water permeation rates in comparison with pre-hydrated samples. We reconcile to the gravimetry outcomes by suggesting the possibilities of both super-permeating channels and void assisted permeation, and the contribution of each of the mechanisms to the permeation flux.

Frost Self-Removal Mechanism during Defrosting on Vertical Superhydrophobic Surfaces: Peeling Off or Jumping Off

Although a superhydrophobic surface has great potential to delay frosting, it tends to become frosted under humid conditions and needs to defrost periodically. So far, the exact mechanism of defrosting still remains unclear. Here, we investigate the frost self-removal mechanism during defrosting on vertical superhydrophobic surfaces. Two self-removal modes are observed: peeling off and jumping off. When the frost thickness is larger than a threshold value, peeling off mode occurs; otherwise, jumping off mode takes place. Compared with the peeling off mode, the jumping off mode is less effective in self-removing frost as jumping is limited by energy transformation. A theoretical model based on frost melting-water permeation mechanism is proposed to determine the threshold value of frost thickness. According to this model, the threshold value of the frost thickness is dependent on the frost porosity and the surface temperature (or heat flux). For our particular experiments, the threshold value of the frost thickness predicted by the proposed model agrees well with our experimental results. Our work may advance the defrosting applications of superhydrophobic surfaces in related engineering fields.

Dissipative Particle Dynamics Simulation for Clarifying Mechanism of Water Permeation in Polymeric Membranes

Polymeric membranes are widely used in engineering fields such as fuel cells and heat storage materials. The polymeric membrane has two properties, which are selective permeability and barrier. The selective permeation mean to let only certain substances pass through porimeric membrane. For example, it is used for fuel cells. The polymeric membrane used in fuel cells allow protons to pass through, but does not water and oxygen. Next, barrier is to not permeate anything. This is used for a microcapsule of a heat storage material and so on. The microcapsule is made of polymers and designed as substances in capsules are not discharged from it. In future, It will be used as CO2 separation technology or seawater desalination, which will be anticipated to solve global issues. However, the prediction and control of penetration into a membrane are still challenging subjects. In this research, we aimed to elucidate the mechanism of the water permeation. because water permeation is most basic. We performed molecular simulations of water transport through a polymeric membrane to clarify the key factors that dominate water permeation. We employed the dissipative particle dynamics (DPD) method. DPD method is a powerful mesoscopic simulation tool which enables the simulation of events occurring in millisecond timescales and on micrometre length scales. Since its introduction a decade ago, this method has been used to model various membrane cell systems, including the self-assembly of amphiphilic molecules interaction with rigid nanoparticles, and the effect of ligands and receptors in the membrane. Therefore, this method is suitable to simulate polymeric membrane system. we used model of actual molecules (COP (membrane polymer), sodium acetate (additive) and water). To determine the interaction parameters of the simulation models, we used the J-OCTA simulation software. We first computed the time-averaged mean square displacement (MSD) of water molecules to evaluate water permeability into a polymeric membrane. Then, MSD was also calculated for the additive system. Comparing of the systems, it was found that the water permeability decrease by adding additives (figure). The results were also reproduced in experiments and we get qualitatively same results. We found that additives enable the reduction of the water permeability into a polymeric membrane. In addition, we discussed polymer synthesis guideline decreasing water permeability. Figure 1

Water permeation across artificial I-quartet membrane channels: from structure to disorder.

Artificial water channels (AWCs) have been designed for water transport across membranes with the aim to mimic the high water permeability observed for biological systems such as aquaporins (∼108-109 water molecules per s per channel), as well as their selectivity to reject ion permeation at the same time. Recent works on designed self-assembling alkylureido-ethylimidazole compounds forming imidazole-quartet channels (I-quartets), have shown both high water permeability and total ionic-rejection. I-quartets are thus promising candidates for further development of AWCs. However, the molecular mechanism of water permeation as well as I-quartet organization and stability in a membrane environment need to be fully understood to guide their optimal design. Here, we use a wide range of all-atom molecular dynamics (MD) simulations and their analysis to understand the structure/activity relationships of the I-quartet channels. Four different types with varying alkyl chain length or chirality have been studied in a complex fully hydrated lipid bilayer environment at both microsecond and nanosecond scale. Microsecond simulations show two distinct behaviors; (i) two out of four systems maintain chiral dipolar oriented water wires, but also undergo a strong reorganization of the crystal shape, (ii) the two other I-quartet channels completely lose the initial organization, nonetheless keeping a water transport activity. Short MD simulations with higher time resolution were conducted to characterize the dynamic properties of water molecules in these model channels and provided a detailed hypothesis on the molecular mechanism of water permeation. The ordered confined water was characterized with quantitative measures of hydrogen-bond life-time and single particle dynamics, showing variability among I-quartet channels. We will further discuss the underlying assumptions, currently based on self-aggregation simulations and crystal patches embedded in lipid bilayer simulations and attempt to describe possible alternative approaches to computationally capture the water permeation mechanism and the self-assembly process of these AWCs.

On the design of graphene oxide nanosheets membranes for water desalination

According to current researches, graphene oxide (GO) membranes show promising desalination properties due to ease of synthesis, low production cost, and high efficiency. There are several experimental works to study ionic sieving properties of GO membranes. However, it is difficult to characterize atomistic mechanism of water permeation and ion rejection by experimental approaches. On the other hand, there exist a few reports in which the atomistic picture of water permeation across GO membranes is investigated by means of molecular dynamics (MD) simulation. In the present work, in addition to water desalination, the atomic scale mechanism of ion rejection is studied using large scale MD simulation. For this purpose, surface color maps based on the potential of mean force (PMF) are computed between GO nanosheets to indicate interaction between functional groups and existing species in saline water. The radial distribution function (RDF) between water molecules and functional groups are measured to study the disordering of water molecules between GO nanosheets. Furthermore, the effect of different layers separation value and applied pressure are examined to explore the optimal design of GO membranes. According to our simulation results, the oxygen atoms in hydroxyl and epoxide groups play an important role in rejection of the Cl ions and attraction of the Na ions. The hydroxyl groups have the most impact on disordering of the water molecules between GO membranes. In addition, our designed GO membrane, showed a water permeability of one to three orders of magnitude higher than commercial reverse osmosis membranes.

