Pore Blocking Effect Research Articles

Effect of pore constriction on adsorption behaviour in nanoporous carbon slit pore: a computer simulation

A Grand Canonical Monte Carlo simulation (GCMC) method is used to study the effects of pore constriction on the adsorption of argon at 87.3 K in carbon slit pores of infinite and finite lengths. It is shown that the pore constriction affects the pattern of adsorption isotherm. First, the isotherm of the composite pore is greater than that of the uniform pore having the same width as the larger cavity of the composite pore. Secondly, the hysteresis loop of the composite pore is smaller than and falls between those of uniform pores. Two types of hysteresis loops have been observed, irrespective of the absence or presence of constriction and their presence depend on pore width. One hysteresis loop is associated with the compression of adsorbed particles and this phenomenon occurs after pore has been filled with particles. The second hysteresis loop is the classical condensation–evaporation loop. The hysteresis loop of a composite pore depends on the sizes of the larger cavity and the constriction. Generally, it is found that the pore blocking effect is not manifested in composite slit pores, and this result does not support the traditional inkbottle pore hypothesis.

Polysilicato-iron for improved NOM removal and membrane performance

The natural organic matter (NOM) removal efficiency of polysilicato-iron (PSI) coagulants and the fouling potential of PSI pretreated waters have been studied using two microfiltration (MF) membrane types: polyvinylidene fluoride (PVDF-2) and polypropylene (PP). The results showed that PSI coagulant with a Si/Fe ratio of 1 (PSI-1) was the most effective, compared to conventional coagulants, in removing dissolved organic carbon (DOC) and in improving the fouling potential. A relative flux of unity through PVDF-2 membrane was achieved for both water sources pretreated with PSI-1. Aluminium-based coagulants, particularly aluminium chlorohydrate (ACH), worked best at lower coagulant dose. Increasing the coagulant dose to improve DOC removal led to increased membrane fouling, possibly due to increased level of unsettleable flocs and pore blocking. For PSI with larger floc size, the advantage of increased DOC removal was not overridden by the adverse effect of pore blocking. In addition, the residual neutral fraction in the waters and/or the presence of a filter cake on the membrane surfaces seemed to have a limiting effect on the fouling rates through both PP and PVDF-2 membranes to the extent that similar rates were obtained, despite substantial differences in DOC removal. In contrast, these limiting factors did not influence the fouling potential of PSI-1 treated waters through the PVDF-2 membrane, as suggested by the relative flux of unity for both water sources. It is suggested that the oxide deposits on the PVDF-2 membrane may act as a ‘screening layer’, acting as pre-filtration by the filter cake. This layer may be effectively removed by backwashing, together with deposited NOM, throughout the experiment to maintain the flux at unity. The hydrophobic nature of the PP membrane may discourage the deposition of the oxides, thus minimising the positive effects of the oxides in the system. The high removal of hydrophobic fractions by PSI-1 may also lead to less association between residual NOM and less binding to the membranes, particularly on the PVDF-2 membrane.

Subtype-specific, bi-component inhibition of SK channels by low internal pH

The effects of low intracellular pH (pH i 6.4) on cloned small-conductance Ca 2+-activated K + channel currents of all three subtypes (SK1, SK2, and SK3) were investigated in HEK293 cells using the patch-clamp technique. In 400 nM internal Ca 2+ [Ca 2+] i, all subtypes were inhibited by pH i 6.4 in the order of sensitivity: SK1 > SK3 > SK2. The inhibition increased with the transmembrane voltage. In saturating internal Ca 2+, the inhibition was abolished for SK1–3 channels at negative potentials, indicating a [Ca 2+] i-dependent mode of inhibition. Application of 50 μM 1-ethyl-2-benzimidazolone was able to potentiate SK3 current to the same extent as at neutral pH i. We conclude that SK1–3 all are inhibited by low pH i. We suggest two components of inhibition: a [Ca 2+] i-dependent component, likely involving the SK β-subunits calmodulin, and a voltage-dependent component, consistent with a pore-blocking effect. This pH i-dependent inhibition can be reversed pharmacologically.

