Nanopore Confinement Research Articles

Effect of Nanopore Geometry in the Conformation and Vibrational Dynamics of a Highly Confined Molecular Glass.

The effect of nanoporous confinement on the glass transition temperature (Tg) strongly depends on the type of porous media. Here, we study the molecular origins of this effect in a molecular glass, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), highly confined in concave and convex geometries. When confined in controlled pore glass (CPG) with convex pores, TPD's vibrational spectra remained unchanged and two Tg's were observed, consistent with previous studies. In contrast, when confined in silica nanoparticle packings with concave pores, the vibrational peaks were shifted due to more planar conformations and Tg increased, as the pore size was decreased. The strong Tg increases in concave pores indicate significantly slower relaxation dynamics compared to CPG. Given TPD's weak interaction with silica, these effects are entropic in nature and are due to conformational changes at molecular level. The results highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.

Effect of Nanopore Confinement on Fluid Phase Behavior and Production Performance in Shale Oil Reservoir

The confinement effects including capillarity and adsorption play a significant role in phase behavior and transport of shale fluids. The effect of capillary pressure has been widely studied and is fully understood. However, the investigation of the adsorption effect on reservoir fluid properties and production performance is still lacking. In this work, an efficient model is proposed to fill this gap and investigate the phase behavior and well performance in the Bakken shale oil reservoir. First, an improved phase equilibrium model is developed based on an adsorption-dependent Peng–Robinson equation of state and a modified Young–Laplace equation. The effect of adsorption and adsorption induced critical property shifts in nature is considered. Second, the phase equilibrium model is used to calculate and compare the black-oil properties of the Bakken oil under different pressures and confinement conditions. Finally, the fluid properties results are combined with the reservoir simulator to assess the confinement effects on the well performance of the Bakken shale reservoir. The result indicates that fluid adsorption makes a more significant contribution to the suppressed bubble point pressure than capillarity. The change in the black-oil properties shows that the confinement effect exhibits a great impact on the physical properties of the oil phase including solution gas–oil ratio, oil formation volume factor, and oil viscosity, but presents a small impact on the physical properties of the gas phase including gas formation volume factor and gas viscosity. In terms of production performance, the presence of confinement increases the cumulative oil production, first increasing, and then decreasing cumulative gas production, which leads to a flatter producing gas–oil ratio. Overall, the effect of adsorption is more significant under high-pressure conditions, and the effect of capillary pressure is more significant under low-pressure conditions.

Nanopore Confinement for Single‐Molecule Measurement of Proteins

Nanopore Confinement for Single‐Molecule Measurement of Proteins

Nanopore Confinement of Electrocatalysts Optimizing Triple Transport for an Ultrahigh-Power-Density Zinc-Air Fuel Cell with Robust Stability.

Metal-air fuel cells with high energy density, eco-friendliness, and low cost bring significantly high security to future power systems. However, the impending challenges of low power density and high-current-density stability limit their widespread applications. In this study, an ultrahigh-power-density Zn-air fuel cell with robust stability is highlighted. Benefiting from the water-resistance effect of the confined nanopores, the highly active cobalt cluster electrocatalysts reside in specific nanopores and possess stable triple-phase reaction areas, leading to the synergistic optimization of electron conduction, oxygen gas diffusion, and ion transport for electrocatalysis. As a result, the as-established Zn-air fuel cell shows the best stability under high-current-density discharging (>90 h at 100mA cm-2 ) and superior power density (peak power density: >300mW cm-2 , specific power: 500 Wgcat -1 ) compared to most reported non-noble-metal electrocatalysts. The findings will provide new insights in the rational design of electrocatalysts for advanced metal-air fuel cell systems.

Effect of Confinement in Nanopores on RNA Interactions with Functionalized Mesoporous Silica Nanoparticles.