High-performance thin-film encapsulation for organic light-emitting diodes

Organic light-emitting diodes degrade rapidly by means of local cathode oxidation when exposed to the ambient atmosphere, resulting in visible non-emissive areas called black spots. High performance inorganic based encapsulations are required to protect the OLED. We have applied a hybrid thin-film encapsulation stack consisting of two inorganic barrier layers of silicon nitride deposited at low temperature with an organic layer in between. The resulting water permeation mechanism into the OLED is solely by means of lateral pinhole-to-pinhole transport. With the application of CaO nanoparticles in the organic layer the lateral water transport rate is reduced and we show that black spot formation in 8 cm2 OLEDs is delayed by 6000 h at accelerated climate conditions of 60°C/90% relative humidity. This is estimated to correspond to 20 years at ambient conditions.

Gas Diffusion Barriers Prepared by Spatial Atmospheric Pressure Plasma Enhanced ALD.

In this work, we report on aluminum oxide (Al2O3) gas permeation barriers prepared by spatial ALD (SALD) at atmospheric pressure. We compare the growth characteristics and layer properties using trimethylaluminum (TMA) in combination with an Ar/O2 remote atmospheric pressure plasma for different substrate velocities and different temperatures. The resulting Al2O3 films show ultralow water vapor transmission rates (WVTR) on the order of 10-6 gm-2d-1. In notable contrast, plasma based layers already show good barrier properties at low deposition temperatures (75 °C), while water based processes require a growth temperature above 100 °C to achieve equally low WVTRs. The activation energy for the water permeation mechanism was determined to be 62 kJ/mol.

Molecular mechanism of water permeation in a helium impermeable graphene and graphene oxide membrane.

Layers of graphene oxide (GO) are found to be good for the permeation of water but not for helium (Science, 2012, 335(6067), 442-444) suggesting that the GO layers are dynamic in the formation of a permeation route depending on the environment they are in (i.e., water or helium). To probe the microscopic origin of this observation we calculate the potential of mean force (PMF) of GO sheets (with oxidized and reduced parts), with the inter-planar distance as a reaction coordinate in helium and water. Our PMF calculation shows that the equilibrium interlayer distance between the oxidized part of the GO sheets in helium is at 4.8 Å leaving no space for helium permeation. In contrast, the PMF of the oxidized part of the GO in water shows two minima, one at 4.8 Å and another at 6.8 Å, corresponding to no water and a water filled region, thus giving rise to a permeation path. The increased electrostatic interaction between water with the oxidized part of the sheet helps the sheet open up and pushes water inside. Based on the entropy calculations for water trapped between graphene sheets and oxidized graphene sheets at different inter-sheet spacings, we also show the thermodynamics of filling.

Structural Determinants of Water Permeation through the Sodium-Galactose Transporter vSGLT

Sodium-glucose transporters (SGLTs) facilitate the movement of water across the cell membrane, playing a central role in cellular homeostasis. Here, we present a detailed analysis of the mechanism of water permeation through the inward-facing state of vSGLT based on nearly 10 μs of molecular dynamics simulations. These simulations reveal the transient formation of a continuous water channel through the transporter that permits water to permeate the protein. Trajectories in which spontaneous release of galactose is observed, as well as those in which galactose remains in the binding site, show that the permeation rate, although modulated by substrate occupancy, is not tightly coupled to substrate release. Using a, to our knowledge, novel channel-detection algorithm, we identify the key residues that control water flow through the transporter and show that solvent gating is regulated by side-chain motions in a small number of residues on the extracellular face. A sequence alignment reveals the presence of two insertion sites in mammalian SGLTs that flank these outer-gate residues. We hypothesize that the absence of these sites in vSGLT may account for the high water permeability values for vSGLT determined via simulation compared to the lower experimental estimates for mammalian SGLT1.

Insight into the Mechanism of Water Permeation through the Sodium-Galactose Transporter vSGLT from Long Molecular Dynamics Simulations

Sodium-glucose transporters (SGLTs) mediate sugar transport as well as water flow across the cell membrane. Using a series of long molecular dynamics simulations generated on the special-purpose Anton supercomputer totaling nearly 16 microseconds in aggregate, we investigated the mechanism of water permeation through the inward-facing state of the bacterial sugar transporter vSGLT from Vibrio parahaemolyticus. Our simulations reveal conformational changes in the extracellular gate that create a transient water channel through the transporter. We developed a novel channel detection algorithm to characterize water pathways through the protein and identify key residues that control water flow. The simulations contain multiple, independent instances in which the galactose, initially bound to the transporter, spontaneously exits to solution. By comparing water flow through vSGLT when the sugar is bound, exiting and free in solution, we show that water flow, while modulated by galactose occupancy, is not coupled to substrate release.

A mechanistic study on amphiphilic block co-polymer poly(butadiene-b-(ethylene oxide)) vesicles reveals the water permeation mechanism through a polymeric bilayer

The mechanistic study of a diblock copolymer reveals that the osmotic permeation mechanism correlates with chain rigidity, specifically bending rigidity modulus.