Toshikatsu Sata

Change of anion exchange membranes in an aqueous sodium hydroxide solution at high temperature

Properties of commercial anion exchange membranes were examined after the membranes had been immersed in an aqueous sodium hydroxide solution, 3.0 to 9.0 N, of high temperature, up to 75°C. The anion exchange membranes used had N-methyl-pyridinium groups or benzyl trimethylammonium groups bonded to a styrene-based copolymer as anion exchange groups, with backing polymers. Alkali deterioration of the membranes was classified: decomposition of backing polymers (added inert polymer and backing fabric) and decomposition of anion exchange groups. N-Methyl-pyridinium groups were especially easy to decompose compared with benzyl trimethylammonium groups, and lost ion-exchange ability. Benzyl trimethylammonium groups also lost ion-exchange ability and changed partially in weakly basic anion exchange groups. The anion exchange membranes with various anion exchange groups, benzyl trimethylammonium groups, triethyl-, tri-n-propyl-, tri-n-butyl-, and 1-azonia-4-azabicyclo-[2,2,2]-octane (different hydrophobicity) were prepared from base membranes of chloromethylated polysulfone, and copolymer of chloromethylstyrene and divinylvenzene with polyethylene fabric and powder. Though the membrane based on polysulfone weakened mechanically by immersion the membrane in the alkali solution, polyethylene reinforced-membrane did not change mechanically. However, with increasing chain length of alkyl groups bonded to ammonium groups, loss of the anion exchange capacity was remarkable and 1-benzyl-azonia-4-azabicyclo-[2,2,2]-octane groups were also not stable. The most stable anion exchange groups among these were benzyl trimethylammonium groups.

Studies on anion exchange membranes having permselectivity for specific anions in electrodialysis — effect of hydrophilicity of anion exchange membranes on permselectivity of anions

Abstract Recent studies on anion exchange membranes and electrodialysis methods to permeate specific anions through the membranes are reviewed. The studies are classified: (1) to increase cross-linkage of the anion exchange membranes, (2) to form tight surface layers on the anion exchange membranes, (3) to decrease hydrophilicity of the anion exchange membranes or their surfaces by introducing specific anion exchange groups in the membranes, (4) to impregnate hydrophilic compounds in the anion exchange membranes to increase hydrophilicity of the membranes, (5) to control permselectivity of anions by photoirradiation using membranes with a photoresponsive group and (6) to control permselectivity of anions through thermally responsive anion exchange membranes with temperature. Permselectivity of specific anions through the anion exchange membranes is governed mainly by the balance of hydration energy of anions with hydrophilicity of the membranes, partially by hydrated ionic size of the anions, except the membranes having an oppositely charged layer on the membrane surface.

Studies on cation-exchange membranes having permselectivity between cations in electrodialysis

Abstract Recent studies on selective permeation of specific cations through a cation-exchange membrane in electrodialysis, modification of the membrane and electrodialysis method, are reviewed. The studies are classified (1) to prepare the composite of the cation-exchange membrane with other polymers such as conducting polymers and cationic polyelectrolytes, (2) to change cation-exchange groups from sulfonic acid groups, which are used in conventional membranes, to other groups such as phosphonic acid groups, and (3) to electrodialyze the mixed salt solution in the presence of chelate-forming agent such as poly(ethylene glycol), crown ethers, etc. Though the formation of the layers of the conducting polymers on the surface of the cation-exchange membrane is effective on selective permeation of monovalent cations to divalent cations through the membrane, mechanism of the selective permeation is different depending on species of the conducting polymers: due to sieving of sodium ions from multivalent cations by a tight polypyrrole layer and due to stronger electrostatic repulsion force of the cationic charge in a polyaniline layer to multivalent cations than to the monovalent, which is similar to the membrane having a cationic polyelectrolyte layer on the surface. When cation-exchange groups such as phosphonic acid groups, which are strongly interacted with divalent cations, are introduced in the membrane, monovalent cations selectively permeate through the membrane. However, current efficiency in electrodialysis decreases due to strong binding of divalent cations to the groups—inactivation of cation-exchange groups. Electrodialysis in the presence of chelate-forming agents is effective on the selective permeation of specific cations that have low complex formation constants with the agents through the membrane without any decrease in the current efficiency.