Amine-functionalized mesoporous silica nanoparticles (MSNPAs) are ideal carriers for oligonucleotides for gene delivery and RNA interference. This investigation examines the thermodynamic driving force of interactions of double-stranded (ds) RNA with MSNPAs as a function of RNA length (84 and 282 base pair) and particle pore diameter (nonporous, 2.7, 4.3, and 8.1 nm) using isothermal titration calorimetry, extending knowledge of solution-based nucleic acid-polycation interactions to RNA confined in nanopores. Adsorption of RNA follows a two-step process: endothermic interactions driven by entropic contribution from counterion (and water) release and an exothermic regime dominated by short-range interactions within the pores. Evidence of hindered pore loading of the longer RNA and pore size-dependent confinement of RNA in the MSPAs is provided from the relative contributions of the endothermic and exothermic regimes. Reduction of endothermic and exothermic enthalpies in both regimes in the presence of salt for both lengths of RNA indicates the significant contribution of short-range electrostatic interactions, whereas ΔH and ΔG values are consistent with conformation changes and desolvation of nucleic acids upon binding with polycations. Knowledge of the interactions between RNA and functionalized porous nanoparticles will aid in porous nanocarrier design suitable for functional RNA delivery.

Pore size effect on selective gas transport in shale nanopores

In shale gas production, gas composition may vary over time. To understand this phenomenon, we use molecular dynamics simulations to study the permeation of CH4, C2H6 and their mixture from a source container through a pyrophyllite nanopore driven by a pressure gradient. For a pure gas, the flow rate of CH4 is always higher than that of C2H6, regardless of pore size. For a 1:1 C2H6: CH4 mixture, however, C2H6:CH4 flow rate ratio is higher than the compositional ratio in the container (i.e., 1:1) when the pore size is smaller than ~1.8 nm. The selective transport is caused by the competitive adsorption of C2H6 over CH4 in the nanopore. The selectivity is also determined by the interplay between the surface diffusion of the adsorbed molecules and the viscous flow in the center of the pore, and it diminishes as the viscous flow becomes to dominate the surface diffusion when the pore size becomes larger than 1.8 nm. Our work shows that compositional differentiation of shale gas in production is a consequence of nanopore confinement and therefore a key characteristic of an unconventional reservoir. The related compositional information can potentially be used for monitoring the status of a production well such as its recovery rate.

Kinetic study of alkyl methacrylate polymerization in nanoporous confinement over a broad temperature range

The effect of nanoconfinement on the free radical polymerization of ethyl methacrylate (EMA) and n-butyl-methacrylate (BMA) with di-tert-butyl peroxide (DtBP) initiator is investigated over a wide temperature range from 80 to 190 °C using differential scanning calorimetry. The effective rates are similar for the two bulk monomers although the BMA reacts approximately 11% faster at 95 °C. For nanoconfined cases, the initial reaction rate for monomer confined in the pores of controlled pore glass is enhanced, with larger effects observed in native pores compared to pores in which the native silanol was converted to trimethyl silyl. The onset of autoacceleration also occurs earlier under nanoconfinement, with decreases in both the conversion and the time required to reach autoacceleration, xgel and tgel, respectively, and larger changes for the native pores. The induction time follows Arrhenius behavior and increases under nanoconfinement. At polymerization temperatures above 160 °C, depropagation becomes important as the ceiling temperature is approached and seems to be more pronounced under nanoconfinement than in the bulk.

Pore confinement and surface charge effects in protein-mesoporous silica nanoparticles formulation for oral drug delivery

Abstract The recent pharmaceutical interest toward confinement effects on the physical state, solubility and release of drugs has paved the way to new material designs for drug delivery. Deeper understanding on how the pore size or the additional functionalities of the nanocarriers can influence the bioavailability of the cargo is nonetheless needed to explore further the potential of nanocarriers in this area. Through the combination of pure or amino-functionalized MCM-48-type mesoporous silica nanoparticles (MSNs) and succinylated beta-lactoglobulin as a protein excipient, we developed versatile tablets for the oral delivery of resveratrol and omeprazole. pH-gating properties of the protein excipient refrained resveratrol from elution in simulated gastric fluid and, owing to effective nanopore confinement, an increased release in simulated intestinal conditions was successfully achieved. Specific effects of positively charged MCM-48-type nanoparticles on the stability of the protein tablets were encountered and appeared to be drug dependent.

