Showing papers on "Permeation published in 1976"

Electrochemical methods for studying diffusion, permeation and solubility of hydrogen in metals

Abstract: Electrochemical methods for the investigation of metal-hydro gen systems are superior to other techniques, e.g., gas-volumetric, because of their simple procedure and their flexibility towards variation of experimental conditions. Moreover, these techniques allow measurements at very low hydrogen equilibrium pressures which occur, for instance, in the V(a)-metal-hydrogen systems in the concentration range of ideal dilute solutions at normal temperatures. The removal of surface layers, which retard the hydrogen passage from the electrolyte into the metal and vice versa, is a necessary condition for electrochemical methods to be applicable. Using ultra-high-vacuum techniques, surface hindrances on base metals like V, Nb, Ta, Ti, and others, can be eliminated. Thus, it is possible to study diffusion, permeation, and solubility of hydrogen (isotopes), in these metals as well as in precious metals such as palladium and its alloys. Various electrochemical hydrogen-diffusion and permeation methods are presented and compared. Their applicability and the limits of application are demonstrated for pre-selected systems. A recently developed electrochemical technique for measuring hydrogen (isotope) solubility in transition metals is described. By means of this method, pressure-concentration equilibrium isotherms of V, Nb, and TaH systems have been obtained recently, at ordinary temperatures, from vanishingly low hydrogen concentrations, corresponding to extremely low equilibrium pressures, up to atomic ratios H/Me of 0.5.

Comments on Exclusion of Polymer Chains from Small Pores and Its Relation to Gel Permeation Chromatography

Abstract: Some criticisms of our theoretical treatment of the partial exclusion of flexible-chain polymers in solution from cavities of macromolecular size and its application to gel permeation chromatography are examined. In other discussion, it is confirmed by simple reasoning that the identification, explicit or implicit in various studies, of the mean projection of a polymer molecule onto a line as a characteristic dimension governing the extent of permeation of simple pores does not depend on specific molecular models. Our previous calculation of permeation by certain random-flight branched-chain species is shown to lead, incidentally, to the mean projection for these structures. From relations between the mean projection and the hydrodynamic volume of a molecule, it appears that the product of intrinsic viscosity and molecular weight is not a common calibration factor for elution of all molecular species from a gel chromatographic column, but theory and experience do support the validity of this correlation among solutes with similar molecular architecture.

Measurement of gas permeability of polymers. II. Apparatus for determination of permeabilities of mixed gases and vapors

Abstract: A gas permeability apparatus has been constructed to facilitate the study of permeation of mixtures of gases and vapors. The apparatus utilizes the carrier gas method with gas-chromatographic analysis to determine individual permeation values and in-line thermal conductivity detectors to provide simultaneous overall diffusion and permeability data. The effects of film type, structure, and morphology on permeability may be studied from atmospheric to 100 psi feed pressure and from ambient to 300°C. Feed stocks may be custom mixed, and a vaporizing bath permits introduction of vapors either as contaminants or as primary permeants in an inert carrier. The experiments which illustrate the use of the equipment suggest that the noninteracting gases H2 and CH4 permeate a number of polymer films independently over a wide concentration range and that water vapor retards their permeation in polyimide films.

Effect of temperature on nonelectrolyte permeation across the toad urinary bladder