Control of ordered structure and morphology of large-pore periodic mesoporous organosilicas by inorganic salt

Highly ordered rodlike periodic mesoporous organosilicas (PMO) were successfully synthesized using 1,2-bis(trimethoxysilyl)ethane as an precursor and triblock copolymer P123 as a template at low acid concentration and in the presence of inorganic salts (KCl). The role of acid and salt as well as the effects of synthesis temperature and reactant mole ratio in the control of morphology and the formation of ordered mesostructure was systematically examined. It was found that the addition of inorganic salt can dramatically expand the range of the synthesis parameters to produce highly ordered PMO structure and improve the quality of PMO materials. The morphology of PMOs was significantly dependent on the induction time for precipitation. The uniform PMO rods can only be synthesized in a narrow range of acid and salt concentrations. The results also show that the optimized salt concentration (1 M) and low acidity (0.167 M) were beneficial to the formation of not only highly ordered mesostructure but also rodlike morphology. Increasing acidity resulted in fast hydrolysis reaction and short rod or plate-like particles. Highly ordered rod can also be prepared at low temperature (35 °C) with high salt amount (1.5 M) or high temperature (45 °C) with low salt amount (0.5 M). Optimum reactant molar composition at 40 °C is 0.035P123:8KCl:1.34HCl:444H 2O:1.0bis(trimethoxysilyl)ethane. Lower or higher SiO 2/P123 ratio led to the formation of uniform meso-macropores or pore-blocking effect.

Effect of the Carbon Surface Layer Chemistry on Benzene Adsorption from the Vapor Phase and from Dilute Aqueous Solutions

We present a complex study of benzene adsorption on chemically modified commercial activated carbons. The porosity of studied carbons is almost the same, whereas the chemical composition and the acid-base properties of surface layers differ drastically from amphoteric (initial de-ashed carbon D43/1, Carbo-Tech, Essen, Germany) and acidic (carbon modified with concentrated HNO3 and fuming H2SO4) to strongly basic (carbon modified with gaseous NH3). Benzene adsorption isotherms measured from aqueous solution at three temperatures (298, 313, and 323 K) and at the neutral pH level are reported. They are supported by studies of water and benzene adsorption from the gaseous phase (volumetric and calorimetric data) and the data of benzene temperature-programmed desorption (TPD). Moreover, the data of the enthalpy of immersion in water and benzene are also presented. Obtained data of benzene adsorption from the gaseous phase are approximated by applying the method of Nguyen and Do (ND) and the Dubinin-Astakhov (DA) equation. The data of adsorption from solution are described by the hybrid DA-Freundlich (DA-F) model. We show that there are similarities in the mechanisms of benzene adsorption from the gaseous phase and from aqueous solutions and that the pore-blocking effect is the main stage of the adsorption mechanism. This effect strongly depends on the polarity of the carbon surface. The larger the ratio of the enthalpy of carbon immersion in water to the enthalpy of immersion in benzene, the larger the reduction in adsorption from solution, compared to that in the gaseous phase, that is observed.

Silane modified inorganic membranes: Effects of silane surface structure

Surface modification of mesoporous, inorganic substrates by organo-silanes to enhance the gas separation properties was studied with an emphasis on understanding the influence of the silane layer structure. Dimethyloctadecylchlorosilane (OCS) was used to create a polymer brush on the surface of mesoporous Vycor glass and octadecyltrichlorosilane (ODS) was used to create a polymerized silane layer. The membranes were characterized by gas permeation as well as indirectly by NMR spectroscopy on silica gel that had undergone same silane treatment as that of the membranes. All the modified membranes were selective for n-butane over nitrogen at 295 K. A variety of reaction and pretreatment conditions were studied to optimize the loading of monochlorosilanes. The effect of varying the silanization reaction temperature was minimal on the selectivity; however, the pure gas selectivity increased by a factor of three by varying the pretreatment conditions. For a typical OCS modified membrane, the pure n-butane/nitrogen selectivity was 7.8 as compared to the mixed gas selectivity of 72.9 for a 50% mixture of n-butane in nitrogen. This difference was probably due to the pore blocking effects of n-butane. In comparison to the OCS membrane, the ODS membrane was seven times more selective for n-butane over nitrogen in the pure gas experiments. However, in the mixed gas experiments, higher n-butane/nitrogen separation factors were observed for the membrane modified with OCS as compared to the ODS-modified membrane.

