Hydrophilic Nanopores Research Articles

Confinement Effects on Proton Transfer in TiO2 Nanopores from Machine Learning Potential Molecular Dynamics Simulations.

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Study on the slip behavior of CO2-crude oil on nanopore surfaces with different wettability

Unconventional reservoirs are characterized by a significant amount of micro and nanopores, which pose a critical challenge in accurately assessing the microscopic fluid transport behaviors to enhance oil and gas recovery in such reservoirs. CO2 injection is a promising recovery method for unconventional resources, hence the transport mechanism of CO2-crude oil at the nanoscale deserves further exploration. In this work, the flow and transport behaviors of CO2 and multicomponent crude oil in hydrophilic, neutral, and hydrophobic nanopores were investigated by the molecular dynamics simulation (MD) method. The CO2-crude oil shows a negative slip flow on hydrophilic surfaces, while it has a slip flow on neutral and hydrophobic surfaces. By illustrating the distribution structure of each component in the fluid and the solid-liquid interaction energy, the inherent causes of the slip mechanism of different wettability surfaces were revealed. The transport capacity and permeability of CO2-crude oil in the pores with different wettability were investigated. Modifying the Hagen-Poiseuille equation based on slip characteristics can effectively improve the prediction ability of nanopore permeability. This study is helpful to understand the flow and slip behavior of CO2-crude oil in nanopores of unconventional reservoirs, which provides a theoretical basis for accurately predicting the migration of reservoir fluid in nanopores.

Highly efficient B(OH)3 removal by single-layered graphene membrane with embedded crown nanopores

Trace boron in seawater, usually presented as boric acid (B(OH)3), should be firstly transformed into ionic state, in desalination plants, which represents high energy consumption and high cost. Therefore, direct removal of B(OH)3 is significant and energy-efficient for industrial boron removal, which is still a great challenge. A very recent work [Desalination, 533 (2022) 115755] provides strong theoretical guidance on making efficient boron removal devices utilizing 2D materials with hydrophilic pores. However, limited effort has devoted to explore other 2D porous membrane aiming at further improving the performance of boron acid removal. Herein, using molecular dynamics (MD) simulation, we investigated the effect of single-layered graphene nanosheet embedded with hydrophilic crown nanopores on the direct removal of boric acid. We construct six typical graphene crown pores with varied pore dimensions. Our MD results demonstrate that the utilized graphene crown pores show high water permeability, simultaneously accompanied by an outstanding B(OH)3 rejection (mostly over 90%). The water permeabilities through the crown pores achieve a highest value of 6.01 L cm−2 day−1 MPa−1. Remarkably, the water fluxes in each pore are evaluated to be 2.15 ∼ 20.1 × 1013 per day per MPa per pore, featuring 1–2 orders of magnitude improvement compared with the previous best report of 2-dimensional nanopores, with the comparable B(OH)3 rejection. Free energy calculations indicate that a water molecule is energetically more favorable to transport through graphene crown nanopore than B(OH)3. Our findings highlight that a porous graphene embedded with hydrophilic crown nanopores can be not only used for efficaciously removing B(OH)3 but also enable the fast water flow, which is desirable in desalination plants.

Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores.

The dynamical properties of water molecules confined in the unidirectional hydrophilic nanopores of AlPO4-54 are investigated with quasi-elastic neutron scattering as a function of temperature down to 118 K. AlPO4-54 has among the largest pores known for aluminophosphates and zeolites (about 1.3 nm), though they are small enough to prevent water crystallization due to the high degree of confinement. Water molecular diffusion into the pore is here measured down to 258 K. Diffusion is slower than in bulk water and has an activation energy of Ea = (20.8 ± 2.8) kJ/mol, in agreement with previous studies on similar confining media. Surprisingly, local hydrogen dynamics associated with water reorientation is measured down to temperatures (118 K), i.e., well below the expected glass transition temperature of bulk water. The reorientational time scale shows the well-known non-Arrhenius behavior down to the freezing of water mass diffusion, while it shows a feeble temperature dependence below. This fast local dynamics, of the order of fractions of nanoseconds, is believed to take place in the dense, highly disordered amorphous water occupying the pore center, indicating its possible plastic nature.