Estimating diffusion coefficients of shale oil, gas, and condensate with nano-confinement effect

Molecular diffusion plays an important role in oil and gas migration in tight shale formations. However, there are insufficient reference data in the literature to specify the diffusion coefficients within porous media. This study aims at calculating diffusion coefficients of shale gas, shale condensate, and shale oil at reservoir conditions with CO2 injection for EOR/EGR. The large nano-confinement effects including large gas-oil capillary pressure and critical property shifts could alter the phase behaviors. This paper estimates the diffusivities of shale fluids in nanometer-scale shale rock from two perspectives: 1) examining the shift of diffusivity caused by nanopore confinement effects from phase change (phase composition and fluid property) perspective, and 2) calculating the effective diffusion coefficient in porous media by incorporating rock intrinsic properties (porosity and tortuosity factor). The tortuosity is obtained by using tortuosity-porosity relations as well as the measured tortuosity of shale from 3D imaging techniques. The results indicated that nano-confinement effects could affect the diffusion coefficient through altering the phase properties, such as phase compositions and densities. Compared to bulk phase diffusivity, the effective diffusion coefficient in porous shale rock is reduced by 102 to 104 times as porosity decreases from 0.1 to 0.03.

Critical Role of Confinement in the NMR Surface Relaxation and Diffusion of n-Heptane in a Polymer Matrix Revealed by MD Simulations.

The mechanism behind the NMR surface-relaxation times (T1S,2S) and the large T1S/T2S ratio of light hydrocarbons confined in the nanopores of kerogen remains poorly understood and consequently has engendered much debate. Toward bringing a molecular-scale resolution to this problem, we present molecular dynamics (MD) simulations of 1H NMR relaxation and diffusion of n-heptane in a polymer matrix. The high-viscosity polymer is a model for kerogen and bitumen that provides an organic "surface" for heptane. Diffusion of n-heptane shows a power-law dependence on the concentration of n-heptane (ϕC7) in the polymer matrix, consistent with Archie's model of tortuosity. We calculate the autocorrelation function G(t) for 1H-1H dipole-dipole interactions of n-heptane in the polymer matrix and use this to generate the NMR frequency (f0) dependence of T1S,2S as a function of ϕC7. We find that increasing molecular confinement increases the correlation time, which decreases the surface-relaxation times for n-heptane in the polymer matrix. For weak confinement (ϕC7 > 50 vol %), we find that T1S/T2S ≃ 1. Under strong confinement (ϕC7 ≲ 50 vol %), we find that T1S/T2S ≳ 4 increases with decreasing ϕC7 and that the dispersion relation T1S ∝ f0 is consistent with previously reported measurements of polydisperse polymers and bitumen. Such frequency dependence in bitumen has been previously attributed to paramagnetism; instead, our studies suggests that 1H-1H dipole-dipole interactions enhanced by organic nanopore confinement dominate the NMR response in saturated organic-rich shales.

Evaluation of nanopore confinement during CO2 huff and puff process in liquid-rich shale formations

CO2 huff and puff (CO2 huff-n-puff) with multistage hydraulic fracture has been proven to be a feasible approach to enhance oil recovery in shale/tight formations. The present study investigates the effect of pore confinement on the incremental recovery of the CO2 huff-n-puff process in shale formation. The results demonstrate that neglecting nanopore confinement has a large impact on production forecasts from unconventional assets especially when the pore size is considerably less than 10 nm. Failure to correct the changes in fluid properties due to pore confinement might lead to errors in simulation results. Fine gridding plays an important role during simulation setup. Moreover, CO2 molecular diffusion is the most important mechanism by which the gas tends to move from fractures into the matrix. Although a short injection time cannot increase oil recovery significantly, a long injection time (more than 5 months) might not be economically or technically feasible.

