Narrow Slit Pores Research Articles

Thermodynamic properties of pinned nanobubbles.

We present molecular dynamics simulations to study the thermodynamics of nanobubbles trapped at the mouth of narrow slit pores. Except when the slit dimensions are comparable to typical molecular sizes, the predictions of macroscopic thermodynamic theory are recovered by our simulations. Our simulations confirm that in this case, the internal pressure of stable nanobubbles is independent of the bubble radius and the surface tension and only depends on the bulk properties of the solute-containing solution, i.e., the chemical potential balance. However, in the case of extreme confinement, the pressure is not a suitable quantity to describe the thermodynamics of the bubbles, while the balance of the chemical potentials is.

Experimental and model analysis of the effect of pore and mineral characteristics on fluid transport in porous soil media

The fluid transport in porous media is a critical property for oil and gas exploitation, construction engineering, and environmental protection. It is profoundly influenced by pore geometry and mineral properties. Currently, the Kozeny–Carman equation serves as the permeability prediction equation for porous media, established on the circular pores model. However, it fails to fully account for the impact of pore shape and mineral properties of the soil, leading to significant deviations between predicted and measured soil permeability results. In this paper, based on scanning electron microscope image and mercury intrusion porosimetry, the pores were divided into circular pores and narrow slit pores according to the ratios of pore area and circumference. Then, the quantitative expression of the two types of pores and their connectivity and tortuosity were given, and the circular and narrow slit composite pore model was used to describe the soil pore. Subsequently, the electrostatic potential of pore water was calculated by the Poisson–Boltzmann equation to consider the adsorption effect of minerals on pore water. Combined with the Navier–Stokes equation, the permeability prediction equation considering pore geometry, pore connectivity, and tortuosity and mineral properties was established. Finally, the experimental results illustrated that the theoretical prediction results were in good agreement with the experimental results. The proposed permeability prediction equation proves valuable for assessing and predicting the fluid transport in porous media.

Encapsulation of Ammoides pusila Essential Oil into Mesoporous Silica Particles for the Enhancement of Their Activity against Fusarium avenaceum and Its Enniatins Production

Essential oils (EOs) that have antifungal activity and mycotoxin reduction ability are candidates to develop bioactive alternatives and environmentally friendly treatment against Fusarium species in cereals. However, their practical use is facing limitations such as high volatility, UV sensitivity, and fast oxidation. Encapsulation techniques are supposed to provide protection to the EOs and control their release into the environment. Ammoides pusilla essential oil (AP-EO) proved to be an efficient inhibitor of Fusarium avenaceum growth and its enniatins (ENNs) production. In the present work, AP-EO was encapsulated, using the impregnation method, into mesoporous silica particles (MSPs) with narrow slit pores (average diameter = 3.1 nm) and coated with chitosan. In contact assays using an agar medium, the antifungal activity of AP-EO at 0.1 µL mL-1 improved by three times when encapsulated into MSPs without chitosan and the ENNs production was significantly inhibited both in coated and non-coated MSPs. Controls of MSPs also inhibited the ENNs production without affecting the mycelial growth. In fumigation experiments assessing the activity of the EO volatile compounds, encapsulation into MSPs improved significantly both the antifungal activity and ENNs inhibition. Moreover, coating with chitosan stopped the release of EO. Thus, encapsulation of an EO into MSPs improving its antifungal and antimycotoxin properties is a promising tool for the formulation of a natural fungicide that could be used in the agriculture or food industry to protect plant or food products from the contamination by toxigenic fungi such as Fusarium sp. and their potential mycotoxins.

Effect of Formation Pressure on Pore Structure Evolution and Hydrocarbon Expulsion in Organic-Rich Marine Shale

Exploring the relationship between formation pressure and shale pore evolution is helpful for the enrichment and development of marine shale gas accumulation theory. The thermal evolution experiment was carried out on the Xiamaling Formation (Pr3x) lowly matured marine shale, which has a similar sedimentary environment to the Longmaxi Formation (S1l) highly matured marine shale. Comparative experiments of open and semi-closed pyrolysis and multiple pore structure characterization techniques, including CO2 and N2 physisorption, mercury intrusion porosimetry, and field emission scanning electron microscopy, were conducted. The marine shale pore evolutionary model under formation pressure is proposed by characterizing pore evolution, and hydrocarbon expulsion and retention for shales under and without formation fluid pressures. The results show that the existence of formation pressure increases the percentage of quartz and reduces the content of clay minerals. The change in formation pressure has no obvious effect on the maturity evolution of shale samples. With the increase of formation pressure, the pore morphology of shale gradually changes from narrow slit pores to ink bottle-shaped pores. The retained hydrocarbons in shale mainly occupy the mesopore space, and the existence of formation pressure promotes hydrocarbon expulsion, especially the hydrocarbon expulsion in the mesopore. In addition, formation pressure improves pore connectivity, especially in the high-over mature stage of shale. With the increase of formation pressure, the micropore volume decreases slightly, the mesopore volume increases significantly, and the macropore volume changes have two stages.