Investigation on gas/water two-phase flow in quartz nanopores from molecular perspectives

Understanding the gas/water two-phase occurrence, flow patterns, and underlying mechanisms in quartz nanopores is crucial for gas transport and production in shale reservoirs. In this study, we investigated the flow characteristics of methane and water in hydroxylated quartz nanopores employing the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations and focused on elucidating the effects of water saturation, pressure gradient and ion concentration in various aperture nanopores. Results show that water molecules preferentially adsorb on the hydrophilic quartz surface to form water film, increasing methane flow friction and reducing its slippage velocity significantly. Water bridges are formed to wrap gas bubbles and hinder the gas flow with increased water saturation. The water phase occupies a larger pore space with increasing water saturation resulting in an increase in methane effective viscosity and a decrease in apparent permeability. The water bridge is broken under the joint action of external force and gas driving when the pressure gradient is much larger, leading to a dramatic growth of gas flow, and the high-speed gas will produce a shear driving effect on the thick water layers. Ions will weaken the formation of hydrogen bonds between water molecules, but the flux of the water phase decreases with the increasing ion concentration due to the combined impact of ion spatial structure and electroviscous effects and counteracts the gas phase, thus impeding its flow. This study shed light on the flow phenomena of gas/water phases in hydrophilic nanopores. The results are expected to provide some insight into energy and environmental issues, such as fracturing fluid optimization, enhanced gas recovery (EGR) by water flooding, and the fabrication of microfluidic chips.

Molecular simulation studies on the water/methane two-phase flow in a cylindrical silica nanopore: Formation mechanisms of water lock and implications for gas hydrate exploitation

The water/gas two-phase flow is a frequently encountered question in the percolation field, and is especially important for the exploitation of natural gas hydrates because their decomposition products are exactly liquid water and natural gas. We studied the water/methane two-phase flow in a hydrophilic cylindrical nanopore by performing molecular simulations, and obtained high-quality nanoflows under different water saturation (Sw) thanks to the newly established nano-manometer method to control pressure difference accurately. With increasing Sw, the methane flow decreases almost linearly until a sudden stop when Sw ≥ 0.52. The formation of the water lock accounting for this phenomenon is observed clearly, and the larger Sw, the earlier formation of the water lock as well as the longer water lock. Based on careful data analysis, a water lock model and its formation mechanism are newly proposed with two pieces of strong evidence — the continuous reduction of the surface area of the water/gas interface when the water lock forms and the existence of maximum thickness of water film for different Sw. Thus, the competition between the surface tension of the water/gas interface and the adsorption of the water/wall interface controls the development of the water lock. These findings are very helpful for understanding the two-phase percolation and optimizing the gas production and water removal schemes during hydrate exploitation. In addition, the nano-manometer can be widely used in other nanoflow simulations for measuring the local pressure accurately.

Occurrence characteristics and influential factors of movable oil in nano-pores by molecular dynamics simulation

CO2-enhanced oil recovery (CO2-EOR) technology has shown great application potential in the development of tight reservoirs. Since the proportion of nano-pores in tight oil reservoirs exceeds 77%, it is necessary to further study the occurrence state of crude oil in nano-pores. In this paper, the molecular dynamics simulation was applied to study the adsorption and diffusion behaviors of multi-component crude oil in quartz (hydrophilic) nano-pores. Besides, the density discretization method was used to investigate the occurrence characteristics of crude oil, the proportion of movable fluid in nano-pores, and the effects of CO2 and polar molecules (C3H6O) on the adsorption characteristics of crude oil. The simulation results showed that the adsorption state of crude oil in the quartz nano-pores was in the form of 4 adsorption layers with a thickness of 0.45 nm each. In nano-pores, the oil molecules with longer molecular chains were more likely to aggregate and adsorb on the quartz surface. Among them, polar oil molecules have the strongest adsorption capacity on the quartz surface. Moreover, the crude oil potential energy and the self-diffusion coefficient gradually increased from the vicinity of the quartz wall and tended to be stable in the free layer. Meanwhile, the content of movable crude oil gradually augmented with the increasement of nanometer pore width and temperature. Furthermore, the pressure had little effect on the density distribution of crude oil in the pores. In contrast, the temperature had a more significant effect on the density distribution of the adsorption layer. At last, due to the more substantial adsorption capacity of CO2 on the rock surface, the crude oil adsorbed initially on the rock surface would be stripped off by CO2, converting from irreducible oil to moveable oil.

Progress of water desalination applications based on wettability and surface characteristics of graphene and graphene oxide: A review

Seawater desalination techniques have been continuously developed to tackle the water scarcity problems. This review article provides comprehensive discussion on the progress of water desalination applications that utilize the unique wettability and surface characteristics of graphene and graphene oxides, which are being employed as ultrafiltration membranes in either a monolayer or multilayer nanosheet configuration. The interaction of water with graphene materials and their wetting characteristics as well as the controlling factors are examined. Particularly, the designs and roles of hydrophilic and hydrophobic nanopores and nanochannels are discussed. A focus is also made on recent developments of graphene membrane with respect to water flow, salt rejection and durability.