Viscous Dissipation and Apparent Permeability of Gas Flow in Nanoporous Media

AbstractThe nanopore confinement effect can increase the mobility for gas flow in shale rocks, and the apparent permeability is usually adopted to account for this effect. However, most of the apparent permeability calculation models are derived based on straight channels (capillaries) and neglect the influence of end effect. The end effect is caused by the bending of streamlines which yields more viscous dissipation and additional flow resistance for gas flow in nanoporous media. In this paper, for the first time the viscous dissipation for gas flow in nanoporous media is analyzed. In addition, an improved apparent permeability calculation model is proposed based on the Beskok‐Karniadakis (B‐K) model by introducing the influence of end effect, which is characterized by the ratio of the length to the width of a short channel (L/H). The accuracy of this model is validated by lattice Boltzmann simulations of gas flow in three different geometries: an infinite‐length straight channel, a finite‐length straight channel, and nanoporous media. For the infinite‐length straight channel, there is no end effect therefore both the B‐K model and the improved model can well predict the apparent permeability. However, for the finite‐length straight channel and nanoporous media, the original B‐K model overestimates the gas apparent permeability as it neglects the influence of end effect, while the improved model can still well predict the gas apparent permeability. In summary, the improved apparent permeability calculation model is more reasonable in predicting gas mobility in nanoporous media.

Fabrication and electrochemical properties of well-dispersed molybdenum oxide nanoparticles into nitrogen-doped ordered mesoporous carbons for supercapacitors

Environment-friendly supercapacitors demand the high-performance electrode materials working with neutral aqueous electrolyte. In this work, the nitrogen-doped ordered mesoporous carbons (NOMCs) have been synthesized and subsequently modified with molybdenum oxide nanoparticles (NPs) by a self-developed temperature controlled impregnation method. The prepared hybrids possess ordered mesoporous structure, high specific surface area (600–1000 m2 g−1), favorable pore size (∼4 nm) and well-dispersed uniform molybdenum oxide NPs. The results demonstrate that the nano-sized molybdenum oxides encapsulated in the ordered mesoporous channels exhibit significant electrochemical activity with an enhanced specific capacitance up to 410 F g−1, which can be attributed to the nanosize effects from the nanopore confinement and efficient surface area of molybdenum oxide NPs under well dispersion. The hybrids exhibit quasi-rectangular CV curves within the enlarged potential window compared with the pristine NOMCs together with pairs of redox peaks owing to the multi-valance changes of molybdenum corresponding to the Faradaic reactions. Thus, the synthesized hybrids demonstrate not only the synergistic contribution of electrical double-layer capacitance and pseudocapacitance but also superior cycling stability in aqueous neutral electrolyte.

A compositional model for gas injection IOR/EOR in tight oil reservoirs under coupled nanopore confinement and geomechanics effects

The nanopore confinement and geomechanics are generally considered as non-negligible in tight oil reservoirs, but their coupled effects are still not well understood for gas injection which is becoming a favorable Improved/Enhanced Oil Recovery (IOR/EOR) technique for tight oil. We present a general compositional model to investigate the complex multiphase and multicomponent behaviors under coupled nanopore confinement and geomechanics in tight oil reservoirs. The in-house simulator developed is validated against a commercial simulator before being applied to simulate dry gas huff-n-puff in Eagle Ford. The simulation reveals that huff-n-puff would improve the recovery factor (RF) of each component versus the depletion. If the reservoir pressure is much higher than the bubble point, the nanopore confinement will have a minimal impact on RF for both the depletion and early cycles of huff-n-puff. Geomechanics is found to be an important factor for RF but not always detrimental, as enhanced compaction drive of matrix could offset the permeability reduction of fracture in certain scenarios. The heavy component would first have a higher RF than the light component in early huff-n-puff cycles, but its RF will be gradually outpaced by the light and finally surpassed after a crossover point which is controlled by the critical gas saturation of matrix. Considering the nanopore confinement in the simulation will delay this crossover and reduce the RF of the light component but increase the RF of the heavy component after huff-n-puff. This work can better assist engineers to understand the complex multiphase and multicomponent behaviors during gas injection in tight oil reservoirs.