Abstract: The permeability of the toad urinary bladder to 22 nonelectrolytes was obtained from measurements of radioactive tracer fluxes. The permeability coefficients (P's), after suitable corrections for unstirred layers, were proportional to the olive oil/water partition coefficients for the majority of the molecules (P α Koil1.3). In the absence of chain branching, inductive effects, and intramolecular hydrogen bonding effects, a hydroxyl group reducedP an average 500-fold and a methylene group increasedP an average four fold. Branched chain solutes were less permeable than their straight chain isomers, and small solutes, polarand nonpolar, exhibited higher rates of permeation than expected from the relationship betweenP and Koil. (Over the molecular size range 18–175 cc/moleP α(Molecular Volume)−2.7.) The high rates of permeation of small molecules are consistent with diffusion through a highly organized lipid structure. Large polar solutes, e.g., sucrose, appear to pass across the epithelium via an extracellular shunt pathway. The apparent activation energies (Ea) for the permeation of 16 select molecules were obtained from permeability measurements over the temperature range 2–32°C. Linear Arrhenius plots (i. e., logP/T−1) were obtained for all molecules after unstirred layer corrections. In the absence of these corrections “phase transitions” were seen for molecules with very highP's (P>300×10−7 cm/sec), but these are simply due to diffusion limited permeation.Ea increased by 2.5–3.6 kcals/mole with the introduction of each additional methylene group into a molecule, and decreased by up to 9 kcals/mole for the addition of a hydroxyl group. Qualitatively similar results were obtained in preliminary studies of olive oil/water partition coefficients. Arrhenius plots of the toad bladder conductance over the temperature range 2–32°C yield apparent activation energies of 4–5 kcals/mole which is identical to that found previously for “leaky” epithelia.

Scopolamine permation through human skin in vitro

Abstract: The sorption and rate of permeation of scopolamine base in human skin have been measured as a function of drug concentration in aqueous solution contacting the stratum corneum surface of the skin. The sorption isotherm is nonlinear, and the apparent penetrant diffusivity computed from steady state permeation data is greater than that estimated from unsteady state (time lag) measurements. By assuming that sorption occurs by both ordinary dissolution and binding of penetrant to immobile sites in the membrane, the experimental sorption isotherm can be predicted, and the disparity between steady state and time lag diffusivities can be reconciled.

Nonelectrolyte diffusion across lipid bilayer systems.

Abstract: The permeability coefficients of a homologous series of amides from formamide through valeramide have been measured in spherical bilayers prepared by the method described by Jung. They do not depend directly on the water:ether partition coefficient which increases regularly with chain length. Instead there is a minimum at acetamide. This has been ascribed to the effect of steric hindrance on diffusion within the bilayer which increases with solute molar volume. This factor is of the same magnitude, though opposite in sign to the effect of lipid solubility, thus accounting for the minimum. The resistance to passage across the interface has been compared to the resistance to diffusion within the membrane. As the solute chain length increases the interface becomes more important, until for valeramide it comprises about 90% of the total resistance. Interface resistance is also important in urea permeation, causing urea to permeate much more slowly than an amide of comparable size, after allowance is made for the difference in the water:ether partition coefficient. Amide permeation coefficients have been compared with relative liposome permeation data measured by the rate of liposome swelling. The ratios of the two measures of permeation vary between 3 and 16 for the homologous amides. The apparent enthalpy of liposome permeation has been measured and found to be in the neighborhood of 12 kcal mol-1 essentially independent of chain length. Comparison of the bilayer permeability coefficients with those of red cells shows that red cell permeation by the lipophilic solutes resembles that of the bilayers, whereas permeation by the hydrophilic solutes differs significantly.

Gas permeation of polymer blends. IV. Poly(vinyl chloride) (PVC)/acrylonitrile–butadiene–styrene (ABS) terpolymer

Abstract: The transport behavior of He, O2, N2, and CO2 in membranes of poly(vinyl chloride) (PVC)/acrylonitrile–butadiene–styrene (ABS) blends has been studied at 25°C. The blends were further characterized by dynamic mechanical measurements, differential thermal analysis (DTA), density measurements, and x-ray diffraction. The equilibrium sorption of CO2 and N2 was measured directly at atmospheric pressure using an electromicrobalance and compared with sorption values obtained as P/D ratios from permeation measurements. The rates of permeation (P) and diffusion (D) increase with increasing ABS content in the blends. The P and D values are not additive, and only slight indications of phase inversion in the blends are observed at 5–10 wt-% ABS in the blends. Experimental densities of the blends are higher than calculated densities assuming volume additivity. The data are interpreted to mean that the PVC/ABS blends form a two-phase system composed of a soft polybutadiene (rubber) phase and a rigid PVC/styrene–acrylonitrile copolymer (SAN) phase of mutually compatible components. DTA and dynamic mechanical measurements also show a two-phase system. Sorption values of CO2 and N2 by equilibrium sorption measurements increase with increasing ABS content in the blends without the large fluctuations which have been observed for the sorption values obtained from the time lag method. Comparison of the two types of sorption values (from direct measurements and from P/D ratios) show larger deviations for CO2 than for N2. This suggests that the time lag method is not valid for permeants with polar character in heterogeneous two-phase systems where chemical immobilizing effect on the permeant molecules occurs.

