Anionic membranes

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Preparation of aqueous HOCl substantially free of CU from either aqueous CI2 or HOCl—salt solutions has been accompHshed by electro dialysis (qv) using semipermeable membranes (130). This method has limited potential because of the unavailabihty of stable anionic membranes. 

Electrodecantation or electroconvec tion is one of several operations in which one mobile component (or several) is to be separated out from less mobile or immobile ones. The mixture is introduced between two vertical semipermeable membranes for separating cations, anion membranes are used, and vice versa. When an electric field is apphed, the charged component migrates to one or another of the membranes but since it cannot penetrate the membrane, it accumulates at the surface to form a dense concentrated layer of particles which will sink toward the bottom of the apparatus. Near the top of the apparatus immobile components will be relatively pure. Murphy [J. Electrochem. Soc., 97(11), 405 (1950)] has used silver-silver chloride electrodes in place of membranes. Frilette [J. Phys. Chem., 61, 168 (1957)], using anion membranes, partially separated and Na, ... 

With eveiy change in ion concentration, there is an electrical effect generated by an electrochemical cell. The anion membrane shown in the middle has three cells associated with it, two caused by the concentration differences in the boundaiy layers, and one resulting from the concentration difference across the membrane. In addition, there are ohmic resistances for each step, resulting from the E/I resistance through the solution, boundary layers, and the membrane. In solution, current is carried by ions, and their movement produces a fric tion effect manifested as a resistance. In practical applications, I R losses are more important than the power required to move ions to a compartment wim a higher concentration. 

This type of cell is limited by the performance of the anion-exchange membrane. The membrane can tolerate only a limited concentration of acid in the anolyte before backdiffusion of protons through the anion membrane becomes rather significant, causing a decrease in the cell s current efficiency, and acid gets into the salt stream. Unconverted salt in the product solutions can be eliminated by the use of the three compartment device, but this option substantially adds to the capital costs and operating complexity of the plant. 

Test cation Membrane anion — Membrane cation... 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

Figure 1.17 Cell voltage and power density vs current density curves for (a) 2 M methanol in 4 M NaOH solution (b) 2 M ethylene glycol in 4 M NaOH solution. Anode and cathode catalysts, laboratory-made Pt (40wt%)/C prepared via the Bdnnemann method, 2mgPtcm commercial anionic membrane, Morgane ADP from Solvay T=20°C.

Figure 4.8—Membrane and electrochemically regenerated suppressors. Two types of membrane exist those that allow the permeation of cations (H+ and Na+) and those that allow the permeation of anions (OH and X ). a) The microporous cationic membrane model is adapted to the elution of an anion. Only cations can migrate through the membrane (corresponding to a polyanionic wall that repulses the anion in the solution) b) Anionic membrane suppressor placed after a cationic column and in which ions are regenerated by the electrolysis of water. Note in both cases the counter-current movement between the eluted phase and the solution of the suppressor c) Separation of cations illustrating situation b).

Bipolar Membrane 0 Cation Membrane 0 Anion Membrane... 

In this process, dissolved electrolytes are removed by application of electromotive force across a battery of semipermeable membranes constructed from cation and anion exchange resins. The cation membrane passes only cations and the anion membrane only anions. The two kinds of membranes are stacked alternately and separated about 1mm by sheets of plastic mesh that are still provided with flow passages. When the membranes and spacers are compressed together, holes in the comers form appropriate conduits for inflow and outflow. Membranes are 0.15-0.6 mm thick. A commercial stack may contain several hundred compartments or pairs of membranes in parallel. A schematic of a stack assembly is... 

C CATION MEMBRANE A ANION MEMBRANE 0 COMPARTMENT NUMBER(SEE TE T)... 

Development of a pair of improved, commercially available cation and anion membranes having a lower electrical resistance than those formerly used but retaining the structural, mechanical, and chemical stability characteristics necessary for successful demineralization at high current densities and rates of throughput. 

Table I lists the typical physical characteristics of the new and old membranes, including a Nepton CR-61 on 9-ounce dynel which was substituted for the 9-ounce glass in production a year or two earlier. Figure 2 shows the electrical resistance of the 4-ounce and 9-ounce membranes. From Table I, it can be seen that the reduction in thickness from 30 mils to 23 mils in both the cation and anion membranes led to reduction in Mullen burst strength to 140 p.s.i. The electrical through resistance (Figure 2) was decreased to approximately two thirds for the cation membranes and about one half for the anion membrane. The Nepton CR-61 9-ounce glass membrane had a much lower resistance than the 9-ounce dynel, because of a difference in the weave pattern in the cloth, so that there was actually little if any difference between the 4-ounce dynel cation and the 9-ounce glass cation in electrical through resistance. However, the superior resistance of the dynel backing to mechanical failures leads to its selection.

A new commercially available anion membrane, Nepton AR-111A on 4-ounce dynel backing, is 23 mils thick and has an electrical through resistance in 0.01 A NaCl of 14 ohms per sq. cm., about one half that of a similar membrane 30 mils thick on 9-ounce dynel. Its Mullen burst strength of 140 p.s.i., stiffness, and resistance to bowing make it suitable for use in membrane stacks operating at pressures of 60 p.s.i. and flow velocities of 60 cm. per second. 

In summary, our findings suggest that cholesterol and certain analogs are a highly valuable neutral lipid component ( helper lipid ) for CL-DNA complexes because they facilitate endosomal escape by reducing the repulsive hydration and protrusion forces. They are thus able to lower the kinetic barrier for fusion of the cationic membranes of CL-DNA complexes with the anionic membrane of the endosome and increase TE, in addition to their beneficial effect on aM. 

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