Estimation of diffusion parameters in functionalized silicas with modulated porosity: Part II: Pore network modeling

In this work, the pore structure of those five (5) silicas SiO 2–X (see Part I) which have suffered gradual functionalization with functional groups X of increasing length (X = Ø, ≡Si–H, ≡Si–CH 2OH, ≡Si–(CH 2) 3OH, ≡Si–(CH 2) 11CH 3), is modeled as a three-dimensional cubic network of cylindrical pores. Those hybrids organic–inorganic SiO 2–X samples are characterized by different extent of pore blocking effects. The pores of samples are represented in a 9 × 9 × 9 lattice by the nodes as well the bonds that are interconnected in a so-called dual site-bond model, DSBM, network. The pore network is developed using a Monte Carlo statistical method where the cylindrical pores (nodes and bonds) are randomly assigned into the lattice, until matching of the theoretical results to the experimental data of N 2 adsorption–desorption measurements. Thus, a visual picture of the porous solid is possible. This realistic network is used next in order to study the steady-state gas transport (Knudsen gas-phase and viscous diffusion) properties for the examined materials and how flow processes depend on the morphology of the pore structure. The pore diffusivity D p and total permeability P of each porous medium is determined based on theoretical calculations and the structural statistical parameters, such as porosity ɛ p, tortuosity factor τ and connectivity c of pores is discussed with the corresponding experimental data described in Part I of this work. The results indicate clearly that the diffusion model made it possible to predict pore effective diffusivity in these porous media in very good agreement with the corresponding experimental results for all the examined solids (Part I). The pore diffusivity increases significantly as the value of the pore connectivity increases but the transport properties of the network are influenced strongly at lowest connectivity. Also the predicted tortuosity factor is related inversely to the extent of interconnection of pores in these solids, which indicates that the influence of pore branching to the tortuosity factor of the pore network decreases, as connectivity increases.

An Inactivation Stabilizer of the Na+ Channel Acts as an Opportunistic Pore Blocker Modulated by External Na+

The Na+ channel is the primary target of anticonvulsants carbamazepine, phenytoin, and lamotrigine. These drugs modify Na+ channel gating as they have much higher binding affinity to the inactivated state than to the resting state of the channel. It has been proposed that these drugs bind to the Na+ channel pore with a common diphenyl structural motif. Diclofenac is a widely prescribed anti-inflammatory agent that has a similar diphenyl motif in its structure. In this study, we found that diclofenac modifies Na+ channel gating in a way similar to the foregoing anticonvulsants. The dissociation constants of diclofenac binding to the resting, activated, and inactivated Na+ channels are ∼880 μM, ∼88 μM, and ∼7 μM, respectively. The changing affinity well depicts the gradual shaping of a use-dependent receptor along the gating process. Most interestingly, diclofenac does not show the pore-blocking effect of carbamazepine on the Na+ channel when the external solution contains 150 mM Na+, but is turned into an effective Na+ channel pore blocker if the extracellular solution contains no Na+. In contrast, internal Na+ has only negligible effect on the functional consequences of diclofenac binding. Diclofenac thus acts as an “opportunistic” pore blocker modulated by external but not internal Na+, indicating that the diclofenac binding site is located at the junction of a widened part and an acutely narrowed part of the ion conduction pathway, and faces the extracellular rather than the intracellular solution. The diclofenac binding site thus is most likely located at the external pore mouth, and undergoes delicate conformational changes modulated by external Na+ along the gating process of the Na+ channel.

Estimation of diffusion parameters in functionalized silicas with modulated porosity: Part I: Chromatographic studies

The diffusion parameters of binary gas mixture He (tracer gas)–N 2 (carrier gas) in hybrid organic–inorganic SiO 2-X porous solids which have suffered gradual functionalization with functional groups X of increasing length (X = ∅, Si–H, Si–CH 2OH, Si–(CH 2) 3OH, Si–(CH 2) 11CH 3) are reported. The effective diffusivities D eff, the Henry law constants K as well as the tortuosity factors τ for the examined solids were estimated by a typical pulse gas chromatographic method. Analysis of the experimental results was carried out by the well-known method of linearization of moments. The moments s analysis provides a powerful means for extracting diffusion parameters from the experimental response curves The proposed methodology is simple compared to other similar studies and provides rapidly reliable data. The results of this work indicate that the effective diffusivity D eff in porous networks drops markedly as the initial porosity of the parent SiO 2 sample is blocked by the functionalization of the pore surfaces with functional groups of increasing size, Si–H, Si–CH 2OH, Si–(CH 2) 3OH and Si–(CH 2) 11CH 3. The low values of the Henry law constants K found indicate that the adsorption of He on the porous surfaces for all the solids is weak. Also, the tortuosity factor τ is proportionally correlated to the pore blocking effects and the percolation phenomena of gases flowing into the porous network.