Tritium Balance in High-Temperature Gas-Cooled Reactors

Abstract: In high-temperature gas-cooled reactors, tritium produced by fission is, for the most part, retained in the coated particle fuel, but 3He- and 6Li-originated tritium takes part in a complex system of sorption, diffusion, and exchange reactions in the core graphite. Permeation through the heat-exchanger tube walls leads to secondary circuit contamination. It was found that tritium is very soluble in graphite and that permeation is significantly reduced by oxide layers on metal surfaces.

Permeabilities to salts and water of macrocyclic polyether–polyamide membranes

Abstract: The polyether–polyamide (PC-6) strongly absorbs sodium salts from their aqueous solutions. Membranes based on PC-6 and on its polymeric alloys with poly(vinylpyrrolidone) (PVP) are, however, much more permeable to water than to salts. The membrane permselectivity is due to the low mobility of the absorbed salts. Desorption experiments were conducted to determine the values of the diffusion coefficients of the sodium ions in the investigated membranes. They were found to vary from 5 × 10-12 cm2/sec in loose PC-6 membranes to 1.7 × 10-9 cm2/sec in the polymeric alloy containing 30% PVP. Water permeation experiments with the alloy membranes yielded values of the diffusion coefficients in the range of 2–5 × 10-7 cm2/sec. The apparent “energy of activation of the diffusion” of sodium ions in such membranes was found to be essentially indentical (∼12 kcal) with the energy of activation of the decomplexation of the sodium–“crown” complex. The ramifications of the proposed “site to site jump” diffusion mechanism were discussed. The permeability characteristics of PC-6 membranes were found to be strongly affected by their “history.”. The observed phenomenon was explained in terms of reversible changes in the structure of the polymeric network, in the presence and in the absence of the absorbed salts. It has been found that PVP has a stabilizing effect on the permeability characteristics of the membranes. Reverse osmosis experiments indicated that their intrinsic osmotic characteristics seem to be superior to those of the commercially used materials. Their salt rejections are in the range of 95–99.5%, and their permeabilities to water are at least one order of magnitude higher than those of the unmodified aromatic polyamides.

Influence of molecular variations of ionophore and lipid on the selective ion permeability of membranes: I. Tetranactin and the methylation of nonactin-type carriers.

Abstract: The manner in which the molecular structure of the carrier and the lipid composition of the membrane modulate the membrane selectivity among monovalent cations has been investigated for nonactin, trinactin, and tetranactin, which differ only in their degrees of methylation, and for membranes made of two lipids, phosphatidyl ethanolamine and glyceryl dioleate, in which “equilibrium” and “kinetic” aspects of permeation, respectively, are emphasized. Bilayer permeability ratios for Li, Na, K, Rb, Cs, Tl, and NH4 have been characterized and resolved into “equilibrium” and “kinetic” components using a model for carrier-mediated membrane transport which includes both a trapezoidal energy barrier for translocation of the complex across the membrane interior and a potential-dependence of the loading and unloading of ions at the membrane-solution interfaces. The bilayer permeability properties due to tetranactin have been characterized in each of these lipids and found not only to be regular but to be systematically related to those of the less methylated homologues, trinactin and nonactin. This analysis has led to the following conclusions: (1) The change in lipid composition alters the relative contributions of “kinetic”vs. “equilibrium” components to the observed carrier-mediated selectivity. (2) Increased methylation of the carrier increases the contribution of the “kinetic” component to the selectivity relative to that of the “equilibrium” component and additionally alters the “equilibrium” component sufficiently that an inversion in Cs−Na selectivity occurs between trinactin and tetranactin. (3) For all ions and carriers examined, the “reaction plane” for ion-carrier complexation and the width for the “diffusion barrier” can be represented by the same two parameters, independent of the ion or carrier, so that in all cases the complexation reaction senses 10% of the applied potential and the plateau of the “diffusion barrier” extends across 70% of the membrane interior.

