Effective Pore Size Research Articles

Water vapor adsorption isotherm expressions based on capillary condensation

Four comprehensive isotherm expressions are proposed for various water vapor adsorption mechanisms. Water vapor adsorption in micropores and on macropore walls is described using the Langmuir and Freundlich models, respectively. Water vapor condensation in the pore core volume of macropores is modeled by assuming that the effective pore size follows a Gaussian and uniform distribution. These expressions are used to fit water adsorption data on various kinds of alumina. The results show that the proposed water vapor adsorption isotherm expressions fit the water adsorption data well.

Preparation and characterization of PVDF-P(PEGMA-r-MMA) ultrafiltration blend membranes via simplified blend method

P(PEGMA-r-MMA) amphiphilic copolymer with Mn of 66,500g/mol and Mw of 34,200g/mol was successfully synthesized via free radical polymerization. And polyvinylidene fluoride (PVDF)-P(PEGMA-r-MMA) blend membranes were fabricated from water and ethanol coagulants via simplified blend method by directly blending PVDF and P(PEGMA-r-MMA) amphiphilic copolymer solution (including the reaction mixture) to form casting solution. The formation of the supramolecular aggregates in PVDF solution containing the copolymer were confirmed by dynamic light scattering and scanning electron microscopy. This contributed to the micro-structure adjustment of PVDF solution and resulted in its decreasing surface tension, accelerating precipitation rate and increasing viscosity with trivial strain thinning behavior. Furthermore, the effects of the variations in dopant contents and coagulant compositions on the performances of those blend membranes were investigated. All PVDF-P(PEGMA-r-MMA) blend membranes possessed narrow distribution mean effective pore size (μ), molecular weight cut off (MWCO), improved recovery water flux after filtration experiments of bovine serum albumin and tuned configurations. Compared with the instantaneous demixing process in water coagulant, the delayed demixing process in ethanol favored the pore-forming and surface segregated of the polar head group of the copolymer, which induced the increasing μ, MWCO, tunable morphologies and hydrophilicity improvement.

Fabrication and characterization of PVDF membranes via an in situ free radical polymerization method

In situ free radical polymerization was proven to be successful for the preparation of PVDF membranes with superior mechanical behaviors and enhanced antifouling properties. Micro-structure adjustment of PVDF solution via in situ polymerization was investigated by dynamic light scattering (DLS) and scanning electron microscopy (SEM). The DLS and SEM results verified the formation of supramolecular polymer–copolymer aggregates. The narrow distribution of the supramolecular aggregates explained the decreasing surface tension, increasing viscosity with trivial strain thinning behavior and accelerating precipitation rate of the PVDF casting solution. The resultant PVDF membranes possessed narrowly distributed pore size and molecular weight cut off (MWCO) of the final filtration properties, which were attributed to the aggregates; additionally, their corresponding mean effective pore size (μ) and MWCO increased with increasing concentration of the monomers via in situ polymerization. Further, in situ polymerization not only enlarged the recovery water flux after filtration experiments with bovine serum albumin (BSA) but also improved the hydrophilicity of both the top-surface and the bottom-surface of the PVDF membranes. Additionally, in situ polymerization modulated the PVDF membrane configurations, varying from stripe-shaped grains to agglomerates of globule (with one of the globules bearing on the surface of another). The tunable morphologies, combined with the progressive enhanced crystallinity of the α and β phase (largely in the β phase form), were used to interpret the superior mechanical properties of the resulting PVDF membranes.

Controlling the Effective Surface Area and Pore Size Distribution of sp2 Carbon Materials and Their Impact on the Capacitance Performance of These Materials

A series of sp(2) carbon materials with different specific surface area (SSA) and controlled pore size distribution (PSD) were synthesized at large scale through a facile and low-cost method. The SSA and PSD of these carbon materials were controlled by using different carbon sources and preparation methods. With different total and effective SSA (E-SSA) and PSD, the impacts on their capacitance performance were investigated thoroughly, which demonstrated that both E-SSA and PSD played the most important roles in their capacitance performance. Furthermore, theoretical modeling was performed, and the results are in agreement with the experimental results for the influence of E-SSA and PSD on their capacitance performance. Based on these, a general model using the slit/cylindrical NL-DFT approach is proposed for the estimation of the specific capacitance of sp(2) carbon materials, which offers a simple but reliable method to predict the capacitance performance of these materials, thus speeding up the design and screening of the materials for high-performance supercapacitor and other surface area related devices.