Anisotropic pressure effects on nanoconfined water within narrow graphene slit pores.

There is an increasing interest toward disclosing and explaining confinement effects on liquids, such as water or aqueous solutions, in slit pore setups. Particularly puzzling are the changes of physical and chemical properties in the nanoconfinement regime where no bulk-like water phase exists between the two interfacial water layers such that the density profile across the slit pore becomes highly stratified, ultimately leading to bilayer and monolayer water. These changes must be quantified with respect to some meaningful reference state of water, the most natural one being bulk water at the same pressure and temperature conditions. However, bulk water is a homogeneous liquid with isotropic properties, whereas water confined in slit pores is inhomogeneous, implying anisotropic properties as described by the perpendicular and parallel components of the respective tensors. In the case of pressure, the inhomogeneous nature of the setup results in a well-defined difference between the perpendicular and parallel pressure tensor components that is uniquely determined by the interfacial tension being a thermodynamic property. For bilayer water constrained in graphene slit pores that are only about 1 nm wide, we demonstrate that there exists a thermodynamic point where the pressure tensor of the inhomogeneous fluid, nanoconfined water, is effectively isotopic and the pressure is thus scalar as in the homogeneous fluid, bulk water. This specific point of vanishing effective interfacial tension is proposed to serve as a well-defined reference state to compare the properties of nanoconfined liquids to those of the corresponding bulk liquid at the same (isotropic) pressure and temperature conditions. In future work, this idea could be applied to assess confinement effects on chemical reactivity in aqueous solutions as well as to other nanoconfined liquids in other pores such as layered minerals.

Pore Structure and Fractal Characteristics of Continental Low Maturity Organic-Rich Shale in the Sha-4 Member of the Liaohe Western Depression

The research on pore structure and heterogeneity of shale reservoirs has always been a hotspot in the study of unconventional reservoir characteristics. China is a country dominated by continental shale. Compared with marine shale, continental shale has lower maturity and stronger reservoir heterogeneity. In this study, Sha-4 shale in the Liaohe Western Depression was selected for low-temperature nitrogen adsorption, scanning electron microscopy and other experiments revealing the pore structure and fractal characteristics of continental low mature organic-rich shale. The fractal dimension was calculated by the FHH model and the effects of TOC and mineral composition on pore structure and fractal characteristics were discussed. The results show that the Sha-4 shale in the study area is mainly mesoporous and the main pore types are inorganic pores with relatively large pore diameters, such as intergranular pores and inter-crystalline pores. The pore morphology is very complex, mainly narrow slit and flat pore, and the pore is often filled with organic matter. The fractal dimensions D1 range from 2.58 to 2.87 and D2 range from 2.18 to 2.55, and the pore structure shows obvious dual fractal characteristics. The pore structure and fractal characteristics of shale are mainly affected by TOC and quartz due to the low degree of the thermal evolution of shale and their effects are different from those of marine shale reservoirs. The increase in TOC reduces the heterogeneity of the shale reservoir. In addition, mineral particles with strong weathering resistance and stability such as quartz can protect the pore structure of shale, improve the pore structure and reduce the reservoir heterogeneity. This study can provide support for the study of low maturity continental shale reservoir heterogeneity in the Sha-4 member of the Liaohe Western Depression.

The structure of a core-softened system in a narrow slit pore

ABSTRACT We have investigated the behaviour of a core-softened system in a narrow slit pore (the width of the pore is equal to three diameters of a particle). In the previous studies, it was shown that strongly confined systems formed crystalline phases which consisted of several triangular or square layers. These phases can also be considered as cuts of FCC or HCP structures. We have shown that the behaviour of a core-softened system is more complex. We have also observed a quasicrystalline phase. Moreover, the phase with two triangular layers appears at lower densities than the phase with two square layers, which is in contrast to the behaviour of the systems studied before. These results demonstrate that the phase behaviour of strongly confined systems can be even more complex than it has been supposed before.

