Electrical pumping in carrier-mediated membrane transport

A model of carrier-mediated pumping induced by electrochemical (redox) reactions is presented. The model is compared with published data for the facilitated transport of nitric oxide in a formamide membrane containing dissolved ferrous and ferric chlorides wherein the flux of nitric oxide is augmented by diffusion of the reversible complex, (NO—Fe²⁺. Passing a current through the membrane drives the reduction of ferric ions at the cathode and the oxidation of ferrous ions at the anode, coupling the charge and mass fluxes within the membrane. Our results indicate that this electrically powered, carrier-mediated membrane can pump permeant up to a concentration 0(10) times greater than that in the feed.     

Phase inversion sulfonated polysulfone membranes

The use of two nonsolvents serving as a cosolvent system, replacing the traditional volatile solvent plus less volatile nonsolvent system, in the formation of asymmetric phase inversion membranes was investigated. Specifically, asymmetric membranes of sulfonated polysulfone were cast from a cosolvent system consisting of tetrahydrofuran and formamide. The nonsolvents and the proportions in which they are mixed to produce the cosolvent system, as well as the gelation medium isopropyl alcohol, were selected based on the three-component solubility parameter concept of Hansen. The structure of each membrane was evaluated using scanning electron microscopy; the performance was evaluated for use in pressure-driven membrane separation processes. The membranes were found to be dependent on the composition of the original casting solution and the composition of the nascent membrane at the instant of gelation. These ideas are clearly represented through the use of a triangular polymer solubility diagram.     

Transport of organic acids through perfluorosulfonate polymeric membranes

Carboxylic acids in aqueous solutions permeate through perfluorosulfonate merebranes. The permeation is significantly enhanced when the sulfonic acid groups are convetted to their sodium salt form. Transport fluxes at atmospheric pressure have been found to vary in the 10^-9—10^-6 mol/cm²-sec range, depending on the nature of the acids. Interesting separation possibilities of carboxylic acids from one another or from nonpolar compounds are discussed.     

On the behaviour of asymmetric membranes in pervaporation

Symmetric and asymmetric membranes of the Loeb type are compared with respect to their performance in pervaporation. The experiments are carried out with water—isopropanol mixtures, employing cellulose acetate membranes of different structure, but of the same total thickness. These results are compared with calculations based on a 2-layer model for asymmetric membranes. Design criteria for optimal asymmetric membranes for pervaporation, as well as the performance characteristics for the two possible modes of installation — active layer facing the feed or facing the permeate — are discussed. Contrary to reverse osmosis, the installation of the membrane with the active layer facing the permeate proves to be superior — at least for low permeabilities of the membrane material. The interdependencies between thickness and permeability of the active layer, and porosity and thickness of the support layer are much stronger than in reverse osmosis.     

Application of ion-exchange reactions between membranes and resins

The separation of a solution of an electrolyte from an ion-exchange resin by an ion-exchange membrane, where the charge sign of the fixed sites is the same, results in a process that is comparable to Donnan dialysis in its overall effect. That is, the counter-ions from the resin can be exchanged for ions of the same charge sign in the electrolyte. The reaction is demonstrated and the efficiency of the process evaluated by monitoring the metathesis of carbonic acid from sodium carbonate. An application of the metathesis of a non-electrolyte to a preconcentration method for ion chromatography is demonstrated.     

Quantitative description of the rejection of polymeric catalysts by ultrafiltration membranes

The rejection capability of ultrafiltration membranes for polymeric catalysts like enzymes was measured under well defined conditions. A simple model has been developed to describe the course of retention during continuous operation. Even at very low concentrations of the polymer, a significant influence of concentration polarization on the retention could be found. The course of retention could be described numerically simply by using distribution parameters for the pore size of the membrane and the particle size of the polymeric catalyst.     

Discrete resistance changes in thin lipid membranes

Semispherical membranes were formed using a mixed lipid preparation extracted from beef brain gray matter. The membrane obtained had a thickness between several hundred angstroms and about one thousand angstroms as determined by the interference color of reflected light. The initial thick membrane and newly formed primary black membrane had a resistance greater than 10¹⁰ ohms, similar to that of the bulk lipid phase. However, when a small (< 15 mV) transmembrane potential was applied, the resistance decreased in a stepwise fashion to a final resistance of about 10⁵10⁶ ohms. The resultant thin membrane exhibited stepwise temperature-dependent fluctuations in conductance similar to those obtained with membranes to which substances such as the “excitability-inducing material” of Mueller and Rudin and certain ionophores have been added. Further, the membrane showed rectification when calcium was present on only one side of the membrane.     