Development of Nanoporous Structure in Carbons by Chemical Activation with Zinc Chloride

Series of activated carbons (ACs) have been prepared from Lapsi (Choerospondias axillaris) seed powder (LSP) by chemical activation with zinc chloride (ZnCI2) and the effects of ZnCl2 impregnation ratio, carbonization time, and precursor sources on the structure and properties of ACs have been systematically investigated. Carbonization was carried out at 400 degrees C and the ratio of LSP and ZnCI2 was varied from LSP:ZnCl2 = 1:0.25 (AC-0.25), 1:0.50 (AC-0.50) 1:1 (AC-1), 1:2 (AC-2), and 1:4 (AC-4). The ACs were characterized by Fourier transform-infrared (FTIR) spectroscopy, Raman scattering, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Surface properties (effective surface areas, pore volumes, and pore size distributions) were studied by nitrogen adsorption-desorption measurements. The electrochemical and vapor sensing properties were investigated by cyclic voltammetry, and quartz crystal microbalance (QCM) method, respectively. All the ACs are amorphous materials containing oxygenated surface functional groups and having nanoporous (microporous and mesoporous) structures. We found that surface properties depend on the LSP:ZnCI2 ratio, carbonization time, and also on the precursor type. The effective surface area increased significantly with increasing LSP:ZnCI2 ratio from 1:0.25 to 1:0.5 and then remain apparently constant. However, total pore volume increased continuously with ZnCI2 ratio. Increase in the carbonization time above 4 h decreased both the surface area and pore volume. ACs prepared from bamboo and coconut shell showed better surface properties compared to AC prepared from sugarcane; surface area and pore volume of the former systems are nearly double of the later system. AC derived from LSP (AC-4) showed excellent electrochemical performance giving specific capacitance value of 328 F/g in 1 M H2SO4 solution demonstrating the potential use of this material for supercapacitor electrodes. Our ACs showed good capability of molecule sensing of toxic solvent vapors such as carbon tetrachloride and pyridine.

Quantum cryo-sieving for hydrogen isotope separation in microporous frameworks: an experimental study on the correlation between effective quantum sieving and pore size

Different microporous frameworks have been experimentally investigated by cryogenic (below 77 K) hydrogen and deuterium adsorption measurements. The D2/H2 molar ratio is strongly dependent on pore size, pressure and temperature. The experiments clearly indicate that the optimum pore diameter for quantum sieving lies between 3.0 A and 3.4 A.

Bi-porous metal–organic framework with hydrophilic and hydrophobic channels: selective gas sorption and reversible iodine uptake studies

A new three-dimensional (3D) bi-porous metal–organic framework (MOF), {[(Me2NH2)2]·[Cd3(5-tbip)4]·2DMF}n (1) (5-tbipH2: 5-tert-butylisophthalic acid) has been synthesized. The 5-tbip ligand containing a hydrophobic t-butyl group and hydrophilic carboxylate groups is used to synthesize the bi-porous framework. This 3D MOF contains two types of channels, a wide mouth hydrophilic channel of dimension 7.448 × 7.676 A2 and a narrow mouth hydrophobic channel of dimension 2.33 × 1.926 A2. Hydrophilic channels are lined with the orderly arranged dimethyl ammonium (DMA) cations which neutralize the anionic 3D framework. The guest-free form of the MOF (1′) showed interesting CO2 selectivity over other gases such as N2, CH4, and H2. Since the effective pore size of the desolvated compound 1′ (∼0.6 nm: from the pore size distribution curve of CO2 adsorption measurement) is much more than the kinetic diameter of all measured gases (CO2 = 3.3 A, CH4 = 3.76 A, N2 = 3.64 A and H2 = 2.8 A), selective capture of CO2 by 1′ could be ascribed to the strong electrostatic interaction of CO2 with the framework. Compound 1′ also shows reversible iodine uptake (1′ ⊂ 4I2: based on the sample weight measurements and TGA data) with visible color change of the compound. Interestingly, the iodine-loaded MOF showed ∼76 times increase in electrical conductivity compared to 1′.

