Decrease In Pore Radius Research Articles

Determination of Fibre Pore Structure: Influence of Salt, pH and Conventional Wet Strength Resins

It has been shown, in the present investigation, that the two methods used to investigate the pore size distribution of unbleached chemical pulps, i.e. inverse size exclusion chromatography (ISEC) and nuclear magnetic resonance (NMR), give different average pore radius for the pores inside the fibre wall. This is due to the way in which these experiments are performed and the sensitivity of the methods to different types of pores in the cell wall. It was also shown that the two methods gave different results when changing the pH and the ionic strength of the pulp suspension. The pore radius, as detected with ISEC, decreased with both increasing ionic strength and decreasing pH, indicating a loose structure of the exterior of the fibrillar network. However, the pore radius as detected with NMR, was virtually unaffected when increasing the ionic strength, indicating a very rigid structure of the interior of the fibre wall. Decreasing pH though, lead to a decrease in pore radius indicating that upon protonation of the carboxylic groups in the fibre wall, the electrostatic repulsion is diminished and the average pore radius decreases. The NMR technique was also used to study wet strength aid penetration into the fibre wall. It was shown that wet strength aids with a small molecular weight, penetrated the fibre wall, as detected by a decrease in pore radius. It was also shown that addition of different wet strength aids increased the tensile index of the sheet and decreased the fibre strength, measured as zero span-strength of the sheets.

Structural and Transport Properties of an SPC/E Electrolyte in a Nanopore

Equilibrium (EMD) and nonequilibrium molecular dynamics (NEMD) simulations were conducted to investigate the effects of confinement on the structural and transport properties of an electrolyte in a nanopore. The extended simple point charge (SPC/E) model was used to model water molecules in a 0.5 M KCl electrolyte. The cylindrical nanopore was smooth, structureless, and hydrophobic with a radius that varied from 4.75 to 15.8 Å. Changes in energies and structures were observed as the nanopore radius varied. The ion−ion, ion−water, and water−water interaction energies, the ion−ion and ion−water pair distribution functions, the density profiles of H and O atoms, and the water orientation about the vertical axis and around an ion were calculated in the simulations. Because of confinement in the radial direction in a narrow pore, there was incomplete solvation of ions, evidenced by less negative ion−water energy and less alignment of water molecules with the field of the ion. With a stronger confinement, H-bonding decreased, whereas the external field had a stronger influence on the orientation of the water molecules. Both EMD and NEMD results showed a decrease of ionic conductivity with decreasing pore radius, but there was an appreciable discrepancy between the conductivities obtained by the two methods for the cases of smaller pore diameters.

Streaming potential through multilayer membranes

AbstractThe fundamental and practical importance of electrokinetic characterization of porous membranes is now well recognized. However, not enough attention has focused on the fact that the most practically interesting membranes have a multilayer structure. Then, standard electrokinetic methods, such as streaming potential measurement performed through these multilayer membranes, lead to a global signal that does not necessarily reflect the electrokinetic behavior of the skin layer that rules the membrane selectivity. The relative influence of membrane layers with different physical (pore length, porosity, pore radius) and electrical (zeta potential) properties on the global streaming potential arising across a two‐layer membrane is studied. Each porous layer plays a role in the streaming potential process, but in a wide range of length fraction of support layer the skin layer contributes more than the support layer to the global signal. The contribution of the skin layer increases as its porosity and pore radius decrease, but is little affected by its zeta potential. Nevertheless, in many practical situations, the support layer is likely to play a nonnegligible role on the global signal due to its huge thickness with respect to the skin layer. Therefore, electrokinetic measurements performed with multilayer membranes should be interpreted carefully.

Diffusivity and Conductivity of a Solvent Primitive Model Electrolyte in a Nanopore by Equilibrium and Nonequilibrium Molecular Dynamics Simulations

Equilibrium and nonequilibrium molecular dynamics simulations are performed to calculate the diffusion coefficient and electric conductivity of ions in a 0.1 M concentration solution confined in neutral cylindrical pores. The applied model is a solvent primitive model (SPM) in which both ions and solvent molecules are soft core spheres and the polar nature of the solvent is represented implicitly as a background with a given dielectric constant. The simulations are carried out in an isokinetic ensemble, and the system, responsing to an applied electric field, is maintained at constant temperature by a Gaussian thermostat. From equilibrium molecular dynamics, diffusion coefficients of ions and solvent decrease with decreasing pore radius or increasing packing fraction of solvent particles. The conductivity determined by nonequilibrium molecular dynamics shows a similar trend, but the pore-size dependence of conductivity does not have a local maximum as was found in the restricted primitive model in which solvent spheres are absent. Using the Nernst−Einstein relation, the ionic conductivity is also calculated from the equilibrium diffusion coefficient and compared with the conductivity obtained from nonequilibrium simulations. The comparison shows that the Nernst−Einsten relation is not valid only at low solvent packing and in very small pores.

