Characterisation of ionic membranes

Ionic membranes are characterised by the presence of charged groups. Charge is, in addition to solubility, diffusivity, pore size and pore size distribution, another principle to achieve a separation. Charged membranes or ion-exchange membranes are not only employed in electrically driven processes such as electrodialysis and membrane electrolysis. There are a number of other processes that make use of the electrical aspects at the interface membrane-solution without the employment of an external electrical potential difference. Examples of these include reverse osmosis and nanofiltration (retention of ions), microfiltration and ultrafdtration (reduction of fouling phenomena), diffusion dialysis and Donnan dialysis (combination of Dorman exclusion and diffusion) and even in gas separation and pervaporation charged membranes can be applied 

If an ionic membrane is in contact with an ionic solution a distribution of ions in 

At a distance of ic (which is referred as the Debye length) the potential has been decreased to a value of exp(-l) = 1/e = 0.37, and this value is frequently taken as the potential which gives the thickness of the double layer. The specific properties of the ionic membranes can be expressed by parameters as surface charge, zeta (Q potential, electrical resistance and ionic permeability. 

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