Sensitive Nanochannel Biosensor for T4 Polynucleotide Kinase Activity and Inhibition Detection

5'-Polynucleotide kinase is a crucial class of enzyme that catalyzes the phosphorylation of nucleic acids with 5'-hydroxyl termini. This process regulates many important cellular events, especially DNA repair during strand damage and interruption. The activity and inhibition of nucleotide kinase have proven to be an evident effect on cellular nucleic acid regulation and metabolism. Here, we describe a novel nanochannel biosensor for monitoring the activity and inhibition of T4 polynucleotide kinase (PNK), a famous member of the 5'-kinase family playing a major role in the cellular responses to DNA damage. On the basis of the functionalized nanochannel system and coupled λ exonuclease cleavage reaction, the nanochannel-sensing platform exhibits high sensitivity and convenience toward kinase analysis. Biotin-labeled dsDNA effectively blocks the streptavidin-modified nanochannel through forming a closely packed arrangement of DNA structure inside the channel. When dsDNA is phosphorylated by PNK and then immediately cleaved by λ exonuclease, the pore-blocking effect almost disappears. This PNK-induced microstructural distinctness can be directly and accurately monitored by the nanochannel system, which benefits from its high sensitivity to the change of the effective pore size. Furthermore, modification convenience and mechanical robustness also ensure the stability of the test platform. This as-proposed strategy exhibits excellent analytical performance in both PNK activity analysis and inhibition evaluation. The simple and sensitive nanochannel biosensor shows great potential in developing on-chip, high-throughput assays for fundamental biochemical process research, molecular-target therapies, and clinic diagnostics.

Material Properties That Predict Preservative Uptake for Silicone Hydrogel Contact Lenses

To assess material properties that affect preservative uptake by silicone hydrogel lenses. We evaluated the water content (using differential scanning calorimetry), effective pore size (using probe penetration), and preservative uptake (using high-performance liquid chromatography with spectrophotometric detection) of silicone and conventional hydrogel soft contact lenses. Lenses grouped similarly based on freezable water content as they did based on total water content. Evaluation of the effective pore size highlighted potential differences between the surface-treated and non-surface-treated materials. The water content of the lens materials and ionic charge are associated with the degree of preservative uptake. The current grouping system for testing contact lens-solution interactions separates all silicone hydrogels from conventional hydrogel contact lenses. However, not all silicone hydrogel lenses interact similarly with the same contact lens solution. Based upon the results of our research, we propose that the same material characteristics used to group conventional hydrogel lenses, water content and ionic charge, can also be used to predict uptake of hydrophilic preservatives for silicone hydrogel lenses. In addition, the hydrophobicity of silicone hydrogel contact lenses, although not investigated here, is a unique contact lens material property that should be evaluated for the uptake of relatively hydrophobic preservatives and tear components.

Seeded growth, silylation, and organic/water separation properties of MCM-48 membranes