The Critical Diameter for Continuous Evaporation Is between 3 and 4 nm for Hydrophilic Nanopores.

Evaporation studies of water using classical molecular dynamics simulations are largely limited due to their high computational expense. This study addresses that issue by developing coarse-grained molecular dynamics models based on Morse potential. Models are optimized based on multi-temperature and at room temperature using machine learning techniques like Genetic Algorithm, Nelder-Mead algorithm, and Strength Pareto Evolutionary Algorithm. The multi-temperature-based model named as Morse-D is found to be more accurate than the single temperature model in representing the water properties at higher temperatures. Using this Morse-D water model, evaporation from hydrophilic nanopores with pore diameter varying from 2 to 5 nm is studied. Our results show that the critical diameter to initiate continuous evaporation at nanopores lies between 3 and 4 nm. A maximum heat flux of 21.3 kW/cm2 is observed for a pore diameter of 4.5 nm and a maximum mass flow rate of 16.2 ng/s for a pore diameter of 5 nm. The observed heat flux is an order of magnitude times larger than the currently reported values from experiments in the literature for water, which indicates that we need to focus on nanoscale evaporation to enhance the critical heat flux.

Complex Coupled Effects of Seawater Ions and Clay Surfaces on CH4 Hydrate Formation in Kaolinite Janus-Nanopores and Bulk Solution

Molecular insights into the kinetic effect of seawater ions and its coupling with the clay surface effect on CH4 hydrate formation help to understand the complex formation process of natural gas hydrate resources in marine sediments. Molecular dynamics simulations are performed to explore CH4 hydrate formation from a homogeneous salty solution containing seawater ions (Na+, K+, and Ca2+, in the form of salts NaCl, KCl, and CaCl2) in the kaolinite Janus double-nanopore and outside bulk phase, by analyzing water order parameter, aqueous CH4 concentration, flows of CH4 and seawater ions, and formation of hydrate-cage clusters. The kaolinite double-nanopore consists of a hydrophilic nanopore with gibbsite surfaces and a hydrophobic nanopore with siloxane surfaces. Simulation results show that in fresh water, the gibbsite surfaces of hydrophilic nanopore do not inhibit hydrate formation, while the siloxane surfaces of hydrophobic nanopore are adverse to hydrate formation due to formation of a large surface nanobubble. By contrast, hydrate formation in saline water is affected by the complex coupled effects of seawater ions and kaolinite surfaces. In the hydrophilic nanopore, most seawater ions quickly adsorb to the gibbsite surfaces and exert very weak effect on hydrate formation, whereas in the hydrophobic nanopore, seawater ions show a certain inhibitory effect on hydrate formation, as few ions adsorb to the siloxane surfaces. Interestingly, more hydrate solids form in saline water than in fresh water, as hydrate solids in the outside bulk solution grow into the throat of the hydrophobic nanopore and decompose the surface nanobubble. Additionally, transport of CH4 molecules and seawater ions between the kaolinite double-nanopore and the outside bulk solution further affects hydrate formation in the systems.

DLVO-Based Analyses of the Water Vapor Adsorption and Condensation in Hydrophilic Nanopores of Low-Rank Coal

DLVO-Based Analyses of the Water Vapor Adsorption and Condensation in Hydrophilic Nanopores of Low-Rank Coal

Different Water Networks Confined in Unidirectional Hydrophilic Nanopores and Transitions with Temperature

Different Water Networks Confined in Unidirectional Hydrophilic Nanopores and Transitions with Temperature

Li+/Mg2+ separation by membrane separation: The role of the compensatory effect

Lithium extraction from salt-lake brines by membrane separation has become a trend to satisfy the global increasing market demand of lithium. Although some progresses have been achieved in membrane fabrication, traditional theories like steric hindrance and Donnan effect struggle to comprehensively explain why Li+/Mg2+ separation factor and Li + permeation could be simultaneously improved, which is also one of the key challenge for this technology. By using uniform nanopores in covalent organic frameworks as the model system, we herein demonstrate that the compensatory effect of pore-wall groups on ionic hydration plays a significant role in Li+/Mg2+ separation: compensation to Li+ second hydration shells is beneficial to Li+ permeation while that to first shells is counterproductive; meanwhile less compensation to Mg2+ hydration is more advantageous to rejecting Mg2+. Via non-equilibrium molecular dynamics simulations, moderately-electropositive nanopores are discovered to be capable of simultaneously enhancing separation factor and Li+ permeation. Balanced ratio of hydrophilic and positively-charged groups on pore walls ensures the nanopores possess suitable compensatory effects that can accelerate Li + permeation while rejecting Mg2+. In contrast, hydrophobic, hydrophilic and strongly-electropositive nanopores are not competent due to their excessively weak or strong compensatory effects. The findings and understandings can be also applicable to other kinds of nanoporous materials and help to inspire other ion-separation applications.