Thermodynamic analysis of nonelectrolyte permeation across the toad urinary bladder.

Abstract: Permeability coefficients (P's) and apparent activation energies (E a ′s) for nonelectrolyte permeation across the toad urinary bladder have been analyzed in terms of the thermodynamics of partition between membrane lipids and water. Particular attention has been paid to the contributions made by −CH2− and −OH groups: on the average, the addition of one −CH2− group to a molecule increasesP fourfold, while the addition of one −OH group reducesP 500-fold. Using these changes inP, we have calculated the incremental free energies (δΔF), enthalpies (δΔH), and entropies (δΔS) for partition, hydration, and solution in membrane lipids. The results for toad bladder have been compared and contrasted with those extracted from the literature for red blood cells, lecithin liposomes, and bulk phase lipid solvents. The partition of −CH2− groups into toad bladder and red cell membranes is dominated by entropy effects, i.e., a decrease in entropy of the aqueous phase that “pushes” the group out of water, and an increase in entropy of the membrane lipid that “pulls” the group into the membrane. This process resembles that in “frozen” liposome membranes. In “melted” liposomes and bulk lipid solvents the free energy of solution in the lipid is controlled by enthalpy of solution. Partition of −OH groups in all systems is governed by hydrogen bonding between the −OH group and water. However, the solution of the −OH group in toad bladder membranes is complex, and processes such as dimer and tetramer formation in the lipid phase may be involved. The results presented in this and the previous paper are discussed in terms of the structure of phospholipid bilayer membranes. Attention is drawn to the possible role of structural defects in the quasi-crystalline structure of the lipid (so-called 2gl kinks) in the permeation of small molecules such as water, urea, methanol and acetamide.

Hydrogen permeation in palladium--chromium alloys

Abstract: As part of a search for new palladium alloys with attractive permeation characteristics for hydrogen isotopes, alloys containing 9.3, 16.8, and 24.2 atom percent chromium were evaluated in the temperature range from 322 to 449 K. The permeation coefficient, diffusion coefficient, and hydrogen solubility were all found to decrease with increasing chromium content. The solubility was found to obey Sievert's law in all alloys. The permeation coefficients deviated slightly from classical diffusion-controlled behavior. These deviations are attributed to slow molecular dissociation and adsorption of hydrogen on the surface at low pressures. In some preliminary experiments at high pressure, the Pd-16.8 atom percent Cr alloy retained its structural integrity while a commercially available Pd-25 atom percent Ag alloy was susceptible to cracking and blistering.

Studies on syntheses and permeabilities of special polymer membranes. II. Permeation characteristics of cellulose nitrate membranes

Abstract: For the purpose of application in artificial kidneys the permeation characteristics of semipermeable membranes obtained from a system of cellulose nitrate/methyl alcohol/1,4-dioxane/barium perchlorate were investigated under various conditions. The optimum composition of the casting solution was found to be the proportion cellulose nitrate: methyl alcohol: 1,4-dioxane as 13:45:42 (wt.-%). It was found that the permeation rate was influenced by the evaporation period, the maximum permeability lay at 30s. The addition of barium perchlorate to the casting solution increased the permeation rate, especially if more than 60 wt.-% of barium perchlorate per cellulose nitrate was used. The permeation rate is increasing with the operating pressure, but decreasing with the feed concentration. When several samples of poly(ethylene glycol) having different average molecular weights were used as feed solute, the permeation rate decreased with an increase in average molecular weight. The permeation fraction was 100% in systems of poly(ethylene glycol) with mol. wt. from 200 to 4000 and was 0% in systems of poly(ethylene glycol) 6000 under 200 mm Hg. In blood filtrations the permeation rate of cellulose nitrate membranes had about four-fold values of cuprophane membranes, and moreover no protein was found in the blood filtrate.