A seeded growth method is reported for synthesis of MCM-48 membranes on α-alumina supports, thereby extending the seeded growth technique used for zeolite membranes to mesoporous silica membrane synthesis. The surface properties of the MCM-48 membranes are then modified by silylation with hexamethyldisilazane. The physicochemical properties of these membranes are characterized by multiple techniques. X-ray diffraction demonstrates that the structure of the MCM-48 membranes is maintained after surfactant removal and silylation. In comparison to MCM-48 membranes previously synthesized by the in situ growth technique, much less silica infiltration into the alumina support is observed by energy dispersive X-ray spectroscopy. The pore structure of the MCM-48 membranes is assessed by direct nitrogen physisorption measurements on membrane samples, demonstrating that a large accessible pore volume is available for molecular permeation and pore modification to tailor selectivity. The gas permeation properties of the calcined and silylated MCM-48 membranes are consistent with a Knudsen-like mechanism, albeit with a substantial influence of gas-solid interactions in the mesopores. The silylated MCM-48 membranes are evaluated for pervaporative separation of ethanol (EtOH), methyl ethyl ketone (MEK), and ethyl acetate (EA) from their dilute aqueous solutions (5wt%). The membranes showed pervaporative separation factors in the range of 5–35 and organic fluxes in the range of 0.03–0.22kg-m−2h−1 at 303K. The intrinsic selectivities (water/organic permeability ratios) of the silylated membranes are 6 (water/EtOH), 10 (water/MEK), and 26 (water/EA) respectively, in comparison to selectivities of 66, 450, and 3900 for the unmodified membranes, at 303K. The selective separation of organic/water mixtures with MCM-48 membranes is attributed to both the organophilic nature of the surface and the effective pore size of the silylated mesopores.

The role of shape selectivity in catalytic fast pyrolysis of lignin with zeolite catalysts

Catalytic fast pyrolysis (CFP) of lignin with four different zeolite catalysts was investigated to determine the role of shape selectivity of zeolites in CFP. These zeolites included ZSM-5, mordenite, beta, and Y zeolites, which have various crystallographically determined static pore sizes between 5.6 and 7.6Å. The molecular dimensions of pyrolysis products, including lignin-derived oxygenates and aromatics, were calculated using quantum chemical computations. The effective pore sizes of the four zeolites at 650°C were then determined by analyzing the molecular size and conversion behavior of the pyrolysis products in CFP. Results suggest that thermal distortion of the zeolite pore structure under high-temperature conditions of CFP effectively enlarge the crystallographically determined pore sizes of the zeolites by 2.5–3.4Å. Therefore, many lignin-derived oxygenates with a molecular size considerably larger than the static pore size were able to enter the pores of the zeolites and become effectively converted in our CFP tests. Bulkier monolignols derived from syringyl lignin, however, could not be effectively converted by ZSM-5 and mordenite zeolites due to size exclusion or pore blockage. Among the four zeolites, ZSM-5 produced the highest aromatic yield, followed in order by beta, mordenite, and Y zeolites. Beta and Y zeolites were the most effective catalysts for deoxygenating lignin-derived oxygenates. This analysis indicates that for CFP of softwood, ZSM-5 is the optimal catalyst because it can achieve satisfactory deoxygenation and aromatic production simultaneously, whereas for hardwood feedstock, beta zeolite may be used to convert bulky oxygenates derived from syringyl lignin.

Nanoporous Activated Carbon Derived from Lapsi (Choerospondias Axillaris) Seed Stone for the Removal of Arsenic from Water

Activated carbons were prepared from Lapsi (Choerospondias axillaris) seed stone by zinc chloride (ZnCl2) activation at three different Lapsi seed powder (LSP):ZnCl2 ratios: 1:0.5 (AC-0.5), 1:1 (AC-1), and 1:2 (AC-2). The properties of these activated carbons (ACs), including effective surface areas, pore volumes, and pore size distributions were characterized from N2 adsorption-desorption isotherms. The ACs obtained were essentially nanoporous (including both micro- and mesoporous) with effective surface area ranging from 1167 to 1328 m2/g. Fourier-transform infrared (FTIR) spectroscopy showed the presence of functional groups on the surface of ACs. Scanning electron microscopy (SEM) images showed a high pore development in the ACs. X-ray diffraction (XRD) patterns showed that, in addition to the amorphous structure, ACs contains crystalline ZnO formed during the carbonization. Presence of amorphous carbon is further confirmed by Raman scattering, where we observed only D and G bands. Iron impregnated nanoporous AC has been found to be very effective for arsenic removal from ground water; amount of arsenic is decreased from ca. 200 ppb to 10 ppb. These experimental results indicate the potential use of Lapsi seed as a precursor material for the preparation of high surface area nanoporous activated carbons.