Impact of surface state on polyethylene glycol conformation confined inside a nanopore.

Solid-state nanopores are a promising platform for characterizing proteins. In order to improve their lifetime and prevent fouling, Polyethylene Glycol (PEG) grafting is one of the most efficient and low-cost solutions. Different models to calculate the PEG thickness do not consider their interaction with the nanopore inner surface nor the effect of confinement. Here, we investigate by molecular dynamic simulation the PEG conformation inside a nanopore in the case of hydrophobic and hydrophilic nanopores. Our results reveal that the nanopore inner surface plays a role in the PEG organization and, thus, in the speed of the salt constituent. The resulting pair interaction between PEG and its environment clearly shows a more important affinity for K+ compared to Li+ cations.

Pore-Scale Perspective of Gas/Water Two-Phase Flow in Shale

SummaryThe transport behaviors of both single-phase gas and single-phase water at nanoscale deviate from the predictions of continuum flow theory. The deviation is greater and more complex when both gas and liquid flow simultaneously in a pore or network of pores. We developed a pseudopotential-based lattice Boltzmann (LB) method (LBM) to simulate gas/water two-phase flow at pore scale. A key element of this LBM is the incorporation of fluid/fluid and fluid/solid interactions that successfully capture the microscopic interactions among phases. To calibrate the model, we simulated a series of simple and static nanoscale two-phase systems, including phase separation, a Laplace bubble, contact angle, and a static nanoconfined bubble. In this work, we demonstrate the use of our proposed LBM to model gas/water two-phase flow in systems like a single nanopore, two parallel nanopores, and nanoporous media. Our LBM simulations of static water-film and gas-film scenarios in nanopores agree well with the theory of disjoining pressure and serve as critical steps toward validating this approach. This work highlights the importance of interfacial forces in determining static and dynamic fluid behaviors at the nanoscale. In the Applications section, we determine the water-film thickness and disjoining pressure in a hydrophilic nanopore under the drainage process. Next, we model water imbibition into gas-filled parallel nanopores with different wettability, and simulate gas/water two-phase flow in dual-wettability nanoporous media. The results showed that isolated patches of organic matters (OMs) impede water flow, and the water relative permeability curve cuts off at water saturation [= 1–volumetric total organic carbon (TOC)]. The residual gas saturation is also controlled by the volumetric TOC, ascribed to the isolation of organic patches by the saturating water; therefore, the gas relative permeability curve cuts off at water saturation (= 1–volumetric TOC).

Preparation of Co-porphyrin catalyst encapsulated in cyclodextrin-based metal-organic framework for coupling reaction

Immobilization of organometallic catalyst were attempted by using cyclodextrin-based metal-organic framework (CD-MOF) with the characteristic nanoporous structure. Water-soluble Co(II)TCPP was prepared as an organometallic catalyst and introduced into hydrophilic nanopores in CD-MOF by co-crystallization method. In order to evaluate the catalytic properties of the obtained Co(II)TCPP/CDMOF, the oxidative coupling reaction of creosol was performed. At a catalyst concentration of 0.2 mol%, the catalytic activity of the heterogeneous Co(II)TCPP/CDMOF catalyst was almost the same as that of the homogeneous Co(II)TCPP catalyst. The conversion for Co(II)TCPP/CD-MOF catalyst reached 92% after 2-hour reaction.

RETRACTED: The increased viscosity effect for fracturing fluid imbibition in shale formation

Abstract Understanding water imbibition behaviors in shale formations plays an essential role in shale gas development and risk assessment on shallow aquifer contamination, as imbibition is the main mechanism for fracturing fluid uptake during a hydraulic fracturing stimulation process. The effective viscosity of water in shale formations is usually different from that of the bulk phase due to confined conditions. In this work, a model considering interactions between water and a solid surface is proposed to predict the effective viscosity of water at the nanoscale. This model is derived based on a classical molecular kinetic theory and incorporates the disjoining pressure. Published experimental data is used to validate the model. It is demonstrated that the effective viscosity of water highly confined in hydrophilic nanopores is much higher than that of the bulk phase, while that in hydropholic nanopores is less than the bulk phase value. Furthermore, geometric factors, including the characteristic length of pores and the pore shape, significantly affect the effective viscosity. Moreover, it is found that for the pores in the shale formation which are open for imbibition, the proposed model can be applied as the extremely small pores are usually filled with initial water. This work establishes a theoretical foundation to understand fracturing fluid imbibition behavior in shale gas reservoirs, which will benefit the simulation of fluid flow within shale formations at the reservoir scale.