Crystal structure of a system with three-body interactions in strong confinement

Crystal structure of the Stillinger-Weber system in a narrow slit pore is studied. It is discovered that depending on the density the system forms several different crystal phases. Importantly, the system splits into several layers, and the symmetry of different layers can be different, which is not observed in systems with pair interaction only.

Nanoconfinement effects on water in narrow graphene-based slit pores as revealed by THz spectroscopy.

The properties of water at interfaces have long been known to differ from those of bulk water in many distinctive ways. More recently, specific confinement effects different from mere interfacial effects have been discovered upon enclosing water in very narrow cylindrical pores and planar surfaces as offered by nanotubes and slit pores, respectively. Using experimental and theoretical THz spectroscopy, we elucidate nanoconfinement effects on the H-bond network of stratified water lamellae that are hosted within graphene-based two-dimensional pores. Characteristic confinement-induced changes of the THz response are traced back to the level of structural dynamics, notably distinct resonances due to intralayer and interlayer H-bonds at correspondingly low and high intermolecular stretching frequencies and impact of dangling (free) OH bonds at the water-graphene interface that enormously broaden the librational band in sufficiently narrow pores. The interplay of these molecular effects causes characteristic changes of the THz lineshape upon nanoconfining water.

Structural and Dynamical Behaviour of Colloids with Competing Interactions Confined in Slit Pores.

Systems with short-range attractive and long-range repulsive interactions can form periodic modulated phases at low temperatures, such as cluster-crystal, hexagonal, lamellar and bicontinuous gyroid phases. These periodic microphases should be stable regardless of the physical origin of the interactions. However, they have not yet been experimentally observed in colloidal systems, where, in principle, the interactions can be tuned by modifying the colloidal solution. Our goal is to investigate whether the formation of some of these periodic microphases can be promoted by confinement in narrow slit pores. By performing simulations of a simple model with competing interactions, we find that both the cluster-crystal and lamellar phases can be stable up to higher temperatures than in the bulk system, whereas the hexagonal phase is destabilised at temperatures somewhat lower than in bulk. Besides, we observed that the internal ordering of the lamellar phase can be modified by changing the pore width. Interestingly, for sufficiently wide pores to host three lamellae, there is a range of temperatures for which the two lamellae close to the walls are internally ordered, whereas the one at the centre of the pore remains internally disordered. We also find that particle diffusion under confinement exhibits a complex dependence with the pore width and with the density, obtaining larger and smaller values of the diffusion coefficient than in the corresponding bulk system.

Variation of pore structure in Zhundong coal particle with stepped K2CO3 loading during supercritical water gasification

Supercritical water gasification (SCWG) is a promising technology for coal utilization and has drawn lots of attention. K2CO3 is an outstanding catalyst for coal gasification and can significantly improve pore structure. The evolution of pore structure in coal particles affects heat and mass transfer efficiency, but little research has investigated the effect of K2CO3 on gasification results from the perspective of pore structure. Hence, this study revealed the variation of pore structure of coal chars with stepped K2CO3 loading during SCWG of coal by conducting experiment in a high-capacity autoclave reactor. Gas products and coal chars were characterized by GC, SEM, and BET to obtain the gasification result, surface morphology, and pore structure characteristics of coal chars. The results show that K2CO3 improved gasification efficiency, gas percentage, and gas yield of H2 and CO2. The time when K2CO3 changes pore structure is during the gasification process rather than before gasification. Macropores are dominant in coal chars. 0 wt%-8 wt% K2CO3 favors the formation of mesopores and macropores, while 10 wt% K2CO3 blocks the pore structure. 0 wt%-4 wt% K2CO3 loading makes a pore structure of the ink-bottle type, and 6 wt%-10 wt% K2CO3 loading makes the narrow-slit pore structure. When K2CO3 increases from 0 wt% to 8 wt%, production rate of C-containing gases has a good linear relationship with specific surface area, and pore volume also has a good linear relationship with carbon gasification efficiency. 10 wt% K2CO3 destroys the linear relationship by the blocking effect.