Characterization of shape selective properties of zeolites via hydroisomerization of n-hexane

In this work we use the hydroisomerization of n-hexane as a catalytic test reaction to investigate the shape selective properties of a series of zeolites (mordenite, Y, ZSM-5, ZSM-12, TNU-9, SSZ-25, SSZ-26, SSZ-32, SSZ-33, SSZ-56, SSZ-57 and SSZ-75). The yield ratio of mono-branched isomers (2- and 3-methylpentane) to di-branched isomers (2,2- and 2,3-dimethylbutane) and the yield ratio of 2,3- to 2,2-dimethylbutane provide a useful tool for characterizing the effective pore sizes of zeolites. The high octane isomer 2,3-dimethylbutane (RON 101.0) can be selectively produced over 2,2-dimethylbutane (RON 91.8) by choosing zeolites (e.g., TNU-9 and SSZ-75) which have the appropriate pore sizes, demonstrating some new examples of the shape selectivities of zeolite catalysis. This catalytic test reaction is complementary to the vapor phase hydrocarbon adsorption for the investigation of the shape selective properties and effective pore sizes of zeolites.

The influence of osmotic pressure on poulticing treatments for cultural heritage objects

The crystallization of salts is widely recognized as one of the most significant causes of irreversible damage to many cultural objects such as wall paintings, stone sculptures, historic buildings. The removal of salts from these objects is however difficult and often poultices are used. In these methods a wet poultice is applied to the surface of the substrate to be treated and is kept in place for some period of time before being removed. Many studies up to now on poulticing have focused on the salt and moisture transport solely in terms of advection and diffusion. The objective of this study is to demonstrate the potential contribution of osmotic pressure to salt extraction during poulticing treatments. To this end we have conducted a series of experiments where we have measured the moisture and salt transport during poulticing for some well defined materials. Here we have used nuclear magnetic resonance to measure non-destructively the moisture and ion transport during these experiments. This study shows that osmotic pressure can exert a significant influence on salt extraction by poulticing methods during drying. Importantly, as salt is transported from the substrate and into the poultice, this results in a build-up of osmotic pressure within the poultice decreasing the effective pore-size of the poultice. Therefore the build-up of osmotic pressure enhances the salt extraction and thus increases the efficiency of the poulticing treatment.

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (∼95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180W/mK. Effective pore size of the CNT wick structure varies between 50 and 180nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (∼50μm wide) passages. Experimental results show a maximum heat flux of 770W/cm2 over a 2mm×2mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ∼140W/cm2 over a 10×10mm2 heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.

Pervaporation of acetic acid aqueous solutions by organosilica membranes

Abstract Organosilica membranes were prepared from bis(triethoxysilyl)ethane (BTESE) by sol–gel processing and applied to the dehydration of aqueous acetic acid (AcOH) solutions in pervaporation. BTESE-derived membranes were found to show high permselectivity with excellent stability. The permeate flux for aqueous AcOH solutions (AcOH: 90 wt%, 75 °C) was 2.0–4.0 kg/(m2 h) with water selectivity of 200–500, corresponding to water permeance of (2.5–5)×10−6 mol/(m2 s Pa). Long-term stability tests in aqueous AcOH solutions revealed that BTESE membranes showed approximately unchanged flux and separation factors, confirming the excellent stability of organosilica membranes. Comparison with gas permeation properties suggests the separation mechanism is dominated by the molecular sieving effect. Concentration dependency was examined for AcOH concentrations ranging from 30 to 90 mol%. An increase in AcOH concentration decreased both water and AcOH permeance, suggesting that adsorbed AcOH on BTESE membranes reduced the effective pore sizes for permeation of the 2 molecules.