Macromolecule transfer through mesothelium and connective tissue.

Diffusional permeability (P) to inulin (P(in)), albumin (P(alb)), and dextrans [70 (P(dx 70)), 150 (P(dx 150)), 550 (P(dx 550)), and 2, 000 (P(dx 2,000))] was determined in specimens of parietal pericardium of rabbits, which may be obtained with less damage than pleura. P(in), P(alb), P(dx 70), P(dx 150), P(dx 550), and P(dx 2, 000) were 0.51 +/- 0.06 (SE), 0.18 +/- 0.03, 0.097 +/- 0.021, 0. 047 +/- 0.011, 0.025 +/- 0.004, and 0.021 +/- 0.005 x 10(-5) cm/s, respectively. P(in), P(alb), and P(dx 70) of connective tissue, obtained after removal of mesothelium from specimens, were 10.3 +/- 1.42, 2.97 +/- 0.38, and 2.31 +/- 0.16 x 10(-5) cm/s, respectively. Hence, P(in), P(alb), and P(dx 70) of mesothelium were 0.54, 0.20, and 0.10 x 10(-5) cm/s, respectively. Inulin (like small solutes) fitted the relationship P-solute radius for restricted diffusion with a 6-nm "pore" radius, whereas macromolecules were much above it. Hence, macromolecule transfer mainly occurs through "large pores" and/or transcytosis. In line with this, the addition of phospholipids on the luminal side (which decreases pore radius to approximately 1.5 nm) halved P(in) but did not change P(alb) and P(dx 70). P(in) is roughly similar in mesothelium and capillary endothelium, whereas P to macromolecules is greater in mesothelium. The albumin diffusion coefficient through connective tissue was 17% of that in water. Mesothelium provides 92% of resistance to albumin diffusion through the pericardium.

Water dynamics in controlled pore silica glasses

Water in porous silica glass is a suitable system for investigating the effect of confinement on translational diffusion. These systems are important because of their relevance in catalytic and separation processes. Two factors are to be considered in the case of confined water: (1) the effects of hydrophilic and hydrophobic surfaces on interfacial water and (2) how the dynamics of the hydrogen bond network changes due to the volume of confinement. Here quasi-elastic neutron scattering experiments at room temperature on water filled controlled pore glasses with radius of 15, 24 and 32 {angstrom}, are presented and analyzed using the random-jump diffusion model. Both the average residence time and the mean jump distance increase with decreasing pore radius.

Raman frequency noncoincidence effect of confined liquid dimethyl sulfoxide

Raman spectroscopy was used to study the short-range structure of liquid dimethyl sulfoxide (DMSO) confined to porous silica glasses in the pore radius range from 15 to 56 Å. By analyzing isotropic and anisotropic Raman spectra of DMSO in the SO stretching vibration region, we found the noncoincidence of the in-phase and out-of-phase SO vibrational wavenumbers increasing with decreasing pore radius. This behavior is in contrast to that found earlier for other confined polar liquids. Using a theory derived by Logan and results of ab initio calculations, the liquid structures of DMSO, acetone, N,N-dimethyl formamide in the proximity of silica surfaces are proposed.

Low-Temperature Phase Transition of Water Confined in Mesopores Probed by NMR. Influence on Pore Size Distribution

The NMR signal intensity vs temperature (IT curve) of water confined in mesoporous materials (pore radius lager than 10 Å) reveals one or more “high-temperature” transitions above 222 K, which are dependent on pore size, and an additional transition temperature below 209 K, which is independent of pore dimension. This latter transition shows no hysteresis effect or discontinuity and contributes to more than 65% of the total water content of the porous materials investigated and is explained as interfacial water in contact with the surface of the matrix and the solid ice phase. The thickness of this interface water is estimated to be 5.4 ± 1.0 Å (cylindrical pores). It is shown that the observed NMR intensity of water associated with the “high-temperature” transition phases has to be corrected in order to present the actual amount of water within these phases. It is further demonstrated that these intensity corrections must be implemented in the pore size distribution functions to give quantitative results. The significance of the correction factors increases with decreasing pore radius.

