Permselectivity exchange membranes

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 ... 

Metal nanotube membranes with electrochemically suitable ion-transport selectivity, which can be reversibly switched between cation-permeable and anion-permselective states, have been reported. These membranes can be viewed as universal ion-exchange membranes. Gold nanotube molecular filtration membranes have been made for the separation of small molecules (< 400 Da) on the basis of molecular size, eg. separation of pyridine from quinine (Jirage and Martin, 1999). 

Fig. 6.1 Distribution of cations and anions in pores of a cation-exchanger membrane depends on pore radius which decreases in the sequence A- B— C. In case C the membrane becomes permselective. (According to K. Sollner)... 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

The PVA/PSSNa membranes evidence a high permselectivity, comparable with the one of commercial ion exchange membrane as it can see in table 14, where were presented the permeability coefficient (P) and the ratio P to D (diffusion coefficient) that express the effect of porosity and of the electrolyte exclusion. 

In 1973, Dupont began to commercialize their first perfluorosulfonic add cation exchange membrane, Nafion. Since then until now, Nafion has been attracting much attention because of its superb chemical and thermal stability, high ionic conductivity, excellent permselectivity and good mechanical strength. Many approaches have been proposed to use this unique material as a modifier of electrochemical electrode surfaces. 

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]). 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Equations (1.52), (1.53) are the quantitative expressions of the previous statements regarding permselectivity of an ion-exchange membrane. Of course, the treatment that led to (1.52), (1.53) is admissible only in the vicinity of equilibrium. Away from equilibrium treatment of the full nonlinear formulation of type (1.37)-(1.42) is required. Examples of such treatments with and without the LEN approximation will be presented in Chapters 4 and 5. 

In this section we shall consider the simplest model problem for the locally electro-neutral stationary concentration polarization at an ideally permselective uniform interface. The main features of CP will be traced through this example, including the breakdown of the local electro-neutrality approximation. Furthermore, we shall apply the scheme of 4.2 to investigate the effect of CP upon the counterion selectivity of an ion-exchange membrane in a way that is typical of many membrane studies. Finally, at the end of this section we shall consider briefly CP at an electrically inhomogeneous interface (the case relevant for many synthetic membranes). It will be shown that the concentration and the electric potential fields, developing in the course of CP at such an interface, are incompatible with mechanical equilibrium in the liquid electrolyte, that is, a convection (electroconvection) is bound to arise. 

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 ... 

Another prototype of an ideally cation-permselective interface would be a cation-exchange membrane (C-membrane). Most practically employed C-membranes are extremely permselective, so that their polarization curves would be expected to coincide with those at electrodes (given the same... 

For the sake of simplicity, we shall assume again an ideal permselectivity of the ion-exchange membrane. As mentioned before, this assumption is practically justified, since the co-ion transport numbers in modern monopolar ion-exchange membranes, employed, e.g., in electrodialysis, are limited to a few percent. 

Formulation. Consider two unity thick diffusion layers of a mixture of 1, 1-, and 1, z-valent3 electrolytes with a common anion, adjacent to a planar ideally permselective cation-exchange membrane. Direct the axis x normally to the membrane and let x = 0 coincide with the outer boundary of the diffusion layer. The diffusion layers will thus be located at 0 < x < 1 and 1 + Aelectro-diffusional transfer of ions across the membrane and the diffusion layers is... 

CP of a binary electrolyte at an inhomogeneous permselective interface. As mentioned before, experiments on ion-exchange membranes point at an electric inhomogeneity of their surface on a mi-... 

It is possible now to explain a number of important membrane properties in a qualitative way. The permselectivity i.e. the increased transference number of cations in a cation-exchange membrane respectively of anions in an anion-exchange membrane compared with the free solutions is due to the fact that the number of counterions is much higher than the number of co-ions. 

In 1939, Manegold and Kalauch assembled a three-compartment ED apparatus consisting of a permselective anion-exchange membrane and a cation-exchange one. It was, however, only in the early 1950s that the manufacture of selective membranes from ion exchangers allowed the multicompartment electrodialysers to be assembled (Shaposhnik and Kesore, 1997). 

The most desired properties of ion-exchange membranes are high permselectivity, low electrical resistance, good mechanical and form stability, and high chemical and thermal stability. In addition to these properties bipolar membranes should have high catalytic water dissociation rates. 

The membrane permselectivity is an important parameter for determining the performance of a membrane in a certain ion-exchange membrane process. It describes the degree to which a membrane passes an ion of one charge and retains an ion of the opposite charge. The permselectivity of cation- and anion-exchange membranes can be defined by the following relations [4] ... 

Here, T is the permselectivity of a membrane, T is the transport number, the superscripts cm and am refer to cation- and anion-exchange membranes, and the subscripts c and a refer to cation and anion, respectively. 

An ideal permselective cation-exchange membrane would transmit positively charged ions only, that is, for a transport number of a counterion in a cation-exchange membrane is T m = 1 and the permselectivity of the membrane is xFcm = 1. The permselectivity approaches zero when the transport number within the membrane is identical to that in the electrolyte solution, that is, for T = Tc is xFcm = 0. For the anion-exchange membrane the corresponding relation holds. 

The permselectivity of an ion-exchange membrane for different counterions is determined by the concentration and the mobility of the different ions in the membrane as indicated earlier. The concentration of the different counterions in... 

Here, E, is the current utilization, / is the membrane permselectivity, T is the transport number, n is the number of cell pairs in the stack, Vw is the partial molar volume of water, and C is the concentration, a, c, s and w refer to anion, cation, solution and water, respectively, and the superscripts cm, am, c, and d refer to cation-exchange membrane, anion-exchange membrane, concentrate and diluate. 

Permselectivity — According to IUPAC A term used to define the preferential permeation of certain ionic species through - ion-exchange membranes. (See also surface-modified electrodes). Discrimination is based on the size or ion charge of the ionic species studied. Permselectivity prevents electrode surface fouling by sample matrix components, e.g., by proteins in biological fluids. 

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