Three-Dimensional Mesoporous Titanosilicates Prepared by Modified Sol−Gel Method: Ideal Gold Catalyst Supports for Enhanced Propene Epoxidation

Mesoporous titanosilicates with 1-12 mol % Ti content and with three-dimensional wormhole-like mesoporosity are prepared by a modified sol-gel technique. Sorption analysis shows that there is little change in the surface properties with increasing Ti concentration in the samples, implying that Ti atoms either are well-dispersed on the walls of the silica matrix or are present inside the framework with no pore blocking effect. Spectroscopic analysis shows that the Ti atoms are atomically dispersed in the silica matrix even at very high Ti concentration and there is no observable Ti aggregate (anatase) present in the samples. These titanosilicate samples after Au deposition followed by trimethylsilylation (for enhanced hydrophobicity) are highly efficient catalysts for vapor-phase propene epoxidation using O2 and H2. It was possible to achieve commercially desirable performance with about 7% propene conversion, >90% propene oxide selectivity, and about 40% hydrogen efficiency.

Corrosion protective properties of nanostructured sol–gel hybrid coatings to AA2024-T3

The work aims at investigating the corrosion behavior of AA2024-T3 substrates pretreated with sol–gel derived hybrid coatings. The hybrid sols were prepared by copolymerization of 3-Glycidoxypropyltrimethoxysilane (GPTMS), tetraethylortosilicate (TEOS) and tetra- n-propoxyzirconium (TPOZ). Coatings with Zr/Si atomic ratios of 1:9 and 1:4 were also prepared. During the synthesis process, there is formation of ZrO 2 nanoparticles, which content depends on the temperature and time of the TPOZ hydrolysis/condensation reactions. The results show that the ZrO 2 nanoparticles have diameters ranging between 40 and 200 nm. These particles are amorphous and they improve the corrosion protection of the sol–gel coatings. The presence of ZrO 2 nanoparticles seems to have a pore blocking effect in the coatings with higher content of particles.

Variation in surface fractal of graphite due to the adsorption of polyoxyethylene sorbitan monooleate

The fractal analysis is carried out to study the influence of adsorption of polyoxyethylene sorbitan monooleate (Tween 80) on the surface properties of graphite. The surface fractal dimension ( d SF ), BET surface area ( S BET ) and pore size distribution (PSD) are calculated from low temperature nitrogen adsorption isotherms. The decline in the d SF of graphite surface is found as the adsorption amount of Tween 80 increases, which suggests that the adsorbed Tween 80 smoothes the graphite surface. Additionally, the observation of atomic force microscopy (AFM) proves that the original slit pores in pure graphite are blocked up and the step defect sites are screened by Tween 80, which may result in the reduction of graphite roughness. The PSD pattern of graphite changes after the adsorption due to the pore blocking effect. S BET of the graphite decreases as the adsorption amount of Tween 80 increases, which is attributed to both pore blocking effect and surface screening effect.

A grand canonical Monte Carlo study of capillary condensation in mesoporous media: effect of the pore morphology and topology.