Hydrogen storage capacity of Li-decorated borophene and pristine graphene slit pores: A combined ab initio and quantum-thermodynamic study

Among the two-dimensional materials of the post-graphene era, borophene has raised an enormous interest due to its unprecedented diversity of structures and the wide variety of potential applications, including its ability for hydrogen storage. In the present paper we use van der Waals-corrected density functional theory in conjunction with a quantum-thermodynamic model to investigate the hydrogen storage capacity of confining Li-decorated borophene sheets in its most stable Pmmn8 configuration. Our theoretical approach surpasses the standard density functional theory calculations only valid at zero temperature and no pressure, thus providing the gravimetric and volumetric capacities as well as the isotherms in real conditions. We show that narrow Li-decorated slit pores of borophene have a good volumetric hydrogen storage capacity particularly at low temperature. Accordingly, nanoporous boron frameworks could be optimal for hydrogen storage in applications at low temperature. We compare the results with those corresponding to pristine graphene slit pores.

Efficient multiscale calculation results for microchannel mass transfer

When the channel height is reduced to a small value such as on the scales of 10 nm or 100 nm, the physical adsorbed layers on the channel walls will participate in the flow, although intermediate between them is a continuum fluid flow. The multiscale simulation results are presented for this multiscale mass transfer in a narrow slit pore based on the derived flow equations. The results are respectively compared with those calculated from conventional continuum flow theory and from the theory based on the solid layer assumption, when the fluid-wall interaction is respectively weak, medium and strong. The total mass flow rate of the two adsorbed layers is also compared with the mass flow rate of the intermediate continuum fluid. The obtained results show the importance of the incorporation of the adsorbed layer flow by the multiscale scheme when calculating the transferred mass in a microchannel.

Tailored Gas Adsorption Properties of Electrospun Carbon Nanofibers for Gas Separation and Storage.

Carbon nanofibers (CNFs) derived from electrospun polyacrylonitrile (PAN) were investigated with respect to their gas adsorption properties. By employing CO2 adsorption measurements, it is shown that the adsorption capacity and selectivity of the fibers can be tailored by means of the applied carbonization temperature. General pore properties of the CNFs were identified by Ar adsorption measurements, whereas CO2 adsorption measurements provided information about the ultramicroporosity, adsorption energies, and adsorption capacities. Ideal adsorbed solution theory (IAST) selectivities under practically relevant conditions were determined by evaluation of single‐component data for N2 and CO2. Especially for low carbonization temperatures, the CNFs exhibit very good low‐pressure adsorption performance and excellent CO2/N2 IAST selectivities of 350 at 20 mbar and 132 at 1 bar, which are attributed to a molecular‐sieve effect in very narrow slit pores. These IAST selectivities are some of the highest values for carbon materials reported in the literature so far and the highest IAST selectivities for as‐prepared, non‐post‐treated carbon ever.

Adsorption of water, methanol, and their mixtures in slit graphite pores.

The behavior of water, methanol, and water-methanol mixtures confined in narrow slit graphite pores as a function of pore size was investigated by Monte Carlo, hybrid Monte Carlo, and Molecular Dynamics simulations. Interactions were described using TIP4P/2005 for water, OPLS/2016 for methanol, and cross interactions fitted to excess water/methanol properties over the whole range of concentrations, which provide a rather accurate description of water-methanol mixtures. As expected for hydrophobic pores, whereas pure methanol is adsorbed already from the gas phase, pure water only enters the pore at pressures well beyond bulk saturation for all pore sizes considered. When adsorbed from a mixture, however, water adsorbs at much lower pressures due to the formation of hydrogen bonds with previously adsorbed methanol molecules. For all studied compositions and pore sizes, methanol adsorbs preferentially over water at liquid-vapor equilibrium conditions. In pure components, both water and methanol are microscopically structured in layers, the number of layers increasing with pore size. This is also the case in adsorbed mixtures, in which methanol has a higher affinity for the walls. This becomes more evident as the pore widens. Diffusion of pure water is higher than that of pure methanol for all pore sizes due to the larger size of the methyl group. In mixtures, both components present similar diffusivities at all pore sizes, which is explained in terms of the coupling of molecular movements due to strong hydrogen bonding between methanol and water molecules. This is particularly evident in very narrow pores, in which pure methanol diffusion is completely impeded on the time scale of our simulations, but the presence of a small amount of water molecules facilitates alcohol diffusion following a single-file mechanism. Additionally, our results indicate that pure water diffusivities display a non-monotonous dependence of pore size, due to effects of confinement (proximity to a fluid-solid-fluid transition induced by confinement as reported in previous work) and the dynamic anomalies of water.