PVDF membranes with inter-connected pores prepared via a Nat-ips process

Polyvinylidene fluoride (PVDF) flat sheet membranes with inter-connected pores were prepared via a non-solvent assisted thermally induced phase separation (Nat-ips) process. Triethylphosphate (TEP) was used as the latent solvent to prepare the metastable PVDF casting solution. The asymmetric ultra- and symmetric micro‐filtration membranes were respectively prepared by adjusting the polymer concentration and quench bath composition. Finger-like pores were totally eliminated and highly inter-connected pores were formed in both membranes. The morphology, crystallization, and mechanical strength of PVDF membranes were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), and tensile strength testing respectively. The effective through-pore size and pore size distribution of PVDF membranes were investigated by a bubble-point method. Pure water flux, protein rejection and fouling resistance were measured to study the filtration performance. All results showed that PVDF membranes with highly inter-connected pores and unimodal pore size distribution could be readily prepared by the Nat-ips process. The ultra- and micro‐filtration PVDF membrane exhibited a pure water flux of 120 and 1860L/m2h with the corresponding effective through-pore size of 0.11μm and 0.26μm respectively.

Helium Tunneling through Nitrogen-Functionalized Graphene Pores: Pressure- and Temperature-Driven Approaches to Isotope Separation

Recently, we showed that nitrogen-functionalized nanopores obtained by removing two rings from a perfect graphene sheet provide suitable barriers for a separation of fermionic helium-3 from its bosonic counterpart helium-4 [J. Phys. Chem. Lett.2012, 3, 209–213]. In this follow-up Article, we provide potential curves for helium passing through several different types of pores, discuss the relation of the barrier height to the effective pore size, give estimations for bound states of helium attached to the pores, and analyze the effects of isomeric and stoichiometric variations of the pore-rim nitrogen-passivation on the gas separation performance. Slight deviations in the tunneling probability for the two helium isotopes can lead to a high selectivity at an industrially acceptable gas flux if the gas temperature is kept sufficiently low. We also recently showed that the mass-dependence of quantum tunneling and zero-point energy differences at the top of the potential energy barrier allow for a classically ...

Hybrid Zeolitic Imidazolate Frameworks: Controlling Framework Porosity and Functionality by Mixed-Linker Synthesis

Zeolitic imidazolate frameworks (ZIFs) are a subclass of nanoporous metal–organic frameworks (MOFs) that exhibit zeolite-like structural topologies and have interesting molecular recognition properties, such as molecular sieving and gate-opening effects associated with their pore apertures. The synthesis and characterization of hybrid ZIFs with mixed linkers in the framework are described in this work, producing materials with properties distinctly different from the parent frameworks (ZIF-8, ZIF-90, and ZIF-7). NMR spectroscopy is used to assess the relative amounts of the different linkers included in the frameworks, whereas nitrogen physisorption shows the evolution of the effective pore size distribution in materials resulting from the framework hybridization. X-ray diffraction shows these hybrid materials to be crystalline. In the case of ZIF-8-90 hybrids, the cubic space group of the parent frameworks is continuously maintained, whereas in the case of the ZIF-7-8 hybrids there is a transition from a...

The roles of alkali metal counter-ions of PFSA play in the formation of PVDF/PFSA-M hollow fiber membranes

The aim of the present study was to thoroughly investigate the effect of alkali metal counter-ions (Li, Na, K and Rb) of PFSA on the formation, morphology and performances of polyvinylidenefluoride (PVDF)/perfluorosulfonic acid (PFSA-M) hollow fiber membranes. It is found that the homogeneous region in the ternary phase diagram of the PVDF–PFSA–M/NMP/H2O systems shrinks and the precipitation rate increases when varying the counter-ions of PFSA in the order of Li, Na and K. The double finger-like structure and the cavities can be evidently developed with increasing the radius of the counter-ions of the spun PVDF/PFSA-M membranes. The pure water permeability (JW) and mean effective pore size of the PVDF/PFSA-M membranes first increase from counter-ion Li, Na to K and then slightly decrease for Rb. In addition, the highest JW value of 96.5L M−2 H−1 bar−1 was obtained for the PVDF/PFSA-K membrane with a MWCO of 86kDa. XRD patterns and FTIR-ATR spectra verify the exclusive formation of β crystalline phase structure in the as-spun PVDF/PFSA-M membranes. The PVDF/PFSA-Li membrane has better Young's modulus and tensile strength than that of other PVDF/PFSA-M fibers.