We study by means of Grand Canonical Monte Carlo simulations the condensation and evaporation of argon at 77 K in nanoporous silica media of different morphology or topology. For each porous material, our results are compared with data obtained for regular cylindrical pores. We show that both the filling and emptying mechanisms are significantly affected by the presence of a constriction. The simulation data for a constricted pore closed at one end reproduces the asymmetrical shape of the hysteresis loop that is observed for many real disordered porous materials. The adsorption process is a quasicontinuous mechanism that corresponds to the filling of the different parts of the porous material, cavity, and constriction. In contrast, the desorption branch for this pore closed at one end is brutal because the evaporation of Ar atoms confined in the largest cavity is triggered by the evaporation of the fluid confined in the constriction (which isolates the cavity from the gas reservoir). This evaporation process conforms to the classical picture of "pore blocking effect" proposed by Everett many years ago. We also simulate Ar adsorption in a disordered porous medium, which mimics a Vycor mesoporous silica glass. The adsorption isotherm for this disordered porous material having both topological and morphological defects presents the same features as that for the constricted pore (quasicontinuous adsorption and steep desorption process). However, the larger degree of disorder of the Vycor surface enhances these main characteristics. Finally, we show that the effect of the disorder, topological and/or morphological, leads to a significant lowering of the capillary condensation pressure compared to that for regular cylindrical nanopores. Also, our results suggest that confined fluids isolated from the bulk reservoir evaporate at a pressure driven by the smallest size of the pore.

Intrusion and extrusion of water in highly hydrophobic mesoporous materials: effect of the pore texture

Understanding of the physics of confined fluids is of major theoretical and practical interest. In the field of materials science, the most popular techniques used to characterise the texture of mesoporous supports are based on the analysis of the properties of a confined fluid:thermoporometry, adsorption technique and mercury intrusion porosimetry. In particular, in the last two techniques the pressure parameter is related to the dimension of the pores. The present study is dedicated to an original analysis of the forced-intrusion of a non-wetting liquid (water) in hydrophobic mesoporous materials presenting different pore topologies. Among these supports, MCM-41 type materials allowed to point out distinct mechanisms for the intrusion and the extrusion of water. Whereas the intrusion process obeys to the Laplace law of capillarity, the extrusion is found to be preferentially governed by the formation of the vapor phase by a mechanism such as nucleation. Therefore, the conventional interpretation given for the hysteresis observed in mercury porosimetry, based on wetting hysteresis and pore-blocking effects is not directly transposable to the case of water intrusion experiments. Data obtained on other supports such as silica gels and Controlled Pore Glass supports are also reported and discussed taking into account the results obtained in MCM-41 type materials. Finally, the mechanisms leading to hysteresis in both phenomena (sorption of a wetting fluid and forced intrusion of a non-wetting liquid) are shown to present strong analogies in model materials as well as in disordered and interconnected porosities.

The surface fractal investigation on carbon nanotubes modified by the adsorption of poly(acrylic acid)

In this paper, surface fractal analysis is carried out to study the surface of carbon nanotubes after the adsorption of poly(acrylic acid) (PAA) using the thermodynamic method. The fractal dimension ( d SF ) of a fractal surface, BET surface area ( S BET) and pore size distribution (PSD) are calculated from low-temperature nitrogen adsorption isotherms. The value of d SF declines as the adsorption amount of PAA increases, which means the adsorption may lower the surface roughness of the carbon nanotubes. The PSD pattern was modified obviously after the adsorption of PAA because of the pore-blocking effect. Additionally, an excellent linear decrease in the BET surface area of carbon nanotubes is found as the adsorption amount of PAA increases, which can be attributed to both the pore-blocking effect and the surface-screening effect. The results of the present work may facilitate our understanding of the interaction between polymer and the carbon nanotubes at the microscale occurring on irregular interfaces.

Experimental Evidence for Pore Blocking as the Mechanism for Nitrogen Sorption Hysteresis in a Mesoporous Material

The theory of “pore blocking” is often used, particularly in combination with percolation theory, to develop methods of analysis for gas sorption data for the structural characterization of porous media. However, the evidence for the existence of the pore-blocking effect is conflicting and equivocal. In the work described here mercury porosimetry experiments were integrated with gas sorption to detect the existence of pore-blocking effects during the desorption of nitrogen from a mesoporous sol−gel silica material at 77 K.

Improved catalytic activity upon Ge incorporation into ZSM-5 zeolites

The catalytic stability of ZSM-5 can be greatly improved by the isomorphous substitution of some Si atoms by Ge. The catalytic properties of Ge-ZSM-5 zeolites have been compared to ZSM-5 in a series of acid-catalysed test reactions (the dehydration of 2-propanol and 2-butanol, the isomerisation of isobutyraldehyde to 2-butanone, and the Friedel–Crafts acylation of anisole). The propene yield in the 2-propanol dehydration reaction at 180 °C is as high as 98% for Ge-ZSM-5 and only 40% for ZSM-5. For the other investigated reactions similar differences are observed. The presence of a considerable amount of mesopores in Ge-ZSM-5 (compared to ZSM-5) is proposed to be the main explanation for the difference. These mesopores are formed upon synthesis and reduce the effect of pore blocking by coke residues that are formed at high reaction temperatures.