Studies on ion exchange membranes with permselectivity for specific ions in electrodialysis

Abstract Studies on ion exchange membranes with permselectivity for specific ions in electrodialysis are reviewed. Studies have been made with respect to: (1) control of permeselectivyt of ions with the same charge on the basis of ionic size difference, (2) separation of ions with the same charge by oppositely charged ion exchange groups on the membrane surface, and (3) control of permselectivity of ions with the same charge by specific interaction of ion exchange groups of the membrane with specific ions. Several methods are used in industrial electrodialysis to separate monovalent ions from divalent ions. However, separation of ions with the same charge and the same valence has not been achieved. It is necessary to find out new concepts for this purpose.

Composite Membranes Prepared from Cation Exchange Membranes and Polyaniline and Their Transport Properties in Electrodialysis

A cation exchange membrane was modified with polyaniline by polymerizing aniline with ammonium peroxodisulfate on the membrane surfaces, producing a membrane with polyaniline layers on both surfaces or a membrane with a single polyaniline layer on the surface. The modified membranes, composite membranes, showed sodium ion permselectivity in electrodialysis compared with divalent cations at an optimum polymerization time. The electronic conductivity of dry membranes showed a maximum (ca. 5 {times} 10{sup {minus}3} S/cm) at the same polymerization time as the time to attain a maximum value of the sodium ion permselectivity. Because emeraldine-based polyaniline is conductive and has a cationic charge, the sodium ion permselectivity is based on the difference in the electrostatic repulsion forces of the cationic charge on the membrane surface of a desalting side to divalent cations and sodium ions. In fact, the selective permeation of sodium ions appeared only when the layer faced the desalting side of the membrane, and was affected by dissociation of polyaniline. Further oxidized polyaniline, pernigraniline-based polyaniline, did not affect the permselectivity between cations, and the diffusion coefficient of neutral molecules, urea, increased with increasing polymerization time. Sodium ion permselectivity was maintained with repeated electrodialysis.

Interaction between Anionic Polyelectrolytes and Anion Exchange Membranes and Change in Membrane Properties

Abstract Electrodialytic transport properties of anion exchange membranes were measured after formation of anionic polyelectrolyte layers on the membrane surfaces: relative transport number of various anions to chloride ions, current efficiency and apparent diffusion coefficients of neutral molecules. The anionic polyelectrolyte layers were formed by immersing the membrane into an aqueous solution of polycondensation product of sodium naphthalene sulfonate and formaldehyde or polystyrene sulfonic acid. The change in the relative transport number between anions was remarkable in the anion exchange membrane with high ion exchange capacity by forming the layer. Results were: the relative transport number of sulfate ions to chloride ions decreased and those of nitrate ions to chloride ions, fluoride ions to chloride ions and bromide ions to chloride ions increased compared with the corresponding membrane. Although the apparent diffusion coefficient of neutral molecules suggested clogging of the membrane pores by the polyelectrolyte, anions with higher hydrated ionic diameter were able to permeate through the membrane easily. This means that difference of electrostatic repulsion force against two anions is effective on the change in the relative transport number of anions.

Permselectivity between two anions in anion exchange membranes crosslinked with various diamines in electrodialysis

A membranous copolymer crosslinked with divinylbenzene reacted with N,N,N′,N′-tetra-methylethylenediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine, and N,N,N′,N′-tetramethyl-1,6-hexanediamine to prepare highly crosslinked anion exchange membranes. More than 80% of both tertiary amino groups of the diamines reacted with chloromethyl groups of the membrane to form crosslinkage. After formation of the high crosslinkage of the membrane was confirmed with dialysis of a neutral molecule, electrochemical properties of the obtained membranes (mainly, relative transport number between two anions in electrodialysis) were evaluated: nitrate ions to chloride ions, sulfate ions to chloride ions, fluoride ions to chloride ions, and bromide ions to chloride ions. Though larger anions, in general, were difficult to permeate through the membranes due to high crosslinkage, the number of methylene groups of the diamines (which means the increase in hydrophobicity of anion exchange groups) also affected the relative transport number between two anions. The lower the hydration of anions, the higher the relative transport number of the anions through the membranes with the hydrophobic anion exchange groups.