Crystallization Kinetics of Poly(ethylene oxide) under Confinement in Nanoporous Alumina Studied by in Situ X-ray Scattering and Simulation.

While a relatively complete understanding of the nucleation and orientation of polymers under confinement in one-dimensional nanochannels has been achieved, crystallization kinetics investigation of confined polymers is still rare. In this work, we investigated the crystallization kinetics of poly(ethylene oxide) confined in anodic alumina oxide templates with different pore sizes using in situ wide-angle X-ray scattering (WAXS). The crystallization kinetics results were fitted with the Avrami equation. The Avrami index was determined by both "isothermal step crystallization" and in situ WAXS. The crystallization process of polymers under one-dimensional nanopore confinement was simulated by a "one-dimensional lattice model". Based on this model, it is shown that homogeneous nucleation with the simultaneous growth of multiple crystal planes with drastically different growth rates could result in Avrami indexes lower than 1.

Microemulsion Effects on Oil Recovery From Kerogen Using Molecular-Dynamics Simulation

Summary Source rocks contain significant volumes of hydrocarbon fluids trapped in kerogen, but effective recovery is challenged because of amplified fluid/wall interactions and the nanopore–confinement effect on the hydrocarbon–fluid composition. Enhanced oil production can be achieved by modifying the existing molecular forces in a kerogen pore network using custom–designed targeted–chemistry technologies. The objective of this paper is to show that the maturation of kerogen during catagenesis relates to the qualities of the kerogen pore network, such as pore size, shape, and connectivity, and plays an important role in the recovery of hydrocarbons. Furthermore, using molecular–dynamics (MD) simulations, we investigated how the transport of hydrocarbons in kerogen and hydrocarbon recovery can be altered with the delivery of microemulsion and surfactant micelles into the pore network. New 3D kerogen models are presented using atomistic modeling and molecular simulations. These models possess important chemical and physical characteristics of the organic matter of the source rock. A replica of Type II kerogen representative of the source rocks in the Permian Basin in the US is used for the subsequent recovery simulations. Oil–saturated kerogen is modeled as consisting of nine different types of molecules: dimethyl naphthalene, toluene, tetradecane, decane, octane, butane, propane, ethane, and methane. The delivered microemulsion is an aqueous dispersion of solvent–swollen surfactant micelles. The solvent and nonionic surfactant present in the microemulsion are modeled as d–limonene and dodecanol heptaethyl ether (C12E7), respectively. MD simulation experiments include two stages: injection of an aqueous–phase microemulsion treatment fluid into the oil–saturated kerogen pore network, and transient flowback of the fluids in the pore network. The used 3D kerogen models were developed using a representative oil–sample composition (hydrogen, carbon, oxygen, sulfur, and nitrogen) from the region. Simulation results show that microemulsions affect the reservoir by means of two different mechanisms. First, during the injection, microemulsion droplets possess elastic properties that allow them to squeeze through inorganic pores smaller than the droplet's own diameter and to adsorb at the kerogen surfaces. The solvent dissolves in the oil phase and alters the physical and transport properties of the phase. Second, the surfactant molecules modify the wettability of the solid kerogen surfaces. Consequently, the recovery effectiveness of heavier oil fractions is improved compared with the recovery effectiveness achieved with surfactant micelles without the solubilized solvent. The results indicate that solubilized solvent and surfactant can be effectively delivered into organic–rich nanoporous formations as part of a microemulsion droplet and aid in the mobilization of the kerogen oil.