Effects of cation exchange on the pore and surface structure and adsorption characteristics of montmorillonite

Ca-montmorillonite was exchanged with the metal cations, titanium, iron, and copper and the ion-exchange effects on the surface area, pore structure and adsorption property of the montmorillonite were studied. The changes in the pore structure and the surface roughness of the montmorillonite were first characterized based on the classical and fractal analyses of the nitrogen adsorption isotherms as well as the XRD and TGA/DSC patterns. Then, the adsorption isotherms of benzene, hexane, and cyclohexane were measured to identify how exchange process affected the adsorption characteristics of the montmorillonite. It was found that the ion-exchange processes could induce enormous changes in the surface and pore structure of montmorillonite and these changes could be interpreted with the coverture of the surface roughness (surface screening effect), the inhibition of the movement of nitrogen molecule into pores (pore blocking effect), and the interlamellar expansion of the montmorillonite (pore opening effect). The effect of alterations of the surface area and the pore structure on the adsorption characteristics of montmorillonite was discussed.

Adsorption of basic dyes onto montmorillonite

Ca-montmorillonite (Ca-Mont) was exchanged with titanium cations and the adsorption equilibrium and kinetics of Basic Green 5 (BG5) and Basic Violet 10 (BV10) on these montmorillonites were measured to examine the ion-exchange effects on the basic dyes adsorption. The relationship between the dye adsorption and the alteration of pore structures of montmorillonite induced by ion-exchange processes was discussed. Moreover, the changes in the surface and pore structure of montmorillonites during adsorption were characterized based on classical and fractal analyses of the nitrogen adsorption isotherms as well as the XRD patterns. The decrease in BET surface area of montmorillonites after adsorption of dyes was interpreted in terms of both the coverage of some surface roughness (surface screening effect) and the inhibition of the movement of nitrogen molecule into some pores (pore blocking effect). The surface fractal dimension D was used to examine whether or not the surface screening effect exists and the pore blocking effect was examined with the changes of mean pore size before and after adsorbing basic dyes.

Separation of organic/water mixtures with silylated MCM-48 silica membranes

MCM-48 membranes were synthesized on an alumina support. The MCM-48 membranes were silylated with trimethylchlorosilane (TMCS) to enhance the hydrothermal stability and hydrophobicity. X-ray diffraction, N 2 adsorption, and permporometry were carried out with the MCM-48 and the silylated MCM-48 (sily-MCM-48) membranes and powders. The XRD patterns indicated that the structure of the MCM-48 membrane was retained after calcination and silylation. The sily-MCM-48 membranes were used for the separation of ethanol (EtOH), methylethyl ketone (MEK), and ethyl acetate (EA) in an aqueous solution. At 303 K, the separation factor and the flux were higher in the order: EA > MEK > EtOH, a finding which was related to the hydrophobicity of organic molecules. The separation factors of MEK/water increased with a decrease in the feed concentration from 85 (20 wt.%) to 318 (1 wt.%). The separation factors of MEK/water (5:95) and EA/water (5:95) were, respectively, 400 and 1000 at 283 K, and 45 and 100 at 323 K. The adsorbed organic molecules must have blocked the pores of the sily-MCM-48 membrane, especially at low temperatures. The pore blocking effect due to the selective adsorption of the organic molecules caused the obstruction of the diffusion of water molecules in the pores. From the results of the permporometry, we found that the average Kelvin diameter of the sily-MCM-48 membranes was 1.9 nm, which is consistent with the intrinsic pore size of sily-MCM-48. We believe that the highly selective separation of organic/water mixtures is attributable not only to the hydrophobic nature of the pore surface but also to the uniform mesopores of sily-MCM-48. Up to 723 K, the sily-MCM-48 membrane was thermally stable; furthermore, repeated calcination and silylation were effective in regenerating it.