Polymers in contact lens applications VII. Oxygen permeability and surface hydrophilicity of poly(4‐methylpent‐1‐ene) and related polymers

Abstract: Some of the problems and advantages in the use of non-hydrogel polymers in contact lenses are discussed together with studies on a series of such polymers which have potential advantages over the established material, poly(methyl methacrylate), in that they are both more flexible and more oxygen-permeable. Of the polymers examined which are all too hydrophobic for direct use, poly(4-methylpent-l-ene) proved to be the most readily modified in such a way that its surface became sufficiently wettable to sustain a coherent tear film without reducing its optical qualities to an unacceptable level. The ‘dissolved’ and ‘gaseous’ oxygen permeability coefficients of this polymer were studied as a function of film thickness, surface hydrophilicity and temperature. A pronounced boundary layer effect was observed in ‘dissolved’ oxygen permeability studies, although this decreased as the surface was treated to make it more wettable (as indicated by the equilibrium advancing water contact angle). The ‘gaseous’ permeability coefficients of oxygen were found to be some 4-6 times greater than those for nitrogen. A discontinuity corresponding to the glass transition temperature was observed at 28°C with both permeants and apparent activation energies for permeation were determined both above and below this temperature.

Studies with Parchment Supported Membranes. VII. Application of Fick’s Diffusion Law and Nernst-Planck Formulae for Electrical Potential—Consideration of Membrane Field Strength and Energetics of Permeation of Cations

Abstract: Electrolytic transport processes occurring across parchment supported membranes have been described by Nernst-Planck flux equation taking into account the membrane resistance Rm, membrane potential Em etc. Em values for various electrolytes display very interesting phenomena. In the case of 1 : 1 electrolyte the Em values are all positive, while in the case of (2 : 1) and (3 : 1) electrolytes surface charge reversal takes place. The diffusion rate sequence and selectivity of the membrane for different uni-, bi-, and tri-valent cations was found to be primarily dependent on the difference in the hydration energies of counter ions in the external solution. On the basis of Eisenman-Sherry theory the diffusion rate sequence of alkali metal cations point towards the weak field strength of the fixed charge groups. Various thermodynamic parameters, ΔH≠, ΔF\eweq, and ΔS\eweq were evaluated by applying the theory of absolute reaction rates to the diffusion process through parchment supported membranes. The values ...

The effect of charges on permeabilities of drugs through collagen membranes.

Abstract: Permeabilities of acetaminophen, bucolome (5-n-butyl-1-cyclohexyl-2, 4, 6-trioxo-perhydropyrimidine), salicylic acid, aminopyrine, sulfamethizole, sulfisomidine, and p-aminobenzoic acid through a collagen membrane were measured at different pH values. The effect of pH of the media on permeation of acetaminophen, an unionizable drug in the pH range studied, was minor. Drugs in molecular form permeated faster than those in ionic form. Amphoteric drugs in cationic form permeated slower than those in corresponding anionic form. Little effect of charges of the drug molecules was observed on drug permeation through a cellophane membrane.

Permeation of hydrocarbon vapors through polyethylene films

Abstract: Permeation of benzene, cyclohexane, n-hexane and toluene through polyethylene films of different densities was studied by a gravimetric method and the values of permeation constant are presented. The values are converted to diffusivity with the aid of sorption equilibria. Activation energy of diffusion is correlated to the cross-sectional area of permeant molecules. Furthermore, the change of energy with the amount sorbed is discussed.

Test of a permeation sampling device for integrated sulfur dioxide concentrations in ambient air

Abstract: Mean ambient and artificially prepared sulfur dioxide concentrations were measured over periods varying from 1 h to 19 days using a new sampling permeation device supplied by P.W. West, Louisiana State University. The analysis was carried out by the West-Gaeke method. Concentrations obtained in this way were compared with those recorded by a continuous conductimetric instrument. For 75 percent of the tests, the ratio of concentrations measured by the device to those recorded by the instrument ranged from 0.80 to 1.17. An attempt is made to explain the results of the remaining tests, which lie outside this range. Statistical tests show that agreement between the two methods is significant.