Separation of metals by liquid surfactant membranes containing crown ether carboxylic acids

A water-in-oil-in-water emulsion liquid membrane system is used to transport alkali metal and alkaline earth cations from an external alkaline aqueous source phase through an organic membrane containing a crown ether carboxylic acid and into the internal acidic aqueous phase of the emulsion droplet. The influence of varying the crown ether carboxylic acid structure upon the selectivity and efficiency of competitive metal ion transport is examined.     

Electrokinetic phenomena in porous membranes: Determination of phenomenological coefficients and transport numbers

Electrokinetic phenomena in porous membranes were studied in the framework of the linear thermodynamics of irreversible processes. Phenomenological coefficients of two commercial porous membranes were determined. The variation of these coefficients with the different concentrations of NaCl used was also studied. Onsager's reciprocal relationship between cross-coefficients holds within the limits of experimental error.Membrane potentials were measured at concentrations ranging from 10^?3M to 75 X 10^?3M. The apparent transport number of the cation,

, was calculated from the diffusion potential expression. An electrometric method was used to obtain the true cation transport number,

, and the water transport number,

, for different solution concentrations, ranging from 15 x 10^?3M to 75 x 10^?3M. The true transport number of the cation,

, was also determined, taking into account water transport across the membrane. A good agreement was found between

and

values.     

Transport of alkali metal cations across liquid membranes by crown ether carboxylic acids

Competitive alkali metal transport from an alkaline aqueous source phase through a chloroform phase to an acidic aqueous receiving phase facilitated by nine crown ethers with pendant carboxylic acid groups has been investigated. Transport selectivity is controlled by the size of the polyether cavity of the carrier. Increasing the lipophilicity of the carrier, while maintaining a constant polyether cavity size, enhances the total transport rate but does not affect the selectivity. There is poor agreement between the results of competitive transport and the behavior anticipated on the basis of single cation transport studies.     

Multicomponent gas separation by an asymmetric permeator containing two different membranes

A numerical analysis of an asymmetric permeator containing two different kinds of membranes and capable of separating a ternary feed gas mixture into three product streams has been carried out. For negligible axial pressure drops in the gas streams, equations have been developed and calculation methods illustrated for four flow patterns: cross, parallel, countercurrent and perfectly mixed. The main parameters used in the analysis are pressure ratios, the ratio of the two membrane areas and the permeabilities. Results have been presented for a 50% H₂10% CO₂40% N₂ feed with a permeator having one cellulose acetate (CA) type membrane and one silicone membrane. For both permeate streams, countercurrent achieves highest permeate qualities and requires least membrane area amongst all four flow patterns. The presence of a silicone membrane produces a richer H₂ permeate from the CA membrane than that from a CA-membrane-alone configuration. Simultaneously, an enriched CO₂ permeate is obtained from the silicone membrane. Under the ideal condition of zero pressure ratios, the asymmetric permeator has also been evaluated against the series configuration of a permeator containing a CA membrane only followed by a permeator containing only a silicone (S) membrane. The asymmetric configuration usually performs better than this series configuration for the chosen range of parameters.     

A Numerical technique for solving carrier-mediated transport problems

A numerical technique and a computer program based on the method of orthogonal collocation on finite elements for solving steady state carrier-mediated transport problems is presented. The method is especially suitable in solving boundary layer type of problems as arise in carrier-mediated transport. The method uses an efficient LU decomposition of a blockdiagonal matrix. A residual criterion is used for the placement of elements which can be easily implemented using an interactive feature in the computer program. Numerical results for three different systems; a system with simple kinetics and equal diffusivities for carrier and its complex, a system with simple kinetics and unequal diffusivities and a system with complex kinetics and equal diffusivities, indicate that the method is very versatile and has very good convergence properties. The method can be used very easily for all these systems by making minor modifications to the computer program.     