Preparation and properties of anion exchange membranes having pyridinium or pyridinium derivatives as anion exchange groups

Anion exchange membranes with pyridinum groups and various pyridinium derivative groups were prepared from a copolymer membrane composed of chloromethylstyrene and divinylbenzene, and pyridine and pyridine derivatives. The anion exchange membranes obtained showed excellent electrochemical properties in electrodialysis. The transport numbers of sulfate ions, bromide ions, nitrate ions, and fluoride ions relative to chloride ions were evaluated in connection with the species of a substituent and the position of the substituent in the pyridinium groups. In general, when a hydrophilic substituent (methanol groups) existed at the 2-position of the pyridinium groups, nitrate ions and bromide ions, which are less hydrated, permeated through the membranes with difficulty, and sulfate ions permeated selectively through the membranes. On the other hand, when hydrophobic groups, for example, ethyl groups, existed at the 2-position of the pyridinium groups, bromide ions and nitrate ionspermeated selectively through the membranes and fluoride ions had difficulty permeating through the membranes. The carbon number of the alkyl chain of 4-alkyl pyridinium groups also affected permeation of nitrate ions and bromide ions due to the change in hydrophilicity of the membranes. Though the hydration of the anions and the species of the substituent at the 2-position of the pyridinium groups were related to selective permeation of the anion through the membranes, permeation of sulfate ions was not as sensitive to the hydrophilicity of the membranes.

Modification of the transport properties of ion exchange membranes. XII. Ionic composition in cation exchange membranes with and without a cationic polyelectrolyte layer at equilibrium and during electrodialysis

Abstract It is considered that the mechanism of specific permselectivity of a cation exchange membrane with a cationic polyelectrolyte layer is due to the different electrostatic repulsion forces of ions relative to the cationic charge on the membrane surface. In order to prove this experimentally, the ionic composition of a cation exchange membrane with a polyethylenimine layer that had been formed by acid-amide bonding and that of a cation exchange membrane without such a layer were determined by using a 1:1 solution of calcium chloride and sodium chloride of various concentrations. p]Donnan adsorbed salt (Δm), ratio of calcium ions and sodium ions in the Δm (DNaCa), ratio of ion-exchanged calcium ions to ion-exchanged sodium ions (KNaCa) and selectivity coefficients were measured when both membranes had been equilibrated with the solutions and also used in electrodialysis. In the case of equilibrium, the ionic composition of both membranes was almost the same except in the low concentration range. However, in the case of electrodialysis, the ionic composition of the membrane with the layer was completely different from that of the membrane without the layer. The content of calcium ions in the membrane with the layer decreased markedly in comparison with that of the membrane without the layer. The effect of current density on the ionic composition in the membrane also showed the same behavior.

Transport properties of ion-exchange membranes in the presence of surface active agents☆

Abstract Transport properties of ion-exchange membranes in the presence of various surface-active agents were observed. Measurements were carried out by the following two methods: (1) electrodialysis using a cation exchange membrane which has been adsorbed or ion exchanged with hexadecyl pyridinium chloride; (2) electrodialysis of a mixed salt solution containing surface-active agents using anion or cation exchange membranes. The transport properties measured in this report were: the relative transport number of two ionic species with the same sign ( P M 2 M 1 ), the current efficiency, the voltage drop by electric resistance of the membrane during the electrodialysis, and the current-voltage curve. P M 2 M 1 of cation exchange membranes was changed remarkably by adsorption of or ion exchange with the cationic surface-active agent above a certain concentration, but was not changed by adsorption of a nonionic or anionic surface-active agent. Similarly, P M 2 M 1 of anion exchange membrane changed remarkably by adsorption of or ion exchange with an anionic surface-active agent. The cation exchange membrane in the presence of cationic surface-active agents and the anion exchange membrane in the presence of anionic surface-active agents were preferentially permeable to monovalent ions over multivalent ions. The current-voltage curves were characteristic of a bipolar ion-exchange membrane. These membranes were preferentially permeable to monovalent ions because the electrostatic repulsion force between the multivalent ions and the surface charge of the membrane is larger than that between the monovalent ion and the surface charge of the membrane. The current efficiency of cation exchange membrane decreased slightly with an increase in the amount of dodecyl pyridinium chloride adsorbed or ion exchanged. However, the current efficiency decreased or increased depending on the species of surface-active agent and ion-exchange membrane. The voltage drop due to the electrical resistance of the cation exchange membrane increased as the electrodialysis proceeded and also as the concentration of cationic surface-active agent was increased.