Low Pressure Hysteresis in Materials with Narrow Slit Pores

Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy jumps by a finite value when touching slit walls are separated from each other. We consider a more realistic adhesion model by introducing a smoothly-varying adhesion force, which depends on the distance between the slit walls. The range of the attraction between the slit walls is found to have a pronounced influence on the shape of hysteresis loops at low vapor pressure. A large interaction range avoids an extraordinarily small relative vapor pressure necessary for pore closing, which is a precondition for low pressure hysteresis. Our extended model allows us to describe a discontinuity, which can appear in the desorption branch of swelling/shrinkage diagrams for hardened cement paste.

Characteristics of pore structure and fractal dimension of low-rank coal: A case study of Lower Jurassic Xishanyao coal in the southern Junggar Basin, NW China

In this study, a series of laboratory experiments were performed to firstly analyze the characteristics of the pore structure in low-rank coal (Ro<0.70%), and then relationships between fractal dimensions and coal rank, coal composition and pore structure, and their impacts on methane adsorbability and seepage ability were studied too. The desorption isotherms of low-rank coal mainly belongs to Type B, which are primarily caused by ink-bottle-shaped and narrow-slit pores that are good for coalbed methane (CBM) enrichment, but bad for seepage. The seepage pores are dominated by primary plant macropores (>1000nm), with a poor development of mesopores (100–1000nm). The fractal features of adsorbed pores and seepage pores were defined and calculated using fractal dimensions D2 (2.563–2.926) and D4 (2.683–3.263), respectively. D2 has a positive correlation with moisture content, while D4 has a positive correlation with Ro and fixed carbon and a negative correlation with the volatile content. Both D2 and D4 have a weaker association with the ash yield and no apparent relationship with coal macerals, indicating that coal-forming materials and environments have little influence on the pore structure fractal characteristics. D2 has a strongly correlation with the BET surface area (R2=0.9026) and the average pore diameter (R2=0.9841), again proving that D2 can be used to characterize the pore structure fractal dimension of adsorbed pores. D2 shows a negative correlation with the Langmuir volume (VL), but no apparent relationship with the Langmuir pressure (PL), indicating that the pore structure of absorbed pores has little effect on methane desorption during the process of CBM exploitation. D4 has weak positive correlation with permeability. Overall, fractal analysis is beneficial for better understanding the pore structure, methane adsorbability and seepage ability of low-rank coal.

Dislocation model for sorption hysteresis in deformable solids

Adsorption and desorption branches of sorption isotherms for soft materials often differ from each other even at low relative vapor pressures of the adsorbate. This low vapor pressure hysteresis may result from elastic deformations produced by the capillary tension that condensates exert on pore walls. A dislocation model for materials with very narrow slit pores is proposed. The shape of adsorption/desorption isotherms of soft solids with slit pores depends on a dimensionless parameter, which is proportional to Young's modulus, the square of pore width, and inversely proportional to the adhesion energy of the solid. The model designed for soft solids can be modified to describe low vapor pressure hysteresis in more rigid materials such as hardened cement pastes.

Sorption Hysteresis of Hardened Cement Pastes

Mesopores in porous solids can produce a pronounced sorption hysteresis at moderate and high reduced vapor pressures of the ambient gas that is condensed in the pores. Unlike to other conventional porous materials, cement pastes often behave exceptionally. The water sorption hysteresis frequently persists at very low humidity. This hysteresis is reflected in a corresponding hysteresis loop of the solid skeleton volume. We discuss a theoretical model based on the strong compression force exerted by a condensate on the walls of narrow slit pores embedded in an elastic solid. This compression force is shown to be capable of shifting walls of narrow slit pores. Humidity-dependent closing and reopening of slit pores can produce hysteresis loops even at low humidity.

Excess Adsorption of Helium in Extremely Narrow Slit Pores

Wave functions of quantum helium in narrow slit pores are strongly restricted; as such, quantum helium condensed in narrow slit pores displays different behaviors from that in bulk. Herein, we report the densities of helium adsorbed on carbon surfaces and in carbon slit pores with average pore widths of 0.7, 0.9, and 1.1 nm at 2–5 K. The density of adsorbed quantum helium in the 0.7-nm slit pores was significantly higher than those in the larger slit pores and bulk. The average layer density of helium in the 0.7-nm pores was also significantly higher than those in the larger slit pores, suggesting solid-like structure formation even under helium vapor condition. The highly dense state of helium in narrow slit pores is due to strong attractive potential effects in such slit pores.