Water in polymer membranes. Part III : Water sorption and pore volume in cellulose acetate films

Changes in waveguide properties of several cellulose acetate membranes and one polyimide membrane were measured as a function of their exposure to varying levels of relative humidity. The volume fraction of water in the films and the occupied pore volumes were determined from refractive index and thickness changes. The dependence of the refractive index on water absorption is related to a competition between two processes: one of filling pores with no film expansion and one of “free expansion” where the film expands to completely accommodate the added water volume. The term “pore” is taken to mean a volume with molecular and not macroscopic dimensions. The hydration properties of these dense cellulose acetate membranes were affected by degree of acetylation, casting temperatures and annealing treatments. Annealing CA398 membranes at 180°C decreased film water concentration by reducing the amount of free expansion. Annealed CA398 membranes that were tested in a reverse osmosis cell were found to have high salt rejection compared to unannealed films. The hydration characteristics of a polyimide membrane are compared to cellulose acetate membranes.     

Discrimination between the permeation of H+ and OH− ions in precipitation membranes

The unique property of precipitation membranes, which allows determination of permeation rates of single ion species by measurement of current—voltage relationships, has been used to measure the transport rates of H⁺ and Oil ions. The permeation of OH^? was found to be approximately 4-5 times higher than that of H⁺ in both BaSO₄ and calcium oxalate precipitation membranes.     

An electron microscopy investigation of tin-molybdenum oxides

Tin-molybdenum oxides prepared by the calcination of precipitates at high temperatures have been investigated by electron microscopy. The solids, which adopt the rutile-related tin(IV) oxide-type structure, are composed of crystalline particles containing planar defects, some of which have been characterized as twin boundaries. Although molybdenum has been found to be concentrated at these planar faults the examination of pure tin(IV) oxide prepared by similar methods shows that their formation is not dependent on the presence of molybdenum. It is suggested that molybdenum segregates to the twin boundaries during the calcination process to accommodate itself in more favorable coordination.     

An automated technique for thermoacoustimetry of solids

An automated technique for thermoacoustimetry of solids is described. The technique is illustrated by measurements on Green River and Kentucky oil shales.     

Bilayer-coated capsule membranes. IV. : Control of NaCl permeability by phase transition of synthetic bilayer coatings,depending on their hydrophilic head groups

Ultrathin nylon capsule membranes coated with synthetic bilayers, the hydrophilic head groups of which had cationic, anionic, zwitterionic and nonionic charges, were prepared. Permeation of NaCl trapped in the inner aqueous phase was reduced by a factor of 10-1000 relative to that of the uncoated, semipermeable capsules, and drastically changed at the phase transition temperature, T_c, of the coating bilayers, depending on the charge of their hydrophilic head group. In the case of capsules coated with positively or negatively charged bilayers, NaCl permeation was enhanced at temperatures above the T_c of the coating bilayers, as expected. On the other hand, NaCl release of capsules coated with neutral charged (nonionic and zwitterionic) bilayers was largely reduced at temperatures above the T_c. From activation energy data of Arrhenius plots, the permeation mechanism of NaCl, depending on the membrane surface charge, below and above the T_c was discussed.     

Dynamic dielectric analysis. A new thermal analysis technique

Applications of dynamic dielectric analysis (DDA) in studies on the thermal decomposition and phase transformations of solid materials are discussed. Three illustrative examples are presented for (a) a system undergoing a chemical reaction or thermal alteration on application of heat, (b) a system undergoing a crystallographic transformation and (c) detection of moisture in naturally occurring materials. The advantages derived by applying a combination of DDA and a conventional thermal analysis technique such as DTA to studies on thermal processes are demonstrated. It is shown that the information obtained from techniques such as DTA or TG is limited in scope unless used in conjunction with a method which sheds light on the mechanistic aspects of the physical or chemical process of interest.     

An automated evolved gas detection apparatus

An automated evolved gas detection apparatus is described consisting of a furnace, furnace temperature programmer, automatic sample changer, thermistor thermal conductivity detector, and a two channel recorder. The samples are automatically introduced into the furnace chamber, pyrolyzed to a preselected temperature limit, and then removed. After the system has cooled to a preselected lower temperature limit, the cycle is repeated. Eight samples can be studied in a sequential manner.