Charged ion-exchange membrane

Figure 10.5 The sodium-to-chloride ion concentration ratio inside a negatively charged ion exchange membrane containing a concentration of fixed negative groups of 3 meq/g as a function of salt concentration. At salt concentrations in the surrounding solution of less than about 1 wt% sodium chloride (0.2 meq/g), chloride ions are almost completely excluded from the membrane...

The BAHLM differs from all bulk liquid membrane systems by application of polyelectrolyte aqueous solutions as carriers and charged (ion-exchange) membranes (lEM) as barriers. As can be seen in Fig. 6.2, the physicochemical aspects of the BAHLM processes are complicated. Transport of solutes or their complexes consist of the following steps (see Fig. 6.2A) ... 

H. Jacobson, Preparation and properties of a variable charge ion-exchange membrane, J. Phys. Chem., 1962, 66, 570. 

Usually, the Donnan potential is of specific importance for polymers with a high density of fixed charges (ion exchange membranes), as polymers with fixed cationic functional groups will exclusively exchange anions and vice versa. Under these circumstances, the Donnan potential is given for a (1 1) electrolyte like NaCl (Doblhofer, 1992 Kutner,1992)... 

Membranes Ion-exchange membranes are highly swollen gels containing polymers with a fixed ionic charge. In the interstices of the polymer are mobile counterions. A schematic diagram of a cation-exchange membrane is depicted in Fig. 22-57. 

Membrane Efficiency The permselectivity of an ion-exchange membrane is the ratio of the transport of electric charge through the membrane by specific ions to the total transport of electrons. Membranes are not strictly semipermeable, for coions are not completely excluded, particularly at higher feed concentrations. For example, the Donnan eqmlibrium for a univalent salt in dilute solution is ... 

An ion-exchange membrane consists of an ionomer, which contains fixed ions that are covalently bound to the polymer backbone. It is electrically neutral because of included counterions . If water-or probably another sufficiently polar solvent - is absorbed and if the fixed and counterions can be separately solvated to an adequate degree, the counterions become mobile and the ion-exchange membrane can work as an ion conductor. Owing to the electric field of the fixed ions coions with the same charge as the fixed ions are rejected and are typically absent inside the membrane. Thus the membrane is selective for the transfer of counterions ( permselectivity = permeation selectivity, e.g. [70]). 

When ions permeate through cellulose acetate their transport pathways will tend to follow the regions where water is most concentrated. Thus they will meet and interact with the dissociated, fixed carboxylate ions. The concentrations of ions absorbed from salt solutions by swollen cellulose acetate are small for reasons connected with the low dielectric constant of the latter (2). The electro-chemical potentials of ions undergoing transport may therefore be influenced significantly by the presence of the fixed charges. Such influences are familiar with normal ion-exchange membranes. 

I assume, the membranes you are referring to are classical ion exchange membranes, the behavior of which is dictated to a large degree by charged groupings of the membrane material. The ion transport mechanism in such membranes is clearly not a carrier mechanism. 

A synthetic ion-selective (ion-exchange) membrane is a dense, nonporous, mechanically stable polymer film about 0.01 —0.04 cm thick. By nonporosity we mean the absence of pores (possibly very tortuous transmembrane channels) with a typical radius above 5 — KM (10-8 cm). Structurally the membrane material is a cross-linked polyelectrolyte. This latter is a polymer containing chemical groups that while in contact with an aqueous solvent are capable of dissociation into charges which remain fixed to the polymer core and counterions which are free to move in the solution. 

Here N(P) (cul/cm3) is the fixed charge density in the membrane, assumed to be a known function of position (negative in a cation-exchange membrane and positive in an ion-exchange one). In most modern ion-exchange membranes IV is typically higher than 102 cul/cm3 ( IV /F > 10 3mol/cm3). 

Essentially the only source of flow in a solid ion-exchange membrane (ion-exchanger) is electro-osmosis. This is a flow induced by the interaction of the electric field with the space charge distributed in the fluid present in the solid. In this respect, electro-osmosis may be regarded as a relative of electro-convection in a hydrodynamically free solution. 

In 4.4 the theory of 4.2 will be applied to study electro-diffusion of ions through a unipolar ion-exchange membrane, separating two electrolyte solutions. This will include the classical treatment of concentration polarization in a solution layer adjacent to an ion-exchange membrane under an electric current. The validity limits of this theory, set by the violations of local electro-neutrality and caused by the development of a macroscopic nonequilibrium space charge, will be indicated. (The effects of the nonequilibrium space charge are to be discussed at some length in Chapter 5.)... 

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

Consider the following simplest prototype problem for stationary electrodiffusion of a univalent symmetric electrolyte through a bipolar ion-exchange membrane with an antisymmetric piecewise constant fixed charge density XN(x). 

They are classified by membrane material into glass membrane electrodes, crystalline (or solid-state) membrane electrodes, and liquid membrane electrodes. Liquid membrane electrodes are further classified into liquid ion-exchange membrane electrodes and neutral carrier-based liquid membrane electrodes. Some examples are shown in Fig. 5.36 and Table 5.3. If the membrane is sensitive to ion i of charge Z and the activities of i in the sample and internal solutions are equal to (i) and a2(i), respectively, the membrane potential, m, which is developed across the membrane, is... 

If. an ion-exchange membrane is placed in a salt solution with the same counter-ion, some salt is absorbed and so in the membrane also some free ions occur with the same kind of charge as the fixed groups (co-ions). 

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