Ions exchange transport through membranes

When a direct current passes through an ion exchange membrane immersed in electrolyte solution, counter-ions are transported through the membrane, accompanied by water molecules, which is the electro-osmotic water. Because the liquid in a membrane pore has the same charge as the counter-ion, the liquid moves in... 

The transport numbers can be calculated using the three-conductor model. The co-ion concentration in gel areas of ion-exchange membrane is far less than in intergel areas, so it would be a valid assumption that co-ions are transported through intergel areas only. As can be seen from the picture of the three[Pg.267]

R.L. Dotson, R.W. Lynch, and G.E. Hillard, Transport of Water Molecules and Sodium Ions Through Nafion Ion Exchange Membranes, In R.S. Yeo and R.P. Buck (eds), Ion-Exchange Transport and Interfacial Properties, PV 81-2, The Electrochemcial Society, Pennington, NJ, (1981), p. 268. 

It is possible to have three-phase combinations of the above-mentioned two-phase systems gas-liquid (l)-liquid (2) (or vapor-liquid (l)-liquid (2)), liquid-ion exchange resin (l)-ion exchange resin (2), etc. We are not going to consider here such three-phase systems. The three-phase (or region) systems of interest here involve a membrane, the feed phase and the permeate phase. Some of the common three-phase systems of this type are liquid-membrane-liquid, gas-membrane-gas, liquid-membrane-gas, gas-membrane-liquid. The two fluid phases on two sides of the membrane are, in general, miscible (exceptions include the liquid-membrane-gas system, etc.). An elementary introduction to species transport through membranes that are either nonporous or porous will be provided here to facilitate development of flux expressions in a few commonly used membrane processes. 

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 ... 

In connection with the content of this section, dynamic features of ion transports through polyvinyl chloride membranes [27,28], ion-exchange resin membranes [29,30], or BLMs [31-36] have been discussed in the light of VCTTMs. For wide and pertinent applications of the VCTTM, however, further investigations have been required on the experimental and theoretical methods to analyze VCTTM quantitatively. 

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. 

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