The increased viscosity effect for fracturing fluid imbibition in shale

Understanding water imbibition behaviors in shale formations plays an essential role in shale gas development. The effective viscosity of water in shale nanopores is usually different from that of the bulk phase because of confined conditions. In this work, a model considering interactions between water and a solid surface is proposed to predict the effective viscosity of water at the nanoscale, which is inserted into the classic Lucas-Washburn (L-W) model to describe the water imbibition behavior in shale formations. This model is derived based on a molecular kinetic theory and incorporates the disjoining pressure. Published experimental data is used to validate the model. It is demonstrated that the effective viscosity of water in hydrophilic nanopores is much higher than that of the bulk phase because of strong interactions between water and a solid surface, and the deviation significantly increases when the separation is below 10 nm. For a hydrophilic capillary tube with a diameter of 10 nm, the water effective viscosity is approximately 2.5 times higher than that of the bulk phase. Moreover, this deviation is larger for a capillary tube compared with a capillary channel, because of the curvature effect. Besides, the effective viscosity for water under a hydrophobic condition is smaller than that of the bulk phase water, because the structural repulsive force dominates under a hydrophobic condition. This work establishes a theoretical foundation to calculate the effective viscosity for water flow at the nanoscale. Furthermore, it helps to understand fracturing fluid imbibition behavior in shale gas reservoirs, which will benefit the simulation of fluid flow at the reservoir scale.

Fluorescence Emission Behaviors of the L-Cysteine/Au(I) Complex in a Cyclodextrin-Based Metal-Organic Framework

Aggregation-induced emission (AIE) molecules are nonemissive in dilute solution but emit intensely upon aggregation in complete contrast to aggregation-caused quenching (ACQ) molecules. The emission of ACQ molecules, such as fluorescein, that have been encapsulated into the hydrophilic nanopores in a cyclodextrin-based metal-organic framework (CD-MOF) was reported to be enhanced due to the disappearance of concentration quenching and the restriction of thermal motion. However, the contribution of the restriction of thermal motion in CD-MOF could not be elucidated. In this study, an AIE-active L-cysteine/Au(I) (L-Cys/Au(I)) complex was synthesized and introduced into the nanopores of CD-MOF via a co-crystallization method. We determined the amount and chemical composition of the L-Cys/Au(I) complex in CD-MOF. The fluorescence intensity of the L-Cys/Au(I)@CD-MOF composite was investigated. The L-Cys/Au(I) complex that was synthesized from Au(III) chloride and L-cysteine was found to be a linear oligomer consisting of Cys5Au4. For the L-Cys/Au(I)@CD-MOF composite with a L-Cys/Au(I) complex of 0.45 per hydrophilic nanopore, the total fluorescence intensity of the isolated L-Cys/Au(I) complex in CD-MOF exceeded that of the L-Cys/Au(I) complex in the solid-state due to the restriction of the thermal motion without the aggregation of the complex.

Effect of hydrophilicity on ion rejection of sub-nanometer pores

Ion rejection by (sub)nanometer-sized pores enables membrane-based desalination. Narrowing the pore size is long considered as the primary choice to increase the ion rejection rate, which is unfortunately at much sacrifice of the water permeance. Using the uniform sub-nanometer pore channels in covalent organic frameworks (COFs) as the model system, we herein demonstrate that hydrophilicity of pore walls plays a significant role in determining ion rejections, and ion rejections can be significantly improved by enhancing hydrophilicity with less loss in water permeance. Via non-equilibrium molecular dynamics simulation, the augmented in-pore effect caused by significant pore-wall-involved hydration effect is discovered to dominate the ion rejections. Furthermore, the desalination mechanism of hydrophilic nanopores is proposed: strong hydrophilicity can make cations enter nanopores more easily than anions by compensation of ionic hydrations, but all ions are not able to transport inside nanopores due to the extreme in-pore effect; hence membranes consequently carry positive net charges after being saturated adsorption by ions and then in turn exclude cations by electrostatic repulsions. These findings and understandings on desalination mechanisms can also be applicable to other kinds of nanoporous materials and thus could provide guidelines for the experimental design of next-generation desalination membranes.