Metal-organic frameworks as emerging platform for supporting isolated single-site catalysts

Abstract Isolated heterogeneous single-site catalysts have been attracted great interest in diverse catalytic reactions because of their uniform and distinct geometric and electronic structure. Among various porous supports, metal-organic frameworks (MOFs), with molecular level structure control and modularity, offer the versatile platforms for supporting isolated single-site catalysts. Owing to the well-defined anchoring sites lying in the nodes, ligands or nanopores of MOFs, the single-site catalysts could be grafted precisely without protection by bulky ligands. More interestingly, together with the nanopore confinement effect of MOFs, single-site catalysts could exhibit the excellent performances in terms of catalytic activity, selectivity and stability. In this review, we focus on summarizing the recent advances in the precise design, synthesis, characterization and catalytic applications of isolated single-site catalysts supported by MOFs and elucidating the relationship between structure and performance of single-site catalysts. Finally, we give a perspective on controllable synthesis of isolated single-site catalysts supported by MOFs as well as their catalytic applications.

Rotational Diffusion of Proteins in Nanochannels

The rotational diffusion coefficient is an essential parameter in determining the mechanistic features of biomolecules in both crowded and confined environments. Understanding the influence of nanoconfinement on rotational diffusion is vital in conceptualizing dynamics of biomolecules (such as proteins) in nanopores. The control of the translational movement of biomolecules is practiced widely in nanopore experiments. However, the restrictions on the translational movement may affect other dynamic properties such as rotational diffusion. In this paper, we use a coarse-grained molecular dynamics approach to study the rotational dynamics of a sample protein under the influence of cylindrical nanopore confinement. Our simulation reveals a 2-fold reduction in magnitude from the bulk rotational diffusion coefficient value as the confinement radius reaches double the size of protein's hydrodynamic radius. However, the changes in the rotational diffusion coefficient are relatively small compared to the changes in the translational diffusion coefficient. Interestingly, the rotational anisotropy also varies considerably when pore radii approach protein dimensions. Our simulations point out that the confinement effects cause the breakdown of small angular displacement theory when the pore radius is close to the protein hydrodynamic radius.

Study of Increasing Pressure and Nanopore Confinement Effect on the Segmental, Chain, and Secondary Dynamics of Poly(methylphenylsiloxane)

The effect of increasing pressure and two-dimensional (2D) confinement on the dynamics of glass-forming polymer poly(methylphenylsiloxane) (PMPS) was investigated with the use of dielectric spectroscopy. We demonstrate that the glass-forming polymer confined to nanoporous alumina might obey the density scaling relation similar to that in the bulk and that the same value of the scaling exponent is used to superimpose the α-relaxation time measured under different thermodynamic conditions. Our comprehensive analysis of the relaxation processes detected in the dielectric loss spectra of PMPS allows us to identify the Johari–Goldstein β-relaxation which for a bulk polymer shows up as a well-resolved peak while under 2D nanoconfinement only as an excess wing. In contrast to previous studies, we provide dielectric evidence of an additional α′-relaxation, slower than the segmental (α-) dynamics, which is related to the chain dynamics of PMPS.

Effect of Surface Modification on the Glass Transition Dynamics of Highly Polar Molecular Liquid S-Methoxy-PC Confined in Anodic Aluminum Oxide Nanopores

In the presence of nanoscale confinement, the role of surface effects come to the fore in determining the glass-transition dynamics of the molecular liquids and polymers. Therefore, by playing with the surface chemistry it is possible to understand better the mechanism which governs the behavior of glass-forming systems at the nanoscale. In this work, we have combined dielectric and calorimetric data to study surface and confinement effects for highly polar glass-forming liquid S-Methoxy-PC constrained within anodic aluminum oxide membranes of different pore sizes. The inner surface of the pores was modified either by silanization or atomic layer deposition (ALD) coatings. For the tested substance in native nanopores, we have observed two glass transition events in the calorimetric response accompanied by a characteristic deviation of the temperature dependence of the α-relaxation time from the Vogel–Fulcher–Tamman law. We found that depending on the hydrophobicity of the ALD layer the glass transition temperature of the interfacial layer tends to decrease, the α-relaxation peak broadens, and the molecular mobility slows down compared to the native pores. These changes are more visible with increasing hydrophobicity of the surface. Silanization was also found to eliminate at least partially the effects caused by nanopore confinement. However, in this study the most pronounced effects were observed only for pores with large diameters.