Raman non‐coincidence effect of the carbonyl stretching mode in confined polar liquids

AbstractThe Raman non‐coincidence effect of the symmetric C=O stretching band was measured in liquid acetone and N,N‐dimethylformamide confined to porous silica glasses prepared by the sol—gel process. The experiments were carried out both on bulk liquids and liquids confined to glasses with pore radii from 17—76 Å at 293 K. The value of the non‐coincidence was decreased with decreasing pore radius (R) and scaled approximately as 1/R. Additional information about the role of pore surface effects was obtained by measurements on the polar liquids confined to surface‐modified glasses when the surface hydroxyl groups were replaced by OSi(CH3)3 groups. The experimental data show that the non‐coincidence effect is independent of the pore size in surface‐modified glasses.

The diffusion limited oxidase-based glucose enzyme electrode: relation between covering membrane permeability and substrate response

Permeability coefficients, P (cm 2-sec −1, for glucose, oxygen, acetaminophen (paracetamol), p-aminophenol (PAP), 5-trihydroxybenzene (phloroglucinol) and ascorbate through polycarbonate (PC) neutron beam track-etch membranes of pore radii 0.015–0.4 μm have been determined at (22 ± 1°C) using a diffusion chamber and the principle of steady state analysis. P values were found to increase with increasing membrane pore radius. The ratio of glucose/O 2 P values for membranes of pore radii 0.4, 0.05, 0.03 and 0.015 μm were determined to be 0.64, 0.36, 0.22 and 0.045 respectively and therefore decreased with decreasing pore radius. The P value ratios for glucose with respect to other organic species were in the range 0.7–1.2, but showed little dependence on membrane pore radius. It has been found that lower glucose/O 2 P ratios characteristic of smaller pore radii membranes, were associated with extended linearity ranges for glucose analysis when utilised as upper membranes in a conventional glucose oxidase enzyme laminate electrode construction.

Effect of porosity on the distribution and reactivity of hydroxyl groups on the surface of silica gel

The distribution and reactivity of the different hydroxyl species on the surface of silica gel are studied using Fourier-transform infrared spectroscopy with photo-acoustic detection and chemical modification with trichlorosilane. The effect of the porosity is investigated using three types of silica gel with different pore distributions. The number of free and bridged hydroxyl groups on the surface of silica gel and their reactivity towards trichlorosilane is quantitatively assessed. It is concluded that smaller pore radii favour the condensation of bridged hydroxyl groups to free ones with increasing pretreatment temperature. Trichlorosilane reacts exclusively with free hydroxyl groups at high pretreatment temperatures, and partially with bridged hydroxyl groups at low pretreatment temperatures. The reactivity of the free hydroxyl groups is not affected by the pore radius, whereas the bridged hydroxyls become less reactive towards trichlorosilane as the pore radius decreases.

Influence of porous support structure and active component composition on deactivation of vanadium catalysts in SO2 oxidation

The effect of porous support structure and active component composition of vanadium catalysts on the crystallization character of this component has been studied in SO2 oxidation at low temperatures and conversion degrees. The tendency to crystallization is shown to drop with decreasing support pore radius and with increasing K/V molar ratio.

和紙抄造工程に及ぼすトロロアオイ粘質物とポリエチレンオキサイドの効果の違いについて

In connection with the paper making properties of tororo-aoi mucin (TM) and polyethylene oxide (PEO) solutions, the permeabilities of the polymer solutions through the rayon pulp bed and the sedimentation velocities of the pulp in the solutions were measured.The permeability of TM solution was significantly smaller than that of PEO solution under the same conditions, and increased rapidly by aging or by the addition of NaCl. The fluent flow of TM solution through the pulp bed was proved to be un-explainable by Darcy's equation.Close correlation was also confirmed among the viscosity and permeability of TM solution or the sedimentation velocity of rayon pulp in the solution and the initial conformational transition of an ordered, weak structure of TM molecule. It was further clarified that the paper making properties described above are remarkably influenced by the dissociation of carboxyl group present in TM molecule and by the surface state of pulp fiber.From the results, the characteristic paper making property of TM is appreciably explained to be occured by a pore radius decrease in the rayon pulp bed, which is due to the thicker immobile layer caused by TM adsorption on the fiber surface, and by the dissociation of carboxyl group present in the mucin molecule. It was also found that a decrease in the effect of TM on paper making is partially inhibited by the addition of PEO in the aqueous solution.

Water permeability of gramicidin A-treated lipid bilayer membranes.

In membranes containing aqueous pores (channels), the osmotic water permeability coefficient, P f, is greater than the diffusive water permeability coefficient, P d. In fact, the magnitude of P f/P d is commonly used to determine pore radius. Although, for membranes studied to date, P f/P d monotonically declines with decreasing pore radius, there is controversy over the value it theoretically assumes when that radius is so small that water molecules cannot overtake one another within the channel (single-file transport). In one view it should equal 1, and in another view it should equal N, the number of water molecules in the pore. Gramicidin A forms, in lipid bilayer membranes, narrow aqueous channels through which single-file transport may occur. For these channels we find that P f/P d approximately 5. In contrast, for the wider nystatin and amphotericin B pores, P f/P d approximately 3. These findings offer experimental support for the view that P f/P d = N for single-file transport, and we therefore conclude that there are approximately five water molecules in a gramicidin A channel. A similar conclusion was reached independently from streaming potential data. Using single-channel conductance data, we calculate the water permeability of an individual gramicidin A channel. In the Appendix we report that there is a wide range of channel sizes and lifetimes in cholesterol-containing membranes.

Pulmonary microvascular filtration and reflection coefficients in two- and four-week-old rabbits

Several researchers have determined that pulmonary capillaries in young mammals are significantly more permeable to water and lipid-insoluble solutes than are the pulmonary capillaries in the adult counterparts. Our purpose was to quantify the microvascular fluid flux parameters in 2- and 4-week-old rabbit lungs and to compare our results with adult and newborn data previously reported. Using an isolated, Ringer-perfused lung preparation, we measured the values for the microvascular filtration coefficient and the reflection coefficients for NaCl and albumin. We found a filtration coefficient value of 2.57 × 10 −4 ml/sec-cm H 2O-m 2 for the 2-week-old population. For the 4-week-old rabbit we evaluated the filtration coefficient to be 3.18 × 10 −4 ml/sec-cm H 2O-m 2. These values are normalized for the estimated exchange area, and neither is significantly different from the adult value. The reflection coefficients for NaCl and albumin were measured using an osmotic transient technique. Correcting the albumin data for flow across the endothelial cell membrane, we arrived at the following values for the pulmonary microvascular reflection coefficient of 2- and 4-week-old rabbits, respectively: σ cap alb = 0.17, and σ cap alb = 0.23. Based on these values we estimated the equivalent pore radii of large pores in the pulmonary microvascular wall to be approximately 175 Å in the 2-week-old, and 145 Å in the 4-week-old rabbits. These results indicate that in the rabbit K f/m 2 has reached the adult value by 2 weeks of age; however, the number of pores increases, and the pore radius decreases, continuously even beyond 4 weeks of age.

Freezing of the water content of porous membranes

A theoretical and experimental study of the freezing of water in a porous membrane is presented. A membrane is considered as a set of very fine capillaries of radius r and thickness e with a thick ice-water interface, consisting of several molecular layers. The effect of the number N of molecules comprising the interface on the melting temperature T is studied in terms of the thermodynamics of small systems. Experimental results on the ice melting point of calibrated membranes are given. It was found that the rate of decreasing of the melting point versus pore radius is different from the previous rate by the Kelvin relation, and it is shown that interfacial phenomena have an important effect when the pore radius decreases. The problems of freezing the water contained in a very fine capillary were studied as a model of the freezing of living cells, and in particular as a model of the effects of freezing on cell membranes, an event that is common to all cryobiological problems. Although the experimental and theoretical results presented here are not directly transposable to actual living membranes, a model of a semipermeable membrane containing distilled water is examined. The characteristic parameter of such membranes is the mean pore radius. Many studies were done in this field (2, 17, 18) in relation to the fundamental function of the cell membrane (3, 4, 16). In a very schematic manner, one can assume that the pores in the biological membranes have a radius of 10 −10 − 10 −6 m (i.e., vegetables stomata). In this type of fine capillary, we have a limited number of water molecules. The thermodynamic relations of the Kelvin type, used to calculate the equilibrium temperature, are not valid in this case because the interface and its radius of curvature are no longer fixed. In a macroscopic system, the interface is defined as the surface which separates two different phases. The solid phase, i.e., ice, is characterized by a certain molecular ordering forming a well-defined crystalline structure (9). The liquid phase is characterized by a different molecular ordering, still imperfectly understood (7, 19). In a system with a small number of molecules, the problem is completely different. There are no more macroscopic effects to average out the phenomena appearing at the molecular level. Since there are no bulk effects, the surface effects, which are negligible in a macroscopic: system, become an essential parameter. In particular, it is difficult to assume that the interface is a discontinuous surface as in the larger systems. On the contrary, the interface is probably composed of a variable number of molecular layers depending on the size of the system under consideration. This assumption is introduced in the theoretical model developed in the following section, a model derived from the thermodynamics of small systems (12).